What is an variable frequency ac drive and how does it work?

An variable frequency drive is a device that is used to control the speed of an electrical motor, either an induction motor or a synchronous motor. AC drives are also known by various other names such as adjustable speed drive (ASD) or adjustable frequency drive (AFD) or variable frequency drive (VFD) or variable speed drive (VSD) or frequency converter (FC) or (FI) or AC drive or ac motor drive.

The word "drive" is used loosely in the industry. It seems that people involved primarily in the world of gear boxes and pulleys refer to any collection of mechanical and electro-mechanical components, which when connected together will move a load, as a "drive". When speaking to these people, an AC drive may be considered by them as the variable frequency inverter and motor combination. It may even include the motor's pulley - I am not sure.

People in the electrical field and electrical suppliers usually refer to a variable frequency inverter unit alone, or an SCR power module alone (when discussing DC drives) as the "drive" and the motor as the "motor".

Manufacturers of variable frequency drives (VFD) used to refer to the drive as just that, a "variable frequency drive". More manufacturers are referring to their drive as an "adjustable speed AC drive". To make matters worse when a motor is included in the package it may be referred to as an "adjustable speed AC drive system".

A variable frequency drive is an adjustable speed drive. Adjustable speed drives include all types; mechanical and electrical. Now is it clear? Don't worry about it. It's not clear to anyone. As you read on, when I refer to the "drive" I  am referring to the variable frequency inverter alone.

The first electrical AC motor was designed in 1899. Electrical motors convert electric energy into mechanical energy by electromagnetic induction. These motors are characterized by:

fixed speed, determined by the frequency of the power supply

fixed torque

Obviously, a fixed speed is not suitable for all processes in all circumstances; thus, the need for adjusting the speed according to need.

Industrial machinery is often driven by electrical motors that have provisions for speed adjustment. Such motors are simply larger, more powerful versions of those driving familiar appliances such as food blenders or electric drills. These motors normally operate at a fixed speed.

If speed control is required, that controller is called a (variable speed, variable frequency) AC drive. AC drives are used in a wide variety of industrial applications. To give an easy example, AC drives are often used with fans to provide adjustable airflow in large heating and air conditioning systems. The flow of water and chemicals in industrial processes is often controlled by adjusting the speed of pumps.

However, variable speed AC drive are commonly used in more complex and difficult environments such as water and wastewater processing, paper mills, tunnel boring, oil drilling platforms or mining.

The technology

The speed is controlled by changing the frequency of the electrical supply to the motor. The 3-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field. An AC drive convert the frequency of the network to anything between 0 to 300 Hz or even higher, and thus controls the speed of motor proportionally to the frequency.

1. Rectifier unit

The frequency inverter is supplied by the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the frequency inverter can accelerate and run the motor by taking energy from the network. If bidirectional, the frequency inverter can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.

2. DC circuit

The DC circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors.

3. Inverter unit

The inverter unit takes the electrical energy from the DC circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed 3-phase AC voltage output for the motor. The frequency can be adjusted to match the need of the process. The higher the frequency of the output voltage is, the higher the speed of the motor, and thus, the output of the process.

Figure 1: The main components of an AC drive: rectifier, DC circuit and inverter

The benefits

The types of motors that AC drives control are normally operating at constant speed. Enabling the user to control the speed of motor potentially gives him various benefits in terms of process control, system stress and energy savings.

Adjusting speed as a means of controlling a process

Smoother operation

Acceleration control

Different operating speed for each process

Compensate for changing process variables

Allow slow operation for setup purposes

Adjust the rate of production

Allow accurate positioning

Control torque or tension

System stress

Reducing the start-up current, which allows use of smaller fuses and supply connections and reduces peak loads on the electrical network

Reducing the mechanical shock in start and stop situations

Saving energy by using AC drives

An AC drive often uses less energy than an alternative fixed speed mode of operation. Fans and pumps are the most common energy saving applications. In these applications, energy savings are typically 20-50%.

When a fan is driven by a fixed speed motor, the airflow may sometimes be higher than it needs to be. Airflow can be regulated by using a damper to restrict the flow, but it is more efficient to regulate the airflow by regulating the speed of the motor.

Case study: AC drives in HVAC applications

40% of all energy in Europe and North America is consumed in buildings. The biggest share of this energy is consumed in heating, ventilation and air conditioning (HVAC) applications.

With the rising energy cost and concerns about the CO2 levels and global warming, it is crucial to use all means available to reduce the energy consumption in HVAC applications. The savings potential is big.

The key thing is to start looking more at lifetime costs of HVAC system, where energy cost plays a big role, rather than the initial investment in HVAC system. To give an example, 90% of the lifetime costs of the pump or fan is energy.

The majority of HVAC applications where AC drives are used are

fans

pumps

compressors.

Figure 2: The main components of an HVAC system: air circulation, water cooling circuits and water heating circuits.

1. Fans and pumps

Using an variable frequency drive to control the fan or pump output rather than using dampers, vanes, valves or on/off control brings substantial energy savings, if the required output is less than nominal most of the time.

The variable frequency drive controls the speed of the pump and fan by changing the electrical energy supplied rather than damping the air- or water flow. It is like reducing the speed of a car by pressing less on the accelerator instead of using the brake to slow down the speed. The payback time of an AC drive is typically one year or less.

Graph 1: Electrical power consumed by pump at partial loads is significantly less with an AC drive than with valve or on/off control.

Other benefits of using AC drive to control the speed of fan or pump are:

Smooth ramp up and ramp down causes less stress to the mechanics of fans and pumps and to air ducts and water piping

Slowing down the speed rather than damping the output will result in lower noise levels

Tuning the HVAC system during and after the commissioning is easier when the flexibility of an AC drive is used

2. Compressors

Compressors in HVAC are often used in chillers for cooling water, which again is used for cooling air. Utilising AC drives in compressor applications will potentially bring energy savings compared to on/off control.

Energy savings are achieved by optimising the system setup of compressor, chilled water circulation and condenser water circulation. The optimum set point for chilled water temperature and condenser water temperature is based on outdoor and indoor temperature and humidity.

The energy savings are most effectively achieved by tuning the system rather than optimizing individual functions. The variable frequency ac drive gives the flexibility of tuning the setup of the system to operate in the most energy efficient operational point.

Other benefits

Reduced number of starts and stops reduces the wear of the compressor

The piping and mechanics are stressed less in ramp up or –down situations

Reduced noise level in low load situations

Possibility to use high speed compressors

How does a variable speed drive work?

Speed Control

Variable speed ac drives for electric motors are able to accurately control the speed and power applied to the motor. The need for precision processing in manufacturing led to the creation and development of variable frequency drives for industry. Sophisticated automation lines for making automobiles utilize hundreds of these types of motor drives in one facility. Variable frequency motor drives have the capability of being electrically tied together so they can operate in concert with one another to move mechanical devices at a precision rate.All Voltage Equal

The pairing of a frequency drive with a motor begins with the voltage of the motor. Although some frequency drives can convert voltage to a lower or higher rating, the most efficient is when the motor and variable frequency drive is of the same voltage. The most common voltage for industry is a three-phase power at 480 volts alternating current (VAC). This type of voltage is able to efficiently send power to an appropriate motor and deliver the horsepower or torque that is required. Torque is a value that designates the amount of "push" or "force" a motor can deliver. Variable frequency drives allow the usage of this torque to deliver power to the electric motor and yet control the speed of that motor.

Hertz Frequency

All electrical power that is sent from the utilities runs at a certain frequency. This frequency is called a Hertz. Most generally, the frequency that is delivered over the large power lines is generated at the power companies in the form of 60 cycles per second, or 60 Hertz (Hz). A Hertz is best described as a full cycle or Sine wave of electrical power. This is where the alternating current goes from zero to a maximum positive voltage, back to zero again and then to a maximum negative voltage and then back to zero. Think of this like a clock face, where 12 o'clock is the beginning zero point of the Sine wave or a single Hertz. At 3 o'clock would be the maximum positive voltage. The 6 o'clock mark would be zero again. At 9 o'clock would be the maximum negative voltage and then back to 12 o'clock is again at zero. Now imagine this clock spinning at a speed where the large dial would make this cycle 60 times of every second. That is the physical characteristic of the electrical power that comes from the wall outlet. All electrical power is generated in this fashion.

Controlling the Frequency

Variable frequency drive take advantage of this generation characteristic of electrical power by being able to slow down or speed up the amount of times that clock face would spin in 1 second. Through a series of capacitors, diodes and an imbedded computer chip, the frequency drive is able to moderate speed while still delivering the full torque of power to the motor. The drive is able to not only vary the amount of frequency, but can also regulate the voltage that is being sent to the motor. It does this by delivering a full current to the motor. The current has a direct correlation to the amount of power that is delivered to the motor. The variable frequency drive may have special software that allows the controller to be tied to main computer. This computer can control a multitude of drives for a factory environment.

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and-like their DC counterparts-control speed, torque, and horsepower.

The first stage of a variable speed drive, AC Drive, or VFD, is the frequency converter. The converter is comprised of six diodes, which are similar to check valves used in plumbing systems. They allow current to flow in only one direction; the direction shown by the arrow in the diode symbol. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing systems) is more positive than B or C phase voltages, then that diode will open and allow current to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close. The same is true for the 3 diodes on the negative side of the bus. Thus, we get six current “pulses” as each diode opens and closes. This is called a “six-pulse VFD”, which is the standard configuration for current variable speed drives.

Let's take a brief look at a drive application. In Fig. 1, you can see a simple application with a fixed speed fan using a motor starter. You could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since you can operate the fan at any speed below its maximum, you can vary airflow by controlling the motor speed instead of the air outlet damper.

Figure 1, Fixed Speed Fan Application

A drive can control two main elements of a 3-phase induction motor: speed and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motors. Fig. 2 shows the construction of an induction motor. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. A motor contains pole pairs. These are iron pieces in the stator, wound in a specific pattern to provide a north to south magnetic field.

Figure 2, Basic Induction Motor Construction

Figure 3, Operating Principles of Induction Motor

With one pole pair isolated in a motor, the rotor (shaft) rotates at a specific speed: the base speed. The number of poles and the frequency applied determine this speed (Fig. 4). This formula includes an effect called "slip." Slip is the difference between the rotor speed and the rotating magnetic field in the stator. When a magnetic field passes through the conductors of the rotor, the rotor takes on magnetic fields of its own. These rotor magnetic fields will try to catch up to the rotating fields of the stator. However, it never does -- this difference is slip. Think of slip as the distance between the greyhounds and the hare they are chasing around the track. As long as they don't catch up to the hare, they will continue to revolve around the track. Slip is what allows a motor to turn. 

Motor Slip:
	Shaft Speed =	120 X FP	- Slip
	Slip for NEMA B Motor = 3 to 5% of Base Speed which is 1800 RPM at Full Load
	F = Frequency applied to the motor P = Number of motor poles
Example:
	Shaft Speed =	120 X 60 Hz4	- Slip
Figure 4, Induction Motor Slip Calculation

We can conveniently adjust the speed of a motor by changing the frequency applied to the motor. You could adjust motor speed by adjusting the number of poles, but this is a physical change to the motor. It would require rewinding, and result in a step change to the speed. So, for convenience, cost-efficiency, and precision, we change the frequency. Fig. 5 shows the torque-developing characteristic of every motor: the Volts per Hertz ratio (V/Hz). We change this ratio to change motor torque. An induction motor connected to a 460V, 60 Hz source has a ratio of 7.67. As long as this ratio stays in proportion, the motor will develop rated torque. A drive provides many different frequency outputs. At any given frequency output of the drive, you get a new torque curve.

Figure 5, Volts/Hertz Ratio

Why should I using variable frequency drive?

A variable speed drive controls the speed of an ac motor, which provides flexibility to the process since speed can be changed easily for process optimization. It takes the fixed power supplied to it and converts it into a variable frequency and variable voltage source which then feeds a motor. This allows the drive to control the speed and torque the motor produces.  

A variable speed drive may enhance the user’s profitability by improving the process, which in turn produces a fast return on investment (ROI). Process improvements may come from better:

Speed control;

Flow control;

Pressure control;

Temperature control;

Tension control;

Torque control;

Monitoring quality; and

Acceleration/deceleration control.

Many applications that use ac motors would benefit from the use of such drives because they can also reduce operating costs while improving the process. Reduced costs come from:

Increased system reliability;

Reduced downtime;

Reduced equipment setup time;

Energy savings;

Lower maintenance;

Smoother operation—less wear and tear; and

Power factor control.

The net result of these improvements is increased profitability.

Reduce Energy Consumption and Energy Costs

If you have an application that does not need to be run at full speed, then you can cut down energy costs by controlling the motor with a variable frequency drive, which is one of the benefits of ac drives. variable frequency drives allow you to match the speed of the motor-driven equipment to the load requirement. There is no other method of AC electric motor control that allows you to accomplish this.

Electric motor systems are responsible for more than 65% of the power consumption in industry today. Optimizing motor control systems by installing or upgrading to variable frequency drive can reduce energy consumption in your facility by as much as 70%. Additionally, the utilization of variable frequency drive improves product quality, and reduces production costs. Combining energy efficiency tax incentives, and utility rebates, returns on investment for variable frequency drive installations can be as little as 6 months.

Increase Production Through Tighter Process Control

By operating your motors at the most efficient speed for your application, fewer mistakes will occur, and thus, production levels will increase, which earns your company higher revenues. On conveyors and belts you eliminate jerks on start-up allowing high through put.

Extend Equipment Life and Reduce Maintenance

Your equipment will last longer and will have less downtime due to maintenance when it’s controlled by variable frequency drive  ensuring optimal motor application speed. Because of the variable frequency drive optimal control of the motor’s frequency and voltage, the variable frequency drive will offer better protection for your motor from issues such as electro thermal overloads, phase protection, under voltage, overvoltage, etc.. When you start a load with a variable frequency drive you will not subject the motor or driven load to the “instant shock” of across the line starting, but can start smoothly, thereby eliminating belt, gear and bearing wear. It also is an excellent way to reduce and/or eliminate water hammer since we can have smooth acceleration and deceleration cycles.

WHAT TYPES OF SYSTEMS ARE BEST SUITED TO A VSD?

Systems that require control of flow or pressure are most suited to the use of a VSD. This is because the consumed power is roughly proportional to the cube of the flow, or speed of the motor . Such applications may include:

Fans and pumps: In applications where the flow of fluid is variable, considerable energy savings can be achieved by replacing existing throttling valves and dampers with VSDs.

Conveyors: For conveyors with varying speed or with varying material flow, a VSD can adjust to the changing load requirements.

Compressors and chillers: In the same manner as fans and pumps, compressors can take advantage of the energy saving that is achieved by varying the flow with a VSD.

ARE SOME SYSTEMS UNSUITED TO A VSD?

Systems with a high static head are unsuited to the use of a VSD. Static head is a measure of the pressure needed to make a fluid flow. This is a constant pressure and is independent of motor speed. When a high motor speed is needed to overcome the static head, the motor speed is no longer proportional to the flow rate. A small reduction in motor speed could result in a large drop in flow and a big reduction in machine efficiency.

In applications where the motor load remains more or less constant, a VSD is unlikely to achieve a significant saving, as the opportunity to reduce power is diminished.

WILL YOU BE ABLE TO RETROFIT A VSD TO AN EXISTING MOTOR?

An existing AC induction motor will be compatible with the application of a VSD. The viability of retrofitting the VSD will depend on:

 the cost

the space available

the suitability of the application to variable speed control

You should also consider the additional energy savings that could be achieved by replacing the existing motor with an equivalent energy-efficient motor.

HOW DO YOU MATCH A VSD WITH AN EXISTING MOTOR?

It’s best to start with the motor and ensure it is suited to its task, i.e. that it’s not overloaded or oversized.

The first step is to define the operating profile of the load. This involves measuring the required torque and current consumption under all operating conditions.

Then, this profile needs to be examined to ensure, first, that the motor is correctly sized; secondly, that the installation is suited to the application of a VSD. The VSD must be sized to the motor load, based on the maximum current requirements under peak torque demands. A VSD cannot be sized using the kilowatt rating of the motor alone.

Suppliers of VSDs included in the ACA specified products list should help you to choose the correct product.

HOW DO YOU PERFORM SIMPLE SAVINGS AND PAYBACK CALCULATIONS?

To perform a useful payback calculation, application-specific data must be gathered from the installation. The load profile under all conditions must be measured or estimated.  As each motor application is unique, only a costing exercise will reveal the potential savings of installing a VSD in a system.

For a simplified example of a payback calculation, see Example of a VSD payback calculation. Note that participation in the ACA scheme further reduces the payback period.

SHOULD YOU BE CONCERNED ABOUT HARMONICS?

All VSDs produce harmonics from pulsed current being drawn from the supply. Harmonics are multiples of the 50Hz supply frequency which become superimposed on the supply system. Harmonics can cause problems such as equipment and capacitor overheating, voltage distortion and equipment malfunction. However, the use of filters, sometimes available as part of a VSD, helps to minimise the effects of harmonics.

HOW CAN YOU FURTHER IMPROVE THE ENERGY EFFICIENCY OF YOUR DRIVE INSTALLATION?

If practical, motors and drives should be switched off when not in use.

For all new installations, energy-efficient motors should be used. The combination of a VSD and an energy-efficient motor ensures an economical and future-proof installation.

For retrofit applications, you should consider replacing standard-efficiency motors with equivalent energy-efficient ones. Energy-efficient motors are also included in the ACA, and those that have qualified included on the specified list.

The mechanical efficiency of the driven equipment and transmission system (i.e. pumps, fans, belts, etc) directly influences the efficiency of the overall drive system. It’s important that this equipment be regularly maintained and lubricated and that wear-and-tear of mechanical parts be monitored.

EXAMPLE OF A VSD PAYBACK CALCULATION

Consider an 11kW 2-pole EFF1 motor driving a product transfer fan for a milk powder processing plant. The fan motor operates 6,000 hours per annum. Air flow is controlled via a manual damper set to 80% open. The motor efficiency is 90.5%.

From the curve representing the system in Figure 2 below, we see how the damper setting reduces the input power requirement by a factor of about 0.9. A cost of electricity of €0.14 per kWh is assumed.

 

Figure 2: Damper vs. VSD control efficiencies (courtesy of The Carbon Trust)

Without VSD

The annual cost of running the motor without VSD is as follows:

ANNUAL RUNNING COST WITHOUT VSD = Input power * Input power reduction factor * Run hours * Electrical cost   =    (11kW/0.905) * (0.9) * (6000) * (€0.14)   =    €9,188.95

With VSD

If the damper is replaced with a VSD, the curve in Figure 2 shows that the input power is now reduced to 58% of maximum when running at 80% of full load. If we assume that the combined efficiency of the motor and the VSD is now 86%, then the annual running cost of the motor combined with VSD can be calculated as follows: 

ANNUAL RUNNING COST WITH VSD = Input power * Input power reduction factor * Run hours * Electrical cost   =     (11/0.905) * (0.58) * (6000) * (0.14)   =    €6,231.63

Thus the annual cost savings achieved by replacing the damper with the VSD are as follows:

Cost savings with VSD = €9,188.95 - €6,231.63 = €2,957.32 p.a.

If we assume a cost of €6,000 to supply and install the VSD, taking support from the ACA scheme into account, this gives us the following payback period:

Payback period = €6,000 / €2,957.32 = 2.03 years

In this simplified example, a payback of two years has been calculated. The load profile has been simplified to a constant 80% of full load. In practice, a more detailed examination of a varying load profile would be needed to calculate the true annual running costs.

Variable speed drive for winding solution to replace torque motor

Technical Requirements:
The features torque motor: softer mechanical feature. When the setting torque is greater than the load torque, it will accelerate automatically, vice versa, when the setting torque is less than the load torque, it will decelerate or reverse, and when the setting toque is equal to the load torque, the rotational speed of motor remains unchanged.

The torque output of torque motor, set by the controller, is applicable to constant tension wind-out and winding control, such as textile, wire, cable, metal processing, papermaking, rubber, plastic and printing machine etc.
The general variable speed drive can only be used for ac motor open loop speed regulation, while the motor torque is out of control, so it doesn't possess the function of torque motor.

The current vector control variable speed drive: Through complicated and accurate vector control algorithm, torque current closed loop control can be achieved for directly setting the output torque of regular induction motor control, replacing the above functions of torque motor and even obtaining higher performance.

Torque Motor and AC Induction Motor


Torque Wind-up of Film Printing Machine



Variable speed drive system configuration:
1. Regular 3-phase induction motor: Select the number of poles and power of 3-phase induction motor based on the needs for the maximum revolution and torque in applications.
2. Open loop variable frequency drive with current vector control, built-in braking unit, braking resistor selected according to motor power.
3. Regular induction motor acts constant torque control. The tension of motor can be directly set and controlled to replace torque motor without speed feedback of encoder. 

Variable speed drive system features:
1. No need for external tension detection mechanism and speed feedback encoder, simple system structure, easy for installation and commissioning.
2. The torque motor system is easily to be damaged by over-heat or improper operation. Present solutions can effectively help to protect the motor and easy for maintenance. The system is safe and stable with a long life-span.
3. The control accuracy of torque is higher than that of torque motor.
4. The taper torque can be set as auto-control mode.
5. With static friction torque compensation and large inertia acceleration torque compensation, the higher winding control performance, easy for operation.

Variable Speed Drive for for Constant Pressure Water Supply

In water supply system, constant pressure water supply refers to the control mode which controls the outlet pressure within the setting value by detecting the pipe and network pressure when the water amount in the pipe and network changes. Usually, when selecting water pump, its rated flow is bigger than the actual flow, and the flow varies a lot when in use, for example, in buildings water supply, the water consumption between day and night varies a lot, valve regulation mode is commonly used. Even the method is simple, however, it is regulated by increasing of resistance manually that results in a lot of waste in electricity and high maintenance required. variable speed ac drive introduced here with built-in PID to perform the automatic constant pressure water supply for saving energy.

VSD for constant pressure water supply wiring:


As illustrated on the wiring diagram, connect with air switch, leakage switch and power, after finishing wiring detection, the air switch is turned on, variable frequency drive is connected, 0.0 shows on the LED.

When the power is turned off, re-connect motor, start/stop switch, remote manometer, and current-limiting resistor, and check if the variable frequency drive and motor is well grounded.

YTZ-150 potentiometer remote manometer is selected, and installed on the outlet pipe of the water pump. The manometer is applicable to regular manometer working condition, which could not only directly measure the value of pressure, but also outputs corresponding electrical signal to the remote controller. There are red, yellow and blue 3 extension wires of manometer.

The Electric Parameter of Manometer:
1. Full scale of resistor: 400Ω(Blue and red)
2. Initial resistor value at zero pressure: ≤20Ω (Yellow and red)
3. Resistor volume at full-scale upper limited pressure: ≤360Ω (Yellow and red)
4. Terminal block external voltage: ≤6V (Blue and red)

Open Loop Commissioning:
Check the wiring is well connected, turn on the air switch and leakage switch, the variable speed drive is electrified 0.0 shows LED. Press JOG button to check the rotation direction of water pump, if reverse, change the phase sequence of motor.

Press RUN button, the indicator turns green, turn the knob on the keypad clockwise.

The output frequency goes up, observe the indicator on the manometer, and check the voltage value between terminals VF and GND on the variable frequency drive by using multi-meter.

The voltage increases with the increase of output frequency and pressure, the feedback voltage between VF and GND increases, record the feedback voltage value corresponding to the expected constant pressure, for example: 5kgs. Press STOP button, the VFD drive decelerates to stop.

VSD parameters setting:


Closed Loop Variable Speed Constant Pressure Running 
Switch-on the start/stop switch, the indicator is on, when the output frequency changes from 0 to 30 Hz, auto-regulation based on the actual water consumption to ensure the outlet pressure is 5kg. Increase the parameter of F7.04, the outlet pressure goes up, vice versa, the pressure goes down.

Variable speed ac drive for air compressor

Air compressor introduction:

Air compressor is a kind of equipment that uses a motor to compress air in the compressing chamber and brings the compressed air to a certain pressure. As a basic industrial equipment, the air compressor has been widely used in almost all industrial fields such as metallurgy, machine manufacturing, mine, electric power, textile, petrochemical, light textile, etc. The electric energy consumption of air compressors accounts for 15% of all large scale industrial equipments (fan, pump, boiler, air compressor, etc.). Most air compressors have obvious inherent technical weaknesses due to their structure and principle. When the output pressure exceeds a certain value, it automatically opens the relief valve and idles the asynchronous motor. This seriously wastes energy. Moreover, the asynchronous motor tends to start and stop frequently. This affects the service life of the motor. The compressor has high starting current in power frequency mode. This has a high impact on the power grid and causes serious wear of the motor shaft and high maintenance of equipment. The working conditions are harsh and the noise level is high. The automation level is low. The adjustment of output pressure is realized by manually regulating the valve opening. This has the disadvantages of low regulation speed, large fluctuation, low stability and low accuracy.

Variable speed ac drive system: 

The electric system is consist of variable frequency drive, pressure sensor and other peripheral devices.
(1) The VFD drive adopts sensorless vector control technology to meet the requirement of high torque when low frequency;
(2) The system adopts PID control, please check the whole control process as follow: Air requirement↑ —— Pipe air compress↓—— The difference between pressure setting value and pressure feedback value↑ —— PID output↑ —— AC drive output frequency↑ —— Air compressor motor speed↑ —— Supply air flow↑—— Pipe air compress to stabilize;
(3) 0~10V (or 4~20mA) signal provided by pressure sensor is used as PID given, input AI from the variable frequency drive analog input;
(4) Start, stop signal is given by central control system, the frequency drive can provide analog output signal via AO, then connect to the central control system to monitor pressure fluctuation. Meantime, fault warming signal can be provided by reactor;
(5) You can configure power frequency by-pass to ensure the continuity of production;

Advantage of the VFD(variable frequency drive) solution:

(1) Active power electricity saving, when air compressor speed reduces, active power electricity saving is more obvious. (The meaning of active power electricity saving is saving energy & lower production cost);
(2) Increase grid side power factor & reactive power electricity saving, the meaning of reactive power electricity saving is that lower the line consumption of workshop power supply bus & the second current of workshop distribution transformer. Meanwhile, reduce of reactive power electricity saving can lower the temperature rise of power transformer to prolong the service life of power transformer;
(3) Stable high load start. Low frequency response of the variable frequency inverter ac drive is good, plus big power margin, can totally meet torque requirement when low frequency. So it is definitely suitable for "high load start";
(4) Prolong mechanical life. Lower air compressor speed, reduce air compressor wear, can prolong the service life of air compressor. Meanwhile, the leakage of air compressor oil way is greatly reduced, relieve the pressure of environmental pollution;
(5) Lower oil temperature increase is a good solution for the shortage of air compressor working pressure decreases under high temperature environment;
(6) Reduce noise. Can prove the working environment obviously to provide a good solution for managing the environment noise pollution in professional health governance;
(7) Excellent soft start, soft stop features. The start & stop time of the ac drive inverter can be adjusted randomly (0~10min), means that acceleration when start & deceleration when stop can be adjusted randomly, meanwhile, you can match with S curve ACC/DEC time to start smoothly, which can minimum the impact of air compressor caused by start & stop, this can hardly achieved by other drive devices.

Frequency Inverter ac motor drive for veneer peeling solution

Introduction of cardless type veneer peeling:

With the continuous improvement of our living standards, the rapid development of construction material, furniture and decoration industry promotes the manufacturing technology of artificial board and glued board developing synchronously. The requirements of wood rotary cutting equipment increased steadily, but most of factories use the backward rotary cutting equipment at present; it has the characteristic of low degree of automation and low production efficiency, so that the research of upgraded equipment becomes an urgent task. 
The traditional structure of wood rotary cutting machine is fixed with a clamper, to hold the both ends of round wood by clamping jaw. For the round wood which the diameter is small, the stress will be asymmetrical due to the clamping jaw, the inflexibility in the middle section is weaker, cutter relieving and knife jumping will happen frequently, there will be a large thickness deviation of processed wood block.
The upgraded AC drive frequency inverter controlled veneer peeling without the clamper, the round wood is not lamped by the clamping jaw, and the clamping has no relationship with the quality of wood. The wood block with uniform thickness and same length can be processed by adjusting the rotary cutting speed and shearing speed, on the premise of precision satisfaction to increase the processing efficiency and utilization rate of wood.

Brief introduction of the frequency inverter ac drive system:

The main control system is the core section of veneer peeling, it is the key system for processing precision and real-time controlling. The main components including PLC, AC drive inverter, communication interface, position stopper and emergency stop button. Every ON/OFF signals and data's storage and transmission is logically control by PLC during the productive process, the feedrate is fed back to PLC as a pulse signal which is converted by online type sensor, PLC will calculate its correspond output frequency and send it to AC inverter drive via Modbus communication to control the feed driving motor, to adjust the speed of peeling manufacture dynamically, the feed speed is monitored and feed back to PLC to constitute a close-loop control system, and the forward and reverse running of feed system is also controlled by PLC, the frequency pulse input signal is processed by control system to realize the real-time control of peeling process. 

Advantage of the frequency inverter ac drive solution:
Process precision is improved, operation is humanized, and wood utilization rate is increased. It saves the crude wood and improves the working efficiency.

Variable Speed Drive Frequency Inverter for Sucker Rod Pumps in Oil Driller

Sucker rod pumps introduction:

Sucker rod pumps are widely used in the oil industry. Sucker rod pump is a modified four-bar linkage, the whole structure just like a balance, one side is pumping load, and another side is the counterweight load for balancing. If the pumping load and the counterweight load have the same torque or their torque change consistently, then the oil driller can work continuously with a small external power. That means the balance performance is the key factor of energy saving. If the balance ration is 100%, the power from electric motor only used to lift 50% weight of liquid column and overcome the friction force, the lower of the balance ratio, the bigger power from the motor is required. The pumping load is changed all the time, but the counterweight cannot make the same change together with the pumping load.
The sucker rod pump is driven by the motor directly in most of systems. The power of motors are always be enlarged too much to ensure safety operation, it increases the reactive power loss and reduces the operational efficiency of the motor. In another hand, the low productivity of some oil wells, dry tap working is existed; all the factors cause the operational efficiency of the complete system is too low. Some of the system's operational efficiency is as low as 10%, and the balance ration of the driller is as low as 40%. According to the production characteristics oil field, to upgrade the system via the solution of "variable speed drive + energy feedback unit" will promote the energy saving performance effectively.

Variable speed drive frequency inverter introduction:

The electric system consists of variable speed drive, travel switch, energy saving unit and some other external components, as shown below:
(1)The system in the multi-step speed control mode: two steps of high speed and low speed, the travel switches are fixed at the up-running end point and the down-running end point, the step speeds are changed by trigger the switches.
(2)The energy feedback unit is fixed to feed the regenerated energy back to the power grid.
(3)The function of the frequency inverter output relay terminal is set as failure warning output.

Advantage of the variable speed drives solution:

(1) Increase the power factor (increase from 0.25~0.5 to greater than 0.9), and reduce the apparent current greatly, lighten the burden of power grid and transformer, reduces the line losses, to save huge expenditure of system capacity enlargement.
(2) To adjust the oil drilling speed dynamically based on the actual liquid feeding capacity of the oil well, increases the crude oil turnout and meanwhile reduces the energy consumption of low production oil well.
(3) Outstanding energy saving performance by using the energy feedback unit;
(4) Soft starting of the system, prevents the mechanical shock of motor, gear box and oil driller, prolongs the working lifetime of the system, reduces the down time and increases the production efficiency.

Frequency Converter for belt conveyor

Belt conveyor introduction:

Utilizing of multi-drive belt conveyor is widely used in the mining transportation system.
During the starting process, if the soft start equipment is not applied, the belt conveyor will be damaged easily due to the big tension vibration. In another hand, if we start the motor directly, the start current will be 4~7times of the motor rating current, this big surge current will change the internal mechanical stress and thermal stress of motor, and cause serious abrasion and damage to the mechanical part. Meanwhile, the big surge current will cause big voltage drop of the grid, and effect other equipment’s operation.

Frequency converter solution introduction: 

The frequency converter controlled system consists of two drives, weight sensor, PG card, encoder etc. Normally, it needs to upgrade the both terminals’ motors, the system diagram is shown as below.
(1)The two frequency converters are common DC bus connected, the regenerated energy can be reused internally. 
(2)One VFD drive set as master, another one set as slave, because the belt conveyor is flexible connection, the master-slave motors can be controlled by speed.
(3)The master drive receives the 4-20mA signal from weight sensor to calculate its output frequency, this frequency signal will output to slave AC drive as it frequency reference, to achieve the synchronous speed and balanced power of the two motors.
(4)Both of the master and slave system are close-loop vector control mode, to get significant starting torque.
(5)The braking unit of energy feedback unit is necessary for down-running belt conveyor.


Advantage of frequency converter in belt conveyor:

(1)Good performance of master-slave speed synchronization, realized the master-slave power balance, and the difference of the running current is very less. 
(2)It reduces the surge current during start process and also reduces the motor thermal loss while start the motor frequently, prolongs the working lifetime of motor.
(3)The belt conveyor can work at 120% load without failure.
(4)The optimized S curve of the AC drive makes the start and stop of belt conveyor smoother.
(5)Energy saving: In normal condition, the mining motors’ power rating are enlarged too much to ensure the safety operation, but they don’t work at full load in most of the time, and it will cause huge electric loss while in light load or without load. After using of the variable frequency drive in the system, the speed of the belt can be adjusted automatically as the change of the load. In addition, the hydraulic coupler will be removed and the gear-box will be connected with motor directly, it improves the transmission efficiency effectively.

Vector Control AC drive for centrifugal dehydrator

Application Technical Requirements:
1. The centrifugal dehydrator produces the centrifugal force through high speed rotation to remove water of materials in the stainless steel cylinder. It is widely used for textile, printing, ceramics, chemical industry, mine, hotel, and etc.
2. Centrifugal dehydrator is a typical low damp big inertia load, it must overcome the over-current during start acceleration, and over-voltage during deceleration.
3. Accelerating and decelerating at the maximum rated current of motor, no skipping in normal situations.
4. Big torque at low frequency, powerful start, and stop without coasting.

AC Drive System Configurations
1. Open loop vector control AC drives
2. Current vector control feature
3. Choose braking resistor according the motor power
4. Braking unit is required if the power is more than 15kW

Electric Schematic Diagram of AC drive system


AC Drive System Features:
1. Simple control program on equipment, and easy for commissioning, operation and maintenance.
2. Open loop vector control ac drive, 150% rated torque output at 1.0Hz. High torque control at low frequency, with stable starting and braking.
3. Current clipping control function is in closed loop mode which is reliable and efficient. Accelerating and decelerating at rated current, no skipping protection. The acceleration/deceleration time automatically prolongs or reduces according to the quality of material in the cylinder.
4. Reliable and efficient DC braking torque, cylinder stops stably without gliding in setting time.
5. No skipping over-current and over-voltage protection at tracking start and stop when the cylinder is gliding.
6. Automatic stop against grid blackout: transfer the big inertia into electricity, then feed back to the frequency converter and consumed by the braking resistor by means of heat, avoid causing dehydrator free-stop when supply grid is blackout.

Variable frequency drive in CNC machine solution

CNC machine Introduction:

The main motion of CNC machine is rotation motion of main spindle via chuck or apex drives work piece, is used to transmission and speed governing via motor drives gearbox. During machining process, usually there is a requirement for a different rotation speed of main spindle, operation staff controls separation and integration of clutch via multiple positions of handle combination, gets multiple combination of gear to obtain multiple speeds, operation is inconvenience and maintenance is large, practices show that the fault rate of electromagnetic clutch using to adjust speed is high. The old feature of main spindle drive cannot meet the requirement which the fast developing economy asks for CNC machine. Currently, more and more inverters match with CNC machine to control main spindles. 

Variable frequency ac drive in CNC machine system:

The whole electrical system is consisting of CNC machine, variable frequency drive, time relay and brake assembly.
(1) AC power supply connects to R, S, T terminals via breaker. The output terminals U, V, W connect with main spindle motor with right consequence. When the running command is inconsistent with the motor rotation direction, you can change any two phase of U, V, W or exchange control circuit terminal FWD/REV ;
(2) Frequency given command is given by CNV with 0~10V or -10V~10V form, connects from AI1 and GND. The rotation direction and running control of motor is decided by the status of DI;
(3) When DI1 and COM are connected, DI1 is high level, the motor runs forward. When DI2 and COM are connected, DI2 is high level, the motor runs reverse. When DI1 and DI2 do not connect with COM, both DI1 and DI2 are low level, the motor stops. The connection or disconnection of DI1 and COM & DI2 and COM is controlled by two relay contacts, the two contacts can be controlled by main spindle runs forward and main spindle runs reverse commands given by the numerical control system. Meanwhile, two digital output terminals are separately set as: TIA and TIC (function setting: running output), T2A and T2C (function setting: fault output). 

Advantages of VFD in CNC machines:

(1) The range of main spindle variable frequency variable speed is very wide, the VFD drive can achieve stepless speed regulation in the range of 0~300Hz, can meet the requirement main spindle asks for speed regulation range;
(2) The variable frequency drive matches with CNC machine not only simplify complex mechanical drive mechanisms, such as gearbox, etc, but also make the operation easy & maintenance convenience;
(3) The variable frequency ac drive can provide 150% torque when low frequency, this can meet the requirement of main spindle motor has harder mechanical features, can provide 150% overload for 60s to make CNC machine has powerful overload ability when low frequency;
(4) The variable speed drive has voltage, current analog input terminals, and the control signal of CNC machine can be matched with inverter very well;
(5) Greatly improve the machining accuracy and production efficiency of CNC machine, play an important role of product quality improve, production increase, cost decrease, automation level increase and maintenance reduce

Frequency Converter in construction elevator industry

Construction elevator system introduction:

In the modern High-rise building construction, the construction elevator is the most important equipment which is used in big-scale tower crane.
Generally, the construction elevator can be divided into three kinds in speed, low-speed elevator in the speed range of 0~40m/min, medium-speed elevator in the speed range of 0~63m/min, high-speed elevator in the speed range of 0~96m/min.
The ordinary elevator is controlled by contactors, it has the disadvantage of:
(1) Single operation speed;
(2) Big surge current in start and stop;  
(3) Heavy damage for structure and mechanism system;
(4) The electric components are easily damaged;
(5) Running speed is always low, in the range of 34~38m/min.
This low-end system will reduce the construction speed and also reduce the benefit of enterprises. If we only increase the operation speed, the concussion of structure and mechanism will be enlarged and increase the abrasion between gear rack and brake disc accordingly, it will reduce the reliability of operation. 
As the layers increase, for the building which the height is more than 400m, the high speed elevator of 96m/min will be used to increase the construction efficiency, It need very big braking resistance to consume the regenerated energy while the elevator going down. It has big security risk and energy losing in the application of long-term braking (length of 400m, braking period more than 5min).

Variable frequency drive adjustable speed system introduction:

The whole system consists of PLC, frequency inverter, energy feedback system, electric motor, reduction box, AC input reactor, transformer, operation hand shank etc., to control the start, stop, up going, down going, brake and every kinds of protection of the elevator. The elevator will work at the condition of high-efficiency, safe and stable, meanwhile, the regenerated energy while the elevator going down will be fed back to the power grid to save energy.

Advantage of the ac drive solution:

(1) High-efficiency, the running speed is as high as 60～70m/min which is double of traditional elevator(34 m/min);
(2) Energy feeds back to power grid, save considerable energy for customer, good braking effect;
(3) High output torque in low speed;
(4) Reduce the driving concussion of mechanism system effectively, prolong the working life of gear, contact roller, bearing and gear rack; The current limitation function of frequency inverter reduces the surge current while starting the motor, and also reduce the interference among the power consumption devices;
(5) The system has perfect safety protect functions such as under voltage, over voltage, over torque, over current and so on; it enhances the reliability and safety for the total system.
(6) The system adopts low-speed braking method, the small rotary friction force prolongs the working life of brake effectively.
(7) Compare with traditional non speed adjustable elevator, the comfortable feeling during operation is wonderful.

Frequency Inverter for glass tempering furnace blower control

Production processes:
Heating the glass in the furnace before softened, it will be removed to the cooling workshop for fulfilling the process of tempering by quick air cooling, and then blowing the glass continuously until it reaches the normal temperature. When glass is tempering and cooling, another batch of glass is sent to the furnace simultaneously.

Glass Tempering Furnace

Inverter System Configurations:
Cooling blower speed control: open loop vector control frequency inverter.

The standard signal of tempering furnace control system output serves as the setting frequency of frequency inverter drive to control the rotation speed of cooling blower.

Frequency Inverter System Features:
1. 120% rated torque acceleration or deceleration, smooth start and acceleration/ deceleration, motor’s current does not exceed the rated current, eliminating the impact on the power grid and equipment.
2. Improve the protection function of motor, and prolong the life span of the motor.
3. Optimize the operational efficiency, and control the rotational speed of cooling blower by the variable frequency drive. The energy saving rate is more than 30%.

AC Drive for Paper Machines

Technical Requirements:
The paper machine consists of head-box, filter, pressing part, drying part, calendering, paper winder, etc. The main drive electric control system is a speed chain coordination system made of multi-transmission points.
A fixed transmission rate is required between transmission points to make the wire speed identical which helps the machine to accelerate, decelerate, and avoid the paper sheet broken due to the wire speed gap between transmission points.
The speed regulation accuracy of transmission system is required to be 0.1%. Normally the frequency accuracy of AC drive inverter should be 0.01Hz.
The analog signal is exposed to electromagnetic interference which impacts the transmission precision and stability of paper machine. In order to enhance the accuracy and stability of system and anti-interference ability, the system takes PLC as the main drive unit, acceleration or deceleration at each step is controlled by button. The acceleration and deceleration of transmission points in the speed chain and the speed transmission rate are calculated and controlled by PLC that outputs speed regulation command to variable frequency inverter drive by RS485 communication in the way of data.

VFD AC Drive System Configurations:
1. Open loop vector control variable frequency AC drives with high transmission accuracy, high dynamic performance, quick response, and etc.
2. System main control unit: Mitsubishi FX2N series PLC with 485 communication module and D/A module.
3. Each transmission point is in speed open loop control mode, and the speed control command is given by PLC.



AC Drive System Features:
1. Electric control system is in open loop control mode, simple structure, clear control logic relation, safe and reliable, easy operation.
2. Current vector control frequency inverter ac drives, flexible speed control, quick response and high dynamic performance.
3. The frequency accuracy is 0.01Hz.
4. Under PLC control, precisely calculating the transmission rate.
5. Thorough numeric communication control mode, strong interference resistance.
6. Automatically store the current speed during the operation. When stop and re-start with no need to regulate the speed.

What is the difference between v/f control & vector control mode?

Using v/f control, also called "volts per hertz" control or scalar control, a drive essentially acts as a power supply of a selected frequency and proportional voltage. At a given speed, the motor performs much as it would when supplied by utility power. For each frequency setting, motor operation is governed by a torque vs speed curve that is similar to the torque vs speed curve that governs utility power operation.

With scalar control, V/Hz tuning adjustments are used to provide a family of torque vs speed curves that are equivalent to the utility power torque vs speed curve over as wide a speed range as possible. The drive's operating point is at the intersection of the selected drive torque vs speed curve and the characteristic torque vs speed curve of the driven equipment.

Acceleration and deceleration ramp time adjustments are used to prevent acceleration and deceleration currents from exceeding safe limits. Current limit adjustments are used to reduce the speed of the motor rather than shut down in the event that the load torque exceeds the safe limit of the drive.

Current measurement can also be used to automatically trim various tuning adjustments to provide enhanced performance.

Properly tuned scalar drives with the best control enhancements can provide 150% of rated torque to overcome static friction at zero speed and to accelerate the load. They can also provide relatively smooth full torque operation at any set speed down to about 10% of base speed.

Vector control drives seek to dynamically regulate motor torque as directly and accurately as possible. Speed is regulated indirectly by providing exactly the torque required to operate the driven equipment at the desired speed. Vector control drives use a mathematical model of the motor to dynamically determine the values of the essential operating and control parameters. They are called "vector control" drives because this analysis is based on a vector representation of current, voltage and magnetic flux.

One of the key elements of vector control is the analysis of the motor current. The current in an induction motor is the combination of a magnetizing current vector and a torque-producing current vector. Vector drives continuously monitor and analyze the motor current to determine what voltage to apply at any given frequency to produce the optimum magnetizing current.

Various drive designs implement vector control in different ways. Some manufacturers consider their designs to be sufficiently unique to be more appropriately identified by terms other than "vector control." The best performance is generally achieved by providing a shaft speed and/or position feedback signal, but "sensorless" vector drives provide performance that is sufficient for many applications without using external feedback devices.

Vector drives, including sensorless models, can often provide significantly more than 150% of rated torque to overcome static friction at zero speed and to accelerate the load. They can also provide smooth full torque operation at any set speed down to zero speed or very close to zero speed. To reliably hold an overhauling load in position at zero speed, speed/position feedback is generally required. Vector drives provide excellent performance in terms of accurate static and dynamic speed regulation and rapid response to sudden changes in load torque. Vector drives can also provide torque regulation as an alternative to speed regulation.

VFD is a v/f drive, with only scalar control and it can not produce torque above the rated value.

In any motor application, the two criteria required are Torque and speed. There are two types of VFDs, v/f drives and vector drives. The latter can develop a maximum of 150-250% torque even at very low rpm. By providing a closed loop control for application like blowers, compressors, pumps, etc.,  savings could be achieved.

Use of a 5-7% AC inductor on primary side reduces generation of harmonics. Other advantages include smooth running by selecting acceleration, deceleration and regenerative-breaking parameters, etc.

ENERGY SAVERS vs VF DRIVES

ES cannot produce torque above rated value. It controls only voltage – the flux reduces with reduction in voltage. ES is meant only for part load operation. It is an electronic version of a simple autotransformer, except that ES can work in closed loop.

Part load efficiency in motors can be improved by reducing the voltage applied using ES rather than the more expensive VFDs. The principle behind this method is :

When the motor operates on a part load, it develops the required torque with a reduced flux; so the applied voltage can be less than the rated. Reducing the voltage causes reduction in iron loss (Iron loss ∞ V2) and therefore the motor efficiency improves; reduction in magnetizing current causes increase in PF too. In fact the motor can be operated at peak efficiency with any load, provided the exact required voltage is applied (in other words, by maintaining the slip at its optimum value). ES does this.

In contrast, a VFD also reduces voltage, along with frequency, and so the speed varies, and the load level varies too. In fact even ES is not essential to reduce the voltage; it can be easily done through an autotransformer(AT). Further, VFDs & ES cause harmonics, but AT does not. AT is much cheaper.

If it is essential to use an oversized motor, one can use AT to supply a matching voltage in case of part load operation.

MOTORS – OVERSIZING

Motors often need to be oversized to take into consideration the initial starting torque, and other unpredictable loads / torques, in rigorous mechanical applications. There are ways in which motors can be selected close to running load / torque conditions, by having fluid couplings / soft starters.

This should ideally be considered while designing the power train, ie the motor, couplings, gear-boxes, and finally the load.

In many applications, particularly in the steel industry, motors are rated much higher than the running load, to take into consideration stalling of the motor.

If the motors are over sized for other extraneous reasons, and

1]    Will run for greater part of the time at lower torques, and / or

2]    Switchgears for S-D starting are not much help,

VFDs would be an ideal solution. Lower torque demand results in substantial energy saving.

For very high ratings and MV/HT motors, the industry also uses variable speed fluid couplings.