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.
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
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.
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%.
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.
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.
A: Advantages of vector control mode have over V/F control are known. Today's manufacturers provide their products with the option of sensorless vector control mode. In this case, why would one use V/F control instead of vector control ? Some say that if V/F control is adequate for your application why do you want to buy more functionality ? But my question is: why would one use V/F control if their drive has sensorless vector control capability ?
B: I often have the same question. Many drives come with V/F selected as default and the first you do after setting the basic parameters is change it to vector control.
One application I know is where you have two motors connected to the same drive. You cannot use sensor less vector in this case. Applications are on overhead cranes.
Another case may be where the cable is long or in poor shape. I have found times where an old PWM V/F drive works just fine but a new sensorless vector drive has high current problems.
C: Sensorless vector control provides extra torque at low rpm. it is almost as good as closed loop vector in this respect. There are many devices such as fans and pumps that do not need loads of torque at low speed. You must remember that in all drives voltage is free and current is what you pay for in terms of running cost so if you can reduce the start up current on a fan or a pump with V/F control then the customer will be pleased. Suggest that you investigate pump and fan output compared with speed. Both consume little power until half speed and only after that do you need to increase the torque.
Customers always like to save money.
