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.
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