Induction Motor Drives Flashcards
What is slip?
Slip is a measure of difference between speed of rotating magnetic field and speed of rotor
What do Variable frequency motor drives aim to do?
keep in linear part of torque speed curve
In this region there in a constant ratio between supply voltage and frequency
How does a VVVF supply operate?
Speed may be varied by altering supply frequency and slip
Most efficient to adjust frequency
This is done by using a power electronic converter
- converts fixed frequnecy mains to DC
- Converts dc to ac at whatever frequency required
- Known as an inverter
Explain role of reactive componets in V/f control
As frequency is varied, magnitudes of reactive components vary
Keeping Im constant equivllent to keeping V/f constant
When omega small, necessary to compensate for drop across starot resistance by boosting v1 at low frequencies, in order to keep Im constant
What is base speed?
Base speed is max speed in rads-1 of motor at rated voltage (Vb) and rated frequency (fb) at which rated flux can be obtained
What is the rotor current limit?
What is stator current limit
Imposed by copper losses in stator winding
Given by
Exaplain Voltage boosting at low speed
At low frequency flux will tend to be lower than desired due to voltage drop across stator resistance R, becoming disproportionaly large, particullarly at higher torque
Simple solution is to fix min voltage at low frequencies
Explain Field weakening
Common to operate IM drives beyond base freqeuncy in order to extend their speed range
Usually rated voltage of IM should not be exceeded
Higher than base speed is therefore accomodated by allowing IM to operate at a reduced flux, lower than rated flux
This means allowing V/f to reduce when operating above base speed
AS motor is then underfluxed, this is known as field weakening
Explain reduced torque due to field weakening when rotor reactance is small
Reduced due to reduced flux at maximum rotor current
Explain Rotor resistance control?
Peak torque of the motor is fixed but the slip at which it occurs increase with increasing rotoe resitance. Speed control is only possible up to base speed of the motor. This method of spped control is only applicable to wound-rotor induction motors and results in large rotor losses and hence loss of drive efficiency
explain voltage control
Speed control up to base speed is possible whilst being apllicable to all types of induction motors. This method results in substantial loss of peak torque with reducing voltage
Thus only usefull for certain classes of load, e.g where torque varies with square of load
Explain VVVF control
Enables efficient operation i.e steep torque-speed curve over a wide range of operating speeds.
Combined with vector control, the stator voltage can be controlled to maintain a 90% angle between rotor current nad the flux, thereby maximizing torque per unit current, and hence efficiency.
Operation beyond base speed of the motor may be obtained with this type of scheme by field weakening.
This can be usefull if load requires a smaller torque at a higher speed than base speed.
