Speed Control of Induction Motor
The speed of the induction motor can be controlled by varying the voltage, frequency, number of poles and by changing the slip by adding external resistance to the rotor winding. The speed of the motor can be expressed by the following mathematical formula.
N=120f/P(1-s)
Where f- frequency, P= No. of Poles and s is the slip of the motor.
First, let us understand what is the significance of the V/f ratio in an induction motor. When the sinusoidal voltage is applied to the stator of the induction motor, the current starts flowing in the winding and the current induces the voltage of opposite polarity to the applied voltage. The magnitude of the induced voltage across the stator (Eb) depends on the frequency and voltage of the applied voltage(Vry).
The magnitude of the induced voltage can be expressed by the following mathematical expression;
In simple expression, the induced EMF across stator is as given below.
Eb=4.44∗Flux∗Frequency∗Number of Turns/phase
Flux= K* (Eb/ f)---------------(1)
Where, K- Constant, Eb- Induced EMF in the stator winding, f- frequency of the stator voltage
From above equation (1) it is clear that the flux in the air gap can be kept constant if the V/f ratio is kept constant.
The VF drive is very popular nowadays because of ease of the speed control. The speed of the induction motor is varied by changing the frequency reference set point of the drive. The frequency is increased if speed is required to be increased. The pulse width modulator (PWM) of the drive takes action to increase or decrease the voltage proportionately with an increase/decrease in the frequency.
PWM Inverter Waveform
Even though the variation in the voltage with the variation in the frequency does not contribute to the speed variation of the motor, the variation in voltage is just done to keep the flux constant.
This way the drive maintains the constant flux in the air gap of the motor.
The adverse effect of Over fluxing/under fluxing of the motor are as follows.
The flux in the core depends on the ratio of voltage and frequency. If the V/f ratio is not maintained the flux in the core would increase/decrease. With increased flux density above the rated core flux density, the temperature of the core will increase, and because of increased core temperature, the motor winding insulation may break down. The application in which high starting torque is required can be met by keeping the V/F ratio higher than the rated flux of the motor during acceleration of the motor. This feature is known as the “ starting torque Boosting” of the drive. The drive maintains the v/f ratio constant after accelerating to the rated speed, and the drive delivers constant torque. This feature can be used for starting high inertia loads like Rotary Kiln, Bucket elevators, belt conveyors, etc.
If the motor is operated at decreased flux density, the torque-delivering capacity of the motor would get decreased, and at full load, the motor may trip with overload. In an application where the running torque requirement is low, the V/f ratio can be reduced from its rated value to minimize the iron losses in the motor.
