Why V/F Ratio Is Kept Constant in VFD?

Why V/F Ratio Is Kept Constant in VFD?

The speed of the induction motor can be governed by regulating the voltage, frequency, number of poles or by changing the slip. The speed of the motor can be calculated with following formula.

N= Ns(1-s)= 120 f /P(1-s)

Where,
Ns= 120 * f/P - Synchronous speed of motor
f- Frequency
S - Slip of the motor-
The difference between synchronization speed and actual speed of the motor,
P - Number of poles.

Speed regulation of induction motor with change in voltage alone has certain limitations, and with this method the speed can be governed in the range of 90% to 100% of the rated speed.

If the voltage is reduced below 90% of the rated voltage, the torque of the motor get reduced drastically as the torque of the motor is proportion to square of the applied voltage, and at this reduced voltage the motor can stall if load demands more torque than the maximum torque delivering capacity of the motor.

Speed regulation by changing the frequency is the best method to govern the speed of the motor. Stator voltage is also increased or decreased proportionately with increase or decrease of frequency with Pulse width modulator(PWM) inverter to maintain the constant flux in the motor.

With slip control, the speed of the slip ring induction motor can be governed. Speed control through slip control cause copper loss in the rotor. The rotor copper loss or the slip power of the rotor can be recovered with slip power recovery system(SPRS).

When the speed of induction motor is regulated by regulating frequency, the stator voltage is also increased/decreased in proportion to change in the frequency.

The increase or decrease in the voltage with frequency does not contribute in the speed variation of induction motor; however it is essential to regulate the voltage with change in frequency. Let us understand the reason of this.

The winding of the stator is wound on the magnetic core. The flux is produced when the sinusoidal voltage is applied at the stator terminal; the  produced magnetic flux in the core that travels through the air gap between stator and rotor gets linked to rotor winding.The flux produced in the core can be calculated with the formula as given below.
             
            Flux = Eb/ 4.44 f*T
Where Eb is the back EMF of stator
Thus, the flux in the stator core is proportional to ratio of V/F.

If speed is increased by increasing frequency, the voltage has to be increased in the same proportion with increase in frequency to maintain the constant flux in the core in order to avoid the overheating of the core.

If voltage is not decreased with decrease in frequency, the flux in the core will go above its rated design capacity and the core of the motor will get saturated. The saturation of core will lead to temperature rise of the core, and eventually the winding insulation may break down.

If the voltage is not increased with increase in frequency the flux in the stator core reduced from its rated average flux and motor will not able to produce the rated torque upto base speed.

In the application where speed is required to be increased above basespeed of the motor , the frequency is increased above 50 Hz., however it is not practically possible to increase the voltage above 440 volt in case of low voltage drives. Speed control above base speed is possible with weakening the main stator flux as V/ F ratio get reduced. This procedure of speed control can be used for increasing the speed of the drives which require less running torque above base speed. Increasing speed above the base speed of the motor will lead to decrease in the torque. With flux weakening the motor speed can be increased above 10 % of the rated speed of the motor. However, the motor design data must be checked before doing the speed regulation through fluxweakening.