Speed control of 3 Phase induction motor by v|f method

Aim: To study speed control of three phase induction motor by v|f method.

Apparatus:
1. Induction motor: 3 HP, 415V, 50 Hz, 4.7 Amp, 1440 rpm, 3 phase delta connected squirrel cage induction motor.
2. Control panel having three phase VFD drive, with
a) Ammeter- (A.C.)-0-30 A
b) Voltmeter-(A.C.)- 0-500V
c) Inbuilt frequency indicator

Theory: A three phase induction motor is practically a constant speed motor like a dc shunt motor. But the speed of the dc shunt motor can be varied smoothly just by using simple rheostats. 

This maintains the speed regulation and efficiency of the dc shunt motor, but in case of three phase induction motor it is very difficult to achieve smooth speed control. 

And if the speed control is achieved by some means, the performance of the induction motor in terms of its power factor, efficiency gets adversely affected. For induction motor,

Nr = Ns (1- S)

From this expression we can say that the speed of the induction motor are often changed either by changing its synchronous speed Ns or by changing the slips. 

So, the parameters like rotor resistance and induced emf are changed than to keep the torque constant for the constant load condition motor reacts by change in its slip, effectively its speed changes. 

The speed of the induction motor can be controlled by two methods:

1. From stator side 

2. From rotor side 

From stator side it includes:- 

a. Controlling supply frequency to control Ns called v/f control. 

b. Supply voltage control. 

c. Controlling number of stator poles to control Ns. 

d. Adding rheostats in stator circuit. 

From rotor side it includes :-

a. Adding external resistance in rotor circuit. 

b. Cascade control.

 c. Infecting slip frequency, voltage into the rotor circuit

Supply Frequency Control or V/F Control: The synchronous speed is given by Ns = 120f/p. Thus by controlling supply frequency smoothly, the synchronous speed can be controlled over a wide range. 

This gives smooth speed control of the induction motor. But the expression for the air gap flux is given by 

Φg = [1/(4.44K1 T1)] * (v/f) 

This is according to the emf equation of the transformer where, K = stator winding constant,T1 = stator turns per phase V = supply voltage f = frequency (supply).

It can be seen that from this expression if the supply frequency f is modified, the worth of air gap flux also gets affected. 

This may result into saturation of stator and rotor cores. Such a saturation leads to sharp increase in the no load current of motor. Hence, it's necessary to care of air gap flux constant when supply frequency is changed.

• In this method, supply to the induction motor required is variable voltage, variable frequency supply and can be achieved by an electronic scheme. 
• The normal supply available is constant frequency, constant voltage supply. The converter converts this supply to the dc supply. 
• This dc supply is then given to the inverter. Inverter converts this to variable frequency variable voltage if selecting proper frequency the v|f ratio is maintained constant and speed control of three phase induction motor is achieved. 
• If “f” is normal working frequency then the figure shows torque slip characteristic for the frequency f1>f and f2<f is for frequency above and below normal frequency. 
• Another disadvantage of this method is that the supply obtained cannot be used to supply other devices which require constant voltage. Hence an individual scheme for a separate motor is required which makes it costly.

  

Result:-Plot Graph between 1.Voltage and Speed 

2.Frequency and Speed

Conclusion:-

Post lab Question:-

1.The power input to a 400 V, 60 Hz, 6 pole, 3-f induction motor running at 1140 rpm is 40 kW at 0.8 p.f. lag. Stator losses are 1 kW and the friction and windage losses are 2 kW. Calculate-: (i) slip (ii) rotor copper loss (iii) brake h.p. (iv) efficiency (v) input current. 

2.Distinguish between squirrel cage and phase wound induction motor. 

3.Explain torque slip characteristics of three phase induction motor. 

4.Explain V/F method of speed control of three phase induction motor.