Conducts Tests on Single Phase Induction Motor

Similar to a three phase induction motor, the various tests can be performed on single phase induction motor. The results of these tests can be used to obtain the equivalent circuit parameters of a single phase induction motor. The tests usually conducted are :

1. No load test or open circuit test
2. Blocked rotor test or short circuit test

1. No load test

       The test is conducted by rotating the motor without load. The input current, voltage and power are measured by connecting the ammeter, voltmeter and wattmeter in the circuit. These readings are denoted as V_o , I_o and W_o .

Now                 W_o = V_o I_o cosΦ

       The motor speed on no load is almost equal to its synchronous speed hence for practical purposes, the slip can be assumed zero. Hence r₂/s becomes ∞ and acts as open circuit in the equivalent circuit. Hence for forward rotor circuit, the branch r₂/s + j x₂ gets eliminated.

       While for a backward rotor circuit, the term r₂/(2 - s) tends to r₂/2. Thus x_o is much higher then the impedance r₂/2 + j x₂. Hence it can be assumed that no current can flow through and that branch can be eliminated.

       So circuit reduces to as shown in the Fig.1.

Fig. 1

       Now the voltage across x_o is V_AB 

But                    V_AB = I_o x_o 
.^..                       x_o = V_AB /I_o 
But                   x_o = X_o /2

   
       Thus magnetising reactance X_o can be determined.
       The no load power W_o is nothing but the rotational losses.

2. Blocked Rotor Test

       In balanced rotor test, the rotor is held fixed so that it will not rotate. A reduced voltage is applied to limit the short circuit current. This voltage is adjusted with the help of autotransformer so that the rated current flows through main winding. The input voltage, current and power are measured by connecting voltmeter, ammeter and wattmeter respectively. These readings are denoted as V_sc , I_sc and W_sc.

       Now as rotor is blocked, the slip s = 1 hence the magnetising reactance x_o is much higher than the rotor impedance and hence it can be neglected as connected in parallel with the rotor. Thus the equivalent circuit for blocked rotor test is as shown in the Fig.2.

Fig. 2

                     W_sc = V_sc I_sc cos Φsc
                      cos Φsc =W_sc /V_sc I_sc 
                                 = blocked rotor power factor
Now               Z_eq = V_sc/ Isc
                       R_eq = W_sc /( Isc)² 
But                  R_eq = R₁ + R₂ 
 .^..                     R₂ = R_eq - R₁ 
                          = rotor resistance referred to stator
                         X_eq =√(Z_eq ² - R_eq ²)
                         X₁ =  X₂  we get,

       The stator resistance is measured by voltmeter-ammeter method, by disconnecting the auxiliary winding and capacitors present if any. Due to skin effect, the a.c. resistance is 1.2 to 1.5 times more than the d.c. resistance.

       Key point : Thus with two tests, all the parameters of single phase induction motor can be obtained.