Conducting Tests on Single Phase Induction Motor

Conducting Tests on Single Phase Induction Motor AU : May-10, Dec.-14, 17

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 V₀, I₀ and W₀. 

Now W₀ = V₀ I₀ cos ϕ

cos ϕ₀ = W₀ / V₀ I₀ = No load power factor

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 X₀  is much higher than the impedance r₂ / 2 + j x₂. Hence it can be assumed that no current can flow through x_m and that branch can be eliminated.

So circuit reduces to as shown in the Fig.8.10.1

Now the voltage across x₀ is V_VB

Thus magnetising reactance X₀ can be determined.

The no load power W₀ is nothing but the rotational losses. 

2. Blocked Rotor Test

In blocked 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 xo 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. 8.10.2.

The stator resistance R₁ 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.

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

Example 8.10.1 A 220 V, 1 phase, induction motor has following test results :

N. L. Test: V = 220 V, I = 6 A, P = 350 W

Locked rotor Test : V = 125 V, I = 15 A, P = 580 W

Stator resistance : 1.2 Ω with direct current

Estimate the p.f. and efficiency when the slip is 0.05.

Solution :

Examples for Practice

Example 8.10.2 The following data shown result on a 230 V, 50 Hz capacitor start single-phase induction motor at standstill :

Main winding : 100 V, 2 A, 40 W

Auxiliary winding, 80 V, 2.0 A, 50 W

Determine the value of capacitance for obtaining the maximum starting torque.

[Ans.: C = 67.52 µF]

Example 8.10.3 The following test results were obtained in case of a 220 V single phase induction motor :

Free running test: 220 V, 5.8 A, 310 W

Blocked rotor test : 120 V, 13.8 A, 530 W

Stator winding resistance = 1.4 Ω

Determine the approximate equivalent circuit of a motor.

[Ans.: X₁ = X₂ = 4.1191Ω R₂ = 1.383 Ω X_a = 31.433, X_m = 62.867 Ω]

Example 8.10.4 A 220-V, 1-phase induction motor gave the following test results :

Blocked-rotor test : 120 V, 9.6 A, 460 W

No-load test : 220 V, 4.6 A, 125 W

The stator winding resistance is 1.5 Ω and during the blocked rotor test, the starting winding is open. Determine the equivalent circuit parameters and core, friction and windage losses.       

[Ans.: Z =12.5 Ω, Req =4.99 Ω,

X = 11.46   5 Ω, R.2 = 3.49 Ω,

X₁= X₂ = 5.78 Ω Zo = 47.826 Ω,

X_o=47A87 Ω, 74.798 W]

Review Questions

1. Explain the no-load test and blocked rotor test for obtaining the equivalent circuit parameters of a single phase induction motor.

2. Draw the equivalent circuit of single-phase induction motor and discuss the experimental procedure to obtain its parameters.