Electrical Engineering ⇒ Topic : No-Load Test or Open-Circuit Test

1. No-load or Open-circuit Test. This test is performed to determine the following :

(i) No-load current I₀

(ii) No-load power factor cos Φ₀

(iii) Windage and friction losses

(iv) No-load core losses

(v) No-load input

(vi) No-load resistance R₀ and reactance X₀.

The test is conducted as follows :

     The motor is uncoupled from its load, and rated voltage is applied to the stator. Since there is no power output, the power supplied to the stator furnishes its copper loss, its core loss, and the friction and windage loss in the rotor. Since slip at no-load is often within one-tenth of one per cent, the rotor current is practically zero, and hence the analogy to the no-load test of the transformer. However the no-load stator current of the and therefore induction motor is comparatively large, copper losses in the stator may not be neglected as they were for the transformer primary. It is thus necessary to calculate this copper loss and subtract from the no-load input in order to obtain the sum of the core and fi-iction and windage losses of the motor.

    Speed and flux vary but little from no-load to full-load, and these losses may be assumed constant for the operating range of the polyphase induction motor.

Knowing the total core losses, P_core no-load current per phase I₀ and applied voltage per phase V (Fig. (A), the values of magnetising current, l_m wattful current no-load resistance R_o and no-load reactance X₀ are determined as follows

                                                                    Figure (a) Efficiency test of induction motor

NO-LOAD TEST OR OPEN-CIRCUIT TEST

This test is similar to the open circuit of a transformer. The motor is uncoupled from its load. The rated voltage at rated frequency is applied to the stator. So the motor runs without load. To measure the input power, two-wattmeter method is used. An ammeter and a voltmeter are connected to measure the no-load current and voltage. Since the motor runs at no load, the total input power is equal to constant iron loss, friction, and windage losses of the motor.

During no-load condition, the power factor of induction motor is less than 0.5. So one wattmeter will show negative reading. The direction of current-coil terminals should be made reverse to take the reading. From this test, R, and X_Φ can be determined. Figure (a) shows the schematic diagram of no-load test of an induction motor.

                                          Figure (a) No-load test of three-phase induction motor.

Let V_L is input line voltage, P is three-phase power, /₀ is input line current, and V_p is input phase voltage