Electrical Engineering ⇒ Topic : NoLoad Test or OpenCircuit Test

Sachin
 
1. Noload or Opencircuit Test. This test is performed to determine the following : (i) Noload current I_{0} (ii) Noload power factor cos Φ_{0} (iii) Windage and friction losses (iv) Noload core losses (v) Noload input (vi) Noload resistance R_{0} and reactance X_{0}. 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 noload is often within onetenth of one per cent, the rotor current is practically zero, and hence the analogy to the noload test of the transformer. However the noload 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 noload input in order to obtain the sum of the core and fiiction and windage losses of the motor. Speed and flux vary but little from noload to fullload, and these losses may be assumed constant for the operating range of the polyphase induction motor. Knowing the total core losses, P_{core} noload current per phase I_{0} and applied voltage per phase V (Fig. (A), the values of magnetising current, l_{m} wattful current noload resistance R_{o} and noload reactance X_{0} are determined as follows
Figure (a) Efficiency test of induction motor  
 
Samual
 
NOLOAD TEST OR OPENCIRCUIT 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, twowattmeter method is used. An ammeter and a voltmeter are connected to measure the noload 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 noload condition, the power factor of induction motor is less than 0.5. So one wattmeter will show negative reading. The direction of currentcoil 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 noload test of an induction motor.
Figure (a) Noload test of threephase induction motor. Let V_{L }is input line voltage, P is threephase power, /_{0} is input line current, and V_{p} is input phase voltage  
 
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