- Blocked Rotor Test Definition: The blocked rotor test of an induction motor is defined as a test to find the leakage impedance and other performance parameters.
- Purpose of Blocked Rotor Test: It determines torque, motor characteristics, and short-circuit current at normal voltage.
- Testing Procedure: During the test, the rotor is blocked, and low voltage is applied to the stator to measure voltage, power, and current.
- Effect on Impedance: Rotor position, frequency, and magnetic dispersion can impact the measured leakage impedance.
- Short Circuit Current Calculation: The test helps calculate short circuit current for the normal supply voltage by measuring specific parameters.
Induction motors are widely used in industries and consume a lot of power. To improve performance, tests like the no-load test and blocked rotor test are used. A blocked rotor test helps find the motor’s leakage impedance.
Additionally, the test can determine parameters like torque and short-circuit current at normal voltage. The blocked rotor test is similar to the short circuit test of a transformer. In this test, the motor’s shaft is blocked, and the rotor winding is short-circuited.
In slip ring motors, the rotor winding is short-circuited through slip rings, while in cage motors, rotor bars are permanently short-circuited. Testing induction motors can be complex because rotor position, frequency, and magnetic dispersion affect leakage impedance. These effects are minimized by conducting a blocked rotor current test on squirrel-cage rotors.
Process of Testing of Blocked Rotor Test of Induction Motor
During the blocked rotor test, the applied voltage to the stator terminals should be low to prevent damage to the stator winding. Low voltage is used to stop the rotor from rotating, making its speed zero and allowing full load current through the stator winding. With the slip at unity, the load resistance is zero. Slowly increase the stator voltage until the current reaches its rated value. Note the readings from the voltmeter, wattmeter and ammeter to determine voltage, power, and current. The test can be repeated at different stator voltages for accuracy.
Calculations of Blocked Rotor Test of Induction Motor
Resistance and Leakage Reactance Values
In the blocked rotor test, core loss is very low because of the low voltage supply, and frictional loss is negligible since the rotor is stationary. However, stator and rotor copper losses are relatively high.
Let us take denote copper loss by Wcu.
Therefore,
Where, Wc = core loss
Where, R01 = Motor winding of stator and rotor as per phase referred to stator.
Thus,
Now let us consider
Is = short circuit current
Vs = short circuit voltage
Z0 = short circuit impedance as referred to stator
Therefore,
X01 = Motor leakage reactance per phase referred to stator can be calculated as
Stator reactance X1 and rotor reactance per phase referred to stator X2 are normally assumed equal.
Therefore,
Similarly, stator resistance per phase R1 and rotor resistance per phase referred to stator R2 can be calculated as follows:
First some suitable test are done on stator windings to find the value of R1 and then to find R2 subtract the R1 from R01
Short Circuit Current for Normal Supply Voltage
To calculate short circuit current Isc at normal voltage V of the stator, we must note short-circuit current Is and low voltage Vs applied to the stator winding.
