- 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 can be tested without driving a mechanical load to estimate their equivalent-circuit parameters. The blocked-rotor test supplies data for combined resistance and leakage impedance. It is normally used with the no-load test, but the calculations must follow an applicable motor test standard.
The results can be used to estimate starting current and torque at rated voltage when the selected method permits that extrapolation. The test resembles a transformer short-circuit test. The motor shaft is mechanically restrained, and the rotor circuit is closed: cage bars are already short-circuited by their end rings, while a wound rotor is connected as specified for the test.
In slip ring motors, the rotor winding is short-circuited through the slip rings under the approved procedure. Blocked-rotor results depend on rotor position, test frequency, winding temperature and magnetic saturation. Rotor construction also matters: deep-bar effects can change the impedance of squirrel-cage rotors. Control these conditions and report them with the readings.
Blocked-Rotor Test Procedure
Mechanically restrain the rotor before energising the stator; reduced voltage does not stop the rotor. Use a guarded test setup, a balanced variable supply and the frequency required by the approved procedure. Starting from zero voltage, raise the supply until the target current, usually near rated current, is reached. At zero speed the slip is one. The mechanical-load resistance term in the equivalent circuit becomes zero, but the winding resistances do not. Record every phase voltage and current and total input power using the specified voltmeter, wattmeter and ammeter connections. Take readings quickly to limit heating. Record winding temperature or resistance, then de-energise the motor before changing the setup.
Blocked-Rotor Test Calculations
Resistance and Leakage Reactance Values
At reduced voltage, core loss is usually small. Friction and windage loss are zero because the rotor is stationary. The measured input power is therefore treated mainly as stator and rotor copper loss, subject to the test method’s corrections.
Denote the combined copper loss by Wcu.
Therefore:
Where Wc = residual core loss included by the selected approximation
Where R01 = combined per-phase stator and rotor resistance referred to the stator.
Thus:
Now define:
Is = blocked-rotor current
Vs = blocked-rotor voltage
Z0 = blocked-rotor impedance referred to the stator
Therefore:
X01 = total motor leakage reactance per phase referred to the stator, calculated as:
The split between stator reactance X1 and rotor reactance referred to the stator X2 is not always equal. Use design data or the allocation specified for the motor type.
Therefore:
Similarly, stator resistance per phase R1 and rotor resistance per phase referred to the stator R2 can be calculated as follows:
Measure R1 with the specified winding-resistance test and apply the required temperature correction. Then obtain R2 by subtracting R1 from R01:
Estimated Current at Rated Supply Voltage
A simple voltage-ratio calculation can estimate blocked-rotor current Isc at rated stator voltage V from measured current Is and applied test voltage Vs.
Treat the result as an estimate. Saturation, rotor position, conductor skin effect and test frequency can make impedance vary with the test conditions.
