The blocked-rotor test of a squirrel-cage induction motor is an important experiment used to determine leakage reactance, resistance, and short-circuit characteristics of the motor. It is performed by locking the rotor and applying a reduced voltage to the stator. The main objective is to determine equivalent circuit parameters such as stator resistance, rotor resistance (referred to stator), and leakage reactance. It also helps estimate short-circuit current and analyze starting performance.
- Determine equivalent circuit parameters
- Estimate short-circuit current
- Analyze starting torque and power factor
In this test, the rotor is first locked to prevent rotation. Then a reduced voltage (about 25–30% of rated value) is applied to avoid overheating. The following quantities are measured:
- Input voltage \(V\)
- Input current \(I\)
- Input power \(W\)
Using these readings, the parameters are calculated as:
\[
Z = \frac{V}{I}
\]
\[
R = \frac{W}{3I^2}
\]
\[
X = \sqrt{Z^2 – R^2}
\]
These values help determine leakage reactance, rotor resistance, and short-circuit current behavior of the motor.
| Parameter | Determined From Test | Importance |
|---|---|---|
| Leakage reactance | From impedance and resistance | Used in torque calculation |
| Rotor resistance | From input power | Affects starting torque |
| Short-circuit current | Extrapolated to rated voltage | Ensures safe motor operation |
This test is widely used in industry for motor design validation, performance prediction, and safety assurance during starting conditions.
- Helps in predicting torque-speed characteristics
- Ensures safe operation under starting conditions
- Useful for equivalent circuit modeling
Some precautions must be taken during the test:
- Apply reduced voltage to avoid overheating
- Ensure rotor is completely locked
- Conduct the test for a short duration only
The blocked-rotor test is complementary to the no-load test. Together, they provide complete information about the motor’s equivalent circuit and performance.