- No Load Test of Induction Motor Definition: A no load test of induction motor is defined as a test conducted when the rotor rotates at synchronous speed without any load torque.
- Purpose of No Load Test: This test helps identify no-load losses like core loss, friction loss, and windage loss.
- Theory of Test: The test assumes that the impedance of the magnetizing path is large, causing small current flow and the applied voltage is across the magnetizing branch.
- Test Procedure: The motor is run at rated voltage and frequency until bearings are fully lubricated, then readings of voltage, current, and power are taken.
- Loss Calculation: Rotational losses are determined by subtracting stator winding losses from the input power, and fixed losses like core loss and windage loss are calculated.
A no-load test provides data for estimating an induction motor’s equivalent-circuit parameters and no-load losses without a mechanical load on the shaft. It is often used with a blocked-rotor test as an indirect method when a full mechanical load test is impractical. The method is similar to an open-circuit test on transformer, but the rotor still turns. An induction motor runs close to synchronous speed at no load, not at synchronous speed. Its small slip makes the rotor-branch impedance very large, so the rotor branch may be neglected in an approximate analysis.
The test measures the combined effect of core loss, friction and windage, as well as the current needed to establish the air-gap flux. Rotor copper loss is usually small because the slip is small. However, no-load current can still be a substantial fraction of rated current. The readings, together with the measured stator resistance, are used to estimate the magnetising branch and no-load losses.
Theory of No Load Test of Induction Motor
The magnetising branch has a high impedance, so most of the applied voltage appears across it. A simplified equivalent circuit may neglect the stator impedance drop, but the input power is not all core loss. It also supplies friction and windage. Because an induction motor has an air gap, its exciting current can be large enough for the no-load stator I2R loss to matter.
Uncouple the driven equipment where the approved procedure requires a true no-load condition. Guard the rotating shaft, use rated frequency, raise the balanced three-phase voltage under control, and stop if current, vibration or temperature exceeds the permitted limit.
Run the motor until its readings stabilise, then record line voltage, every line current, total input power and speed. Subtract the stator I2R loss from input power to obtain the remaining no-load loss. This remaining value combines core loss with friction and windage unless the test method includes an additional loss-separation procedure.
No-Load Test Calculations
Let the total measured input power to the induction motor be W0 watts.
Where:
V1 = line voltage
I0 = no-load line current
Combined core and mechanical loss = W0 – S1
Where:
S1 = stator winding loss = Nph I2 R1
Nph = number of phases
This result contains core loss plus friction and windage. A single rated-voltage reading does not separate those components.
Stator winding loss = 3Io2R1
Where:
I0 = no-load phase current
R1 = per-phase stator resistance at the required reference temperature
Core loss = 3GoV2 for the per-phase equivalent circuit, where Go is core-loss conductance and V is phase voltage. Use this relation only after the selected test method has separated mechanical loss.
