- 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.
The efficiency of large motors is determined by measuring their input and output powers under load. However, arranging loads for large motors can be challenging and result in significant power loss. Therefore, no load and blocked rotor tests are used. The no load test is performed when the rotor spins at synchronous speed without load torque, similar to the open circuit test on transformer. Achieving exact synchronous speed in an induction motor is impossible, so it is assumed by considering the slip as zero, creating infinite impedance in the rotor branch.
This test provides information about no-load losses like core loss, friction loss, and windage loss. Rotor copper loss at no load is negligible due to its minimal value. A small current is needed to generate enough torque. Also known as the running light test, it evaluates the resistance and impedance of the motor’s magnetizing path.
Theory of No Load Test of Induction Motor
The impedance of the motor’s magnetizing path is high, which limits current flow. Thus, a small current is applied, causing a minor drop in stator impedance and ensuring the rated voltage across the magnetizing branch. This drop and the power loss due to stator resistance are negligible compared to the applied voltage. Hence, it is assumed that all drawn power converts into core loss. The air gap in the magnetizing branch in an induction motor increases the exciting current, making the no-load stator I2R loss recognizable.
It’s crucial to ensure the current does not exceed its rated value to prevent the rotor from accelerating beyond safe limits.
The test is conducted at poly-phase voltages and rated frequency applied to the stator terminals. After the motor runs for a while and the bearings are fully lubricated, readings of applied voltage, input current, and input power are taken. Rotational loss is calculated by subtracting the stator I2R losses from the input power.
Calculation of No Load Test of Induction Motor
Let the total input power supplied to induction motor be W0 watts.
Where,
V1 = line voltage
I0 = No load input current
Rotational loss = W0 – S1
Where,
S1 = stator winding loss = Nph I2 R1
Nph = Number phase
The various losses like windage loss, core loss, and rotational loss are fixed losses which can be calculated by
Stator winding loss = 3Io2R1
Where,
I0 = No load input current
R1 = Resistance of the motor
Core loss = 3GoV2
