No-Load Test and Blocked Rotor Test of Three-Phase Induction Motor, Losses and Efficiency

 

No-Load Test and Blocked Rotor Test of Three-Phase Induction Motor, Losses and Efficiency

These tests are performed to determine the parameters, losses, and efficiency of a three-phase induction motor without loading it fully.

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1. No-Load Test (Open Circuit Test)

Purpose

The no-load test is conducted to determine:

• Iron (core) losses

• Friction and windage losses

• Magnetizing current

• No-load power factor

Test Circuit

      3-Phase Supply
           │
       ┌─────────┐
       │ Ammeter │
       └─────────┘
           │
        ┌───────┐
        │ Motor │
        └───────┘
           │
      Wattmeter & Voltmeter

Procedure

1. Run the motor at rated voltage and frequency.

2. Keep the motor without mechanical load.

3. Record:

   • Line voltage $V_0$

   • No-load current $I_0$

   • Input power $W_0$

Power Measured

$$P_0 = W_0$$

This input power mainly represents:

• Iron losses

• Friction and windage losses

• Small stator copper loss

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2. Blocked Rotor Test (Short Circuit Test)

Purpose

The blocked rotor test is used to determine:

• Total copper losses

• Equivalent resistance

• Leakage reactance

• Starting current

Test Circuit

      3-Phase Supply
           │
        ┌───────┐
        │ Motor │
        │Rotor  │
        │Blocked│
        └───────┘
           │
     Ammeter, Voltmeter
        and Wattmeter

Procedure

1. Lock the rotor so it cannot rotate.

2. Apply reduced voltage to circulate rated current.

3. Measure:

   • Voltage $V_{sc}$

   • Current $I_{sc}$

   • Power $W_{sc}$

Power Measured

$$P_{sc} = W_{sc}$$

This power mainly represents stator and rotor copper losses, since iron loss is very small due to low applied voltage.

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3. Losses in Three-Phase Induction Motor

Total losses in an induction motor are classified as follows:

1. Stator Copper Loss

Loss due to resistance of stator winding.

$$P_{stator} = 3 I^2 R$$

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2. Rotor Copper Loss

Loss due to current in rotor conductors.

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3. Iron (Core) Loss

Occurs in the stator core due to alternating magnetic field.

Types:

• Hysteresis loss

• Eddy current loss

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4. Mechanical Loss

Includes:

• Friction loss (bearings)

• Windage loss (air resistance)

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5. Stray Load Loss

Small additional losses due to:

• Leakage flux

• Harmonics

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4. Efficiency of Three-Phase Induction Motor

Efficiency is defined as the ratio of output power to input power.

$$\eta = \frac{Output\ Power}{Input\ Power} \times 100$$

or

$$\eta = \frac{P_{in} - \text{Total Losses}}{P_{in}} \times 100$$

Where:

• $P_{in}$= Electrical input power

• $P_{out}$ = Mechanical output power

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5. Summary Table

Test	Purpose
No-Load Test	Determines iron loss and mechanical losses
Blocked Rotor Test	Determines copper losses and equivalent parameters