No load and block rotor test of Three Phase Induction motor.

Experiment No.: – 04

Title of the Experiment: –

No load and block rotor test of Three Phase Induction motor.

Objectives: –

To be written by student.

Apparatus Required: –

Sl. No.NameSpecificationQuantity
01Three phase Induction Motor400/440V, 50Hz, 6.9 A1 nos.
02Voltmeter(0-500)V, MI1 nos.
03Ammeter(0-5/10)A, MI1 nos.
03Wattmeter(0-750)W, 5/10A, 600V2 nos.
04TachometerDigital Type1 nos.
05Connecting wirePVC insulated copperas per required
Table No: 3.1

Circuit Diagram: –

Fig. No. 3.1: Circuit diagram for No-load and block rotor test of three phase Induction Motor.

Theory: –

No-Load Test:

The no load test is similar to the open circuit test on a transformer. It is performed to obtain the magnetizing branch parameters (shunt parameters) in the induction machine equivalent circuit. In this test, the motor is allowed to run with no-load at the rated voltage of rated frequency across its terminals. Machine will rotate at almost synchronous speed, which makes slip nearly equal to zero. This causes the equivalent rotor impedance to be very large (theoretically infinite neglecting the frictional and rotational losses). Therefore, the rotor equivalent impedance can be considered to be an open circuit which reduces the equivalent circuit diagram of the induction machine (Fig. 3.1) .Hence, the data obtained from this test will give information on the stator and the magnetizing branch. The no load parameters can be found from the voltmeter, ammeter, and wattmeter readings obtained when the machine is run at no load.

Fig. No. 3.1: Equivalent circuit of a three phase Induction Motor at No-Load.

Block Rotor test:

Blocked rotor test is similar to the short circuit test on a transformer. It is performed to calculate the series parameters of the induction machine i.e., its leakage impedances. The rotor is blocked to prevent rotation and balanced voltages are applied to the stator terminals at a 25% of the rated voltage where the rated current is achieved. Under the reduced voltage condition and rated current, core loss and magnetizing component of the current are quite small percent of the total current, equivalent circuit reduces to the form shown in below (Fig. No. 3.2).

Fig. No. 3.2: Equivalent circuit of a three phase Induction Motor at Blocked Rotor Condition.

The slip for the blocked rotor test is unity since the rotor is stationary.

Precautions: –

  1. Ensure the motor is unloaded to minimize mechanical losses.
  2. Supply rated voltage to avoid saturation of the core.
  3. Ensure proper locking of the rotor to avoid accidents.
  4. Use reduced voltage to prevent damage from high currents.
  5. Maintain safety measures as high currents can cause overheating.

Procedures: –

For No-Load test:

  1. Connect the circuit as per circuit diagram.
  1. Keep the variac to be at zero output.
  2. First switch on the 3Ñ„ supply.
  3. Gradually increase the voltage applied to the machine to the rated voltage. Motor runs at a speed quite close to its synchronous speed.
  4. Take the reading of voltmeter, ammeter ,wattmeter & speed on rated voltage.

For Block Rotor test:

  1. Connect the circuit as per circuit diagram.
  1. Keep the variac to be at zero output.
  2. First switch on the 3Ñ„ supply.
  3. Now, keeping the rotor still (block the rotor from running), slowly increase the autotransformer output until rated current flows (Typically, this happens at 25% of the rated voltage).
  4. Take the reading of voltmeter, ammeter ,wattmeter on rated current.

Observation Table: for No-Load test-

Sl. No.V0 ( in volts.)I0 (in Amps.)W1 (in watts.)W2 (in watts.)P0= W1+W2 (in watts.)
01
Table No: 3.2

Calculation: –

From No-load test data:

\[\color\green{No-load\;\;power\;=}\] \[\color\red{Windage \;and \;friction \;losses\;+\;core \;losses}\] \[\color\green{P_0}\;=\;\color\red{P_{wf}\;+\;P_i}\]
\[\color\green{windage \;and \;friction \;losses\; is \;small.}\] \[\color\magenta{No-load \;power \;factor\;=}\] \[\color\green{\cos\phi_0\;=}\color\red{\dfrac{P_0}{\sqrt{3}*V_0*I_0}}\] \[\color\magenta{No-load \;energy \;component\;current\;=}\] \[\color\green{I_e\;=}\color\red{I_0*cos\phi_0\;=I_0*\dfrac{P_0}{\sqrt{3}*V_0*I_0}\;=\;\dfrac{P_0}{\sqrt{3}*V_0}}\] \[\color\magenta{No-load \;magnetising \;component\;current\;=}\] \[\color\green{I_m\;=}\color\red{\sqrt{I_0^2-I_e^2}}\] \[\color\magenta{No-load \;Impedance\;=}\] \[\color\green{Z_0\;=}\color\red{\dfrac{\dfrac{V_0}{\sqrt{3}}}{I_0}}\] \[\color\magenta{No-load \;Resistance\;=}\] \[\color\green{R_0\;=}\color\red{\dfrac{\dfrac{V_0}{\sqrt{3}}}{I_e}\;=\;\dfrac{\dfrac{V_0}{\sqrt{3}}}{\dfrac{P_i}{\sqrt{3}*V_0}}\;=\;\dfrac{V_0^2}{P_i}}\] \[\color\magenta{No-load\;Reactance\;=}\] \[\color\green{X_0\;=}\;\color\red{\sqrt{Z_0^2-R_0^2}}\]

Observation Table: for Block rotor test-

Sl. No.Vs ( in volts.)Is (in Amps.)W1 (in watts.)W2 (in watts.)Ps= W1+W2 (in watts.)
01
Table No: 3.3

Calculation: –

From Blocked rotor test data:

\[\color\green{Short \;circuit \;power \;factor\;=}\] \[\color\magenta{cos\;\phi_s\;=}\color\red{\;\dfrac{P_s}{\sqrt{3}*V_s*I_s}}\] \[\color\green{Short \;circuit \;power\;=}\] \[\color\magenta{P_s\;=}\color\red{3*I_s^2*R_{01}}\] \[\color\green{Motor \;equivalent\; resistance\; per\; phase,\; as\; referred\; to\; stator,}\] \[\color\magenta{R_{01}\;=}\color\red{\dfrac{P_s}{3*I_s^2}}\] \[\color\green{Motor \;equivalent\; impedance\; per\; phase,\; as\; referred\; to\; stator,}\] \[\color\magenta{Z_{01}\;=}\color\red{\dfrac{\dfrac{V_s}{\sqrt{3}}}{I_s}}\] \[\color\green{Motor \;equivalent\; reactance\; per\; phase,\; as\; referred\; to\; stator,}\] \[\color\magenta{X_{01}\;=}\color\red{\sqrt{Z_{01}^2-R_{01}^2}}\]

Conclusion: –

To be written by student.

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