Shri Vile Parle Kelavani Mandal’s
INSTITUTE OF TECHNOLOGY
Dhule (M.S.)
Subject: ELECTRICAL MACHINE-II LABORATORY. (BTEEL408) Remarks
Name: __________________________________________ Roll No: _______________
Class: _________________ Branch: _______________ Division: ______________
Exp. No: _______________ Date : / /20 Signature
Title: Load and block rotor tests on squirrel cage induction motor
AIM: To determine the equivalent circuit parameters of a given three phase induction motor by
performing no load and blocked rotor test.
APPRATUS:
Sr. No Apparatus Specification / Range Quantity
Name
1 Voltmeter
2 Ammeter
3 Wattmeter (LPF)
4 Wattmeter(UPF)
5 3-Phase Autotransformer
Induction Motor Name Plate Details:
Power rating: HP, Voltage: V,
Current: A , Speed: (RPM), Frequency: Hz
THEORY:
The induction motor can be treated as generalized transformer. Transformer works on the
principle of electromagnetic induction. The induction motor also works on the same principle. The
energy transfer from stator to rotor of the induction motor takes place entirely with the help of a flux
mutually linking the two. Thus stator acts as a primary while the rotor acts as a rotating secondary when
induction motor is treated as a transformer.
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. 1) to the circuit as shown in Fig. 2. Hence,
the data obtained from this test will give information on the stator and the magnetizing branch. The
connection circuit diagram of no load test is shown in Fig. 3. The no load parameters can be found from
the voltmeter, ammeter, and wattmeter readings obtained when the machine is run at no load as shown
below:
Readings Obtained:
1. Line to line voltage at stator terminals: V0L volts
2. Stator Phase Current: I L amps
3. Per phase power drawn by the stator: P0 watts
Fig. 1. Per phase equivalent circuit of 3-phase induction Fig. 2. Approximate Equivalent Circuit for No-
motor Load Test
BLOCKED ROTOR TEST
Blocked rotor test is similar to the short circuit test on a transformer. It is performed in the 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 frequency of 25 percent of the rated
frequency at a 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 Fig. 3.
Fig. 3. Equivalent Circuit for Blocked Rotor Test
The slip for the blocked rotor test is unity since the rotor is stationary. The resulting speed-
dependent equivalent resistance r2’{(1/s)-1} goes to zero and the resistance of the rotor branch of the
equivalent circuit becomes very small. Thus, the rotor current is much larger than current in the
excitation branch of the circuit such that the excitation branch can be neglected. Voltage and power
are measured at the motor input.
Readings Obtained:
Line to line voltage at stator terminals : Vsc volts
Stator Phase Current : Isc amps
Per phase power drawn by the stator : Psc watt
CIRCUIT DIAGRAM:
NO LOAD TEST ON THREE-PHASE INDUCTION MOTOR
BLOCKED ROTOR TEST ON THREE PHASE INDUCTION MOTOR
PRECAUTIONS:
The three- p h a s e autot r a n s f o r m e r kept at minimum position, when the motor
is starting or stopping time.
If wattmeter reading shows negative, switch off the supply and interchange the
wattmeter terminal M & L connections.
For no load test ensure that there is no load in brake drum For blocked rotor test,
the belt should be tightened to prevent the rotation of rotor.
PROCEDURE:
NO – LOAD TEST:
1. Connections are given as per the circuit diagram.
2. Close the TPST switch.
3. At the time of starting the three phase auto transformer is kept at the minimum
position.
4. Adjust the three phase auto transformer to get the rated voltage.
5. Note down the readings of ammeter, voltmeter and wattmeters and tabulated.
6. Bring the three phase auto transformer is brought to original position.
7. Open the TPST switch.
BLOCKED ROTOR TEST:
1. Connections are given as per the circuit diagram.
2. At the time of starting the three phase variac is kept in minimum position
3. The rotor is locked by tightening the belt over the brake drum.
4. Supply is given by using the TPST switch.
5. The three phase variac is adjusted to get the full load or rater current in the stator.
6. Note down the readings of ammeter, voltmeter and wattmeters and tabulated.
7. The three phase variac is brought to the original position.
8. Open the TPST switch
9. Switch off the power supply.
OBSERVATION TABLE:
NO – LOAD TEST:
No Load Line No Load Power (P0 OR W0)
No Load
Voltage (VOL)
SI. current (IL) W1 W2 W0 = W1 + W2
NO. (Volt) (Amp) (Watt) (Watt) (Watt)
BLOCKED ROTOR TEST:
Short circuit Short circuit Power (PSC OR WSC)
Short
Voltage
SI. circuit W1 W2 WSC = W1
(VSC))
current + W2
NO.
(ISC)
(Volt) (Amp) (Watt) (Watt) (Watt)
EQUIVALENT CIRCUIT FOR A THREE-PHASE INDUCTION MOTOR:
FORMULA USED:
Here Stator Resistance (R) & Slip (S) is given.
CONCLUSION: Hence the parameters found
