What is No Load Test of an Induction Motor and Block Rotor Test of

Electric Motors?
The circuit model of an induction motor is similar to the transformer. Therefore, the circuit
parameter of a model is also similar to the circuit parameter of a transformer. The No-load test
of an induction motor is similar to the open-circuit test of a electrical transformer which is
performed to determine the efficiency of motor and related circuit parameters of three phase
electric motors.

This test is performed in an induction motor to determine no-load current I0, no-load power
factor cosϕ0, fiction and windage loss Pwf, no-load core loss Pi, no-load power input Po, and
no-load resistance R0 and reactance X0.

For small size motors, we can determine performance parameters by directly applying to load.
But in the case of a large motor, it is inconvenient to apply a large load in the laboratory.
Hence, for this type of motor, the no-load test is convenient to determine performance
parameters.

Theory of No-load Test

As the name suggests, this test was performed without loading condition. The rated
voltage and rated frequency three-phase supply are given to the stator winding of three
phase induction motor. The connection diagram of the no-load test of an induction motor
is shown in the figure below.

An ammeter A and voltmeter V are connected as shown in the figure to measure the no-load
current and the rated supplied voltage respectively. To measure the input power two-wattmeter
method is used. Hence, two watt-meters P1 and P2 are connected as shown in the figure to
measure the input power.
In a no-load test, the motor is running without load. Hence, the power factor is less than 0.5
and the total power input is equal to the sum of two wattmeter readings. One wattmeter will
show negative reading. So, it is necessary to reverse the direction of current coil terminals to
take a proper reading.

Pconstant = P1 + P2
At no-load, input power is equal to the core loss, stator copper loss, and friction and windage
loss. In no-load condition, the current that passes through the circuit is very small (approx. 20-
30% of rated current) and the slip is extremely small (in order of 0.001). Therefore, the I 2R
loss in stator winding can be neglected as it varies with the square of the current.

The load is not connected with the rotor winding. And also, for the wound rotor, the external
resistance should be cut out completely. The rotor resistance in the circuit model is . In this
equation, the slip is very small. Therefore, the value of rotor resistance is extremely high. In
no-load conditions, the equivalent circuit is shown in the figure below.

Separation of Losses
By determining the stator resistance R1, we can calculate the stator copper loss by;

PC1 = 3I02 + R1
So, we can calculate the stator copper loss from the above equation.

To find the value of a core loss, stator copper loss, and friction and windage loss are subtracted
from the input power. If we have the value of no-load current I0, applied voltage V0, and total
core loss, we can determine the magnetizing component Im and energy component Ie, no-load
resistance R0, and no-load reactance X0 from the following calculations.
Block Rotor Test of Induction Motor
The block rotor test is performed to determine the short circuit current I sc power factor at short
circuit cos фsc, total equivalent resistance R01 and reactance X01 referred to the stator. This test
is the equivalent of short-circuit test in a transformer.

In this test, the rotor is locked and it cannot move. Hence, this test is also known as the locked
rotor test. In the case of a slip ring induction motor, the rotor winding is short-circuited using
slip rings and in the case of a wound rotor motor, the rotor bars are short-circuited.

Three-phase supply of variable voltage is given to the stator winding. Generally, an auto-
transformer is used to get variable supply voltage. The connection diagram of a block rotor
test is the same as the no-load test. The only difference is that the supplied voltage and
frequency are reduced and the instruments used in this test must change to record high power
and current indication.

At starting, zero voltage is supplied to the stator, and gradually increase the voltage in step
with the help of an autotransformer. The voltage is increased till the full-load current flows in
the stator. At this stage, note down the record of the voltmeter, ammeter, and wattmeter. The
reading must be taken as quickly as to avoid overheating. The temperature of stator winding
was also measured during the test to increase the accuracy of the result and made appropriate
corrections can be done for the recommended temperature of 75˚C.

As the rotor is blocked, the mechanical losses (friction and windage losses) are zero. And the
reduced voltage is supplied to the motor. Hence, the flux produced during the block-rotor test
is very less. So, the core loss is very small and it can be ignored during this test. Therefore,
the power input is considered to fulfill the stator and rotor copper loss.

The no-load resistance is also ignored in this test. The magnetizing reactance X 0 is small in
the case of an induction motor compared to the transformer. But it cannot be ignored. So, the
equivalent circuit of this test is shown in the figure below.

The supplied voltage (line to line) is Vs. And due to this voltage, the current cause in the stator
winding is Is and the total input power during the short circuit test is P s. In this condition, the
short-circuit current with normal voltage V (line to line) applied across the motor is given by;

The input power required during this test is to supply the rotor and stator copper loss. The
input power is;

PC1 = 3IS2 + R01

Equivalent Resistance per phase (referred to stator side) R01;

Equivalent Impedance per phase (referred to stator side) Z01;

Equivalent Reactance per phase (referred to stator side) X01;

Now, from the above value, we can find the equivalent circuit of the block rotor test as shown
in the figure below.

Generally, stator reactance per phase X1 is assumed to be equal to the rotor reactance per phase
X2’.
In the case of a wound rotor motor, the stator and rotor resistances are separated by dividing
equivalent resistance R01 in the ratio of DC resistance of the stator and rotor windings. In the
case of a squirrel cage induction motor, the rotor resistance per phase (referred to as a stator)
is derived by subtracting R1 from R01.

Blocked Rotor Test of Induction Motor

Key learnings:
 Blocked Rotor Test Definition: The blocked rotor test of an induction motor is defined
as a test to find the leakage impedance and other performance parameters.
 Purpose of Blocked Rotor Test: It determines torque, motor characteristics, and short-
circuit current at normal voltage.
 Testing Procedure: During the test, the rotor is blocked, and low voltage is applied to
the stator to measure voltage, power, and current.
 Effect on Impedance: Rotor position, frequency, and magnetic dispersion can impact
the measured leakage impedance.
 Short Circuit Current Calculation: The test helps calculate short circuit current for
the normal supply voltage by measuring specific parameters.

Induction motors are widely used in industries and consume a lot of power. To improve
performance, tests like the no-load test and blocked rotor test are used. A blocked rotor test
helps find the motor’s leakage impedance.

Additionally, the test can determine parameters like torque and short-circuit current at normal
voltage. The blocked rotor test is similar to the short circuit test of a transformer. In this test,
the motor’s shaft is blocked, and the rotor winding is short-circuited.

In slip ring motors, the rotor winding is short-circuited through slip rings, while in cage
motors, rotor bars are permanently short-circuited. Testing induction motors can be complex
because rotor position, frequency, and magnetic dispersion affect leakage impedance. These
effects are minimized by conducting a blocked rotor current test on squirrel-cage rotors.
Process of Testing of Blocked Rotor Test of Induction Motor
During the blocked rotor test, the applied voltage to the stator terminals should be low to
prevent damage to the stator winding. Low voltage is used to stop the rotor from rotating,
making its speed zero and allowing full load current through the stator winding. With the slip
at unity, the load resistance is zero. Slowly increase the stator voltage until the current reaches
its rated value. Note the readings from the voltmeter, wattmeter and ammeter to determine
voltage, power, and current. The test can be repeated at different stator voltages for accuracy.

Calculations of Blocked Rotor Test of Induction Motor

Resistance and Leakage Reactance Values

In the blocked rotor test, core loss is very low because of the low voltage supply, and frictional
loss is negligible since the rotor is stationary. However, stator and rotor copper losses are
relatively high.

Let us take denote copper loss by Wcu.

Therefore,

Where, Wc = core loss

Where, R01 = Motor winding of stator and rotor as per phase referred to stator.
Thus,
Now let us consider
Is = short circuit current
Vs = short circuit voltage
Z0 = short circuit impedance as referred to stator

Therefore,
X01 = Motor leakage reactance per phase referred to stator can be calculated as

Stator reactance X1 and rotor reactance per phase referred to stator X2 are normally assumed
equal.
Therefore,

Similarly, stator resistance per phase R1 and rotor resistance per phase referred to stator R2
can be calculated as follows:
First some suitable test are done on stator windings to find the value of R1 and then to find
R2 subtract the R1 from R01

Short Circuit Current for Normal Supply Voltage

To calculate short circuit current Isc at normal voltage V of the stator, we must note short-
circuit current Is and low voltage Vs applied to the stator winding.
Some questions on Induction motor:
1. What is a three-phase induction motor and how does it operate?
2. What are the main components of a three-phase induction motor?
3. What are the different types of three-phase induction motors?
4. How is the speed of a three-phase induction motor determined?
5. What is slip in a three-phase induction motor, and how does it affect
performance?
6. What are the advantages of using a three-phase induction motor over single-
phase motors?
7. How do the starting methods of a three-phase induction motor work?
8. What are the common causes of failure in three-phase induction motors?
9. What role does the stator and rotor play in the functioning of a three-phase
induction motor?
10.How is torque developed in a three-phase induction motor?
11.What is the significance of power factor in a three-phase induction motor?
12.What is the difference between squirrel-cage and wound-rotor three-phase
induction motors?
13.How can the efficiency of a three-phase induction motor be improved?
14.What are the typical applications of three-phase induction motors in industries?
15.How does voltage and frequency affect the performance of a three-phase
induction motor?