THREE PHASE INDUCTION MOTOR EXPERIMENTS

Outline

1. Experiment 1: Introduction to three phase induction motor

2. Experiment 2: Applying DC test on three phase induction motor
3. Experiment 3: Applying open circuit test on three phase induction motor
4. Experiment 4: Applying short circuit (blocked rotor) test on three phase induction
motor
5. Experiment 5: Applying load test on three phase induction motor
6. Experiment 6: Performance and power flow of three phase induction motor

Instructions:
- By the end of each experiment, report should be submitted individually by each student to
the lab’s instructor
- The experiments require at least three sessions of 2 hours each to be completed

EMU
Title InductionMotor
Designed by Assist. Ahmad AL Ahmad

Done by Assist. Ahmad AL Ahmad

a Ia R1
R2
+
10k
Experiment (1) R3
Introduction to Three-phase Induction Motor
10k
1) What areVa
the input and output of Electric Motors?

M M M
Electric 1 3 Mechanical

- DC Motor Single phase motor Three phase motor

ure Circuit
Fig.1. Different types of Electric motors
2) Check the construction of the three phase induction motor in your lab. Write a paragraph about the
construction and principle work of induction motor according to the given process and
illustrations in Figure 2.

Done by Assist. Ahmad AL Ahmad

Done by Assist. Ahmad AL Ahmad
Done by Assist. Ahmad AL Ahmad
 Fig.2. Construction and theory of operation of three phase induction motor.

3) Define the 4 parameters given on the name plate of three phase induction motor and fill them in
table 1.

Fig.3. Three phase induction motor name plate

Table 1: parameter of name plate of three phase induction motor
PN
UN
nN
IN

4) What is the relation between angular speed, Torque and ouput mechanical power.

5) Find rated torque of the three phase induction motor

6) According to the equivalent circuit diagram of three phase induction motor given in Fig.4 (a) and
Fig.4. (b). Study the equivalent circuit and Define every parameter in table 2.

Done by Assist. Ahmad AL Ahmad

 (a)

(b)
Fig.4. Equivalent circuit diagram of three phase induction motor

Table 2: The components of equivalent circuit diagram of the three phase induction motor

Parameters Definitions
and values
V1
E1
E2
Is
I2
I’2
I0
R1
R2
RC
X1
X2
X’2
Xm
s

Done by Assist. Ahmad AL Ahmad

 7. Connect the armature circuit of the asynchronous motor to the three phase supply. What do you
notice?

8. Is Induction motor a self-starter? Why?

9. Why is it called asynchronous motor?

10. What is the relation between synchronous speed, frequency, and number of poles?

11. What is the relation between synchronous speed, motor speed and motor slip?

12. Calculate the value of the slip at full load

13. Are asynchronous motors more applicable than synchronous motors? If yes, Why?

14. Compare the equivalent circuit of asynchronous motor to that of the transformer

15. Give another name for the induction motor

16. What are the required Devices in your lab to test the three phase induction motor

Three phase AC Supply U IM Mechanical Load

Ammeter T
Shaft
V A
W
Resistive Load

DC Generator Voltmeter
V
DC Supply Field
Lf Display

A B

E Armature

Speed Torque Mechanical Power

r.p.m N.m watt

Fig.5. Required devices for testing three phase induction motor

Done by Assist. Ahmad AL Ahmad
 Experiment (2)
Stator Resistance Test of Three-phase Induction Motor
(DC Test of 3-phase IM)

Objective:
To measure the value of the stator resistance of 3-phase IM.
Theory:
Basically, a DC voltage is applied to the stator windings of a 3-phase IM. Because the current is DC, there
is no induced voltage in the rotor circuit and no resulting rotor current. Also, the reactance of the motor is
zero at direct current. Therefore, the only quantity limiting current now in the motor is the stator resistance,
and that resistance can be determined. The basic circuit for the DC test is shown in Figure 1. This figure
shows a DC power supply connected to two of the three terminals of a -connected 3-phase IM.
The measured value of resistance of 3-phase IM should be multiplied by a factor (Skin effect)
ranging from (1.05-1.25) in order to convert its value from DC value to AC value. The skin effect factor
depend on the temperature of motor before test (𝑡𝑠 ) and the temperature of motor at the moment of DC test
(𝑡𝑡 ).

𝑡𝑠 + 𝑘
𝑆𝑘𝑖𝑛 𝑒𝑓𝑓𝑒𝑐𝑡 =
𝑡𝑡 + 𝑘

K=234.5 for copper.

Procedure:

1- Connect the circuit shown in the Figure below.

Fig.1. DC Resistance Test

2- Switch on the DC supply after checking connection by concerned teacher.

3- Increase the supply input voltage slowly from 5V to 40V with step equal to 5V.
4- Record the readings of the measurements in table (1).

Done by Assist. Ahmad AL Ahmad

 Table (1): reading of DC test of 3-phase IM
Vdc (v) Idc (A)
5
10
15
20
25
30
35
40
Calculations:

After applying DC voltage to the terminals of the stator winding and taking the reading of the voltage and
the current, the relationship between them should be plotted, then the DC resistance can be determined as
follows:

Vdc V2  V1
Rdc  
I dc I 2  I1
Rdc
R1dc  perphase for Y connection
2
R 3
R1dc  dc perphase for  connection
2

Skin effect = 1.01 or 1.15

R1, AC  R1,DC  Skin effect

where R1, AC is the AC value of the stator resistance per phase of three phase induction motor

Report:

1- Calculate the stator resistance R1, AC according to the above equations.

Done by Assist. Ahmad AL Ahmad

 Experiment (3)
No load Test of Three-phase Induction Motor

Theory:
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 (a) to the circuit as shown in Fig. 1 (d).
Hence, the data obtained from this test will give information on the stator and the magnetizing branch.

R1 jX1 R2' jX2' R1 jX1 R2' jX2'

Io Io

Vph Ic Im R2'*((1-s)/s) Vph Ic Im

Rc jXm Rc jXm

(a) (b)

Io R1+R2' j(X1+X2') Io

Io Io

Vph Ic Im Vph Ic Im
Rc jXm Rc jXm

(c) (d)
Fig.1. Development of equivalent circuit of induction motor in open circuit test

Procedure:
1. Connect the circuit as shown in the connection diagram in Figure 2.

Done by Assist. Ahmad AL Ahmad

Fig. 2. Induction motor open circuit test

2. Start the motor by ensuring the shaft is at no load condition.

3. For starting, increase the voltage to reach rated voltage.
4. Note the readings of voltmeter, ammeter and wattmeter by carefully and fill them in table 1
5. Reduce the voltage to zero and turn the main switch off.

Table 1: The measured parameters during open circuit test of the induction motor
Parameter Measurement
Value
VLL
IL
P1
P2
Pinput

Discussion:
1. Calculate the machine parameters that is can be obtained from No-Load test.
2. What is the power factor of the machine? Comment on its value.
3. Comment on the slip of the machine when operated at rated voltage.
4. How to obtain the no-load input power to an induction motor when two-wattmeter method of
measuring power used?
5. What is the nameplate reading on the machine? What inferences can be drawn from it?
6. What are the different losses that are present in an induction machine?
7. Which loss in the machine is significant in no load test and why

Done by Assist. Ahmad AL Ahmad

 Experiment (4)
Blocked Rotor Test of Three-phase Induction Motor

Theory:

For this test, the motor shaft is clamped so that it cannot turn. The motor terminals are connected to a 3-
phase supply. The rotor becomes the secondary of a transformer operating at the supply frequency. So, the
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 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. (1).

R1 jX1 R2' jX2'

+
Isc

Vph

Fig. 1: 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.

Procedure:
1. Connect the circuit as shown in the connection diagram in Figure 2.

Fig.2. Short circuit test of induction motor

2. Before starting, ensure that the shaft is blocked completely from rotating.
3. Increase the voltage to the machine such that it draws rated current.
4. Note the readings of voltmeter, ammeter and wattmeter by carefully and fill them in table 1

Done by Assist. Ahmad AL Ahmad

 Table 1: The measured parameters during open circuit test of the induction motor

Parameter Measurement
Value
VLL
IL
P1
P2
Pinput

5. Take the readings quickly so that the machine does not heat up due to full load copper losses
taking place at this condition.
6. Reduce the voltage to zero and turn the main switch off.

Discussion:

1. Calculate the R2' , X1 , X 2'

2. When r2/s is split into a series connection of r2’ and r2’{(1/s)-1} in the rotor equivalent circuit of
an induction machine, what do the power absorbed by the individual resistors physically represent?
3. How does the equivalent circuit of an induction motor simplify to under blocked rotor conditions?
Justify.
4. Which loss in the machine is significant in blocked rotor test and why?

Done by Assist. Ahmad AL Ahmad

 Experiment (5)
Load Test of Three-phase Induction Motor

Aim:
(a) Perform load test on 3-phase induction motor.
(b) Compute Torque, Output power, Input power, Efficiency, Input power factor and Slip for every load
setting and to determine how speed, efficiency, power factor, stator current torque, and slip of an induction
motor vary with load.
(c) Plot the following performance curves: (i) Efficiency Vs. Output power, (ii) Torque Vs. Output power,
(iii) Line current Vs. Output power, (iv) Power factor Vs. Output, (v) Slip Vs. Output power, and (vi)
Torque Vs. Speed

Theory:
The load test on induction motor is performed to compute its complete performance i.e. torque, slip,
efficiency, power factor etc. During this test, the motor is operated at rated voltage and frequency and
normally loaded with resistive load after coupling with DC permanent magnet generator, the performance
can be calculated, following the steps given below.

1) Slip (s)
The speed of rotor (Nr) droops slightly as the load on the motor is increased. The synchronous speed
(Ns) of the rotating magnetic field is calculated, based on the number of poles, P and the supply
frequency (f).
120 f
Ns  in rpm
p
Ns  Nr
s
Ns
Normally, the range of slip at full load is from 2 to 5 percent.

2) Torque (Tm)
The torque of the motor depends on the load. The induction motor is coupled to DC permanent
generator which is connected to a resistive load and a meter.
The meter has the ability to measure the output torque, rotor speed and output mechanical power.
In this case, the torque will be measured by this type of meters.
3) Output Power (Pout)
The output power in watts developed by the motor is given:
Output power = Torque x Speed

Done by Assist. Ahmad AL Ahmad

  2 Nr 
Pout  Ts   
 60 
The output power can be measured also using the same meter that measured the torque
4) Speed (Nr)
When the induction motor is on NO-LOAD speed is slightly below the synchronous speed. The
current due to induced emf in the rotor winding is responsible for production of torque required at
NO-LOAD.
As the load is increased the rotor speed is slightly reduced. The emf induced in the rotor causes the
current increased to produce higher torque, until the torque developed is equal to torque required by
load on motor.

The rotor speed (motor speed) can be measured also using the same meter that measured the output
power and torque.

5) Input Power (Pin)

Input power is measured by the two wattmeter (W1 and W2), properly connected in the circuit:
Pin  P1  P2

Where Pin is the total input power of the induction motor. P1 and P2 are the measured values of
two wattmeter’s’.
6) Power factor (cosΦ)
The power factor of induction motors varies with load, typically from around 0.85 or 0.90 at full
load to as low as about 0.20 at no-load, due to stator and rotor leakage and magnetizing reactance’s.
The input power factor of induction motor can be determined as follows:
Pin  3  Vline  Iline  cos( )

Pin
cos( ) 
3  Vline  Iline
7) Efficiency:
Percentage efficiency of the motor is
Pout
%   100
Pin
Full load efficiency of 3 phase induction motor lies in the range of 72 % (for small motors) to 82 %
(for very large motors).

Done by Assist. Ahmad AL Ahmad

Inductive Pick Up Tachometer
Purpose: It is a sensor used to measure the speed (rpm) of the three phase induction motor. It is mounted
on the shaft of the motor.
Construction & Working:

 Small permanent magnet with a coil wound around it. This magnetic pick up is placed near a
metallic toothed rotor as shown in Figure 1
 As the shaft rotates, teeth passes by pick up & change magnetic flux Inductive voltage generates &
disappears.
 Frequency of pulses depends on no. of teeth & rpm of shaft.
 No. of teeth is known & RPM can be found
 Speed N = Pulse per minute / No. of teeth rpm

Fig.1. Inductive Pick Up Tachometer

Done by Assist. Ahmad AL Ahmad

Procedure:

1- Connect the circuit as per Fig. 2.

Fig. 2: Schematic diagram for load test on Three Phase Induction Motor

2- Switch-on 3 phase AC mains and start the motor at reduced applied voltage.

3- Increase the applied voltage, till its rated value.

4- Take-down the readings of all the meters and the speed under no load running in table (1).

5- Increase the load on the motor gradually using the variables resistive load

6- Record the readings of all the meters at every setting of the load in table (1).

7- Observation may be continued upto the full load current rating of the motor.

8- Reduce the load on the motor and finally unload it completely.

9- Switch-off the supply to stop the motor.

Report: plot the following characteristics according to table 1

1. Torque vs speed

2. Torque vs output power

3. Efficiency vs output power

4. Input Current vs output power

5. Output (load) current vs Torque

6. Slip vs output power

7. Power factor and output power

Hint: You can use excel or matlab for your plotting

Done by Assist. Ahmad AL Ahmad

Table 1: Readings of the load test of three phase induction motor

Load
Set 0 Set 1 Set 2 Set 3 Set 4 Set 5 Set 6 Set 7 Set 8
settings
ILine (A)
VLine-Line
(v)
Measured values at each setting of the resistive load

P1 (watt)
P2(watt)
ILoad(A)
VLoad (v)
Tm
(mN.m)
Nr
(rev/min)
Pout (watt)
Slip s
Calculated values

Pin(watt)
Cos(Φ)
%η

Done by Assist. Ahmad AL Ahmad

 Experiment (6)
The performance and power flow of three phase induction motor

Objective: This test is performed to study the power flow and performance of the three phase induction
motor.

Theory: for this test, the three phase induction motor IM is full loaded (full load test), that’s when the input
current of the loaded three phase IM equals the rated input current given on its name plate. At the end of
the full load test, many parameters which determines the performance of the induction motor is to be
calculated.

Procedure:

1- Connect the circuit as per Fig. 1.

Fig. 1: Schematic diagram for load test on Three Phase Induction Motor

1- Switch-on 3 phase AC mains and start the motor at reduced applied voltage.

2- Increase the applied voltage, till its rated value.

3- Increase the load on the motor gradually using the variable resistive load until the input current of
the three phase IM equals its rated current given on its name plate. In this case, the full load test is
achieved.

4- Record the required readings at full load test in table (1).

5- Reduce the load on the motor and finally unload it completely.

6- Switch-off the supply to stop the motor.

Done by Assist. Ahmad AL Ahmad

Table 1: Full load test readings
P1 P2 VLL Nr Tout
Parameters
(watt) (watt) (volt) (r.p.m) (mN.m)
Measured
values

Calculations:

Using table1, equivalent circuit in figure 1, power flow diagram in figure 2 and the per phase values of
resistances and reactance’s of the three phase IM that are calculated in the previous tests, find the
following parameters in the purpose of studying the performance and the power flow of the three phase
induction motor:

1. Find the synchronous speed Ns, motor slip S and slip speed ns of the three phase IM given that the
motor has four poles fixed at the stator.
2. Find the frequency of the rotor
3. Find the input power factor of the three phase induction motor
4. Find I1, I0 and I2’ using equivalent circuit in Figure 1 (use current divider for I0 and I2’).

R1 jX1 R2' jX2'

Io

Vph Ic Im R2'*((1-s)/s)
Rc jXm

Fig.1. Per phase equivalent circuit diagram of induction motor

5. Find stator copper losses Pstator-Cu

6. Find core losses Pcore
7. Find air gap power Pg
8. Find rotor copper losses Protor-Cu
9. Find the developed power Pd
10. Find the induced torque Tinduced

Done by Assist. Ahmad AL Ahmad

 11. Find the rotational losses Prot
12. Find the output power using table 1
13. Find the efficiency of this motor

Fig.2. Power flow diagram of the three phase induction motor

14. Find the starting current Ist and starting torque Tst
15. Using thevenin’s theorem, find maximum slip Smax and maximum torque Tmax
16. State the values of the slip S in three cases:
- Min value of s (No load)
- Max value of s (at maximum torque)
- Value of slip at full load
17. Draw the Torque-speed characteristic by your hand showing the values of speed, slip and torque
(min, max, starting and the values at full load).

Done by Assist. Ahmad AL Ahmad