Study of Characteristics of Three Phase Induction Motor

SCHOOL OF ELECTRICAL AND ELECTRONICS ENGINEERING

LAB REPORT

THREE PHASE INDUCTION MOTOR

SUBJECT

CONTROL AND ELECTRICAL SYSTEMS

YEAR

2011

2

EXPERIMENT 1

STUDY OF CHARACTERISTICS OF THREE PHASE INDUCTION MOTOR

Aim

To study the no load characteristics of three phase induction motor

Objectives

i) To investigate the no load characteristic of a three phase induction motor.

ii) To study the relation between the current and the voltage of three phase induction

motor in both star and delta connection.

iii) To modify the sense of rotation.

Name plate details

3 phase induction motor

Rated voltage = 41.5 V.AC

Rated power = 50 W

Rated current = 1.4 A

Rated speed = 1320 RPM

3

Introduction

Induction motor is a rotating electrical machine. Induction motor can be operated by one

phase alternating voltage source and by three phase alternating voltage source. The

induction motor consists of ‘Stator’ and ‘Rotor’, stationary part and rotational part,

respectively. The rotor consists of cylindrical iron that laminated with copper strips. The

stator consists of a three phase windings that are placed on the slots of laminated core,

these windings apart by 120 degrees. These windings can be connected be configured in a

star connection or in delta connection.

4

i) Star connection

Circuit diagram

Figure 1.0: Star connection diagram

Procedure

Connections were mad as shown in (Figure 1.0).

The AC supply voltage adjusted to minimum and the maximum rated voltage of

the motor is noted.

The AC supply is turned on and the voltage increased to the maximum voltage

rated for the motor which is 41.5 volts. I been ware to not exceed the limit.

The no load current of the maximum rated voltage for the motor is tabulated.

The voltage is decreased gradually by 5 volts and the no loads current is tabulated

and repeat by decreasing the voltage and tabulate the no load current and voltage.

When all readings are tabulated I decreased the AC supply voltage to 0 volts and

off the supply.

The no load characteristics of the induction motor is sketched.

5

Experimental results of star connection

SI

No.

Star connection

Voltage Current

1 41.5 0.4

2 35 0.35

3 30 0.3

4 25 0.25

5 20 0.2

6 15 0.15

7 10 0.1

Star connection IV characteristics graph

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 10 20 30 40 50

Cu

rre

nt

(A)

Voltage (V)

Figure 1.1: Star connection I versus V graph

Table 1.0: Star connection IV table

6

ii) Delta connection

Circuit diagram

Figure 1.2: Delta connection diagram

Procedure



the motor is noted.



The no load current of the maximum rated voltage for the motor is tabulated.

The voltage is decreased gradually by 5 volts and the no loads current is tabulated

and repeat by decreasing the voltage and tabulate the no load current and voltage.

When all readings are tabulated the AC supply voltage is decreased to 0 volt and

off the supply.

The no load characteristics of the induction motor is sketched.

7

Experimental results of delta connection:

SI

No.

Star connection

Voltage Current

1 41.5 1.4

2 30 0.9

3 25 0.7

4 20 0.6

5 15 0.5

6 10 0.4

7 5 0.3

. Delta connection IV characteristics graph

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25 30 35 40 45

Cu

rre

nt

(A)

Voltage (V)

Figure 1.2: Delta connection I versus V graph

Figure 1.2: Delta connection IV table

8

Analysis

1. Difference in phase and line current in star and delta connections.

Figure 1.3: Star connection

(Figure 1.3) shows that for a star connected loads the potential difference across

any load is the potential difference between the phase line connected to it and the

neutral, however the line voltage will be greater than this because it is the

potential difference between two phase lines. The current through the load will be

the same as the line current. Thus for balanced star connected loads:-

ELine = √3EPhase

And

ILine = IPhase

IP = IL

9

Figure 1.4: Delta connection

(Figure 1.4) shows that for delta connected systems of loads the phase voltage is

equal to the difference between two lines. The phase current for each load is

coming from two lines (because each load is connected to two lines) and will be

more than the line current. Thus for delta connected loads:-

ELine = EPhase

And

ILine = √3 IPhase

2. Reasons for the motor to change the rotation direction.

When exchanging two phases the motor rotation direction changes due to the

change in flux direction produced by the windings in the motor.

√3 IP = IL

10

3. Explanation on how rotating magnetic field is produced in the stator of a

three phase induction motor.

The premise for motor operation is that if you can create a rotating magnetic field

in the stator of the motor, it will induce a voltage in the armature that will have

magnetic properties causing it to 'chase' the field in the stator. This premise

applies to AC motors that employ a squirrel cage rotor, and it is probably the most

simple and basic of all motor designs. The three phase motor is widely used in

industry because of its low maintenance characteristics. Due to the nature of three

phase power, creating a rotating magnetic field in the stator of this motor is simple

and straight forward.

The stator windings are arranged on the stator poles in a way that results in

magnetic flux lines that seem to rotate where , the number of pole pairs must be

the same as or a multiple of the number of phases in the applied voltage. The

poles must then be displaced from each other by an angle equal to the phase angle

between the individual phases of the applied voltage and the strength of the

magnetic field changes, as the current flow in the coils of wire around the stator

poles change. Just as the current in the field windings rise and fall 120 electrical

degrees apart, so does the resulting magnetic field on the pole.

11

4. Safety requirements when dealing with high power rated three phase

induction motor.

Safety is needed at a time of installation, after the installation and at the time of

servicing.

Safety requirement at time of installation:-

Check for any damage which may have been incurred in handling.

The motor shaft should turn freely by hand. Repair or replace any loose or

broken parts before attempting to use the motor.

Check to be sure that motor has not been exposed to dirt, grit, or excessive

moisture in shipment or storage before installation.

Measure insulation resistance.

Clean and dry the windings as required. Never start a motor which has

been wet without having it thoroughly dried.

Safety requirement after installation:-

Ensure that safety guards are in use.

Keep hands and clothing away from moving parts.

Never attempt to measure the temperature rise of a motor by touch.

Temperature rise must be measured by thermometer, resistance, imbedded

detector, or thermocouple.

12

Safety requirement at time of maintenance:-

All power sources to the motor and to the accessory devices should be de-

energized and disconnected and all rotating parts should be at standstill.

Suitable protection must be used when working near machinery with high

noise levels.

Safeguard or protective devices must not be by-passed or rendered

inoperative.

The frame of this machine must be grounded in accordance with the

National Electric Code and applicable local codes.

A suitable enclosure should be provided to prevent access to the motor by

other than authorized personnel. Extra caution should be observed around

motors that are automatically or have automatic re-setting relays as they

may restart unexpectedly.

Shaft key must be fully captive or removed before motor is started.

Provide proper safeguards for personnel against possible failure of motor-

mounted brake, particularly on applications involving overhauling loads.

13

Conclusion

All in all, the three phase synchronous induction machine consist of winding

either connected in delta configuration or Star configuration but for each

configuration the relation between the phase and line in voltage and current varies.

And the rotating direction depends on the flux produced by the windings.

14

References

i. Donald G. Beaty, H. Wayne Fink, (1978). Standard Handbook for Electrical

Engineers. 11th Edition. The Kingsport Press.

ii. Galileo Ferraris, (1885). Electromagnetic rotation with an alternating current,

Electrican, Vol 36. p 360-75.

iii. Benjamin Franklin Bailey, (1911). The induction motor. McGraw-Hill.

15

EXPERIMENT 2

LOAD TEST ON THREE PHASE INDUCTION MOTOR

Aim

The load test on 3-phase induction motor is performed to obtain its various characteristics

including efficiency.

Objectives

i) To investigate the performance characteristics of three phase Induction motor by

applying eddy current load

ii) To determine the efficiency of a three phase Induction motor

Name plate

3 phase induction motor

Rated voltage = 41.5 V.AC

Rated power = 50 W

Rated current = 1.9 A

Rated speed = 1320 RPM

16

Circuit Diagram

3 phase squirrel cage induction

motor

Eddy current load

Stator star connected

3-phase winding

Eddy

current

load

3 phase

41.5V 50Hz

AC supply

Figure 2.0: Motor to Eddy current load connection

17

Procedure



the motor is noted.



The no load current, power factor, power and torque of the maximum rated

voltage for the motor are tabulated.

Eddy current load was connected to the induction motor and supplied by a DC supply.

The Eddy current load is increased gradually by increasing the DC supply.

The load current, power factor, power and torque are tabulated for the resultant

until the load current reaches the maximum rated current.

When all readings are tabulated the AC and DC supplies are decreased to 0 volt

and off the supplies.

The load characteristic of the induction motor is sketched.

Experimental results

V1 V2 I1 I2 P.F1 P.F2 T N

41.3 V 41.5 V 0.5 A 0.5 A 0.995 0.82 0 1412.5

40.6 V 40.8 V 0.6 A 0.5 A 0.995 0.81 0.114 1389

38.8 V 39.1 V 0.7 A 0.6 A 0.995 0.78 0.1444 1332.4

35.9 v 36.2 V 0.8 A 0.8 A 0.998 0.74 0.2052 1223.1

35.5 V 35.8 V 0.9 A 0.8 A 0.999 0.72 0.2584 1191.1

34.4 V 34.7 V 1 A 0.9 A 0.999 0.69 0.3192 1082.3

Table 2.0: Tabulated experimental results

18

Analysis

1. Determine the input power, output power and efficiency.

i. For V1 = 41.3, I1 = 0.5 and Cos1 = 0.995

W1 = V1 x I1 x Cos1

= (41.3) (0.5) (0.995)

= 20.55W

For V2 = 41.5, I2 = 0.5 and Cos2 = 0.82

W2 = V2 x I2 x Cos2

= (41.5) (0.5) (0.82)

= 17.02W

Power Input = W1 + W2

= 20.55 + 17.02

= 37.57W

Power output = T

= 0

= 0 W

Efficiency% =

x 100

19

=

x 100

= 0 %

Synchronous speed (Ns) =

=

= 1500rpm

Thus, slip =

=

= 0.06

ii. For V1 = 40.6, I1 = 0.6 And Cos1 = 0.995

W1 = V1 x I1 x Cos1

= (40.6) (0.6) (0.995)

= 24.24W

For V2 = 40.8, I2 = 0.5 and Cos2 = 0.81

W2 = V2 x I2 x Cos2

= (40.8) (0.5) (0.81)

= 16.52W


= 24.24 + 16.52

= 40W

20

Power output = T

= 0.0114

= 16.58W

Efficiency% =

x 100

=

x 100

= 40.68%

Slip =

=

= 0.074

iii. For V1 = 38.8, I1 = 0.7 and Cos1 = 0.995

W1 = V1 x I1 x Cos1

= (38.8) (0.7) (0.995)

= 27.02W

For V2 = 39.1, I2 = 0.6 and Cos2 = 0.78

W2 = V2 x I2 x Cos2

= (39.1) (0.6) (0.78)

= 18.3W

21


= 27.02 + 18.3

= 45.32W

Power output = T

= 0.1444

= 20.15W

Efficiency% =

x 100

=

x 100

= 44.46%

slip =

=

= 0.112

iv. For V1 = 35.9, I1 = 0.8 and Cos1 = 0.998

W1 = V1 x I1 x Cos1

= (35.9) (0.8) (0.998)

= 28.66W

For V2 = 36.2, I2 = 0.8and Cos2 = 0.74

22

W2 = V2 x I2 x Cos2

= (36.2) (0.8) (0.74)

= 21.43W


= 28.66 + 21.43

= 50.09W

Power output = T

= 0.2052

= 26.3W

Efficiency% =

x 100

=

x 100

= 52.5%

Slip =

=

= 0.184

23

v. For V1 = 35.5, I1 = 0.9 and Cos1 = 0.999

W1 = V1 x I1 x Cos1

= (35.5) (0.9) (0.999)

= 31.79W

For V2 = 35.8, I2 = 0.8 & Cos2 = 0.72

W2 = V2 x I2 x Cos2

= (35.8) (0.8) (0.72)

= 20.62W


= 31.79 + 20.62

= 52.41W

Power output = T

= 0.2548

= 32.23W

Efficiency% =

x 100

=

x 100

= 61.5%

Slip =

=

= 0.206

vi. For V1 = 34.4, I1 = 1 and Cos1 = 0.999

24

W1 = V1 x I1 x Cos1

= (34.4) (1) (0.999)

= 34.37W

For V2 = 34.7, I2 = 0.9 and Cos2 = 0.69

W2 = V2 x I2 x Cos2

= (34.7) (0.9) (0.69)

= 21.55W


= 34.37 + 21.55

= 55.92W

Power output = T

= 0.3192

= 36.18W

Efficiency% =

x 100

=

x 100

= 64.7%

Slip =

=

= 0.28

25

The calculated input power, output power and efficiency are tabulated as shown in

(Table 1.1).

W1 W2 Efficiency

20.55w 17.02w 37.57 0 14.9%

24.24w 16.52w 40.76 16.58 40.68%

27.02w 18.30w 45.32 20.15 44.46%

28.66w 21.43w 50.09 26.3 52.5%

31.79w 20.62w 52.41 32.23 61.5%

34.37w 21.55w 55.92 36.18 64.7%

1. Can the speed of the induction motor be equal to synchronous speed?

At idle the rotor almost reaches the synchronous speed of the rotary field,

since only a small counter-torque (no-load losses) is present. If it were to turn

exactly synchronously, voltage would no longer be induced, current would

cease to flow, and there would no longer be any torque. During operation the

speed of the rotor drops to the load speed n. The difference between the

synchronous speed and the load speed is called slip s. Based on this load-

dependent slip s, the voltage induced in the rotor winding changes, which in

turn changes the rotor current and also the torque T. As slip s increases, the

rotor current and the torque rise. And that resultant drops in speed.

Table 2.1: Input power, output power and efficiency

26

3) Construction of squirrel cage rotor and slip ring rotor.

The squirrel cage rotor consists of a slotted cylindrical rotor core sheet package

with aluminium bars which are joined at the front by rings to form a closed cage. The

rotor of three-phase induction motors sometimes is also referred to as an anchor. The

reason for this name is the anchor shape of the rotors used in very early electrical

devices. In electrical equipment the anchor's winding would be induced by the

magnetic field, whereas the rotor takes this role in three-phase induction motors as

shown in (Figure 2.1).

Figure 2.1: Squirrel cage rotor

27

The slip ring induction motor rotor has similar construction as the squirrel cage

rotor, unless it consists of slip rings and it has three carbon brushes connected

externally to a 3-phase start connected rheostat as shown in (Figure 2.2). Thus

these slip ring and external slip rings rheostat makes the slip ring induction motors

possible to add external resistance to the rotor circuit, thus enabling them to have

a higher resistance during starting and thus higher starting torque. When running

during normal condition, the slip rings are automatically short-circuited by means

of a metal collar, which is pushed along the shaft, thus making the three rings

touching each other. Also, the brushes are automatically lifted from the slip-rings

to avoid frictional losses, wear and tear. Hence, under normal running conditions,

the wound rotor is acting as same as the squirrel cage rotor.

Rotor windings

Slip rings

Carbon brushes

Rheostat

Figure 2.2: Slip ring rotor

28

4) Sketching torque, speed and slip characteristics of the induction motor

Figure 2.3: Slip and Speed versus torque graph

29

5) Sketching torque, speed and slip characteristics of the induction motor

by using MATLAB software.

MATLAB commands is as shown in (Figure 2.4):-

The plotted slip versus torque graph is shown in (Figure 2.5) and the speed

versus torque graph is as shown in (Figure 2.6).

Figure 2.4: MATLAB commands

30

Figure 2.5: Slip vs torque graph by using MATLAB

Figure 2.6: Speed vs torque graph by using MATLAB

31

Conclusion

According to the plotted graphs we observe that the slip having a proportional

relation with the torque as the torque is increasing the slip is increasing as well,

while the relation between the torque and speed is non-proportional as the speed is

increasing the torque is decreasing.

32

References

i. B.L. Theraja, (2008). Textbook of Electrical Technology. Chand (S.) & Co

Ltd ,India.

ii. Edward Hughes, (2001). Electrical & Electronic Technology. 8th

Edition.

Longman Group United Kingdom.

iii. William D Stevenson, (1975). Elements of power system analysis

(McGraw-Hill electrical and electronic engineering series). 3rd

Edition.

McGraw-Hill.