Motor Testing by Test Lamp

 Motor Testing by Test Lamp

Preamble

This is regarding testing of three phase squirrel cage induction motor. As known stator has winding whereas rotor has only conductor bars connected to end rings at both ends. So probability of fault is negligible due to sturdy construction. Any damage is detectable by visual inspection only. Numbers of coils are coils placed in different slots of core in stator as per design. These coils are interconnected in three groups for three phases. Six ends of three groups are taken out in terminal box for external connection with supply. They are marked like A1-A2,  B1-B2 and C1-C2  or R1-R2,  Y1-Y2 and B1-B2

Motor is subjected to abnormal operating conditions like high voltage or voltage surge, over heating due to over load, low supply voltage or single phasing. So there is possibility of damage or degrading of insulation resulting in current leakage to body, between phases or between turns of the coil. Over heating may melt and break connection leaving open circuit.

Also entry of insects or moisture, contaminated atmosphere or mishap during servicing and aging can cause fault. So possibilities of such fault have to be checked.

 Importance

Now industries are following individual drive system having separate motor for each machine. So any industry may have many motors depending upon type of industry. Generaly electrical maintenance staff and equipments may be bare minimum except in big industry. Such simple testing method may be much useful to them as it does not require special testing instrument and expert for its operation.

Setup

Small three phase squirrel cage induction motor.  (2HP, 3HP 0r 5 HP)

This motor requires small modified as under for simulation of various faults.


Schematic stator winding partial layout

Five wires A, B, C, D & E are to be tapped from stator winding.

Tapping is to be done from the joints of the coils without disturbing windings.

 

Requires five one meter long wires of different colors ( for identification.)

Open joint of coils of any phase (say R phase)

Separate both ends and connect wire to each ends. Identify wire as A and B

Find other joint of the same phase of next coil.

Connect one wire to joint with coil ends together as it is. Identify wire as D

Find joint of other phase (say B phase)

Connect one wire to joint with coil ends together as it is. Identify wire as C

Connect one wire to the body of motor(from inside). Identify wire as E

Cover all connections properly with insulation tape.

Take out all the five wire out from air vent of end cover.

Other ends of five wires to be connected to switch board.

Switch Board

Switch Board will have four switches as shown.

Five wires A, B, C, D & E connected as shown.

Internal loops between switches as shown by broken lines.

Switch 1 is kept upside down. (so that close circuit – no fault- in normal position)

Board have marking for switch numbers as 1, 2, 3 and 4 and also switch position as Normal and Fault as shown.

Cover with locking arrangement is provided on the board to prevent unauthorized switching.

The board remain with motor as set.

This motor functions as normal and can be used for any purpose with all switches in normal position.

Procedure

Before start of testing of motor, remove all supply connection to terminal.

Toggle one switch at a time to simulate faults as under.

Switch              1   >   Open circuit fault.

Switch              2   >   Phase winding short circuit.

Switch              3   >   Phase to phase short circuit.

Switch              4   >   Ground (body) fault.

 

Test lamp has two probes as under.

One wire from supply phase through lamp is w1

Other wire from supply neutral is w2

 Part 1 Continuity Test

First connect probes W1 and W2 to terminals A1 and A2.

Glowing of test lamp indicates the circuit has no fault.

Not glowing of test lamp indicates open circuit (break) in concern phase.

Repeat the procedure for other phases with pair of terminals B1-B2 and C1-C2.

 Observation

Sr.

No.

Terminal

Pair

Test Lamp Glow

Yes/No

Type of Fault

Phase Affected

1

A1-A2

 

 

 

2

B1-B2

 

 

 

3

C1-C2

 

 

 

Part 2 Winding Short Circuit Test

 Observation of Part 1 is useful.

Compare the brightness of glowing lamp in three cases.

Equal brightness in all the three cases indicates no fault.

More brightness in any case compared to others indicates short circuit of winding in concern phase. Part of the phase winding is bypassed by inter turn short circuit due to insulation failure.

 Observation

Sr.

No.

Terminal

Pair

Test Lamp Brightness

Type of Fault

Phase Affected

1

A1-A2

 

 

 

2

B1-B2

 

 

 

3

C1-C2

 

 

 

 Part 3. Phase to Phase Short Circuit Test

Connect test probes to pair of terminals A1- B1.

Check test Lamp. Not glowing of test lamp indicates no fault.

Glowing of lamp indicates short circuit between concern pair of phases.

Repeat same way with other pairs of terminals A1-C1  and B1-C1.

 Observation

Sr.

No.

Terminal Pair

Test Lamp Glow

Type of Fault

Phase Affected

1

A1-B1

 

 

 

2

A1-C1  

 

 

 

3

B1-C1

 

 

 

Part 4. Ground (body) Fault Test

Connect test probes W1 to terminal A1 and W2 to earth terminal E on the body of motor.

Check test Lamp. Not glowing of test lamp indicates no fault.

Glowing of lamp indicates concern phase to body (ground) fault.

Repeat same way with other terminal pairs B1-E and C1-E.

Observation

Sr.

No.

Terminal

Pair

Test Lamp Glow

Type of Fault

Phase Affected

1

A1-E

 

 

 

2

B1-E

 

 

 

3

C1-E

 

 

 

Conclusion

Fault detection of induction motor is possible by this simple method with test lamp only.

This method of motor fault detection is very simple and short. This may be convenient for laboratory practical examination of student because of simple and fast. Examiner may simulate fault by secretly operating one of the switch and student may be asked to find the type of fault and phase effected.