Seminar PresentationOn

Study and repair of motors

PRESENTED BY:

Krishanu Ghosh.7th semester.Department of EEE.

Contents: 1. Introduction2. Classification of motors3. Constructional features4. Insulation used for motors5. Why do motors fail?6. Rewinding Process7. Replacement of Bearings8. Repair of a 1550 kW, 6.3 kv HT motor9. Benefits10. Conclusion

Introduction:

An electric motor is a machine which converts electrical energy into mechanical energy. Its action is based on the Principle that when a current carrying conductor is placedin a magnetic field, it experiences a mechanical force whosedirection is given by Fleming’s Left Hand rule & whoseMagnitude is given by F=BIl Newton. Motors never fail at good times. Production stops; workers stand idle; deliveries are missed. The pressure to get back in operation quickly oftenleads to snap decisions to repair or replace the failed motors.Rewind or otherwise repair a motor when it is cheaper thanbuying a new motor.

Classification of motors:

On general purpose there are three types of motor available:

DC motors AC motors Universal motors

DC motors are of three types:

Shunt wound Series wound Compound wound

As regards the principle of operation AC motors areclassified into two categories:

Synchronous motors Asynchronous / Induction motors

Induction motors are also of two types:

Squirrel cage Induction motors Slip-ring Induction motors

Constructional Features:

The essential parts of a DC motor are yoke, pole shoes, field coils, armature core, armature windings, brushes and bearings. When its field windings are excited and its armature conductors are supplied with current from the supply mains, it will be seen that each armature conductor experiences a force which tends to rotate the armature in anticlockwise direction.

AC motors commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field. In AC motors the rotor receives its electric power by induction. The rotor receives its power from its windings in which power is delivered through induction from the stator windings and thus the rotor rotates freely.

Insulation used for motors:

Class Maximum Temp. Materials used

Y 90˚C Cotton, silk, paper, wood without impregnated

A 105˚C Cotton, silk, paper suitably

impregnated

E 120˚C Wire, enamel

B 130˚C Mica, glass fibre, asbestos with

suitable coating

F 155˚C Mica, glass, asbestos(25˚C

higher than Class B)

H 180˚C Combination of materials mica,

glass fibre, asbestos

C Above 180˚C Mica, porcelain, glass, quartz and

asbestos

Why do motors fail?Motors don’t fail just because of age or operating hours. Typical failures are caused by:

Heat Power supply Anomalies Humidity Contamination Improper Lubrication Unusual Mechanical loads

Primary causes of overheating are overloading, too frequent starts, high ambient temperature, unbalanced voltages & Inadequate ventilation. Problems in power supply occurs due to Harmonics, Overvoltage, Voltage unbalance, Bearing damage from shaft currents.

Humidity becomes a problem when the motor is de-energized long enough to drop near the dew point temperature. Moisture weakens the dielectric strength of electrical varnish and other insulating materials, contributes to corrosion of bearings. Contamination destroys motors by abrasion, corrosion and overheating. Different motors pose different requirements for the introduction of lubricant and removal of old lubricant. Improper lubrication may lead to the damaging of motor. A variety of mechanical conditions can either overstress bearings, leading to early failure, or distort the motor frame causing asymmetric air gap, which in turn can cause vibration and bearing failure or winding overheating.A failed motor whose

Stripping is being done.

Rewinding Process:

Damaged Winding Rewinding in process

Rewindingcomplete

Replacement of Bearings:

SLIP RING / BEARING PULL OUT WITH HYDRAULIC PULLER

Repair of a 1550 kW, 6.3 kvHT motor:

Problem Statement:

After opening this motor, thorough checking of stator winding and it’s faulty part was done.

• One burnt portion was found in stator coil overhang portion caused by Phase - Phase flashover and molten copper particles is also visible there.

• Stator to Ground IR value was 50 Meg-ohm. and Phase - Phase IR value between two phases were zero but with third phase IR value was 50 Meg-ohm.

Actual Problem -

Insulation between two adjacent HT coil belongs to two different phases, failed.

Goal statement:

In normal condition, complete rewinding of such type of HT stator

takes 6 months time and total cost will be approximately 20

lakhs. For a 2-pole motor rewinding with new copper is also very

difficult and more time consuming.

But this case was different. as it was a very critical condition,

Customer wanted to get back the motor after repair as early as

possible.

Keeping the urgency in mind, the risk of part repair was discussed

and decided to go for Part Repair of the Stator with all

precautions.

Part Repair, in this case, means that defective portion of stator

coil will be cut off, new copper strip will be brazed to the both

ends of the conductors turn by turn, insulate individual conductors

and finally overall insulation will be provided over the coils.

Reasons for failure and identifying corrective actions:

Problem - Insulation between two adjacent HT coil belongs to two different phases failed.

Probable reasons -

• Excessive dirty stator• High resistive phase to phase fault occurred.• Localized heat took place due to resistive fault.• Inter phase insulation failed & phase fault occurred.• Molten copper drops out.

Corrective actions -

• Damaged portion of conductors to be cut-off at both ends• New pieces of copper conductors are to be brazed at both ends of cut off coils• Insulate individual conductors• Shaping up the repaired portion and insulate completely• Do varnishing, heating and conduct testing of stator.

Repair job is in progress

Final taping job completed Repair job completed after varnishing and heating

Confirmation of job -

The Part Repair of the stator was done based on above plan and following tests were conducted for ensuring correct healthiness of the repaired stator.

• Insulation Resistance value between phase - phase and phase - ground was measured.• Inductance and Resistance values was measured.

All test results found OK.

The Repaired Stator can be put in service now.Motor could be delivered after part repair avoiding complete rewinding

Benefits:Tangible:

Saved the cost of rewinding, approximately 20 lakhs.

Intangible:

1.Availability Improved. 2.Quality Improved. 3.Customer Satisfied. 4.Creadibility improved. 5. Moral of people enhanced.

Conclusion:Hence we can draw out a conclusion that regardless the type of motor, it should be studied and examined carefully before its repairing or replacement. It should not only serve the purpose but also it should be economical from every aspect. Better materials should be used for insulation. Proper bearings should be installed. Proper rewinding is required at the time of repair of motor. Efficiency should be improved.