AC Motors:Classification:• Synchronous Motors• Induction motors

– Squirrel cage • Single cage • Double cage

– Slip ring induction motors. • Induction motors are also available which work in single-phase.

– Split phase induction motor – Capacitor induction motors, – Universal induction motors, – shaded pole induction motors, – Repulsion motors.

• Motors are also classified – According to speed– According to mounting– According to duty cycle– According to enclosure

Induction Motor:

Operating principle: • Works on the induction principle, so is called an

induction motor. • It has a rotor and a stator. • AC supply is given to the stator. • Stationary coils in the stator produces magnetic flux

rotating in nature when supplied with polyphase supply. • Rotating flux induces currents when linked with shorted

rotor conductors and produce a current. • This current interacts with the rotating magnetic field of

the stator, which produces a mechanical force.

Constructional features:

Squirrel Cage Induction Motor:• Stator:

– The stationary part. – Consists of silicon steel laminations insulated with

varnish coating or paper. – Slots are cut all around the periphery of the

laminations for housing the winding. – The laminations are fitted in the frame or body of

the motor with nut and bolts. – The symmetrical winding is wound for 2,4,6 poles

depending the speed required. – Winding terminals are brought out and terminated

in the terminal box. .

Rotor:

– Rotating part of the motor supported at the two ends with the bearings.

– Made up of silicon steel laminations with slots near the periphery.

– Copper or alluminium bars are housed inside the closed slots and are riveted / welded to solid metallic rings on either side called end rings.

– Some rotors have double cage winding. – Generally the outer cage is built with brass and inner

cage is built with copper conductor with larger cross section compared to the outer cage.

Slip ring Induction Motor:

• Regular winding on the rotor of insulated wires housed inside the slots wound for the same no. of poles as the stator.

• Three terminals are brought out on to the slip rings and the other ends connected in star.

• Slip rings mounted on the rotor shaft but insulated from it. • Brushes rest on the slip ring and enable for connection to

the external circuit like resistance for starting purposes.• This type of motors are used to obtain high staring torque,

i.e. when the motors are to be started with load like rolling mills large pumps lifts, line shafts, overhead cranes etc.

Faults that can occur in Induction Motors:

Electrical faults in a motor are• Short circuit in the winding

– It reduces the resistance value of one phase– In star connected motor the healthy phases will have line voltages

instead of phase voltage– In delta connected motor it is effectively shorting the two phases– In either cases the healthy phases will draw higher current that the rated

one– Overheat the windings and burn if the protective device fails to act– The short circuit in one phase produces a strong unbalance of he field

polarity of the motor since there is absence of the field poles in the unhealthy phase winding. It fails to start or starts very slowly with noise.

– Leads to heating of main wires leading to burning of the wires.– Mainly due to ageing of the insulation.– Can be located with measuring the resistance of each phase winding– Complete rewinding of the motor is the only solution

• Open circuit in the winding– An open in one of the phase windings results in shifting of the star

point leading to other two healthy phases getting different voltage. – This condition causes unbalance in the field– Motor fails to start.

• Failure of insulation between the winding and the body of the motor– The current instead of returning to the mains partly flow to ground

through the motor frame.– Aging and deterioration of insulating varnish– Bad workmanship while inserting the oils into the slots of the

stator. – Abrasion of winding wires during assembly of motors – Testing is by insulation tester.

• Short circuit between the phase windings.– Generates an unbalance of currents in the machine

windings and improper distribution of magnetic field generated by the stator poles.

– Motor cannot start or may run with heavy noise.

– Causes heating of the windings, supply leads

– Aging of insulation between the phase windings .

– Also may be due to humidity, vibration etc

– Can be detected by using a multimeter and checking for the continuity across the windings of different phases.

– In a healthy motor there should be no continuity between the windings of different phases.

• Single Phasing– Discontinuity in one of the supply phases

• Rotor faults– Mechanical damage in the squirrel cage rotor

bars or end rings– Winding problem in the case of wound rotor

Protection of induction motor:

• Over load Protection:– Thermal or magnetic overload relays are incorporated in the

motor starter,10-15 seconds transient overload during starting is taken into account.

– Whenever the overload relay trips due to overload, it has to be reset.

– Both manual and automatic resetting is possible by suitably adjusting the tripping mechanism.

– But due care should be taken while choosing the one so as to avoid any damage to the motor.

• The common causes for overloads – Excessive belt tension, – Rubbing of rotor and stator, – Single phasing, machine jam, – Low voltage, – Hard bearings.

Short circuit protection:

– Short circuits in the motor winding or external to the winding causes excess drawal of current as the short circuit practically reduces the load resistance to zero.

– Overload results in the overcurrents depending on the extent of overload

– Since the load resistance is zero in short circuit condition leads to very high value of overcurrents

– May cause damage to the associated equipments– Protection is provided by Fuses in the circuit of

every motor.

• Low Voltage Protection:– For protecting the motors from low voltages – This protective device will isolate the motor

when the voltage goes below the permissible limit or total failure of voltage.

– This is also called ‘No Volt Protection’– Once the motor is isolated, it will not start itself

unless it is started once again

• Single phasing in induction motor:– This causes the motor windings to be heated up and burnt. – Causes:

• Blown out fuse in transformer primary, • Blown out fuse in the motor branch circuit, • Loose or poor connection in the switch/starter/contactor. • Badly soldered winding connections in the motor. • Open circuit within the winding.

– Standstill motor will not start in single phasing condition and gives humming sound.

– If the fault occurs when the motor is running, it will continue to run with squeezing sound till it gets isolated by the protective device.

– Generally bimetallic overload relay provides phase failure protection also since this also results in overcurrent.

– Protection against single phasing is provided in the cases of large motors and critical duty motors.

• Phase reversal protection:– Provided when motor is required to be

protected against reversal of rotation.– Provided when the phase reversal causes

damage like in a lift.

Checks before commissioning of a motor:

• Mechanical checks:– Alignment of the driving and driven equipment.– Flexible coupling, – Air gap between the stator and the rotor.– Condition of grease – Free rotation of the rotor by hand.

• Electrical checks:– All connections and contacts are tightened.– Earth connections are in order– Earth resistance is within the limits.– Ratings of the fuses– Setting of overload relays.– Supply voltages in all the phases.

Maintenance of Motors:

• Cleaning of motors: – Dirt, metal dust etc get into motors with cooling air and weaken the

insulation – Clog the cooling ducts and the air gap – Blow motors periodically with low pressure dry air. – Blowing should be in a direction opposite to that of the cooling air.

• Insulation resistance: – Check periodically – Depends on temperature, moisture cleanliness of motor, its age,

test voltage and duration of its application. – Observations to be made under similar conditions and at regular

intervals. – The recommended test voltage is 500 volts dc. applied for a period

of one minute. – To avoid damage to insulation, low test voltage may be applied in

the case of low voltage motors which also happen to be dirty and wet.

– The windings to be tested should be allowed time to discharge any residual charge.

– The recommended values for insulation resistance in mega ohms for 220 Volts motor is 0.2 and for 440 V motor is 0.4.

Rotor Maintenance: • Check Rotor bars for loose or break in the slots• Jarring noise while staring the motor is the indication of broken rotor bars• Where the visual inspection is not possible, application of low voltage to

the stator and rotating the rotor manually can detect the fault. The variation more than 3% in the current is an indication of the fault.

• Repair of bars is only by welding, brazing though it is tough since it is copper to copper aluminium to aluminium welding.

• Check windings similar to the methods adopted for stator winding.Bearings: • Ball bearing, Sleeve bearings, Roller bearings are normally used for

motors.– Ensure Lubrication

• Frequency of lubrication• Maintain oil level • Oil change at regular intervals. • Maintenance of Lubricating wick wherever applicable. • Correct grade of oil should be used. • Follow standard procedure while mounting and removing.

Bearing replacement:

• Use of bearing puller

• Application of force to only to the races (inner or outer depending on the case)

• Bearing should be removed from the carton just before the assembly.

• Bearing fit should be adhered. Only tapping force should be applied.

• Shaft should be free from rust, dirt etc.

• Attention should be paid to end clearance so that the rotor can take a position exactly in the magnetic centre of the winding. Otherwise there will be end thrust on the bearing leading to overheating.

• The normal temperature of a correctly fitted bearing will be around 50 degree centigrade.

• Air Gap measurements: – Check air gap with a tapered feeler gauge.– The air gap may vary 0.15mm to 1.5mm depending on the

size of the motors.– Quiet operation, adequate torque, good power factor and

efficiency depend upon a reasonable uniform air gap all round.

– Measurement to be taken at lowest position and the side– The deference of more that 10% should be viewed seriously.

• Couplings and Pulleys: – Check for their alignment. – Fixing should be carried out properly with checking in

deferent positions. – Can be heated to about 130 degrees while fixing if needed.

• Motor overhauling: – Overhaul periodically, depending upon the severity of

service. Motor windings should be cleaned with solvent like carbon tetra chloride, after blowing away all dirt etc.

– Dry after the cleaning, if necessary, apply varnish

• Installation of Motors:• Location:

– Locate the motors in clean ventilated place, where water, oil, steam, carbon and metallic dust etc. cannot find access into them.

– Ensure free flow of cooling air.– Place on the solid concrete foundation or on a rigid steel structure. If

necessary place shock absorbent rubber pads or springs. – Grout the bolt holes and concrete top surface after leveling and final

alignment of the motor with the driven machine,

• Alignment of Motors: – Check air gap all round the rotor. – Measure the gap at 12.00 and 6.00 clock positions in vertical plane and

3.0 clock and 9.0 clock positions in horizontal plane. – Turn one shaft through 90o check the gap in all the four positions. – The difference in corresponding readings should not exceed 0.05mm. – Turn the same shaft is turned further though 90o and the gaps are

measured once again. – The difference between the readings should be the same as before. – In the case of belt driven loads the two shafts should be level as well

as parallel.– Final checking of the level should be done with a level indicator.

Storage:

• Store in a clean and dry place.

• The machined parts have a protective coat of anti rust preservative which should not be taken off during normal storage.

• In case of long storage periodic examinations should be carried out and fresh preservative be applied if necessary.

• Insulation resistance of Motors: – Insulation resistance should be be minimum of o.2 mega-

ohm for a 220 volts motor and 0.4 mega-ohm for a 440 Volts motor (As per IEEE)

– If less the motor should be dried out before full voltage is applied.

• Drying:– In a drying oven– Blocking the rotor and circulating current at low voltage of

about 10% of the rated voltage, – Using incandescent lamps. – Glow heaters.

Single-phase Induction motors:• Used for smaller capacities and for fractional hp. • Consist of a stator and rotor. • Stator carries windings of insulated wires. • Windings are connected to 1-phase supply. • Rotor has short-circuited cage winding. • 1-phase supply produces a alternating flux • Rotor produces a pulsating torque and cannot start

rotating by itself. • Therefore, single phase motors are not self starting. • Once a torque for starting is applied to the rotor in any

direction the pulsating torque changes into the unidirectional torque and the rotor continues its rotation.

According to the starting methods the single phase motors can be classified as follows:• Split Phase Motors

– Plain split phase motor– Capacitor start induction run motor– Capacitor start capacitor rum motor– Permanent capacitor motor– Shaded pole motor

• Repulsion motors– Plain repulsion motor– Repulsion start induction run motor– Repulsion induction motor

• Universal Motor

Split phase induction motor: • Consists of two windings – main winding with high reactance and

auxiliary winding with high resistance. • Current in two windings differ in phase. • Two sets of poles produce a kind of rotating flux, which starts the

motor.• When rotor comes to speed, a centrifugal device opens the switch and

disconnects the auxiliary winding.

Two pole induction MotorConnection of single phase induction motor

• Capacitor induction motor: – Capacitor is used for splitting the phase.

• capacitor start and

• capacitor start and run

Capacitor start induction motorSingle value capacitor Motor

Two pole shaded pole induction motor. Universal Motor

• Universal induction motor:– Works on both ac and dc. – It has stator and rotor both made of laminated sheet steel. – Rotor has a commutator connected in series with the field. – Extended rotor shaft drives mechanical gear unit.

• Shaded pole induction motor:– Starting torque is obtained by shading the flux of a portion of the

pole and thus allowing the flux to shift towards the area of lower flux density.

• Repulsion motor:– It is a form of series motor with the rotor energized inductively. – Rotor winding is made for a low working voltage, and its brushes

are joined by a short circuiting connector to provide a closed current path.

• Single-phase series motor:– This is similar to dc series motor in construction.

What to inspect What to inspect for PeriodSurroundings

Sleeve Bearings

Mechanical condition

Ball or roller bearings

Brushes and commutators or rings.

Rotors and armatures

Winding

Dripping liquids, blockage for ventilation

Lubrication oil level and associated arrangement

Unusual noise, unusual odour,

Bearing housing vibration unhealthy noise, creepage of grease etc.

Sparking, Colour, surface condition of commutator , pig tail, connections, black spots, roughness on commutator surface

Air gap ,

Mild blowing, cleaning of dust Observe for any moisture.

Weekly

Maintenance and Check programme of Motors

Mechanical inspection

Windings

Brushes AC and DC

Commutators

Ball and Roller bearings

Belt for suitable slack and surface condition, couplings.

Check shunt, series and commutating filds for tightness

Check field spools machine cable connections etc.

Check holders for fit, free play, brush spring pressure, replace worn out brushes.

Commutator surface, mica insulations,

Lubrication and Greasing

Every 1-2 months

Sleeve bearings

Enclosed gears

Couplings and other drive detail.

Loads

Windings

Bearing wear, end play, bearing surfaces.

Check at the drain plug and oil flow for presence of metal scale, sand, or water. Take suitable action

Replacement of belts if needed.

Check loads for changed conditions and whether correction needed.

Check insulation resistance, clean surface and check moisture, varnishing, baking, drying of windings etc.

Once a year

Air gap and bearings

Squirrel Cage motors

Rotors wound

Armature

Mechanical parts

Loads

Check thoroughly, and replace the bearings if needed. Replace the wicks for lubrication of bearings

Check the rotors, check for any local heating, check fan blades

Clean the collector rings, washers and connections, clean the rings on lathe if necessary.

Clean thoroughly all air passages check for surface condition of bars, high mica eccentricity turn the commutator if necessary

Check belt , couplings, replace if necessary, align the couplings,

Read load on motor with instruments at no load, fu; load, or through cycle as check on mechanical conditions of the driven machine.

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