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SECTION 4 ELECTRIC MOTORS UNIT 20 TROUBLESHOOTING ELECTRIC MOTORS PowerPoint Presentation
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SECTION 4 ELECTRIC MOTORS UNIT 20 TROUBLESHOOTING ELECTRIC MOTORS - PowerPoint PPT Presentation


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SECTION 4 ELECTRIC MOTORS UNIT 20 TROUBLESHOOTING ELECTRIC MOTORS. UNIT OBJECTIVES. Describe different types of electric motor problems List commonly encountered electric problems with motors Identify various mechanical problems with electric motors Explain how to field test a capacitor

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slide1

SECTION 4

ELECTRIC MOTORS

UNIT 20

TROUBLESHOOTING ELECTRIC MOTORS

unit objectives
UNIT OBJECTIVES
  • Describe different types of electric motor problems
  • List commonly encountered electric problems with motors
  • Identify various mechanical problems with electric motors
  • Explain how to field test a capacitor
  • Explain the difference between troubleshooting hermetic
  • and open motors

After studying this unit, the reader should be able to

electric motor troubleshooting
ELECTRIC MOTOR TROUBLESHOOTING
  • Problems are either electrical or mechanical
  • Mechanical problems may appear to be electrical
  • Electrical problems may appear to be mechanical
  • Technicians must be able to diagnose system problems correctly
  • Technicians must locate the cause of the problem, which is not always the resulting effect
mechanical motor problems
MECHANICAL MOTOR PROBLEMS
  • Most common problems involve the bearings or drive connections
  • When fractional hp motors have bearing damage the best way to repair them is to replace the entire motor.
  • Bearing failure
    • Tight or worn bearings
    • Lack of lubrication
    • Excessive grit in the bearing
    • Overtightened belts
    • Not often repaired by the field technician
slide7

TOOL TO REMOVE SLEEVE BEARINGS

TOOL IS STRUCK WITH A SOFT FACE HAMMER

slide8

TOOL TO REMOVE SLEEVE BEARINGS

ONCE THE BEARING HAS BEEN REMOVED, THE SAME TOOL CAN BE USED TO INSTALL THE NEW BEARING

removing drive assemblies
REMOVING DRIVE ASSEMBLIES
  • Pulley, coupling or blower or fan must be

carefully removed from the motor shaft

  • The assembly must not be damaged
  • Special pulley pullers can be used
  • Set screws are tightened to the motor shaft
  • Set screws are made of hardened steel
  • The end of the motor shaft should never be hammered in order to keep the shaft perfectly round
slide10

PULLEY

SHAFT

TOOL TO REMOVE PULLEYS FROM THE MOTOR SHAFT

slide11

AS THE SCREW IS TIGHTENED, THE PULLEY IS REMOVED FROM THE SHAFT

TOOL TO REMOVE PULLEYS FROM THE MOTOR SHAFT

belt tension and pulley alignment
BELT TENSION AND PULLEY ALIGNMENT
  • Overtightened belts can cause bearing damage
  • Loose belts can result in slippage
  • Belt tension gauges should be used
  • Drive and driven pulleys should be aligned
    • Drive mechanisms can become damaged
    • Belt life can be reduced
    • Belts can slip off the pulleys
slide14

BOTH THE DRIVE AND DRIVEN PULLEYS MUST BE PERFECTLY ALIGNED OR BELT WEAR WILL OCCUR

Grooves of the belt will be shiny if belt is slipping.

On belt drive evaporators the cfm’s can be better adjusted than on direct drive applications.

MATCHED SET OF BELTS ARE BELTS OF THE SAME SIZE

BLOWER

MOTOR

electrical motor problems
ELECTRICAL MOTOR PROBLEMS
  • Relatively easy to diagnose
  • Motor may smell burned
  • Common motor problems include
    • Open motor windings
    • Short circuit from winding to ground
    • Short circuit from winding to winding
open motor windings
OPEN MOTOR WINDINGS
  • Can be checked with an ohmmeter
  • There should be a measurable resistance between Common, Start and Run terminals
  • An infinite resistance reading indicates an open winding
  • If the motor is hot, it should be permitted to cool in the event that an internal thermal overload is open
slide17

OPEN WINDING

START

L1

RUN

L2

shorted motor windings
SHORTED MOTOR WINDINGS
  • Results from holes in the winding insulation
  • Alternative path for current is created
  • Current flow through the winding increases
  • If the short is in the run winding, the motor may start, but the current draw will be high
  • If the short is in the start winding, the motor may not start, but excessive current will be drawn
short circuit to ground
SHORT CIRCUIT TO GROUND
  • Should be no continuity from a winding to ground
  • Megohmmeters can be used to detect small, high resistance paths to ground
  • In damp conditions, dirty motors may have a resistance reading from windings to ground
  • If refrigerant oil is dirty, hermetic compressors may have resistance readings from a winding to ground
slide20

SHORTED WINDING

START

L1

RUN

L2

motor starting problems
MOTOR STARTING PROBLEMS
  • Relatively easy to troubleshoot
  • Check for full power to the motor
  • Motor may hum but not start
  • Problem with starting components
  • Open winding
  • Insufficient supply voltage
  • Motor may run for a short time and then shut down
  • Motor may not attempt to start at all
  • If motor turns freely, examine electrical circuits
  • If motor hums but does not start, the starting components should be checked
motor problems
MOTOR PROBLEMS
  • If run winding resistance decreases the motor will draw too much current while it’s running.
  • A megger is a test instrument that measures very high resistance.
  • When a motor grounds out it will usually trip a breaker.
  • Motors and compressors should be checked at the motor or compressor terminals.
checking capacitors
CHECKING CAPACITORS
  • Capacitors can be checked with analog ohmmeters
  • Always discharge capacitors before testing, the best way to discharge a capacitor is to use a 20,000 ohm 5 watt resistor that has insulated leads soldered to it.
  • A good capacitor will register a resistance reading toward zero ohms and then the needle will move back towards infinity
  • If there is no reading, reverse the meter leads
  • No needle movement indicates an open capacitor
  • A reading of zero ohms indicates a short circuit
capacitor identification
CAPACITOR IDENTIFICATION
  • Run capacitors
    • Encased in metal, oil-filled
    • Swollen capacitors should be replaced
    • Range from 2 to 60 microfarads
  • Start capacitors
    • Dry type, encased in bakelite
    • Overcurrent can cause the “vent” to pop
wiring and connections
WIRING AND CONNECTIONS
  • Wiring must be in good condition
  • Loose connections can cause system problems
    • Can result in oxidation on electrical contacts
    • Oxidation increases circuit resistance
    • Resistance increases the amount of heat generated
    • Can result in low voltage being supplied to the motor
    • Low voltage can result in increased motor amperage
troubleshooting hermetic motors
TROUBLESHOOTING HERMETIC MOTORS
  • Can only be checked electrically from outside the compressor shell
  • Experience the same problems as open motors
    • Grounded circuit
    • Open circuit
    • Short circuit
    • Starting component problems
unit summary
UNIT SUMMARY
  • Electrical motor problems can appear to be mechanical
  • Common mechanical motor problems include defective bearings or drive connections
  • Belt tension and pulley alignment must be correct
  • Electrical motor problems include open windings, short circuits from winding to winding and short circuits from windings to ground
  • Defective starting components can prevent motor starting
  • All wiring and connections must be in good order to ensure proper motor operation