Upload
adil-rezouk
View
219
Download
0
Embed Size (px)
Citation preview
7/29/2019 Electric Motor Trblsht
1/27
SECTION 4
ELECTRIC MOTORS
UNIT 20
TROUBLESHOOTING ELECTRIC
MOTORS
7/29/2019 Electric Motor Trblsht
2/27
UNIT OBJECTIVESAfter studying this unit, the reader should be able to
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 hermeticand open motors
7/29/2019 Electric Motor Trblsht
3/27
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
7/29/2019 Electric Motor Trblsht
4/27
MECHANICAL MOTOR PROBLEMS Most common problems involve the bearings or
drive connections When fractional hp motors have bearing damagethe best way to repair them is to replace the entiremotor.
Bearing failure Tight or worn bearings
Lack of lubrication
Excessive grit in the bearing
Overtightened belts
Not often repaired by the field technician
7/29/2019 Electric Motor Trblsht
5/27
TOOL TO REMOVE SLEEVE BEARINGS
SLEEVE BEARING
7/29/2019 Electric Motor Trblsht
6/27
TOOL TO REMOVE SLEEVE BEARINGS
7/29/2019 Electric Motor Trblsht
7/27
TOOL TO REMOVE SLEEVE BEARINGS
TOOL IS
STRUCK
WITH ASOFT FACE
HAMMER
7/29/2019 Electric Motor Trblsht
8/27
TOOL TO REMOVE SLEEVE BEARINGS
ONCE THE
BEARING HAS
BEEN REMOVED,
THE SAME TOOL
CAN BE USED TOINSTALL THE
NEW BEARING
7/29/2019 Electric Motor Trblsht
9/27
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
7/29/2019 Electric Motor Trblsht
10/27
TOOL TO REMOVE PULLEYS FROM THE MOTOR SHAFT
PULLEYSHAFT
7/29/2019 Electric Motor Trblsht
11/27
TOOL TO REMOVE PULLEYS FROM THE MOTOR SHAFT
AS THE SCREW IS
TIGHTENED, THE
PULLEY IS REMOVED
FROM THE SHAFT
7/29/2019 Electric Motor Trblsht
12/27
TOOL TO REMOVE PULLEYS FROM THE MOTOR SHAFT
7/29/2019 Electric Motor Trblsht
13/27
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 alignedDrive mechanisms can become damaged
Belt life can be reduced
Belts can slip off the pulleys
7/29/2019 Electric Motor Trblsht
14/27
BLOWER
MOTOR
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 cfms
can be better adjusted than on directdrive applications.
MATCHED SET OF BELTS ARE
BELTS OF THE SAME SIZE
7/29/2019 Electric Motor Trblsht
15/27
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
7/29/2019 Electric Motor Trblsht
16/27
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 openwinding
If the motor is hot, it should be permitted to cool in
the event that an internal thermal overload is open
7/29/2019 Electric Motor Trblsht
17/27
L2
L1
START
RUN
OPEN WINDING
7/29/2019 Electric Motor Trblsht
18/27
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 maystart, 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
7/29/2019 Electric Motor Trblsht
19/27
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
7/29/2019 Electric Motor Trblsht
20/27
L2
L1
START
RUN
SHORTED WINDING
7/29/2019 Electric Motor Trblsht
21/27
MOTOR STARTING PROBLEMS Relatively easy to troubleshoot
Check for full power to the motor
Motor may hum but not start
1. Problem with starting components
2. Open winding
3. 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 bechecked
7/29/2019 Electric Motor Trblsht
22/27
MOTOR PROBLEMS If run winding resistance decreases the motor will draw too much
current while its 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.
7/29/2019 Electric Motor Trblsht
23/27
CHECKING CAPACITORS
Capacitors can be checked with analog ohmmeters
Always discharge capacitors before testing, the best wayto discharge a capacitor is to use a 20,000 ohm 5 wattresistor 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 towardsinfinity
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
7/29/2019 Electric Motor Trblsht
24/27
CAPACITOR IDENTIFICATION
Run capacitorsEncased 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
7/29/2019 Electric Motor Trblsht
25/27
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
7/29/2019 Electric Motor Trblsht
26/27
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
7/29/2019 Electric Motor Trblsht
27/27
UNIT SUMMARY Electrical motor problems can appear to be mechanical
Common mechanical motor problems include defective bearings ordrive connections
Belt tension and pulley alignment must be correct
Electrical motor problems include open windings, short circuits fromwinding 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 propermotor operation