Troubleshoot Motors

8/12/2019 Troubleshoot Motors

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Troubleshoot motors

Limited number of components means

fewer problems in three-phase motors

Three-phase motors have fewer components that may malfunction than other motortypes. Therefore, three-phase motors usually operate for many years without any

problems.

If a three-phase motor is the problem, the motor is serviced or replaced. Servicing

usually requires that the motor be sent to a motor repair shop for rewinding. If the

motor is less than 1 H and more than five years old, it is replaced. If the motor is

more than 1 H, but less than ! H, it may be serviced or replaced. If the motor is

more than ! H, it is usually serviced.

Troubleshooting Three-Phase Motors

The e"tent of troubleshooting a three-phase motor is dependent upon the motor#sapplication. If the motor is used in an application that is critical to the operation or

production, testing is usually limited to chec$ing the voltage at the motor. If the

voltage is present and correct, the motor is assumed to be the problem.

%nless it is very large, the motor is usually replaced at this time so production can be

resumed. If time is not a critical factor, further tests can be made to determine the

e"act problem.

Figure 1 Troubleshoot three-phase motors with an ohmmeter.

To troubleshoot a three-phase motor, apply the following procedure&

1. %sing a voltmeter, measure the voltage at the motor terminals. If the voltage is present and at the correct level on all three phases, the motor must be chec$ed. If the

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voltage is not present on all three phases, the incoming power supply must be

chec$ed.

'. If voltage is present but the motor is not operating, turn the handle of the safety

switch or combination starter ()). *oc$ out and tag the starting mechanism per

company policy.

+. isconnect the motor from the load.. fter the load is disconnected, turn power (/ to try restarting the motor. If the

motor starts, chec$ the load.

!. If the motor does not start, turn it ()) and loc$ out the power.

0. ith an ohmmeter, chec$ the motor windings for any opens or shorts. Ta$e a

resistance reading of the T1-T coil. This coil must have a resistance reading. If the

reading is 2ero, the coil is shorted. If the reading is infinity, the coil is opened. Since

the coil winding is made of wire only, the resistance is low. However, there is

resistance on a good coil winding. The larger the motor, the smaller the resistance

reading.

fter the resistance of one coil has been found, the basic electrical laws of series and

parallel circuits are applied. hen measuring the resistance of two coils in series, the

total resistance is twice the resistance of one coil. hen measuring the resistance of

two coils in parallel, the total resistance is one half the resistance of one coil. See

Troubleshooting guides in additional three-phase motor troubleshooting article below.

Single Phase Motors

Centrifugal, thermal, and capacitor switches

cause most single-phase motor malfunctions

3ost problems with single-phase motors involve the centrifugal switch, thermal

switch, or capacitor4s5. If the problem is in the centrifugal switch, thermal switch, or

capacitor, the motor is usually serviced and repaired. However, if the motor is more

than 16 years old and less than 1 H, the motor is usually replaced. If the motor is less

than 178 H, it is almost always replaced.

Troubleshooting Split-Phase (Single Phase) Motors

The split-phase motor has a starting and running winding. The starting winding is

automatically removed by a centrifugal switch as the motor accelerates. Some split- phase motors also include a thermal switch that automatically turns the motor ())

when it overheats. Thermal switches may have a manual reset or automatic reset.

9aution should be ta$en with any motor that has an automatic reset, as the motor can

automatically restart at any time. See )igure 1.

'



Figure 1 Troubleshoot split-phase motors with an ohmmeter.

To troubleshoot a split-phase motor, apply the following procedure&

1. Turn power to motor ()). :isually inspect the motor. ;eplace the motor if it is

burned, the shaft is <ammed, or if there is any sign of damage.

'. 9hec$ to determine if the motor is controlled by a thermal switch. If the thermal

switch is manual, reset the thermal switch and turn motor (/.

+. If the motor does not start, use a voltmeter to chec$ for voltage at the motor

terminals. The voltage should be within 16 = of the motor#s listed voltage. If the

voltage is not correct, troubleshoot the circuit leading to the motor. If the voltage is

correct, turn power to motor ()) so the motor can be tested.

. Turn the handle of the safety switch or combination starter ()). *oc$ out and tag

the starting mechanism per company policy.

!. ith power ()), connect the ohmmeter to the same motor terminals the incoming

power leads were disconnected from. The ohmmeter will read the resistance of the

starting and running windings. Since the windings are in parallel, their combined

resistance is less than the resistance of either winding alone. If the meter reads 2ero, ashort is present. If the meter reads infinity, an open circuit is present. In either

case, the motor should be replaced. /ore& The motor si2e is too small for a repair to

be cost efficient.

0. :isually inspect the centrifugal switch for signs of burning or bro$en springs. If

any obvious signs of problems are present, service or replace the switch. If not, chec$

the switch using an ohmmeter.

3anually operate the centrifugal switch. 4The endbell on the switch side may have to

be removed.5 If the motor is good, the resistance on the ohmmeter will decrease. If the

resistance does not change, a problem e"ists. 9ontinue chec$ing to determine the problem.

+





To troubleshoot a capacitor motor, apply the following procedure&

1. Turn the handle of the safety switch or combination starter ()). *oc$ out and tag

the starting mechanism per company policy.

'. %sing a voltmeter, measure the voltage at the motor terminals to ma$e sure the power is ()).

+. 9apacitors are located on the outside frame of the motor. ;emove the cover of the

capacitor. 9aution& good capacitor will hold a charge, even when power is

removed.

. :isually chec$ the capacitor for lea$age, crac$s, or bulges. ;eplace the capacitor if

present.

!. ;emove the capacitor from the circuit and discharge it. To safely discharge a

capacitor, place a '6,666 ohm, ' resistor across the terminals for five seconds.

0. fter the capacitor is discharged, connect the ohmmeter leads to the capacitor

terminals. The ohmmeter will indicate the general condition of the capacitor.

capacitor is either good, shorted, or open.

Set your voltmeter to measure capacitance. The capacitance value read should be

within ?7- '6= of the value on the capacitor label.

Single Phase Continued

Troubleshooting Guide or Single-Phase

(Split-Phase) Motors

Problem Possible CauseSuggested Test

ToolCorrecti!e "ction

3otor will

not startThermal cutout

switch is open

;eset the thermal switch. Caution#

;esetting the thermal switch may

automatically start the motor.

@lown fuse or

open 9@

@asic electrical

tester, 33,

clamp meter,

megohmmeter

Test the (9. If voltage is present at the input, but not the

output of the (9, the fuse is

blown or the 9@ is open. 9hec$

the number of the (9. It should

be at least 1'! = of the motor#s

)*9.

3otor overload

on starter tripped

llow overloads to cool. ;eset

overloads. If reset overloads do not

start the motor, test the starter.

*ow or novoltage applied

@asic electricaltester, 33,

9hec$ the voltage at the motorterminals. The voltage must be

!



to motor clamp meter

present and within 16 = of the

motor nameplate voltage. If voltage

is present at the motor but the

motor is not operating, remove the

motor from the load the motor is

driving. ;eapply power to themotor. If the motor runs, the

problem is with the load. If the

motor does not run, the problem is

with the motor. ;eplace or service

the motor.

(pen control

circuit between

incoming power

and motor

@asic electrical

tester, 33

clamp meter

9hec$ for cleanliness, tightness,

and brea$s. Test the circuit starting

with the incoming power and

moving to the motor terminals.

:oltage generally stops at the problem area.

Starting winding

not receiving

power

@asic electrical

tester, 33

clamp meter

9hec$ the centrifugal switch to

ma$e sure it connects the starting

winding when the motor is ()).

)use, 9@, or

overloads

retrip after

service

@lown fuse or

open 9@

@asic electrical

tester, 33,

clamp meter,

megohmmeter

Test the (9. If voltage is

present at the input, but not the

output of the (9, the fuse is

blown or the 9@ is open. 9hec$ the

rating of the (9. It should be at

least 1'! = of the motor#s )*9.

3otor overload

on starter tripped

llow overloads to cool. ;eset

overloads. If reset overloads do not

start the motor, test the starter.

*ow or no

voltage applied

to motor

@asic electrical

tester, 33,

clamp meter

9hec$ the voltage at the motor

terminals. The voltage must be

present and within 16 = of the

motor nameplate voltage. If voltage

is present at the motor but the

motor is not operating, remove the

motor from the load the motor is

driving. ;eapply power to the

motor. If the motor runs, the

problem is with the load. If the

motor does not run, the problem is

with the motor. ;eplace or service

the motor.

(pen control

circuit between

incoming power

and motor

@asic electrical

tester, 33,

clamp meter

9hec$ for cleanliness, tightness,

and brea$s. Test the circuit starting

with the incoming power and

moving to the motor terminals.:oltage generally stops at the

0



problem area.

3otor shaft does

not turn

isconnect the motor from the

load. If the motor shaft still does

not turn, the bearings are fro2en.

;eplace or service the motor.

3otor

produces

electric

shoc$

@ro$en or

disconnected

ground strap

9onnect or replace ground strap.

Test for proper ground.

Hot power lead

at motor

connecting

terminals is

touching motor

frame

isconnect the motor. (pen the

motor terminal bo" and chec$ for

poor connections, damaged

insulation, or leads touching the

frame. Service and test motor for

ground.

3otor winding

shorted to frame ;emove, service, and test motor.

3otor

overheats

Starting

windings are not

being removed

from circuit as

motor

accelerates

hen the motor is turned ()), a

distinct clic$ should be heard as the

centrifugal switch closes as the

motor slows down

Improperventilation

Infraredthermometer

9lean all ventilation openings.

:acuum or blow dirt out of motorwith low-pressure, dry, compressed

air.

3otor is

overloaded

@asic electrical

tester, clamp

meter or 33

with clamp

accessory

9hec$ the load for binding. 9hec$

shaft straightness. 3easure motor

current under operating conditions.

If the current is above the listed

current rating, remove the motor.

;emeasure the current under no-

load conditions. If the current is

e"cessive under load but not whenunloaded, chec$ the load. If the

motor draws e"cessive current

when disconnected, replace or

service the motor.

ry or worn

bearings

ry or worn bearings cause noise.

The bearings may be dry due to

dirty oil, oil not reaching the shaft,

or motor overheating. (il the

bearings as recommended. If noise

remains, replace the bearings or the

motor.

A



irty bearings 9lean or replace bearings.

>"cessive

/oise>"cessive end

play

9hec$ end play by trying to move

the motor shaft in and out. dd

end-play washers as required.

%nbalanced

motor or load

n unbalanced motor or load

causes vibration, which causes

noise. ;ealign the motor and load.

9hec$ for e"cessive end play or

loose parts. If the shaft is bent,

replace the rotor or motor.

ry or worn

bearings

ry or worn bearings cause noise.

The bearings may be dry due to

dirty oil, oil not reaching the shaft,

or motor overheating. (il the

bearings as recommended. If noiseremains, replace the bearings or the

motor.

>"cessive grease

@all bearings that have e"cessive

grease may cause the bearings to

overheat. (verheated bearings

cause noise. ;emove any e"cess

grease.

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Troubleshoot Motors