First a little note.
Most of the smaller single phase and 3 phase motors can be replaced cheaper than can be repaired. Check with your local motor shop. Sometimes it just isn’t worth the time and effort to bother with repairing or rebuilding them.
That
being said, it may also be the middle of the night or an unusual situation
where you need to fix what you have.
You must make sure it is not a control,
load , voltage or environment problem, as nothing makes you look bad
like changing something only to find the replacement does the same thing as the
one we took out.
Warning:
A) Always Disconnect power to the motor before performing any work.
B) Always follow all local codes and manufacturers guidelines
C) Always keep away from moving parts.
D) Discharge all capacitors before servicing motor.
E) Be sure all safety guards are in place.
Motor Probable Cause Corrective
Action
|
Motor fails to start after initial installation. |
Motor is wired wrong Motor damaged or rotor is striking stator Rear Fan stuck on Guard |
Verify motor is wired correctly. Replace Motor or Disassemble and repair Remove cover and repair Always check that the Motor (Rotor) turns
freely before it is installed on a load |
|
Motor used to run but now it fails to start. |
Fuse or circuit breaker tripped. Stator is shorted or grounded. Motor will make
a humming noise and the circuit breaker or fuse will trip. Motor overloaded Or stuck load Capacitor (on single phase motor) may have
failed. Since this is a foggy area to a lot of people,
I have included a section at the bottom of this page on how to test
capacitors. Starting
switch has failed. |
Replace fuse or reset the breaker. Disassemble motor and inspect windings and
internal connections. A blown stator will show a burn signs. Motor
must be replaced or the stator rewound Inspect
to see that the load is free. Verify current (amps) draw of motor versus
nameplate rating. First discharge capacitor. To check capacitor,
set ohm meter to RX100 scale and touch its
probes to capacitor terminals. If capacitor is OK, needle
will jump to zero ohms, and drift back to high. Steady zero ohms indicates a
short circuit; steady high ohms indicates an open circuit. Very Common- Check the start switch-
Disassemble motor and inspect both the centrifugal and stationary switches.
The weights of the centrifugal switch should move in and out freely. Make
sure that the switch isn’t sloppy on the shaft. Inspect contacts and
connections on the stationary switch. Replace switch if the contacts are
badly burned or pitted. |
|
Motor runs but dies down. |
Voltage drop. Load increased. |
If voltage is less than 10% of
the motor’s rating check incoming voltage Verify
the load has not changed. Verify equipment hasn’t got tighter. If fan
application verify the air flow hasn’t changed. |
|
Motor takes too long to accelerate |
Defective capacitor Faulty stationary switch. Bad
bearings. Voltage
too low. |
Test capacitor per above and at the end of this
page Inspect
switch contacts and connections. Verify that switch reeds have some spring in
them. Noisy
or rough feeling bearings should be replaced. Make
sure that the voltage is within 10% of the motor’s nameplate rating. Very
Common--Wire size feeding the motor may be too small (Extension Cord?) |
|
Motor runs wrong way |
Incorrect wiring |
Rewire motor according to wiring schematic. |
|
Motor overload protector continually trips. |
Load
too high. Ambient
temperature too high. Protector
may be defective. Winding
shorted or grounded. |
Motor
overloaded or load jammed. Verify that the load is not jammed. If motor is a
replacement, verify that the rating is the same as the old motor. If previous
motor was a special design, a stock motor may not be able to duplicate the
performance. Remove the load from the motor and inspect the amp draw of the
motor unloaded. It should be less than the full load rating stamped on the
nameplate. Verify that the motor is getting enough air for
proper cooling. Most motors are designed to run in an ambient temperature of
less than 40°C. (Note: A properly operating motor may be hot to the touch.) Replace
the motor’s protector with a new one of the same rating. Inspect
stator for defects, or loose or bare wires that may cause it to go to ground. |
|
Motor vibrates. |
Motor misaligned to load. Load
out of balance. Motor
bearings defective. Motor
may have too much endplay. Rotor
out of balance. Winding may be defective |
Realign
the load. Remove motor from load and inspect motor by
itself. Verify that motor shaft is not bent. Rule of thumb is about
.001" runout per every inch of shaft length Test
motor by itself. If bearings are bad, you will hear noise or feel roughness.
Replace bearings. Add oil if a sleeve of bearing. Add grease if bearings have
grease fittings. With
the motor disconnected from power turned shaft. It should move but with some
resistance. If the shaft moves in and out too much freely, this may indicate
a preload problem and the bearings may need additional shimming. Inspect motor by itself with no load attached.
If it feels rough and vibrates but the bearings are good, it may be that the
rotor was improperly balanced at the factory. Rotor must be replaced or rebalanced Test
winding for shorted or open circuits. The Current (amps) may also be high.
Replace motor or have stator rewound. |
|
Bearings continuously fail. |
Load to motor may be excessive or unbalanced. High ambient temperature |
Inspect
drive belt tension to ensure it’s not too tight An unbalanced load will also
cause the bearings to fail. If
the motor is used in a high temperatures, a different type of bearing grease
may be required. You may need to consult the factory or a bearing distributor. |
|
The motor makes a loud rubbing or grinding noise |
Rotor may be rubbing on stator Fan rubbing on housing |
Ensure that motor was not
damaged. It may not be repairable. If you can’t see physical damage, inspect the motor’s rotor and stator for rub marks. If signs of
rubbing are present, the motor should be replaced. Sometimes simply Dis &
Re -assembly eliminates rubbing. Usually it’s the bearings. This is the usual cause. Check the fan and housing |
|
Start capacitors keep failing |
The motor is not coming up to speed quickly
enough. The motor is being cycled too frequently Voltage May be too low Starting
switch may be defective, preventing the motor from coming off the start winding.
Usually hums and gets hot |
Motor
may not be sized properly. Verify how long the motor takes to come up to
speed. Most single phase capacitor start motors should come up to speed
within about three seconds or the capacitors may fail. Verify duty cycle. Capacitor manufacturers
recommend no more than 20, three-second starts per hour. Install capacitor
with higher voltage rating, or add bleed resistor to the capacitor. Check
that voltage to the motor is within 10% of the nameplate value. If the motor
is has a voltage range the deviation must be calculated from the highest Replace
switch |
|
Run capacitor fail. |
Ambient
temperature too high. |
Make sure its not hotter than motor’s
nameplate. |
Return to B.C. Electrician Home Page
NEMA Standards
--Single Phase Terminal Markings Identified By Color:
1-Blue 2- White 3- Orange
4-
Yellow 5-Black 6- No Color Assigned 7- No Color Assigned 8- Red
Single Voltage
|
Rotation |
L1 |
L2 |
|
CCW |
1,8 |
4,5 |
|
CW |
1,5 |
4,8 |
Dual Voltage: (Main Winding Only)
|
Voltage |
Rotation |
L1 |
L2 |
Join |
|
High |
CCW |
1 |
4,5 |
2&3&8 |
|
|
CW |
1 |
4,8 |
2&3&5 |
|
Low |
CCW |
1,3,8 |
2,4,5 |
------- |
|
|
CW |
1,3,5 |
2,4,8 |
------- |
Dual Voltage: (Main & Auxiliary Winding)
|
Voltage |
Rotation |
L1 |
L2 |
Join |
|
High |
CCW |
1,8 |
4,5 |
2&3,6&7 |
|
|
CW |
1,5 |
4,8 |
2&3,6&7 |
|
Low |
CCW |
1,3,6,8 |
2,4,5,7 |
--------- |
|
|
CW |
1,3,5,7 |
2,4,6,8 |
--------- |
What does a start capacitor do in a motor
An electric motor is basically composed of windings around a magnet.
Motors are either multi-phase or single-phase. Multi-phase motors generate
starting torque along the various windings by applying out of phase voltages to
each winding in a pattern that generates a torque force in the desired
direction. Single-phase motors must generate the same starting torque however
they have only one phase to work from. This means they have to have a method to
generate a shifted version of the single phase voltage to send to one of their
windings.
There are three common methods of creating single-phase electric motors: capacitor start, split-phase, and shaded pole. Each other these motors has some method to provide starting torque to the motor by shifting the voltage given to one of the windings on the motor by some angle. This phase shift corresponds to one winding of the motor having a voltage before another coil. The difference in time between when one coil has a voltage and when a second coil has a voltage causes the torque force and begins the movement of the motor.
To solve why capacitive start motors work we can generalize Ohm's Law,
V = IR, and say that V = IZ where Z is a generalized impedance. The impedance
is composed of inductance, capacitance,
and resistance. Inductance will cause the current to lag the voltage,
capacitance will cause the current to lead the voltage, and resistance has no
effect on the timing between the current and voltage
In all motors the windings of a motor are highly inductive so the current
always arrives after the voltage. The capacitor changes the relative impedance
of the circuit on one winding causing the shift in the relationship between the
voltage and current on one winding and the other winding. This difference in
the time the electrical energy is dispersed in a winding allows the motor to
rotate
The capacitor provides a delay in the energy given to one of the
windings. This delay causes the forces in the motor to be unbalanced and the
motor then starts. Capacitor start motors are often more expensive because of
the capacitor but they have the most starting torque.
CAPACITOR
When connected in an alternating-current circuit, causes the current to lead
the voltage in time phase. The peak of the current wave is reached ahead of the
peak of the voltage wave. This is the result of the successive storage and
discharge of electric energy. They are used in single phase motors to start or
in 3 phase for power factor correction.
CAPACITOR MOTOR
A motor with a main winding arranged for direct connection to the power source,
and auxiliary winding connected in series with a capacitor. There are three
types of capacitor motors: capacitor start, in which the capacitor phase is in
the circuit only during starting, permanent-split capacitor, which has the same
capacitor and capacitor phase in the circuit for both starting and running;
two-value capacitor motor, in which there are different values of capacitance
for starting and running.
CAPACITOR START
The capacitor start single phase motor is basically the same as the split phase
start, except that it has a capacitor in series with the starting winding. The
capacitor provides a more ideal phase relation and results in greater starting
torque with much less power input. Like a the split phase motor, this type can
be reversed at rest, but not while running unless special starting and
reversing switches are used. When properly equipped for reversing while
running, the motor is much more suitable for this service than the split phase
start as it provides greater reversing ability at less watts input.
CENTRIFUGAL CUTOUT SWITCH
A centrifugally operated automatic mechanism used in split phase and other
types of single phase induction motors. Centrifugal cutout switches will open
or disconnect the starting winding when the rotor has reached a pre-determined
speed, and reconnect it when the motor speed falls below it. Without it, the
starting winding would be susceptible to rapid overheating and subsequent
burnout.
SPLIT PHASE START
Motor which employs a main winding and an auxiliary winding, which is
called the starting winding. The windings are unlike and thereby
"split" the single phase of the power supply by causing a phase
displacement between the currents of the two windings, producing a rotating
field. After the motor has reached about 75% of rated speed, the starting
winding is automatically disconnected by
a centrifugal switch or by a relay. The motor then runs on a single
oscillating field, which in conjunction with the rotation of the rotor, results
in a rotating field effect. Since there is no rotating field, after the
starting winding is de-energized, the rotation cannot be changed until the
motor has come to rest or at least slowed down to the speed where the automatic
switch closes. Special starting switches are available as well as special
reversing switches which have a means for shunting the open contacts of the
automatic switch while the motor is running and thus permits the split phase
motor to be reversed while rotating. This type of starting is found typically
on single phase fractional motors.
Link to
a Very good description of single phase motors (it’s really long!)