Trouble shoot and replace motors.pdf

Trouble shooting and Replacement of
LT motors
J Niranjan

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Motor Malfunctions
With a few simple troubleshooting techniques
motor problems can usually be identified
quickly because there is little complex circuitry
and virtually no control devices within the
motor

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Recognize Faulty Operations
The table below shows some of the serious problems
associated with the motor and their causes
Condition Probable Cause
Motor does not run
does not hum
• Blown fuse
• Open circuit breaker
• Loss of supply
• Open lead to motor
• Motor is burned out
• Motor overload device tripped

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Motor fails to start does
not run but hums possibly
trips overload heater
Open phase
Low supply voltage
Mechanically overloaded
Motor overheats while
running under load
Overloaded
Low voltage
High voltage
Low frequency
Clogged ventilating ducts
Shorted stator coils
Worn bearings
Rotor rubbing on stator
Motor vibrates excessively Load/coupling misaligned
Shaft bent
Worn bearings
Broken rotor bar

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Motor runs slow Overloaded
Low voltage
Low frequency
Broken rotor bars
Unbalanced line
current on polyphase
motors during normal
operation
Line voltage
Stator coils incorrectly connected
Poor connection of feeder for that phase
Motor trips ground
fault relay
Grounded connection in motor junction
box
Grounded feeder conductor
Stator coil shorted to stator core or
frame

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Trouble shooting Procedures
Condition Probable Cause
Motor does not run
does not hum
• Blown fuse
• Open circuit breaker
• Loss of supply
• Open lead to motor
• Motor is burned out
• Motor overload device tripped

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Blown Fuse
Check to see that all line fuses are good. Fuses may be
associated with a line circuit breaker or fused disconnect
switch. Fuse checking must be done in accordance with
the Praxair fuse test procedure, which includes selecting
the proper PPE, de-energizing the circuit, verifying the
circuit dead, lockout, tagout of the disconnect.
If the fuse is discovered to be open, then the cause of the
blown fuse should be investigated. Also, the motor should
be rotated by hand to see that it is not mechanically
locked.

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Open Circuit Breaker
Check all circuit breakers supplying the motor circuit. If the
circuit breaker is found off, take action to determine why the
breaker was turned off. If the breaker is found in the tripped
position, check for conditions that would cause an overload or
fault as above.
Loss of Supply
Verify the source voltage is present at the motor starter, or if
the starter is operational, check voltage leaving the starter.
This is diagnostic work and must be conducted under control
of an HWP with appropriate PPE.
If proper voltage is present on the leads leaving the motor
starter the motor or motor circuit is open. Proceed with the
next steps

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Open Lead to Motor
This is done with the motor starter or disconnect switch open,
locked and tagged out. Use the motor checker. (Operation of
motor checker explained in future slides)
If a lead shows open (high resistance) open the motor junction
box and look for a burned open or loose connection.

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Motor Is Burned Out
This is usually evident by the color of the enclosure. A burned
out motor will have blistered or heavily discolored paint
If the motor is in fact burned out, check for the reason.
Occasionally, a motor will simply burn up due to defective
construction, or deterioration of insulation, but the overload
device should have tripped off.
In any case, investigate the cause of the burn out.

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Motor Overload Device Tripped
This occurs as a result of three conditions: (1) the motor is
overloaded; (2) the overload relays (thermal elements) are
undersized; (3) the motor is partially shorted.
The first step is to lockout the motor and try, then check the
motor nameplate for rating, then be sure the shaft turns free.
Check the resistance and inductance using Motor Checker.
Next, re-set the overload and restart the motor, then check the
current on all three legs. (Difference shall not be more than
5%). Also look for evidence of a loose connection in the motor
starter.

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Motor fails to start
does not run but
hums possibly trips
overload heater
Open phase
Low supply voltage
Mechanically overloaded

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Open Phase
Verify all 3 phases are at the proper voltage leaving the motor
starter or disconnect switch. Loss of one phase can result from a
blown fuse, defective circuit breaker, contactor or overload device.
Also suspect would be bad connections, particularly in a wet and
damp environment.
Low Supply Voltage
When supply voltage is low, a motor may not start under load.
Voltages lower than 20% of the nominal 440 volts may not be
sufficient to start the motor, consequently the overload relay usually
trips.
Causes of low supply voltage range from utility problems,
overloaded circuits or panels, or undersized conductors to the motor.

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Mechanically Overloaded
A good example of this condition is a compressor that
has stopped with high head pressure. Often, a
compressor motor cannot start against head pressure, so
it appears that the motor is locked up. Investigate
mechanical loads before attempting to restart the motor

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Motor overheats
while running
under load
Overloaded
Low voltage
High voltage
Low frequency
Clogged ventilating ducts
Worn bearings
Rotor rubbing on stator

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Overloaded
The overloaded motor will draw excessive current. Check the
current under load and compare with the nameplate data. If
the motor is undersized, replace the motor or reduce the load.
Low Voltage
Low voltage, as discussed earlier will cause the motor to draw
excessive current for the load, thus causing overheating. After
the current is checked (above) then check the voltage to the
motor.
High Voltage
High voltage, in excess of 20% can create an overheating
problem due to higher magnetic losses. Look for an over-
voltage condition when checking voltage above.

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Low Frequency
Low frequency, below 45 Hz will cause overheating due
to loss of inductive reactance. The motor will also run
slower. Low frequency usually occurs when the system is
running on a generator that is not adjusted for the
proper speed.
Check the supply frequency and take action accordingly.
Clogged Ventilating Ducts
Most industrial motors run fairly hot under load. But
they run hotter with the ventilation ducts clogged with
dust or other material. Every l0 degC over the rated
temperature can reduce insulation life by 50%!
Carefully inspect and clean the vent ducts if necessary.

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Shorted Stator Coils
Stator coils can short in two ways: (l) the insulation between turns
can break down, thus decreasing the inductance of that winding,
(2) turns can make contact with the laminated core also due to
insulation breakdown.
Shorted turns appear as higher current on one phase with balanced
voltage, or as uneven heating of the motor. Turns shorted to the
stator core are more serious because the live conductor places
voltage on the core. The ground fault relay will trip in this case
Shorted turns may be detected by the use of a low resistance
ohmmeter or Motor checker, stator to winding shorts may be
detected with a conventional ohmmeter or a megger.

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Worn Bearings
Bearing wear is detected by noise, vibration, heating and
complete seizure of the motor in extreme cases. Bearing
wear is caused by several factors:
• Misalignment
• Lack of lubrication
• Moisture and dirt
• Excessive temperatures
Eventually, an excessively worn bearing will fail to the
extent that the rotor will begin rubbing the stator, the
shaft will break or the motor will seize up.

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Rotor Rubbing On Stator
On most motors, the air gap between the rotor and stator
is only 0.06 ” on small motors up to 0.20” on larger
motors. Although not common, the motor end shields
may be come distorted, the shaft bent, or the bearings so
worn that the rotor actually rubs the stator of the motor.
Rotor-stator rub is detected by noise, heating, sparks
flying out of the vents, and loss of power.

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Motor runs
slow
Overloaded
Low voltage
Low frequency
Broken rotor bars

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Overloaded
The Overloaded motor tends to slow down proportionate
to the torque vs. Rpm curve, until the level of stall torque
is reached, then the motor speed will drop dramatically.
If the overload device does not activate, the motor will
burn out.
Check the voltage and current supplying the motor and
compare to the nameplate rated current. Also check the
rating of the overload relays if the motor seems too hot.

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Low Voltage and Low Frequency
Both will cause a motor to run slowly. Under heavy loads,
however, the motor will begin to overheat. Low voltage can
result from utility conditions or undersized feeders, while low
frequency usually only comes from a generator.
Check voltage, then check frequency with the multi-meter.
Shorted Stator Coils
As discussed earlier cause loss of power consequently the
motor will slow down under load, but again current drain and
heating will increase

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Broken Rotor Bars
Though rare, occasionally a shorting ring will work loose
from the ends of the rotor bars. When this happens, a portion
of the secondary of the transformer is opened, thus reducing
the rotor current flow and eliminating rotary torque.
Mis-wiring
Typically does not occur in the field. On some occasions, a
motor may come from the factory or back from the motor
shop with a particular coil in a winding connected in reverse.
The reversed coil opposes the magnetic flux generated for that
pole and acts as a brake for the rotor. A reversed coil within a
winding may or may not create excessive current draw or
heating, depending on the characteristics of that particular
motor.

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Unbalanced
line current on
poly-phase
motors during
normal
operation
Line voltage
Stator coils incorrectly
connected
Poor connection of
feeder for that phase

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Unbalanced line current on a motor is more a symptom of
another condition as opposed to being a malfunction of its
own. Some current imbalance may be inherent in a motor due
to anomalies in the construction, but this imbalance should
not exceed 5% between phases.
Shorted Stator Coils or Mis-wired Coils should also be
considered if no other conditions can be detected. The motor
may appear to run properly with this condition, as long as
there is minimal load.
A Poor Connection in any point in the feeder conductor can
create resistance sufficient to cause a drop in current for that
phase. However under load, the poor connection will manifest
itself by creating heat at the poor connection.

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Motor trips on
ground fault
relay
Grounded connection in
motor junction box
Grounded feeder
conductor
Stator coil shorted to
stator core or frame

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A Grounded Connection in the motor junction box is the most
common cause of shorts to ground in a motor. This is due to
the tight proximity of wires in the box and the sometimes
questionable methods of taping connectors. This is the first
area that should be checked.
A Grounded Feeder Conductor anywhere in the motor circuit
will create a ground fault. Water in an underground conduit
can also create a ground fault.
As discussed previously, a Stator Coil Shorted to the Core will
result in ground fault relay operation

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CAUSES OF PROBLEMS AND FAILURES
Heat—High temperature causes degradation of
insulation. Whether caused by high ambient
temperature, overloading, or poor air circulation this
condition accelerates aging.
Moisture and corrosive substances can damage the
motor, even though it may be rated for hazardous duty.
Dirt, dust, abrasive and corrosive materials can enter the
motor and attack windings and switch contacts in single-
phase motors.
Under-voltage and under-frequency create excessive
current drain and subsequent heating.
Voltage imbalance on larger motors (400 — 500 HP)

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CAUSES OF PROBLEMS AND FAILURES
Misalignment between the motor and its load can create
excessive stress on bearings and distort the shaft
Improper application, for example an open motor is
installed in a hostile environment, or a motor is
undersized for the load that creates an overheating
condition.
Lack of compatibility with inverter power where a VFD
(variable frequency drive) is installed as a replacement
for a conventional starter. Spikes on voltage waveform
stress insulation, resulting in failure.
Lack of Maintenance—Dry bearings, air passages
clogged with sawdust, dirt or fibers cause excessive
heating

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Replacement Procedures
•MEGGER THE REPLACEMENT MOTOR
•LOCATE THE POWER SOURCE AND REMOVE POWER
Be sure the motor is stopped before opening the main disconnect—this will
prolong the contact life of the disconnect switch contacts. If the motor is still
running, stop it by the normal method.
Note the direction the motor is turning as it comes to a stop. If the driven load
has no directional arrow, this step can save time later.
Be sure that any other sources of energy cannot turn the motor or the driven
shaft when the motor coupling is disconnected. Examples of such energy are:
Air, gas or fluid pressure against a pump, Spring or hydraulic pressure, Wind
blowing through a set of large fan blades, a winch or cable winder with
tension on the rope.

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• UTILIZE LOCKOUT/TAGOUT PROCEDURES
• BREAK THE ELECTRICAL AND MECHANICAL
CONNECTIONS
Remove the shaft coupling, drive shaft or belts. Before removing
belts, be sure to release the tension.
Open the motor junction box and carefully unpack the wires.
They may be literally jammed into the junction box and brittle
from heat. Note the connection configuration. If no metal tags or
wire numbers are around the stator leads, then tag or label the
leads

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•UTILIZE LOCKOUT/TAGOUT PROCEDURES
•BREAK THE ELECTRICAL AND MECHANICAL
CONNECTIONS
Remove the shaft coupling, drive shaft or belts. Before removing
belts, be sure to release the tension.
Open the motor junction box and carefully unpack the wires.
They may be literally jammed into the junction box and brittle
from heat. Note the connection configuration. If no metal tags or
wire numbers are around the stator leads, then tag or label the
leads
Disconnect and label heater leads, RTD leads where applicable.
Disconnect the conduit/cable gland from the motor junction box
and move it and the wires out of the way.

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• REMOVE THE MOTOR
Loosen the motor on the mounts
Remove the mounting bolts. If the coupling is still in place, be careful to
slide the motor away from the coupling. Do not allow stress to be put on
the coupling or the driven shaft, as this may create alignment problems
later.
Lower the motor to the ground and put it in a safe place. Before
removing, any accessories take a measurement of the distance from the
end of the shaft so that you can put the pulley back on for proper
alignment. Remove any accessories such as pulleys, couplings or other
hardware and install them on the new motor
• INSTALL THE NEW MOTOR
Place the new motor into position using the same sling or lifting
arrangement as with the removal. Never allow hands, feet or other parts of
the body to be under a suspended load.
Once the motor is in place lightly tighten mounting bolts.

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• TAPE CONNECTIONS ACCORDING TO PRAXAIR
SPECIFICATIONS
• MEGGER THE FEEDER
With all covers and guards installed, the feeder to the motor should be
meggered again. Because all motor connections are made up, the most
practical place to megger the feeder from is the output of the motor
starter (the load side of the feeder). This will test the motor feeders and
windings all at the same time
This step is critically important in case a motor junction box screw has
pierced a connection, or a feeder has become shorted with the movement
of the conduit/due to the glanding.

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• RECONNECT THE WIRES
First, review the connection diagram on the motor nameplate for the
applied voltage (it is possible that the motor being replaced was a single-
voltage motor with only three leads). Be sure the leads are properly
connected for the voltage or starter configuration. Remember to reconnect
any auxiliary leads, such as heaters or thermocouples.
Make sure the motor frame is properly grounded
• VERIFY CONNECTIONS ARE CORRECT FOR VOLTAGE
AND ROTATION
Make a final check for voltage and rotation connections. If you reconnected
the new motor exactly the same way as the old, then there should be no
problem. If the wires were not marked on the old motor as to rotation,
then you have only a 50% chance of getting it right.

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TEST OPERATION AND RESTORE TO
SERVICE
• CLEAR THE AREA AND REMOVE THE LOCKOUT
• RESTORE POWER
• TEST ROTATION
One person should go to the motor controller and prepare to
operate the motor momentarily while another person watches
the motor shaft. The person at the controller then “bumps” the
motor . The observer then notes whether or not the motor is
running in the right direction. The observer also notes any
irregular sounds when the motor starts. The motor should
coast down normally.
If the motor starts in the wrong direction, then reversing any two
wires from the starter will reverse the motor

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SERVICE
• CONDUCT FINAL ALIGNMENT
• TEST OPERATION
Again, any affected personnel should be notified that the starter is
about to be tested and that the motor will be starting. The motor
should be brought on in the normal mode by remote controls. The
motor should be checked for excessive vibration by feel. If targets are
installed, a baseline set of vibration readings should be taken.

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SERVICE
RESTORE SERVICE
For the final time, notify affected personnel that the equipment
will be restored to normal service. When the motor has been
started in the normal mode, it should be again observed proper
operation. The motor should be checked for heating by feel. If
the motor feels hotter than normal, then the current should be
checked at the MCC by a clamp-on ammeter and compared to
the nameplate rated current.
It is also a good practice to check the motor more frequently
within the first 24 hours after the replacement has been
completed, since problems may surface when the motor has
come up to operating temperature, or vibration has set in.

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SERVICE
DOCUMENT THE REPAIR, CORRECT DRAWINGS
No job is complete until the documentation is done. The replacement
should be documented as a “Corrective Maintenance” in EAM 7i.
Any changes made in wiring to accommodate a new type of motor
should be noted and documented.
In cases of a sudden or unexpected motor failure, a RCA (root cause
analysis) should be conducted immediately.

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Using the ‘Motor Checker’ – EMC 22-
Introduction

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Using the ‘Motor Checker’ - INTRODUCTION

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Using the ‘Motor Checker’ - INTRODUCTION
Fault Analysis
A difference in the resistance and/or inductance of
individual windings which is greater than 10% will
indicate a fault in any motor.
The resistance and inductance readings on the motors
connected in star or delta will show approximately
half the effect of a fault in a single winding. Thus a
spread of more than 5% measured on a connected
motor corresponds to a deviation by more than 10%
in an individual winding

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Using the ‘Motor Checker’ Instrument

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Using the ‘Motor Checker’ Instrument –
Connections between various phases

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Using the ‘Motor Checker’ Instrument – Resistance
measurement

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Inductance
Measurement

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Using the Range Chart
For Obtaining the approximate resistance and Inductance,
values, note the reading obtained on the percentage scale and
the range selected. From the Range chart calculate the value

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Trouble shoot and replace motors.pdf