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Motor Protection: Preventing the Burn-out
Copyright © 2002 Francis J. Martino
Overload Relay Class Time Delay
American industry has standardized on Class 20 overload protection
for motor control. The Europeans have standardized on Class 10.
Class 20 will give a nominal 590 second trip (9.83 minutes) at an
overload of 125% of full load amps, a 29 second trip at a 500% overload,
and a 20 second trip at a 600% overload. Thus, a motor that is stalled and
drawing locked rotor amperage will be taken off line in 20 to 29 seconds.
However, a motor that draws a continuous locked rotor current can be
expected to burn out before 20 seconds.
Class 10 will give a nominal 230 second trip (3.83 minutes) at 125%
overload, 15 seconds at 500% overload, and 10 seconds at 600% overload.
Class 30 has a longer time delay to be used on high inertia loads that
require a long acceleration or have shock loading that causes repetitive
The Model CRP series overload relays by Cerus Industrial are infinitely
adjustable for Class 1 to 30 by setting the trip time from 0 to 30 seconds.
The set time is the trip time at six times the set current. The digital,
solid-state CDP series is adjustable for Class 1 through Class 60.
The wide range of class adjustments with the Model CDP will allow the relay
to be adjusted for the maximum protection of the motor. Thus, if a motor is
normally up to running speed in six seconds then the overload relay may be
adjusted for Class 1 with a nominal trip time of 12 seconds at 125% overload.
A typical overload event is that of a motor bearing failure in which a
bearing begins to seize the motor shaft but yet still allows the shaft to
turn. This type of bearing failure causes a continuous drag on the motor
rotor, causing a high motor current. If the bearing causes, for example, a
125% overload, the overload will be sustained for the 590 second delay of
Class 20 or the 230 second delay of Class 10 and may end in winding burn
out before the overload relay takes the motor off line. A lower Class
protection will take the motor off line in less time. Model CDP relays
will then give an indication that an over current condition took
place rather than a phase loss or other fault. The bearing problem can
then be detected and the bearings replaced before the motor burns out.
The CDP relay also has a bar graph which indicates load is approaching
thermal limit. The graph indicates 60 to 110% limit which, if monitored
regularly, can give a warning that a problem is developing.
In addition, the CDP relay will give a digital readout of amperage
on each of the three motor legs.
Model CDP will sense a stalled or locked rotor condition and will shut
down the motor independently of the Class time delay. The Stalled Rotor
Protection will trip when load current is more than 180% of FLA for 5
seconds after start. The Locked Rotor Protection will trip when the motor
draws locked rotor current for .5 second. Stalled or Locked Rotor detection
is the only type of protection against load jam or bearing seizure that will
allow a motor to be shut down with any possibility of preventing a winding
For large motor applications, Resistance Temperature Detectors (RTDs)
for sensing motor winding and bearing temperatures may also be used.
A motor with RTDs will have two RTDs on each of the three windings plus
one on each bearing. General Electric Model RRTD module will accept
up to twelve RTDs.
Single-phasing Protection via Current Sensing
The standard melting alloy or bimetallic heating element relays,
such as the Cerus Industrial Model CTK, offer single-phasing
protection by virtue of the remaining two motor legs drawing more
current to compensate for the lost leg. The relay will then sense
the increased heat that is generated by the current and see an
overload. Shut down of the motor, however, will be subject to the time
delay that is inherent within the heating element type relays.
Many electronic single phasing protectors are voltage sensitive. The
inherent problem that arises is that a three phase motor that looses one
phase will begin to act like a transformer. The unpowered winding then
generates an output voltage that is sensed by the protective relay. As a
result, the relay, if not adjusted properly to the characteristics of the
specific motor, may not sense a loss of power and the motor will continue
to operate rather than be taken off line.
The only positive solution is a relay that senses motor current. When
current in any motor leg ceases, the relay will shut down the motor in
three seconds without being affected by either thermal time delay or
motor transformer action. Cerus solid state overload relays Models
CRP and CDP are current sensing relays. For high amperage motors, use
Models CRP22-3S or CDP06-S. Set the current trip for 5 amps and use
three current transformers which have 5 amp secondaries.
Fault Indication, Electronic Relays, and Inductive Spikes
The Model CRP solid state relays require a power supply of 100
to 260 VAC, 50/60 Hz, and the Model CDP requires either
110 VAC nominal or 190 to 240 VAC, 50/60 Hz.
There are current sensing overload relays on the market which are
assembled to the motor contactor so as to have the internal electronics
powered by the same power that feeds the motor. Those relays are then
subjected to the continuous inductive spikes that are generated by
electric motors. Without sufficient internal transient suppression, the
relays could fail after a period of time due to the electronic components
being degraded by the repetitive motor generated transients.
An advantage of using a control transformer input with the Models CRP
and CDP is that it will provide isolation which will protect against
external transients that would enter the power input of the relay. If the
240 VAC used on the CRP or CDP is taken from the motor power source,
then the relays will be subjected to the motor transients. Although
suppression is built within the relay, an external suppressor
is available for extra protection and can be wired in parallel with
the input terminals of the relay. Use Cerus suppressor Part Number AS-3.
Voltage Sag Protection
The input power requirement of the CRP relay is a range from 100 to
260 VAC. If a 240 VAC power input is used, it will have the added
advantage of voltage sag ride-through. The line which supplies the 240
volts could drop to 100 volts, a 58% sag, and the relay will continue to
operate without shutting down the motor. If the sag affects both the
motor and relay, the motor will be shut down by the relay and the relay
will then give an indication of a phase imbalance or other appropriate fault.
The model CDP has available a ground fault sensing option and
also a kit to mount the display in a panel door.
Advantages of a Comprehensive Motor Protection
A motor winding has a generally accepted life expectancy of 18 years *
when operated at rated internal temperatures. However, bearings often
fail before the winding. Thus the objective of providing a quick response
with motor overload protection is to have the benefit of the maximum
lifetime of the motor winding.
Consider the case of the bearing failure in which the bearing places a
drag on the motor shaft, as was mentioned above. If the failure is detected
by an overload Class less than Class 10 or by RTD sensors that are
imbedded within the motor, then that early detection will allow the motor
to be taken out of service for perhaps a one day bearing replacement
rather than a three or four day period for rewind and repair of mechanical
damage caused by bearing collapse.
Enhanced motor protection will therefore not only lower motor
rewinding and repair costs but will also lower over-all down time and
yield an increase in production.
Overload Relays often should not be used on the output
of a variable frequency drive as nuisance tripping will result under 30 HZ.
When using Current Transformers as noted in the relay descriptions, use
standard units with 5 amp output. Prices of current transformers are
available upon request.
* Tom C. Lloyd, Electric Motors and Their Applications, John Wiley & Sons,
Inc., New York, 1969, page 235-236. Lloyd places the life expectancy at
20 years based on accelerated testing of motor insulating materials.
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