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Unbalanced Voltages & Single Phase Operation
Of Squirrel Cage Induction Motors
Introduction
From time to time we hear of cases where motor damage has occurred because it was
“single phased,” and its overloaded devices did not act in time to prevent damage.
Depending upon who furnished what, the supplier of either the motor or the control can
be accused of furnishing equipment that did not do what the customer expected or
believed it was supposed to do.
A communication challenge begins with answering the question “ Will the starter’s
overload devices protect motor if it is single phased?” The answer varies from “Usually
yes” to “ most of the time ‘yes’, some of the time ‘no’.” Hardly the answer an upset
customer expects to hear.
Let’s go through the subject of unbalancing and single phasing (single phasing is a
special case of unbalancing ) a three phase squirrel cage motor. For sake of brevity, we’ll
use the term “ Type K motor” to apply to any squirrel cage induction motor operating
from three phase power.
Unbalanced Voltages & Single Phasing- How Does it Happen?
Unbalanced voltages in a power system can occur due to a number of reasons. Some of
the more common reasons are:
Open Delta transformers
Unbalanced loading
Unequal trap settings on transformers,
High resistance connections
*Open Delta Transformers
Open delta transformers will produce different individual phase voltages as they are
loaded down by a balanced three phase load with a power factor typical of Type H
motors.
*Unbalanced Loading
Mixed single and three phase loading may produce some unbalance in line voltages.
*Unequal Tap Settings
Frequently, individual single phase transformers are used to provide a three phase power
supply of the desired distribution voltage. If transformer tap settings are unequal,
unbalanced voltages will be distributed to the utilization equipment/
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*High Resistance Connections
High resistance connections are equivalent to inserting resistors in the affected phases. A
voltage drop will occur. However, so will rapid heating of the improper connection. The
motor may not be the first element in the circuit to be in real trouble.
Single phasing is an extreme example of unbalancing a three phase motor. Single phasing
usually occurs because one of the phase conductors breaks ( opens), or one fuse in a three
fuse set opens. There are three fairly common types of single phasing situations a motor
can see:
Common Forms of Motor “Single Phasing”
Figure 1A shows the most basic form of single phasing where one of the phase
conductors to the motor has opened. Figure 1B is typical of a fuse opening one primary
lead in a unit or captive transformer. Figure 1C is typical of cases involving two or more
motors on the same bus, where one of the bus feeders is open circuited. Each of these
single phase conditions produces different and undesirable results. We will examine the
situations in Figures 1A and 1B in some detail, and pass over Figure 1C quickly.
Motor Heating Due to Unbalanced Voltages
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A frequently discussed method of estimating the effect of unbalanced voltages is to state
that the temperature rise in the affected phase of the motor is increased about twice the
square of the percent unbalance, where the percent unbalance is calculated as follows:
Percent Unbalance = 100 x Max Deviation from Average Voltage
Average Voltage
Example: Assume a motor has line voltages 452, 470 and 4888 volts. Estimate the
increase in temperature rise of the phase carrying the highest current.
•
•
•
•
Average voltage = 470V
Max. deviation from average = 18V
Unbalance % = 100 x (18 ) = 3.83%
470
Increase in rise = 2 x (3.83) ^ 2 = 2 x 14.67 = 29 % (Approximately)
This simple relationship is useful for estimating the effect relatively small values of
unbalanced voltage upon motor heating. As the motor of unbalance increases beyond
about 5%, its accuracy declines.
Effects Upon Motor Performance
As a type K motor power supply is unbalanced, its ability to start and carry load is also
affected. Figure 2 shows three speed torque curves for the same motor, depending upon
the amount of voltage unbalance.
Figure 2
Typical Speed torque Curve
Type K Motor, 25-200 HP
With balanced and unbalanced power supply
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With balanced voltages, a type K motor will deliver a starting torque of 100% to 150%
depending upon rating. Breakdown torques will be 200%. Larger motors ( e.g., 1500HP
@ 900 RPM) typically have lower values of starting and breakdown torque. Starting
torques of 60% and breakdown torques of 175% are typical for large pump and fan
motors.
As a motor becomes exposed to unbalanced voltages, a “negative sequence current”
component begins to increase in importance.
A motor can be considered to have two poly phase power systems operational within the
motor. A “positive sequence” system provides toque that is in the same direction as the
direction of rotation. The “negative sequence” system revolves in the opposite direction
and develops “negative” torques.
When a motor’s voltages are unbalanced, or it is single phased, negative sequence
currents are large. Moderate amounts of unbalance reduce the motor’s starting and
breakdown torques. If the motor is single phased, it loses all of its starting torque, its slip
is increased and its breakdown torque is further decreased. Both situations are shown in
Figure 2.
If a motor is single phased while running, it will continue to run as long as load torques
do not exceed the reduced breakdown torque. Typically, a single phased motor will have
a breakdown torque of 40% to 50% of full voltage values. This means that a motor
without a normal breakdown torque of 200% may sustain loads near 100% torque
without stalling.
However, serious overheating of the motor does occur, even if the motor does not stall.
We should never confuse load carrying ability with thermal capability of a single phased
motor.
Let’s summarize performance effects. If a Type K motor’s power supply is unbalanced,
its starting and breakdown torques will be reduced and its slip increased. If the motor is
single phased, it will have zero starting torque. However, if it is single phased while
running, its slip will increase, it will overheat, and its breakdown torque will be cut about
in half, but the single phased Type K motor may be able to carry nearly full load torque
without stalling.
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Currents in a Single Phased Motor
If a wye or delta connected motor is operating under moderate load from a balanced
power supply, line and winding currents will distribute as shown in Figures 3A and 3B.
However, if the same motors are single phased while running as shown in Figure 1A, line
and winding currents divide as shown in Figure 4A and 4B. with moderate loading, the
active phase line current will increase to 173% of balanced values. Individual motor
windings divide the line currents according to how the motor is connected internally.
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If the motor is operating near full load torque, motor line current may increase to 225%
of full load, balanced voltage values.
Delta/ Wye Transformer Input—Single Phase Primark
Figure 1B showed a motor served by a delta/wye transformer with one primary phase
lead open circuited.
This transformer connection is commonly used in load center unit substations, or unit
(captive) transformers where the motor is supplied by a dedicated step down transformer.
Figure 5 shows the division in line currents in the transformer secondary after opening
primary phase lead 3. If 100 amps were flowing in each secondary lead before the open
circuit, either 115 amps or 230 amps would flow in the lines to the motor afterwards.
Two motors on the same bus-feeder single phased
Figure 1C illustrated the case where two or more motors are connected to a common bus,
and one feeder phase conductor opens. All of the motors on the bus act as phase
converters to keep phase three energized. The current supplied by smaller motors on the
bus cause them to overheat more rapidly than the larger motors.
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Protection of the Motor
There are three basic techniques used to protect a motor from winding damage through
overheating. Let’s ignore motors with unusual ventilization schemes that may have
supplementary protection. Basic protective schemes look at individual phase current, or
individual phase winding temperature, or the current balance.
Phase Current Devices
Low voltage motors typically use overload devices consisting of heaters and bimetallic
trip elements in theirs starters that attempt to duplicate motor heating characteristics.
Overload devices measuring line current have inverse time characteristics that call for
quick tripping at high values of line current. Present codes and standards require an
overload device in each phase lead to provide good motor branch circuit protection.
Stall Protection
However, if a motor stalls, its heating rate increases rapidly. The actual temperature of a
motor is a function of many variables, and line current is one of these variables. For
example, a motor operating in a high ambient or with blocked ventilation may be
experiencing high winding temperatures even though its overload devices may be
detecting acceptable conditions. If a motor, operating hot, is then stalled, there is a
possibility that its overload devices may not trip the motor prior to winding damage.
Phase Winding Temperature Detectors
Thermocouples, RTD’s (resistance temperature detector), Thermo Tectors , and
thermistors are imbedded detectors. Except for Thermo Tectors, usually only one detector
is active at any one time. If an unsensed phase winding is heated due to single phasing,
total motor protection is obviously reduced.
Thermo Tectors are available for most GE integral horsepower low voltage motors. One
Thermo Tector is imbedded in each phase winding, and all three Thermo Tectors are
wired in series so that either excessive heating or a high rate of temperature rise in any
one or all three phase windings is sensed.
Thermostats are not imbedded detectors. They are usually taped to the end turns of the
stator winding. They offer a level of motor stator winding protection that is generally less
than that of an imbedded detector.
Lightly Loaded Motors and Rotor Heating
If a motor is lightly loaded (approximately 50% load) and is then single phased, its line
current in the active leads to the motor will increase to near nameplate full load values.
Motor heating under these conditions is about 10% higher than when operating normally
at full load.
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Total rotor heating loss is the sum of positive and negative sequence rotor I^2R losses.
As current unbalancing increases, negative sequence rotor I^2R losses increase rapidly.
Rotor heating due to negative sequence I^2R losses is just as real as heating due to
positive sequence current.
Rotor heating of the lightly loaded, single phased motor is more likely to pose problems
for motors in frame sizes above NEMA 447/449 frame ( e.g.. Custom 8000 motors), or
with motors having high resistance rotor designs, such as the high slip Tye KR (NEMA
Design D) motor.
Current Balance Relaying
A positive way to pick up unbalanced operation is to use current balance relaying. If the
voltages are unbalanced, currents will be unbalanced. In some cases, voltages may
remain reasonably balanced, but currents are unbalanced. Overload protection is still
required, since it is something other than current balance.
Current balance relaying is relatively expensive. A LodTrak phase unbalance relay lists
for $666 plus current transformers, and it is still a pilot device. A 25 HP motor (drip
proof. 1800 RPM) lists for $616.
Obviously, the engineering solution is poor economics for a small motor.
Summary and Recommendations
The electrical system designer is the individual charged with the responsibility to
evaluate the risks of unbalanced operation and estimate the costs of the consequences of
losing a motor.
Three Phase Protective Devices
Fuses can age. Phase conductors can be broken. On the other hand, if protective devices
operate on all three phases simultaneously, singling phasing may be a remote possibility.
Overload Sizing
Properly sized overload devices offer excellent protection against single phasing. The
level of protection is not 100%, but the value provided for the cost of the protection is
outstanding. There are three things a customer can do for himself to further increase the
odds in his favor.
Be sure There Are Three Overloads
Prior to the 1975 National Electric Code, an overload device was not required by the
Code for each phase conductor. Existing older installations should be checked to be
certain there is an overload device for each phase of the three phase motor.
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Use Actual Amps, Not Nameplate Full Load Current.
Many motors are applied to continuous loads that are much lower than nameplate full
load levels. A motor can be given excellent single phase protection by selecting an
overload heater based on actual measured loaded line current rather than nameplate
values.
Recheck Overloads After Power Factor Improvement
Power factor improvement is gaining management attention as a means of reducing the
billed costs of electrical power in many locations. Always recheck motor overload
devices after power factor improvement capacitors have been added. A common
technically sound practice is to use the starter for a single speed, non-reversing motor as
the contactor for the motor’s power factor improvement capacitors.
If the capacitors are connected “downstream” of the motor’s overload devices ( so that
capacitor current does not flow through motor overload devices), the motor overload
devices will either require new overload heaters of readjustment of existing overload
settings to compensate for the reduced current flowing through the overloads.
This last point is vital to providing even reasonable overload protection under balances
three phase operating conditions. Reductions in apparent full load line current to fully
improved motors of from 10% to 40 % are common. Always review motor overload
protection after power factor improvement capacitors have been added.
Sense of Perspective
The system designer’s judgment is necessary to weigh the costs of a full protective
package against the risks of encountering an unbalanced operating condition that may
escape detection by the relatively inexpensive overloads and stator winding temperature
sensors.
In general, current balance relaying is not cost justifiable for stall low voltage motors.
Large, medium voltage motors tend to be more vulnerable to motor damage than small
motors, and the motor investment is larger. Consequently, large motors frequently will
have current balance relaying as one of several functions included in the machine’s
protective array.
Timing
The best time to think through motor protection from unbalanced/ single phase operation
is while the electrical system is on paper. The next best time is prior to encountering an
unbalance that sneaks past the motor’s overload devices.
If you believe this discussion may be helpful to those who specify, buy, or use motors
and motor controllers, then share it with them. If you have questions on this or other
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aspects of motor protection, let us know. Part of our job is to help make your selling time
as productive as possible.
R.F. Cota
Bridgeport, Ct.
January, 1978
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