Distribution Automation Handbook
Section 8.11 Motor Protection
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
2
Contents
8.11 Motor Protection............................................................................................................................ 3
8.11.1 Introduction .............................................................................................................................. 3
8.11.2 Thermal Behavior and Thermal Protection .............................................................................. 3
8.11.2.1 THERMAL MODEL ................................................................................................................................................................ 3
8.11.2.2 FREQUENT STARTUP SUPERVISION ....................................................................................................................................... 6
8.11.2.3 OVERLOAD PROTECTION ...................................................................................................................................................... 8
8.11.2.4 STARTUP SUPERVISION AND STALL PROTECTION .................................................................................................................. 9
8.11.2.5 OVERHEATING PROTECTION ............................................................................................................................................... 11
8.11.3 Protection against External Network Disturbances ............................................................... 12
8.11.3.1 UNBALANCE PROTECTION .................................................................................................................................................. 12
8.11.3.2 VARIATION IN SUPPLY VOLTAGE AND FREQUENCY ............................................................................................................ 13
8.11.3.3 OUT-OF-PHASE RE-ENERGIZING PROTECTION .................................................................................................................... 13
8.11.4 Protection against Insulation Failures ................................................................................... 15
8.11.4.1 STATOR EARTH-FAULT PROTECTION .................................................................................................................................. 15
8.11.4.2 SHORT CIRCUIT AND INTER-WINDING FAULT PROTECTION ................................................................................................. 16
8.11.5 Additional Protection Functions for Synchronous Motors ..................................................... 16
8.11.5.1 POLE SLIPPING PROTECTION ............................................................................................................................................... 16
8.11.5.2 LOSS-OF-EXCITATION PROTECTION .................................................................................................................................... 17
8.11.5.3 DIODE FAILURE SUPERVISION ............................................................................................................................................ 18
8.11.5.4 ROTOR EARTH-FAULT PROTECTION .................................................................................................................................... 19
8.11.6 Examples ................................................................................................................................. 19
8.11.6.1 HV-INDUCTION MOTORS ................................................................................................................................................... 19
8.11.6.2 SYNCHRONOUS MOTOR WITH BRUSHES .............................................................................................................................. 20
8.11.6.3 BRUSHLESS SYNCHRONOUS MOTORS ................................................................................................................................. 22
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
3
8.11 Motor Protection
Motor protection is described in references [8.11.1] to [8.11.9].
8.11.1 Introduction
Electric motors are exposed to many kinds of disturbances and stress. Part of the disturbances is due to im-
posed external conditions such as over- and undervoltage, over- and underfrequency, harmonics, unba-
lanced system voltages and supply interruptions, for example autoreclosing that occurs in the supplying
network. Other possible causes of external disturbances are dirt in the motor, cooling system and bearing
failures or increase of ambient temperature and humidity. Stress factors due to abnormal use of the motor
drive itself are frequent successive startups, stall and overload situations including mechanical stress. The
above stress and disturbances deteriorate the winding insulation of the motor mechanically and by in-
creased thermal ageing rate, which may eventually lead to an insulation failure.
The purpose of the motor protection is to limit the effects of the disturbances and stress factors to a safe
level, for example, by limiting overvoltages or by preventing too frequent startup attempts. If, however, a
motor failure takes place, the purpose of the protection is to disconnect the motor from the supplying net-
work in due time.
8.11.2 Thermal Behavior and Thermal Protection
Motor overload condition is mainly a result from abnormal use of the motor, harmonics or unbalanced
supply voltages. They all increase the motor losses and cause additional heating. As the temperature ex-
ceeds the rated limits specified for the insulation class in question, the winding insulation deterioration ac-
celerates. This will shorten the expected lifetime of the motor and may lead in some point to an electrical
fault in the winding. Thus, the thermal overload protection can be considered being the most important pro-
tection function in addition to the short circuit protection of the motor. Usually also authorities require that
motors are equipped with thermal overload protection.
8.11.2.1 Thermal Model
The thermal behavior of the stator and the rotor during startups and during constant overload situations dif-
fers significantly from each other. Due to this fact, the dynamics of the motor heating and cooling is typi-
cally modeled separately for the stator and for the rotor. Implementing the thermal overload protection in
this way, it can be set to follow the thermal state of the motor optimally, and good and accurate protection
against both short and long-time overload conditions can be accomplished, which allows the full use of the
available capacity.
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
4
The protection operates according to the model that is the most critical one in the prevailing operating con-
ditions. Thus, the thermal model for the stator and for the rotor can be written as:
(8.11.1)
( ) ( ) ( )RtNRNRRtNRNRR eIIpeIIp 2/2)1/2 1)/(1(1)/( ττ −− −⋅∆Θ⋅⋅−+ −⋅∆Θ⋅⋅=∆Θ (8.11.2)
where
RS ,θ∆ is the temperature rise of the stator or the rotor (ºC)
RSp , is the weighting factor for the short time constant of the stator or the ro-
tor (winding conductors)
I is the measured phase current, typically the highest phase current
NI is the rated current of the protected machine
NRNS ,θ∆ is the temperature rise (ºC) with the rated current under sustained load,
the stator or the rotor
RS 1,1τ is the short heating / cooling time constant of the stator or the rotor
(winding conductors)
RS 2,2τ is the long heating / cooling time constant of the stator or the rotor (sta-
tor or rotor body)
t is the time
The estimated temperature of the stator and the rotor can then be obtained by adding the prevailing ambient
temperature to the calculated temperature rises. For the ambient temperature, either a constant maximum
value can be given as a setting value or it can be measured by the IED with the related functionality.
The model must also take into account the fact that a running motor heats up and cools down according to
the same time constant but in a standstill the long time constant for cooling can be much longer than that
for heating, depending on the cooling system. This is important especially in providing adequate protection
against too frequent starting.
The values of τ1, τ2, ∆θN, IN and p are set according to the motor type in question. The time constants and
weighting factors for the stator and the rotor can also be extracted from the thermal limit curve of the ma-
chine by assuming that the heating is proportional to the square of the load current. The value of the rated
temperature rise depends on the insulation class of the machine, and it is obtained by subtracting the as-
sumed ambient temperature from the highest allowed temperature. Table 8.11.1 shows the temperature lim-
its for insulation classes B, F and H.
Table 8.11.1: Insulation classes in accordance with IEC 60034-1
Definition
Insulation Class
B F H
Highest “hot spot” temperature (oC) 130 155 180
( ) ( ) ( )StNSNSStNSNSS eIIpeIIp 2/2)1/2 1)/(1(1)/( ττ −− −⋅∆Θ⋅⋅−+ −⋅∆Θ⋅⋅=∆Θ
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
5
Highest operating temperature (oC) 120 145 165
Highest operating temperature when short-time am-
bient temperature does not exceed 40 (oC)
80 105 125
With typical squirrel gage induction machines, the maximum temperature is according to class F, but the
maximum temperature rise according to class B. The purpose of this is to obtain additional thermal margin,
and to extend the lifetime of the insulation. Also, a typical squirrel gage rotor of an induction machine is
not insulated, so the temperature rise can usually be higher than in the stator without increasing the risk of
damage. However, a rotor temperature of 250oC must not be exceeded.
However, adequate thermal modeling can also be accomplished by using a single-time-constant model but
treating the normal loading and overload situations in a different way. In this kind of modeling, the thermal
behavior has two different thermal characteristic curves; one describing short and long-time overloads, that
is, modeling the motor ”hot spot” behavior, and the other keeping track of the thermal background, that is,
modeling the motor parts with a slower heat absorption, such as the stator body. Equations 8.11.3, 8.11.4
and 8.11.5 represent this kind of modeling. Equations 8.11.3 and 8.11.4 are valid when the measured motor
current exceeds the rated current IΝ:
( ) %1001
05.1
/
2
⋅−⋅
⋅
=Θ − τt
N
A eI
I
(8.11.3)
( ) %1
05.1
/
2
pe
I
I t
N
B ⋅−⋅
⋅
=Θ − τ (8.11.4)
where
AΘ is the thermal level describing motor parts having fast heat absorption,
that is, ”hot spot” behavior
BΘ is the thermal background history level describing motor parts having
slow heat absorption, for example stator body
I is the measured phase current
τ is the selected time constant
p is the selected weighting factor for the thermal history
When an overload situation ends, that is, the measured current becomes lower than the rated current, the
thermal level describing the ”hot spot” behavior is taken down to the thermal background level in a rela-
tively short period. This constitutes the thermal behavior modeling of the motor when the hot spot tempera-
ture will cool down due to the effect of the other parts of the motor, after which Equation 8.11.5 is valid.
( ) %1
05.1
/
2
p
t
p
N
B epI
I
Θ+−⋅
Θ−⋅
⋅
=Θ − τ (8.11.5)
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
6
where
pΘ is the calculated thermal level when the current goes below the rated
current setting, for example after a startup
Figure 8.11.1 illustrates the behavior of the single-time-constant modeling in accordance with Equations
8.11.3, 8.11.4 and 8.11.5. A thermal level of 0% corresponds to the ambient temperature, and 100% the
maximum allowed temperature.
Figure 8.11.1: The behavior of the single-time-constant thermal model
The purpose of the single-time-constant modeling is not to try to model the protected motor as accurately as
possible. Typically, its accuracy is sufficient for supervising the thermal state of the motor and in this way
allowing the normal use of the motor drive for which it is designed to with a suitable margin. The parame-
ters of the model are then selected based on this assumption, and they are typically easily derived from the
basic data of the motor.
8.11.2.2 Frequent Startup Supervision
In order not to shorten the expected lifetime of the motor, there must be an adequate time interval between
successive startups. Therefore, a certain starting frequency, that is, the number of startups per hour speci-
fied for the motor, must not be exceeded.
Especially during successive startups, the temperature of the rotor rises and drops rapidly whereas the tem-
perature of the stator changes much more slowly. At rated load, the temperature of the rotor is much lower
than the temperature of the stator.
If after a startup the motor is running for some time before stopping, the rotor has enough time to cool
down. Then whether a restart can be done or not depends totally on the stator temperature, which can be a
limiting factor. On the other hand, if the initial start had been done from a cold condition and it failed for
some reason, then whether a restart can be done or not depends now totally on the rotor temperature, which
can be a limiting factor instead. If the initial start had been done from hot condition, then the limiting factor
is again the stator temperature. Figure 8.11.2 illustrates this kind of thermal behavior simulated with the
two-time-constant model for the stator and for the rotor.
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
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Figure 8.11.2: Temperatures of the rotor and stator according to the two-time-constant model
when two startups have been done from cold condition (top) and when one
successful startup and one unsuccessful startup have been done from hot condition
(bottom).
Supervision of successive startups can be done by using a simple counter-function: a certain amount of
starting seconds is allowed in certain period. The disadvantage of this method is that it does not assort
stalled or undervoltage startups from normal ones. Figure 8.11.3 shows an example of this kind of counter-
function, the setting values of which are the sum of the allowed starting seconds and the countdown rate.
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
80 %
90 %
100 %
0 50 100 150 200 250Time [s]
0
1
2
3
4
5
6
7
8
9
10
×
IN
Current
Stator
Trip level
Rotor
Th
er
m
al
le
ve
l(
%
)
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
80 %
90 %
100 %
0 50 100 150 200 250Time [s]
0
1
2
3
4
5
6
7
8
9
10
×
IN
Current
Stator
Trip level
Rotor
Th
er
m
al
le
ve
l(
%
)
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
80 %
90 %
100 %
0 500 1000 1500Time [s]
0
1
2
3
4
5
6
7
8
9
10
×
IN
Trip level
Stator
Rotor
Current
Trip (unsuccesful start)Th
er
m
al
le
ve
l(
%
)
0 %
10 %
20 %
30 %
40 %
50 %
60 %
70 %
80 %
90 %
100 %
0 500 1000 1500Time [s]
0
1
2
3
4
5
6
7
8
9
10
×
IN
Trip level
Stator
Rotor
Current
Trip (unsuccesful start)Th
er
m
al
le
ve
l(
%
)
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
8
Figure 8.11.3: Operation and setting of a restart inhibit counter when six consecutive startups are
performed. The starting time is 2.5 s, and no more than 6 starts per hour are
allowed.
A better and more accurate way to protect the motor against too frequent starting is to make use of the
thermal model and to determine and set a restart inhibit temperature limit: if the estimated rotor or stator
temperature exceeds this limit, a further restart attempt is inhibited. For example, a restart inhibit level for
the example of Figure 8.11.2 could be set at 55%, which is based on the estimated 45% increase in thermal
level for a single startup.
8.11.2.3 Overload Protection
A minor overload does not cause a motor failure immediately but it will eventually shorten the expected
lifetime. On the other hand, a constant overload can be a sign of some kind of disturbance in the process in
which the motor drive is being used. Thus, a two-stage overload protection is preferred. The alarming stage
gives an indication that the rated load of the motor with a possible margin has been exceeded. This function
can be implemented by a pre-warning temperature level setting in the thermal model. The pre-warning
alarm gives the operator some time to find out the possible source of the overload and to attempt to remove
it. If the overload becomes higher, for example 10-15%, the tripping stage starts and trips the motor feeder
in due time unless the source of the overload has disappeared before that. According to the thermal model,
tripping takes place when the estimated thermal level exceeds 100%. Figure 8.11.4 shows an example of
the thermal behavior when the motor is running on a constant cyclic overload. These curves have been si-
mulated using the two-time-constant model for the stator and for the rotor. In this case, the pre-warning
level is exceeded and the operator is notified. As a result, the tripping is prevented as the loading of the mo-
tor becomes suitably reduced. It can also be concluded from Figure 8.11.4 that a comprehensive overload
protection in fact requires the use of the two-time-constant model for the rotor and for the stator, and in this
way, the full utilization of the available capacity of the motor is ensured. However, adequate protection can
also be implemented using the single-time-constant model, which is set to allow the normal use of the mo-
tor drive with a suitable margin.
starts inhibited
Distribution Automation Handbook (prototype)
Power System Protection 8.11, Motor Protection
1MRS757291
9
Figure 8.11.4: Temperatures of the rotor and the stator according to the two-time-constant model
when the motor is running in constant cyclic overload. Pre-warning level is set to
90%.
The ambient temperature has a high impact on the motor loadability as shown in Table 8.11.2. For this to
be fully utilized, the protection scheme must be equipped with a sensor for measuring the ambient tempera-
ture, which is then used for compensating the thermal model. Alternatively, the ambient temperature can be
taken into account when selecting the start current setting of the protection.
Table 8.11.2: Effect of ambient temperature on the loadability of the motor
Ambient temperature Allowed output
oC % of rated output
30 107
40 100
45 96
50 92
55 87
60 82*
70 65*
80 50*
*) Output power and lubrication according to the contract.
8.11.2.4 Startup Supervision and Stall Protection
The starting current of induction motors varies between 5…7·IN whereas with synchronous motors that are
started as induction motors the current range is between 3…4·IN. Slip ring induction motors that are started
with the help of controllable external rotor resistors the current does not exceed the rated current. The start-
ing current decays differently depending on the resistance of the rotor circuit. If the resistance is relatively
high like with LV-motors, the st
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