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November 2003
Understanding Motor Temperature Rise Limits
By Tom Bishop, P.E.
EASA Technical Support Specialist
EASA
MEMBER
BENEFIT
How do we know if a motor is operating
within its temperature rating? The simple answer,
and a good one, is that the National Electrical
Manufacturers Association (NEMA) has defined
temperature rise for electric motors in Motors and
Generators, NEMA standard MG 1-1998. In this
article we will focus on temperature rise and temperature sensing of three-phase induction motors.
We will begin by identifying some key terms.
Temperature rise is the increase in temperature
above ambient. Ambient temperature is the temperature of the air (or other cooling medium) in the area
surrounding the motor, frequently termed “room
temperature.” The sum of the ambient temperature
and the temperature rise is the overall, or “hot,” temperature of a component. Insulation temperature
classes are based on the overall temperature. For example, a Class B winding system is rated 130º C.
The normal maximum ambient, per NEMA, is
40º C. The temperature rise limit for the Class B
winding would be estimated at 90º C (130-40).
in MG1-12.43, and
shown in Table 1. The
temperature rise values
are in degrees Celsius
(º C) and are based on a
maximum ambient of
40º C. In the most common speed ratings the
NEMA designation of
medium motors includes horsepower
ratings from 1/2 horsepower (hp) (0.37 kW) up to
500 hp (370 kW) for 2 and 4 poles, and up to 350
hp (260 kW) for 6 poles.
Temperature rise limits for large motors, those
above medium motor ratings, differ based on the
service factor. Table 2 is taken from MG1-20.8.1
and lists the temperature rise for motors with a 1.0
service factor (SF) and Table 3, taken from MG120.8.2, applies to motors with 1.15 SF.
Measuring Resistance
The resistance method is useful for motors that
do not have embedded detectors such as thermocouples or resistance temperature detectors (RTDs).
Note that temperature rise limits (Table 1) for meOther Factors To Consider
dium motors are based on resistance. The
However, there are other factors that NEMA
temperatures of large motors can be measured by
uses in establishing temperature rise for motors.
resistance or by embedded detectors. Resistance
Therefore, our estimate is not precise and not applitesting is performed by measuring the lead-to-lead
cable to motor windings. The other factors are
resistance of line leads of the winding. An initial
primarily allowances for hot spot temperatures.
test is done with the motor “cold,” i.e., room (amThat is, a safety factor is built in to the rating to acbient) temperature. Verify that the motor is at room
count for parts of the winding that may be hotter
temperature by checking the winding temperature
than the location at which temperature is measured.
directly if possible.
Temperature rise limits for medium motors are
Alternative checks that are
Table 1: Temperature rise by resistance method for medium
not as precise would be checking the temperature of the
induction motors.
stator core or the external
frame. The motor winding hot
Medium Induction Motors
Insulation Class and
resistance is tested after the
Temperature Rise º C
winding temperature stabilizes
with the motor operating at
Motor Type
A
B
F
H
rated load and the change in
resistance is used to determine
1 Motors with 1.0 service factor (SF)
the hot temperature. (Note: It
other than those in 3 or 4.
60
80
105
125
may take as long as 8 hours at
2 All motors with 1.15 or higher SF
70
90
115
--rated load for the winding tem3 Totally-enclosed nonventilated
perature to stabilize.) The
motors with 1.0 SF
65
85
110
130
ambient temperature is subtracted from the hot winding
4 Motors with encapsulated windings
temperature to determine the
and with 1.0 SF, all enclosures
65
85
110
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Continued on Page 4
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EASA CURRENTS
Understanding Motor Temperature . . . Continued From Page 1
Table 2: Temperature rise for large motors with 1.0 service factor.
Large Motors with 1.0 Service Factor at Rated Load
Motor Rating
Insulation Class and
Temperature Rise º C
Method of
determination
A
B
F
H
1 All horsepower (or kW) ratings
Resistance
60
80
105
125
2 1500 hp (1120 kW) and less
Embedded
detector
70
90
115
140
3 Over 1500 hp (1120 kW)
and 7000 volts or less
Embedded
detector
65
85
110
135
4 Over 1500 hp (1120 kW)
and over 7000 volts
Embedded
detector
60
80
105
125
Table 3: Temperature rise for large motors with 1.15 service factor.
Large Motors with a 1.15 Service Factor at Service
Factor Load
Insulation Class and
Temperature Rise º C
Motor Rating
Method of
determination
A
B
F
H
1 All horsepower (or kW) ratings
Resistance
70
90
115
135
2 1500 hp (1120 kW) and less
Embedded
detector
80
100
125
150
3 Over 1500 hp (1120 kW)
and 7000 volts or less
Embedded
detector
75
95
120
145
4 Over 1500 hp (1120 kW)
and over 7000 volts
Embedded
detector
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temperature rise. An example will help illustrate
how this is done. An un-encapsulated, open dripproof medium motor with a Class F winding and a
1.0 SF has a lead to lead resistance of 1.02 ohms
at an ambient temperature of 25º C, and a hot resistance of 1.43 ohms. The formula to determine
temperature is:
Th = [ (Rh/Rc) x (K + Tc) ] – K
The meanings of the terms are:
Th hot temperature
Tc cold temperature
Rh hot resistance
Rc cold resistance
K 234.5 (a constant for copper)
The hot winding temperature for the example
is calculated as follows:
Th = [ (1.43/1.02) x (234.5 + 25) ] – 234.5
= 129.3º C
The temperature rise is the hot winding tem-
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perature minus the ambient. The temperature
rise for the example is
129º C (rounded
value) hot temperature
minus the 25º C ambient, or 104º C (129 –
25) rise. The temperature rise limit for Class
F in Table 1 is 105º C.
The example value is
only one degree below
the temperature rise
limit. That is acceptable, but it also
indicates that any increase in load would
result in excessive
temperature rise and
consequent thermal
degradation. Further, if
the ambient at the motor installation were to
go above 40º C, the
motor load would have
to be reduced so as not
to exceed the total
temperature (hot winding) capability.
Embedded
Detectors Monitor
Temperature
Motors equipped with temperature detectors
embedded in the windings are monitored by directly reading the output of the detectors with
appropriate instrumentation. Typically, the motor
control center has panel meters indicating the
temperatures sensed by the detectors. If the detectors are in the windings but not connected to the
controls, a hand held temperature meter can sense
the output of the detector leads. The output temperature displayed is the hot winding temperature
at the location of the sensor. If the detector read
129º C as in the example above, the same temperature concerns would apply. From a practical
perspective, it is easier to measure a detector output rather than the winding lead-to-lead
resistance. Further, the detector resistance can be
measured when the motor is operating. That can’t
be done with the lead-to-lead resistance test.
What if we want to determine the winding
90
115
135
Continued on Page 3
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November 2003
Understanding Motor Temperature . . . Continued From Page 2
temperature of a motor that does not have embedded detectors and that can’t be shut down? If the
motor voltage is rated 600 volts or less it may be
possible to (following all applicable safety rules)
open the terminal box and access the back of the
stator core iron laminations. The stator lamination
temperature will not be the same as the winding
temperature, but it will be closer to it than any
other readily accessible part of the motor. If the
lamination temperature minus the ambient exceeds the temperature rise for the motor, it can be
assumed that the winding is operating beyond its
rating. For example, if the stator core temperature
measured 136º C and the motor was the same one
as in the example above, the stator core rise would
be 136 – 25, or 111º C rise. That is above the
105º C limit for the winding, and the winding can
be expected to be hotter than the laminations.
Hot Spot Temperature Is Key
The critical limitation on winding temperature
is the hot or overall temperature. That is the sum of
ambient plus rise. In large part, the load determines
the rise. The other key factor that must be dealt
with is ambient. The NEMA standard limit for ambient temperature is 40º C. Operation above that
temperature may require de-rating of the motor so
as not to exceed the hot winding limit. The limit for
the temperature rise must be reduced by the number of degrees the ambient exceeds 40º C. If the
ambient were 50º C, and the motor had a temperature rise limit 105º C from one of the tables above,
the temperature rise limit would have to be reduced
10º C (50º – 40º C ambient difference) to 95º C.
The total temperature in both cases is limited to the
same amount. That is, 105 plus 40 equals 145º C,
and 95 plus 50 also equals 145º C. Regardless of
the method used to sense winding temperature, it is
the total, or hot spot, temperature that is the real
limitation. Further, winding life can be halved if the
temperature increases 10º C, so the lower the hot
winding temperature, the better.
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