Wheatley & Hall
Small Electric Motors
Design
Manufacture
Delivery
Wheatley & Hall Small Electric Motors Ltd
Unit 5A, Coulman Street, Thorne,
South Yorkshire, DN8 5JS
T. 01405 740445 F. 05603 435805
MOTOR TEMPERATURE RATINGS
A critical subject relating to Electric Motors is its Insulation Class and Temperature Ratings.
This document aims to explain the temperature relationships of standard AC Induction
Motors.
DEFINITIONS
Ambient Temperature - is the temperature of the air surrounding the motor. This is the
temperature that the entire motor would be at when it is switch off and completely cool.
Temperature Rise – is the change in temperature of critical electrical parts within a motor
when it is being run at full load. e.g. if a motor is located in an ambient of a 20°C,
started and operate continuously at full load, the winding temperature would rise from
20°C to a higher temperature. The difference between these two temperatures is the
motors temperature rise.
Insulation Class – insulations are standardised and graded by their resistance to thermal
Aging and failure. There a four insulation classes in common use which are designated by
the letters A, B, H and H. The temperature capabilities of these four classes are separated
from each other by 25°c increments. The temperature capability of each insulation class is
defined as being the ‘maximum temperature’ at which the insulation can be operated to
yield 20,000 hours life. The maximum temperature rating for each class is –
INSULATION CLASS
A
B
F
H
MAX TEMPERATURE RISE
65 °C
90 °C
105 °C
140 °C
Insulation System – there are several insulating materials used in the construction of a
motor. Like a chain the insulation system is only a strong as its weakest link. The
classification of an insulation system is based on the temperature rating of the lowest rated
material in the system e.g. if a single Class B material issued along with Class H materials,
the entire system can only be classified as Class B.
Registered in England No.6869738. Registered Office: 29 Somerton Drive, Hatfield Woodhouse, Doncaster, South Yorkshire, DN8 5JS
Wheatley & Hall
Small Electric Motors
Design
Manufacture
Delivery
Wheatley & Hall Small Electric Motors Ltd
Unit 5A, Coulman Street, Thorne,
South Yorkshire, DN8 5JS
T. 01405 740445 F. 05603 435805
TEMPERATURE AND MOTOR RATING
The base ambient temperature rating point of all electric motors 40°C. This means that a
motor, rated for 40°C ambient, is suitable for installation in applications where the normal
surrounding are temperature does not exceed 40°C. This is the base point.
When the motor is operated at full load, it has a specific temperature rise. The rise is
always added to the ambient temperature.
For example, a motor designed for Class A insulation and a maximum temperature rise
by resistance is 55°C would have a total average winding temperature rise of 40°C (ambient)
+ 55°C (rise) or 95°C. The 10°C difference between 95°C and 105°C Class A insulation
system is used to handle the ‘hot spot allowance’. If you now use the same motor but
change the insulation system to Class B, there is an extra 25°C of thermal capability
available which can be used handle –
Higher than normal ambient temperature
Higher than normal temperature rise brought on by overloads
Extend the motor life and be more tolerant of overheating factors (high/low voltages,
voltage imbalance, blocked ventilation, high inertia loads, high starting frequency, etc.)
MOTOR INSULATION IN PRACTICE
Over time improvements have been made in insulating materials. In tandem with these
improvements come cost reductions. As a result of these changes most motor manufactures
use a mixture of materials in the design of their motors, many of which have a higher than
required temperature rating. This means that many fractional horsepower motors are design
for Class A temperature rise, the practical insulation class can be Class B or higher.
As a rule of thumb, insulation life will be doubled for each 10°C of unused insulation
temperature capability. The same 10°C rule equally applies to motors operating at above
rated temperature. In this case the insulation life is halved for each 10°C of over-temperature.
Registered in England No.6869738. Registered Office: 29 Somerton Drive, Hatfield Woodhouse, Doncaster, South Yorkshire, DN8 5JS
Wheatley & Hall
Small Electric Motors
Design
Manufacture
Delivery
Wheatley & Hall Small Electric Motors Ltd
Unit 5A, Coulman Street, Thorne,
South Yorkshire, DN8 5JS
T. 01405 740445 F. 05603 435805
MOTOR SURFACE TEMPERATURE
As can be expected motor surface temperature is of concern. The motor surface temperature
will never exceed the internal temperature of the motor. This should not be ignored because
the motor surface temperature can be high enough to cause human skin to burn (a first
degree burn injury occurs at 48°C) when touched. Although high, this temperature does
not mean overload or imminent motor failure.
SUMMARY
One of the main factors in motor life is the insulation system. The key to the insulation and
motor life is the maximum temperature that the insulation system experiences and the
temperature capabilities of individual insulation components.
Registered in England No.6869738. Registered Office: 29 Somerton Drive, Hatfield Woodhouse, Doncaster, South Yorkshire, DN8 5JS