Message Board Thread - "motor temp. "

Back to Threads | Back to Forums

TitleByPosted On
motor temp. rodger 1/26/2001
RE:motor temp. Gary Orlove 2/5/2001
The abbreviations AMB or TEMP on a motor nameplate indicate the maximum ambient temperature environment for motor operation. Ambient means the temperature of the air surrounding the motor. In general, maximum ambient temperature for motors is 40 Degrees C or 104 Degrees F. This general rule holds unless the motor is specifically designed for a different temperature.

Operation of a motor at ambient temperatures exceeding that given on the nameplate may affect the life of the motor, depending on whether the motor is operating at or near its rated full load. Under these conditions, motors will have reduced life.

Gary Orlove
Infrared Training Center
RE:motor temp: Part II Gary Orlove 2/5/2001
Temperature Rise

Temperature rise is the increase in the motor's internal operating temperature. The ambient temperature plus the temperature rise is the maximum temperature at which the motor should operate at full load.

The heat causing the temperature rise is a result of electrical and mechanical losses, and a characteristic of a motors design. The ambient temperature has very little, if anything to do with the motors actual temperature rise. Certain inaccessible spots of a motor winding are considered to be the hottest spots of the insulation system. These areas are called hot spots. For this reason, a 10 Degree C allowance is made for uneven heating in the motor. This is called a hot spot allowance. The total temperature rise equals the rise due to the load on the motor plus the hot spot allowance. Typically there is no control over temperature rise due to the load or hot spot allowance.

By selecting a motor that can produce enough horsepower without being overloaded, and by ensuring the motor runs in ambient temperatures at or below the nameplate rating, one can limit motor temperature and maximize life.

If a one horsepower motor with a service factor of 1.0 is required to continuously deliver 1 1/2 horsepower, the internal temperature of the motor would probably exceed the rating of the insulation material. This will result in premature motor failure. A good rule to remember is that for every 10 Degrees C that the operating temperature increases over rated temperature, motor life will be cut in half.

Insulation Class

A critical factor in the reduced life of electrical equipment is heat. The type of insulation used in a motor depends on the operating temperature that the motor will experience. Average insulation life decreases rapidly with increases in motor internal operating temperatures.

NEMA has established safe maximum operating temperatures for motors based on an average 20,000 hour lifetime. These maximum temperatures are the sum of the ambient and maximum temperature rise ratings of the motor.

There are four NEMA insulation classes based thermal endurance of the system for maximum temperature rating purposes. These are listed on the motor's nameplate and are either A, B, F, or H. These codes indicate the maximum temperature the motor insulation can withstand without failure.

Class A insulation was the standard insulation used on older U Frame motors between 1952 and 1964. Since 1964, T Frame motors use class B insulation as the standard insulation. Most common fractional horsepower motors use either insulation class A or B.

Class B is used on most integral horsepower motors. Classes F and H are generally used for NEMA motor designs which are special applications. Do not confuse the NEMA insulation classes with the NEMA motor designs which are also given by letters.

NEMA insulation ratings assume the motor is operating within its rated ambient temperature. Ambient temperature is the air temperature surrounding the motor and is also indicated on the nameplate.

Motors should be replaced by motors with the same or higher insulation class to avoid reductions in motor life and nuisance tripping of the motor overload device. Each 10 Degree C rise above the motor's rating can reduce motor life by one-half.

Class A insulation has a recommended temperature limit of 105 Degrees C or 221 Degrees F. Class B goes to 130 Degrees C or 226 Degrees F, Class F to 155 Degrees C or 311 Degrees F, and Class H to 180 Degrees C or 356 Degrees F. Insulation classes are directly related to motor life.

For example: A motor operating at 180 Degrees C will have an estimated life of only 300 hours with a Class A insulation system. If Class B insulation is used, estimated life is increased to 1,800 hours. If Class F insulation is used, 8,500 hours of life can be expected from the motor and with Class H insulation motor life will increase to tens of thousands of hours.

IEC insulation winding classes parallel those of NEMA and in all but very rare cases, use the same letter designations.

Gary Orlove
Infrared Training Center
Winding Temp/Surface Temp Eddie Hindmarsh 11/26/2003
Can anyone give the relationship between the winding temperature and the surface temp which we can measure. Using installed instrumentation I have often found winding Temp to be 20C hotter than the surface. In practice, I would suspect any motor running in excess of 90C,(surface temp) let alone the fact that the insulation can take higher temps. I am interested in a response
Motor Temperature oj 1/8/2004
I agree with the 20C rise difference for normal large HP. Smaller horsepower motors that I have read have shown a 40F or less difference depending on the motor type and size (open drip, TEFC, etc.)

I have found issues with motors or the associated equipment with temperatures lower than 90C. Hot Spoting or heating due to an eccentric air gap can occur below those temperature. Once case found a bent shaft. Another was an indication of loose bearing a gearbox with a C-face motor. The biggest thing to remember is the class insulation, actual load and the effects of exceeding that max NEMA temp. 10C can happen happen easily after you exceeded that max temp and result in great motor life loss. There are always indications before then of the problem. Follow-up with other PdM technologies is paramount- to include vibration and MCA.
The mechanical clinic given at Inframation 2003 gave excellent examples for the best techniques used for mechanical inspections. The best technique I have found is watching both sides of the motor and regular trending of no more than quarterly intervals.

  • Back to Threads
  • Back to Forums


  •   Copyright © FLIR Systems, Inc 2012