What are insulation classes of motors?

Feb 22
07:56

2012

Henry Steve James

Henry Steve James

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You have heard a lot about class A and class F classification, while purchasing motors. What are these classifications? What do these classes indicate...

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You have heard a lot about class A and class F classification,What are insulation classes of motors? Articles while purchasing motors. What are these classifications? What do these classes indicate? This article gives an answer to all your queries.
 
NEMA is the association for electrical goods manufacturing industry. NEMA stands for National Electrical Manufacturers Association, who has set the standards for the electrical industry. It is a commonly seen that an electrical motor will fail when the internal temperature of the motor raises beyond the insulation temperature limit. Therefore, NEMA specifies certain alphabets that indicate motor temperature and insulation rates. They also specify a temperature limit at maximum load for motors. That is to note that, a motor must not operate at temperatures above the maximum limit. For any motor, the reference ambient temperature is 40°C.
Maximum temperature limit is ambient temperature + allowed temperature +allowance for “hot spot “.

Below listed are the different classes, as coined by NEMA, IEC standards, and Underwriters Laboratories.

Class A

Class A accepts a 60°C rise with a hot spot of 5°C, which means, Class A can withstand the temperature limit up to 105°C. You can use organic materials like silk, paper, cotton, and certain synthetic fibers at this temperature.

Class E

As per the IEC class E, maximum temperature allowed for PET (Poly Ethylene Terephthalate), Polyurethane, some resins, etc is 120°C.

Class B

In Class B, maximum hot pot temperature allowed is 130°C. This accepts a temperature rise of 80°C and a hot spot of 10°C. This is the right temperature for substances like glass fiber, asbestos, mica, and few other inorganic materials.

Class F

This is the most common for International and 50 Cycles motor and suitable for temperatures up to 155°C. Class F accepts a temperature rise of 105°C and a hot spot of 10°C. Materials with usable lifetime and Class B materials with binders that are stable at higher temperatures require Class F motors.

Class H

Class 130 inorganic materials that uses high temperature binders or other materials that has usable lifetime are the usual materials at this temperature. Temperature allowed is 180°C, which means, it accepts a temperature rise of 125°C and a hot spot of 15°C.

Class N

Class N is recommended for a motor that is suitable for Class B materials and Teflon. Maximum temperature allowed is 200°C.

Class R

The temperature limit acceptable is 220°C. The materials recommended are just the same as for Class 200 (Class N)

Class S

There is no proper material data for class S. However, the temperature limit is 240°C.

As the above classes indicate, for every 10°C rise in temperature, the operating life of a motor is degraded or halved. To be more precise, the insulation of the motor is weakened when there is a high temperature rise. Therefore, reducing the temperature will increase the life span of your motor.