Understanding electric motor insulation & temperature 

By on

Anyone specifying or using electric motors should have a basic understanding how the insulation is related to temperature. Three classes of insulation are in common use (with 'F' being the most common):

  • class B - with a maximum operating temperature of 130 oC
  • class F - with a maximum operating temperature of 155 oC
  • class H - with a maximum operating temperature of 180 oC

The image (which is form an ABB catalogue for their low voltage performance motors), shows how temperature rise is distributed across the insulation.

Typically motors are designed for a maximum ambient temperature of 40 oC.

The difference between the average winding temperature and any hot spot is limited and it is usual to allow a 10 oC margin for class 'B' and 'F' insulation and a 15 oC margin for class 'H'.

Considering the ambient temperature and hot spot allowance gives the maximum temperature rise within which the motor must be designed to operate (105 oC for class 'F' for example).

When specifying (buying) a motor there are a couple of options. An insulation class could be specified and the motor specified as designed to run within that class. Alternatively the motor could be specified for an insulation class, but be design to run at a low class (for example insulation class 'F', temperature rise 'B').

The advantage of the second method is that there is an inherent 25 oC safety margin - useful if you are in a region with high ambient temperatures or need to date the motor for some other reason. Running motors at a reduced temperature will also significantly extend the useful life.



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus



Network Theory – Introduction and Review

In electrical engineering, Network Theory is the study of how to solve circuit problems. By analyzing circuits, the engineer looks to determine the various...

Earth Electrode Resistance

Earthing of electrical systems is essential for the correct functioning and the protecting of life and equipment in the event of faults.  The earth electrode...

Power Transformers - An Introduction

One of the fundamental requirements of an alternating current distribution systems it to have the ability to change the magnitude of voltages.  It is more...

What are you reading!

Reading is a bit of a hobby of mine and I"ve done a few off-topic posts in the past on this. Rather than continue doing the occasional post I thought ...

Fault Calculation - Per Unit System

Per unit fault calculations is a method whereby system impedances and quantities are normalised across different voltage levels to a common base.  By removing...

Generator Sizing & Operation Limits

When selecting a generator, there are inherent limits on the active and reactive power which can be delivered. Generators are normally sized for a certain...

Understanding Circuit Breaker Markings

IEC 60947 is the circuit breaker standard and covers the marking of breakers in detail. Any manufacturer following this standard should comply with the...

Frame Leakage Protection

While not as popular as it once was, frame leakage protection does still have some use in some circumstances.  In essence frame leakage is an earth fault...

The ac resistance of conductors

In a previous article I looked at the dc resistance of conductors and in this article we turn our attention to ac resistance. If you have not read the...

How to Check a Circuit is Dead

If you want to check a circuit is dead (not live), you should always use the three point method. First check a known live circuit, then check the dead...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note