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



Maxwell's Equations - Introduction

Maxwell's Equations are a set of fundamental relationships, which govern how electric and magnetic fields interact. The equations explain how these fields...

What is a rectifier transformer?

I've recently come across this question a couple times browsing the internet. Decided to give a quick answer here. A rectifier transformer is a transformer...

How to Size Power Cable Duct

Some colleagues had an issue earlier in the week on sizing conduits to be cast in concrete for some power cables . It became clear that none of us had...

1,000 kV UHV First for China

At the beginning of the year China put the world's first 1,000 kV UHV transmission system into operation. Transmitting power at over a million volts is...

IEC 61439 - The Switchgear Standard

The new standard IEC 61439 replaces the old 60439. Compared to the old standard, the new 61439 is a more clearly defined and takes into account the assembly...

Robots - Interesting Videos

The robot folding towels post below was interesting enough at the time to post a link.  Recently I’ve come across a couple of other interesting videos...

Our internet address and Vanity URLs

Visitors who like to type web address rather then click menus may be interested in how our URL structure works.

Tips for a better Low Voltage Protection Discrimination Study

Carrying out a protection system discrimination study is critical to ensure the correct functioning of  the electrical system in the event of faults. ...

Voltage Levels – Confused?

I was having a conversation the other day about voltage levels.  While everyone was in agreement that low voltage was 1000 V and less, there was more confusion...

IEC 60287 Current Capacity of Cables - Rated Current

In the previous note we looked at the approach taken by the standard to the sizing of cables and illustrated this with an example.  We then looked at one...

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