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



Latest Questions:

  1. how to select MCB
  2. 9.5 Conversion 3Phase to 1Phase
  3. Neutral CTs in HV/LV transformer
  4. current transformer
  5. MCCB Selection
  6. Electrtical Relay
  7. Arc flash calculator
  8. Current on each phase

most popular notesMost Popular Notes:







newsletter logo

Our Newsletter

Receive updates on new posts by email
down arrow

Solar Energy: The Physics and Engineering of Photovoltaic Conversion, Technologies and Systems
Solar Energy: The Physics and Engineering ...
Olindo Isabella, ...
Paperback - 488 pages
$24.51
Solar Electricity Handbook - 2015 Edition: A simple, practical guide to solar energy - designing and installing solar PV systems.
Solar Electricity Handbook - 2015 Edition: ...
Michael Boxwell
Paperback - 204 pages
$17.09
Electric Power Distribution Engineering, Third Edition
Electric Power Distribution Engineering, ...
Turan Gonen
Hardcover - 1061 pages
$104.97
Independent Energy Guide: Electrical Power for Home, Boat, & RV
Independent Energy Guide: Electrical Power ...
Kevin Jeffrey
Paperback - 280 pages
Handbook of Solar Energy: Theory, Analysis and Applications (Energy Systems in Electrical Engineering)
Handbook of Solar Energy: Theory, Analysis ...
G. N. Tiwari, Arvind ...
Hardcover - 824 pages
$279.00
Energy Systems Engineering: Evaluation and Implementation, Second Edition
Energy Systems Engineering: Evaluation and ...
Francis Vanek, Louis ...
Hardcover - 672 pages
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 712 pages
$102.30
Photovoltaic Design and Installation For Dummies
Photovoltaic Design and Installation For ...
Ryan Mayfield
Paperback - 384 pages
$16.99
Do It Yourself 12 Volt Solar Power, 2nd Edition (Simple Living)
Do It Yourself 12 Volt Solar Power, 2nd ...
Michel Daniek
Paperback - 128 pages
$14.24
Solar Engineering of Thermal Processes
Solar Engineering of Thermal Processes
John A. Duffie, ...
Hardcover - 936 pages
$108.50
Power System Monitoring and Control
Power System Monitoring and Control
Hassan Bevrani, ...
Hardcover - 288 pages
$119.00
Wind Turbine Operation in Electric Power Systems: Advanced Modeling
Wind Turbine Operation in Electric Power ...
Zbigniew Lubosny
Paperback - 262 pages
$199.00
Solar PV Engineering and Installation: Preparation for the NABCEP PV Installation Professional Certification
Solar PV Engineering and Installation: ...
Sean White
Paperback - 248 pages
$37.27
Solar Energy Engineering: Processes and Systems
Solar Energy Engineering: Processes and ...
Soteris A. Kalogirou
Hardcover - 778 pages
Electric Energy: An Introduction, Third Edition (Power Electronics and Applications Series)
Electric Energy: An Introduction, Third ...
Mohamed A. El-Sharkawi
Hardcover - 606 pages
$110.68
Large-Scale Solar Power System Design (GreenSource Books): An Engineering Guide for Grid-Connected Solar Power Generation (McGraw-Hill's Greensource)
Large-Scale Solar Power System Design ...
Peter Gevorkian
Hardcover - 704 pages
$95.62
Large-Scale Solar Power Systems: Construction and Economics (Sustainability Science and Engineering)
Large-Scale Solar Power Systems: ...
Dr Peter Gevorkian
Paperback - 400 pages
$72.74
Large-Scale Solar Power System Design (GreenSource Books): An Engineering Guide for Grid-Connected Solar Power Generation (McGraw-Hill's Greensource)
Large-Scale Solar Power System Design ...
Peter Gevorkian
Hardcover - 704 pages
$95.62
Large-Scale Solar Power Systems: Construction and Economics (Sustainability Science and Engineering)
Large-Scale Solar Power Systems: ...
Dr Peter Gevorkian
Paperback - 400 pages
$72.74
Electrical Power System Essentials
Electrical Power System Essentials
Pieter Schavemaker, ...
Hardcover - 340 pages
$82.56
Submarine Power Cables: Design, Installation, Repair, Environmental Aspects (Power Systems)
Submarine Power Cables: Design, ...
Thomas Worzyk
Paperback - 296 pages
$189.00
Power System Relaying
Power System Relaying
Stanley H. Horowitz, ...
Hardcover - 398 pages
$123.84
Power Generation and the Environment
Power Generation and the Environment
Anco S. Blazev
Hardcover - 1333 pages
$53.49
Hydro Plant Electrical Systems
Hydro Plant Electrical Systems
David M. Clemen
Hardcover - 194 pages
$89.00
Energy Systems Engineering: Evaluation and Implementation, Third Edition
Energy Systems Engineering: Evaluation and ...
Francis Vanek, Louis ...
Hardcover - 736 pages
$91.63
Control of Power Inverters in Renewable Energy and Smart Grid Integration
Control of Power Inverters in Renewable ...
Qing-Chang Zhong, ...
Hardcover - 438 pages
$133.00
Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design, Second Edition (Power Electronics and Applications Series)
Modern Electric, Hybrid Electric, and Fuel ...
Mehrdad Ehsani, Yimin ...
Hardcover - 557 pages
$106.10


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