Understanding electric motor insulation & temperature 

By on April 2nd, 2012

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


comments powered by Disqus



Latest Questions:

  1. Inspection of Cathodic Protection Transformer Rectifier
  2. Per Unit & Faults
  3. relay testing
  4. UPS battery sizing
  5. VSD/ VFD/ softstarters current supply to motor
  6. On load RPM of DC series motor
  7. Motor Current Calculation
  8. Protection Settings Training Course

most popular notesMost Popular Notes:







newsletter logo

Our Newsletter

Receive updates on new posts by email
down arrow

Energy Systems Engineering: Evaluation and Implementation, Second Edition
Energy Systems Engineering: Evaluation and ...
Francis Vanek, Louis ...
Hardcover - 672 pages
$60.99
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
$16.60
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 712 pages
$110.70
Power System Relaying
Power System Relaying
Stanley H. Horowitz, ...
Hardcover - 398 pages
$122.30
Electric Power Distribution Engineering, Third Edition
Electric Power Distribution Engineering, ...
Turan Gonen
Hardcover - 1061 pages
$137.08
Photovoltaic Design and Installation For Dummies
Photovoltaic Design and Installation For ...
Ryan Mayfield
Paperback - 384 pages
$19.14
Submarine Power Cables: Design, Installation, Repair, Environmental Aspects (Power Systems)
Submarine Power Cables: Design, ...
Thomas Worzyk
Hardcover - 296 pages
$151.20
How to Solar Power Your Home Everything You Need to Know Explained Simply (Back-To-Basics Conserving)
How to Solar Power Your Home Everything You ...
Martha Maeda
Paperback - 336 pages
$15.53
Do It Yourself 12 Volt Solar Power, 2nd Edition (Simple Living)
Do It Yourself 12 Volt Solar Power, 2nd ...
Michael Daniek
Paperback - 128 pages
$13.41
Photovoltaic Systems Engineering, Third Edition
Photovoltaic Systems Engineering, Third ...
Roger A. Messenger, ...
Hardcover - 528 pages
$107.04
Power System Monitoring and Control
Power System Monitoring and Control
Hassan Bevrani, ...
Hardcover - 288 pages
$108.04
Large-Scale Solar Power Systems: Construction and Economics (Sustainability Science and Engineering)
Large-Scale Solar Power Systems: ...
Dr Peter Gevorkian
Paperback - 400 pages
$49.05
Electric Power Substations Engineering, Third Edition (Electrical Engineering Handbook)
Electric Power Substations Engineering, ...
Hardcover - 536 pages
$125.96
Photovoltaics: Design and Installation Manual
Photovoltaics: Design and Installation ...
Solar Energy ...
Paperback - 336 pages
$36.74
Electric Energy: An Introduction, Third Edition (Power Electronics and Applications Series)
Electric Energy: An Introduction, Third ...
Mohamed A. El-Sharkawi
Hardcover - 606 pages
$107.04
Convex Optimization of Power Systems
Convex Optimization of Power Systems
Joshua Adam Taylor
Hardcover - 209 pages
$90.77
Smart Power Grids 2011 (Power Systems)
Smart Power Grids 2011 (Power Systems)
Hardcover - 696 pages
$103.40
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
$98.33
Power Generation and the Environment
Power Generation and the Environment
Anco S. Blazev
Hardcover - 1333 pages
$191.21
Wind Turbine Operation in Electric Power Systems: Advanced Modeling
Wind Turbine Operation in Electric Power ...
Zbigniew Lubosny
Paperback - 262 pages
$209.00
Design of Smart Power Grid Renewable Energy Systems
Design of Smart Power Grid Renewable Energy ...
Ali Keyhani
Hardcover - 592 pages
$116.96
Renewable Fuel Standard: Potential Economic and Environmental Effects of U.S. Biofuel Policy
Renewable Fuel Standard: Potential Economic ...
Committee on Economic ...
Paperback - 250 pages
$68.00
Optimization of Power System Operation (IEEE Press Series on Power Engineering)
Optimization of Power System Operation ...
Jizhong Zhu
Hardcover - 664 pages
$126.66
Power Quality: Problems and Mitigation Techniques
Power Quality: Problems and Mitigation ...
Bhim Singh, Ambrish ...
Hardcover - 600 pages
$131.32
Renewable and Efficient Electric Power Systems
Renewable and Efficient Electric Power ...
Gilbert M. Masters
Hardcover - 680 pages
Small Geothermal Energy Systems and Geothermal Heat Pumps: Guide for the Do-it-Yourselfer (DIY), Ground Source Heat Pumps, Information Survival Kit for Heat Pump Owners, Energy Program Successes
Small Geothermal Energy Systems and ...
U.S. Government, ...
Kindle Edition - 226 pages
The Homeowner's DIY Guide to Electrical Wiring
The Homeowner's DIY Guide to Electrical ...
David Herres
Paperback - 336 pages
$19.08
Smart Grid Standards: Specifications, Requirements, and Technologies
Smart Grid Standards: Specifications, ...
Takuro Sato, Daniel ...
Hardcover - 425 pages
$117.35
Waste-to-Energy: Advanced Cycles and New Design Concepts for Efficient Power Plants
Waste-to-Energy: Advanced Cycles and New ...
Lisa Branchini
Hardcover - 145 pages
$99.99


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