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Insulation Class?

Insulation Class?

Can anyone help me define Class of Insulation based on temperature? I mean what's the difference between Class B and Class F insulation? Does it depend on the application?
Thank you for your help.



RE: Insulation Class?

Yes. Class of insulation is closely related to winding temperature since the insulation materials used in windings are temperature sensitive.

Class B - Maximum operating temp 120 deg C
Class F - Maximum operating temp 155 deg C
Class H - maximum operating temp 180 deg C

However, due to difficulty in pin pointing the place where the maximum temp occurs, lower limits are used. For eg, with winding embedded RTD's, for class F machines, the recommended maximum temp is 120 deg C.

RE: Insulation Class?


Thank you for your reply. However I would appreciate if you could elaborate further.
1- How do we define the class of material, on what basis?
2- How is it dependant on the application?
3- Is it possible that a material could be class B for one application and at the same time class F for another application?

Thank you


RE: Insulation Class?

Edison, further to my above reply, I am a confused on your explaination of the RTD example. Does it mean that for every class of insulation there is a range of temperature? could you please send me the ranges for all the classes?


RE: Insulation Class?

I suggest reading NEMA MG-1 for the definitions of these ratings. Also, there should be similar definitions for other types of wound equipment (in another standard).

RE: Insulation Class?

Reference standards IEC 34-1, BS4999, AS 1359.32
'Life' refers to the life of the windings before they require rewinding. The temperature rise of the windings (and the insulation materials), of an electric motor is critical to the life expectancy of the motor and is a function of the design of the motor. The insulation materials age over time and this aging process is directly related to temperature. Eventually, the materials lose their insulating properties and break down causing a short circuit.
The increase in temperature of a motor is due to the losses that occur in the motor. These losses are mainly made up of copper and iron losses. The temperature inside the motor will depend on how effectively this heat can be removed by the cooling system of the motor. It should not be assumed that a motor that appears to be hot externally is not internally.
If the cooling system is efficient, the thermal gradient through the motor will be small and the difference between the winding temperature and the external temperature low.
Some standards estimate the life of the insulation materials as 25,000 hours if operated continuously at their rated temperature and the external temperature low.
Western Electric motors are built with Class F insulation and designed for Class B rise, and most of the motors only have a Class E rise. This 'Thermal Reserve' greatly increases the life of the motor, so that it is not of concern, especially when most motors do not operate at less than full load, are not not in a continuous ambient temperature of 40 degrees. A life of 20 to 30 years under normal conditions can confidently be expected.
Insulation class     A     E     B     F     F
B rise     H
Temperature rise    105     120     130     155     155     180
Max temp of the winding    100     115     120     140     140     165
Ambient temperature    40     40     40     40     40     40
Allowance of hot spots    5     5     10     15
(10)     15     15
Max temp of rise of winding    60     75     80     100
(105)     80     125
Thermal reserve    0     0     0     0     20     0
The permitted temperature rise of the windings of an electric motor are subdivided into different insulation classes and temperature limits. The above table applies to motors in an ambient temperature up to 40°C and an altitude of less than 1000 meters above sea level.
The difference between the 'Maximum Temperature of the Winding' and the 'Temperature Limit' is because there will be hot spots in the winding which are not measured by the 'Resistance Method' which only measures the mean temperature of the whole winding. An allowance is made for this difference to ensure that no part of the winding is operating at its full thermal rating. It is not considered practical to try to locate and measure the hottest spot in the winding.
The temperature rise of the winding is measured by the resistance method using the following formula:
DT = (R2 - R1)/*(235 + T1) + (T1 - T2)
DT = temperature rise in deg K
R1 = cold resistance of the winding atT1
R2 = hot resistance of the winding atT2
T1 = ambient at start-up deg C
T2 = ambient at finish in deg C
235 = reciprocal of the temperature coefficient of the resistance of Copper at 0 deg C.
For a winding to comply with Class F insulation requirements, all the materials must be to Class F specification or better.
There are a number of advantages in buying motors with a thermal temperature reserve apart from an anticipated long service life.
* Service factor: this is really an American (NEMA) term that is not covered in IEC standards. It means that the motor can be overloaded without serious damage of overheating.
Typically, NEMA specifications call for Service Factor of 1.1 or 1.15, meaning a 10% or 15% overload Service Factor is in fact using up the thermal reserve of the motor and allowing it to operate at its full Class temperature rise.
Although IEC does not acknowledge Service Factor in the same way, it certainly allows motors to operate to their full class rating and in fact most motors with a generous thermal reserve will easily match the NEMA requirement for 1.1 or 1.15.
Service Factor and duty rating (eg S1, S2 etc) should not be confused. Duty ratings are clearly covered in IEC standards for different repetitive short term overloads which can be defined and simulated to ensure the motor still meets the requirements for temperature rise.
*Voltage of Frequency Variations: In some installations, especially with their own power generation, or a very weak grid, large fluctuations in voltage and/or frequency are possible which can cause increases in temperature rise of the motor. Motors with a large thermal reserve can operate in these conditions, usually without exceeding their Class rating, by using some or all of their thermal reserve, depending on the size of the fluctuation.
Effects of temperature and insulation on motor performance and life
It is a fact that you could take just about any electric motor and hook it up to any appliance and it would work. As would be expected though, some motors would perform better and last longer than others.
The one fundamental characteristic which determines whether a motor survives or dies is quite simply how hot it gets. Motor overheating can be caused in a number of ways including:
•    overloading
•    ambient temperature too high
•    incorrect supply voltage and/or frequency
•    frequency of starting
All of these can seriously affect motor performance and/or life.
On the other hand, a motor operating in an ambient temperature below freezing could be overloaded and run quite happily for years.
The controlling factor is always the temperature of the motor.
Perhaps the best starting point is to understand that any motor is designed such that when operating at its rated output under known conditions, its windings will experience a set increase in temperature (temperature rise) above the ambient temperature around the motor.
The temperature rise is a function of two criteria:
1. The amount of heat generated in the rotor and stator per unit time
2. The efficiency of heat transfer from the motor by the type of cooling system employed.
Providing the motor output, power supply, location and other relevant conditions remain constant, then the resulting temperature rise will also remain constant.
The motor manufacturer having established the design temperature rise and assuming a maximum ambient temperature that is likely to be encountered, then decides upon the use of particular grade or 'class' of insulating material which will be sufficient to ensure no thermal breakdown that would lead to short circuiting between phases or between phase and earth as well as providing for an acceptable lifetime for the motor.
Each 'class' of insulation has its own maximum recommended temperature. If this critical temperature is drastically exceeded, failure will occur in a very short time. If it is marginally exceeded, then the lifetime of the insulation (and the motor) will be reduced by the order of 50%, leading to premature burn-out.
An increase in load of about 4% will result in an increase in temperature rise of 10%, which makes selection of the correct motors for the job absolutely vital for long term cost-effective operation.

RE: Insulation Class?

sabap, thank you for taking the time to reply to my thread, by I think you have't picked the right frequency!

Thank you any way.


RE: Insulation Class?

"1- How do we define the class of material, on what basis?"

We test the insulation system in thermal aging tests using motorette samples.

"2- How is it dependant on the application?"

In some applications as was mentioned above, there is difficulty in determining the true hot spot in the application.  We measure at one point and it may be hotter at another point. Therefore we establish a lower limit at the measured point to ensure insulation temperature limits won't be exceeded at another point.  Also, in some applications reliability is more important, and purchaser may specify some thermal margin to be built into the system. A common example is to specify Class F insulation materials, but that the actual temperature rise be limited as if it were class B materials.

"3- Is it possible that a material could be class B for one application and at the same time class F for another application?"

Yes.  An insulation system consists of many materials.  The classification of the system depends on how the whole system acts under test.  In general it is only as strong as it's weakest link.

"I am a confused on your explaination of the RTD example. Does it mean that for every class of insulation there is a range of temperature?"

No. Class identifies a maximum temperature.  The motor may not be designed/operated to that maximum for reasons ientified in #2.

Please feel free to ask again if we still haven't hit the wavelength.

Eng-tips forums: The best place on the web for engineering discussions.

RE: Insulation Class?

When I have a motor rewound for an application that requires motor thermostats (poor ventilation, variable frequency drive, dirt accumulation) I have the thermostats placed at the end opposite the fan. In the case of motors that are direct coupled to gearboxes that end of the motor will be very hot because of heat from the gearbox.

In one instance of a gearmotor on a variable frequency drive I had the motor rewound with thermostats AND installed a standard motor overload relay and recommended by SquareD's book on VFDs. You want to use the standard relay for primary protection and to protect mechanical components against overload. The thermostat should always be treated as backup protection against loss of cooling and so forth. Sure enough, it turned out that I had to turn down the standard overload relay from 3 Horsepower down to 2.5 Horsepower because of what the gearbox could handle because of lack of ventilation not what the motor could handle.

Mike Cole,

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