Electric Motor Insulation, (Read This First!)

Electric Motor Insulation, (Read This First!)
Motor insulation class!

What does insulation Class mean on a Motor?

It is the ability of motor insulation windings to handle the heat. Using class, we can specify the allowable temperature rise from an ambient temperature of 40°C.

For more details about motor data, Read also my article: motor specs and name plate understanding

The insulation of a motor are divided into classes based on the thermal designation and evaluation. There are 5 Classes of insulation on the motor. These Insulations are named A, E, B, F, and H. To get a piece of proper knowledge of why wedivided the insulations into classes, we must know why we actually need the insulation on a motor.

We have a number of windings in our motor. Each winding is insulated from the other so the effect of current related to the magnetic field does not disturb the motor working. How the current affects the conductor in the motor is another discussion.

Each class of insulation refers to an upper limit of the temperature that it can tolerate to work normally. From the lowest temperature to the limit of the class, the insulation lets the motor work normally. Let’s call the highest temperature that insulation can tolerate as temperature limit of that class.

Temperature beyond that temperature limit may lead a motor to a dysfunctional state or an accidental condition. Because that temperature leads to a shortening of the life of the motor. Now let’s see what these classes actually denote.

Why is Motor Insulation Class Important?

Motor insulation class has an important role in the safety of devices. As we know that, with the flow of current electrical conductor heats up, and if we don’t make proper attention it can cause heavy damage to the system.

The same is the case for motor windings.  If it overheats besides other parts insulation can damage. And as a result, the motor is no more protected. For this purpose, a standard insulation classes system has been introduced.

How to determine motor insulation classes?

We can divide it into different classes. This process is carried out according to the NEMA STANDARD. Major and most commonly used motor insulation classes include A, B, F & H. However, N, R, and S classes are also in use.

Class: A 105 Degrees C
Class: B 130 Degrees C
Class: F 155 Degrees C
Class: H 180 Degrees C

Before we go in detail about the table data we need first to understand some terms. Which are :

  1. NEMA allowable rise (maximum permissible winding temperature). This is the maximum temperature that the winding can handle.
  2. maximum temperature rise, This is the maximum temperature rise that the motor generates at full load and a certain service factor.
  3. Temperature margin (or hot spot Over Temperature), This is the subtract of NEMA allowable rise and  Winding maximum temperature rise.

Let’s now go in details and then we will give an example.

Class A:

Class A insulation of the motor can tolerate the temperature of 60 degrees Celsius around the motor. If the room temperature is 60 degrees around the motor with insulation type Class A, the motor can work just okay.

Beyond that it may will motor will be affected. While the temperature of motor windings must not exceed 105-degree Celsius. There is a limit of 5 degrees Celsius above 105 degrees just for the purpose of protection.

Class A motor insulation has:

  • Maximum permissible winding temperature of 105°C.
  • While the Maximum Temperature Rise is 60°C.
  • The allowed Hot-spot Over Temperature is 5°C.
  • In this type of class insulation materials like silk, cotton, or paper as the dielectric material.

Class B:

For Class B type motor insulations, the motor can tolerate a maximum temperature of 130 degrees Celsius. While the ambient temperature range is up to 80-degree Celsius.

In this class, we can have a limit of 10 degrees above the temperature limit. This gives us more facility to activate the cooling system until the temperature is 10 degrees beyond the limit.

Class B motor insulation has:

  • maximum permissible winding temperature of 130°C.
  • maximum Temperature Rise is 80°C.
  • The allowed Hot-spot Over is 10°C.
  • Class B insulation materials consist of glass fiber, mica, and asbestos.

Class E motor insulation

For Class E insulation, the motorcan tolerate a temperature of 75 degrees Celsius around the motor (ambient temperature). The Motor will work just fine if the room temperature is 75 degrees around the motor with insulation type Class E.

While the temperature of motor windings must not exceed 120-degree Celsius. Like class A, there is a limit of 5 degrees Celsius above 105 degrees just for the purpose of protection.    

Motor insulation Class F:

For Class F, the insulation can stand winding temperatures up to 155 degrees Celsius. While the ambient temperature range is up to 80-degree Celsius. In this class, we can have a limit of 10 degrees above the temperature limit. This gives us more facility to activate the cooling system until the temperature is 10 degrees beyond the limit.

Class F motor insulation has:

  • maximum permissible winding temperature of 155°C.
  • maximum Temperature Rise is 105°C.
  •  allowed Hot-spot Over Temperature is 10°C.
  • This class also consists of dielectric materials like mica. Asbestos and glass fiber.

Motor insulation Class H:

For Class H, the insulation can stand winding temperatures up to 180 degrees Celsius. While the ambient temperature range is up to 125-degree Celsius. In this class, we can have a limit of 15 degrees above the temperature limit. This gives us more facility to activate the cooling system until the temperature is 15 degrees beyond the limit.

Class H motor insulation has:

  • maximum permissible winding temperature of 180°C.
  • The maximum Temperature Rise is 125°C.
  • And the allowed Hot-spot Over Temperature is 15°C.
  • This type of dielectric material consists of silicon elastomers combined with glass fiber and asbestos.

Some other classes are available such as N, R, and S. But these are rarely used.

Read my other detailed articles:

Motor insulation class example

If We have a motor with insulation class F, this motor is running at 110°C. Then the important question now is, Is this motor running too hot?

The short and direct answer is No, This motor is not running too hot. Let’s find out why!

From NEMA insulation class table above, this motor has maximum Temperature Rise of 105°C, adding this temperature to the ambient temperature of 40°C gives us 145°C.

The surface of the motor is usually 30°C lower than the winding temperature, So this motor winding temperature is 110+30 = 140°C.

Which means that this motor is running at 5°C lower than its maximum Temperature Rise (Which is 145), and 15°C lower than its maximum permissible winding temperature (Which is 155). This is why this motor is not running too hot however its temperature is 110°C

Which insulation class is better?

Class H has the maximum permissible temperature i.e 180°C, So its the best of the classes. But as an electrical engineer I say, This class is the best of the classes, but not the best choice!

The best choice is the motor of class suitable to the application or the working needs. This is because the higher the class the higher the motor price and cost.

Every motor has its own range of temperature during its operation, This is called motor temperature rise. NEMA determined this rise at full load and service factor based upon a reference ambient temperature of 40°C. The higher the difference between the motor rise and the insulation temp. the longer the motor life, but also the higher the price of the motor.

  • allowable temperature rises at full load and 1.0 S.F.
A= 60°C
B= 80°C
F= 105°C
H= 125°C
  •  NEMA allowable temperature rises For a 1.15 S.F. motor
A= 70°C
B= 90°C,
F= 115°C.

For a motor with F class, the temperature rise is 105 + 40 = 145°C at service factor 1, F class insulation has 155°C maximum temperature, So this motor has 10°C margin.

If we have a motor with insulation class F and a temperature rise B this motor is normally refereed to as F/B, this means that it has temperature rise of 80 + 40 = 120°C, While it maximum allowable temperature is 145°C. This motor has 145 – 120 = 25°C


  • Each motor has insulation class, which determine the maximum temperature that the insulation can handle.
  • Each motor has temperature rise, which is the maximum rise of the motor based on the 40°C ambient.
  • Choosing the motor insulation class is based on the motor rise, service factor and the latitude. When choosing motor class, the higher the class the longer the motor life time also the higher its price.
NEMA Insulation Temperature Ratings Motor Temperature Rises
  1.0 SF 1.15 SF
Insulation Class Temperature Ambient Hots pots Rise Rise
A 105 40 5 60 70
B 130 40 10 80 90
F 155 40 10 105 115
H 180 40 15 120

How to measure electric motor insulation resistance?

To perform insulation resistance for a motor you need a high resistance range ohmmeter, DC voltage of 500 to 1000V is applied between winding and motor body the reading should be in mega-ohm (1MOhm and larger is accepted).

This process is widely known as MEGGER a motor. I have written a detailed article about How to megger a motor? Check it out.

How do I choose an insulation Class?

So, based on the temperature, you may choose an Insulation Class. Actually, the class of the insulation type is already written on the motor. Instead of choosing the insulation class temperature we need to observe the ambient temperature of the place where we need to set the motor.

For example, If the sum of ambient temperature and motor temperature is within or equal to the range of an Insulation Class, you can use it. Let us take a hypothetical the ambient temperature is 40-degrees Celsius. And the motor being in use generates 50-degrees Celsius. Then it is OK to use Class A.

This Section will become clearer through the chart.

Motor Insulation Chart:

ClassMaximum Operation allowance Degree CelsiusMaximum Operation allowance Degree FahrenheitAllowable Temperature at Service factor 1.0 Degree CelsiusAllowable Temperature at Service factor 1.5 Degree Celsius
H180356125115 and above
Motor insulation class chart

What is Insulation Varnish?

Motor insulation varnish
I use this motor insulation varnish for motors and generators

Insulation varnish is a coating or a layer applied over the conductors to protect them from short-circuiting. Insulation varnish is a coating that insulates the conductor so that if it comes in contact with another conductor, the varnish does not let the electric current transfer from one conductor to the other.

An Insulation Varnish possesses good chemical, electrical, thermal, and electrochemical properties. It is a paint or epoxy kind of material that is poured over the motor windings, transformer windings, and other devices to prevent shorting.

The composition of Insulation varnish consists of pigment (mostly red), base material, curing agent, filler, solvent, and flame retardant. The base material is composed of different kinds of resins.

Such as polyurethane resin, Bismaleimide resins, silicone resin, epoxy resin, and polyamide resins. There are other resins added to these resins but that is rarely done. Resins bind the Insulation varnish with the conductors.

Some commonly used pigments are talc powder, barium sulfate, calcium carbonate, silica, mica powder, iron oxide red, chrome yellow, and white.

The flame retardants can be either additive flame retardants or reactive ones. Reactive fire retardants are preferred over active flame retardants and if the reasons are discussed here, it will drag the already discussed topic too far away.

Some additives that are added in the Insulation Varnishes are Auxiliary agents that are stabilizers, anti-aging agents, curing accelerators, rheology modifiers, and surfactants.

Curing accelerators are added to the Insulation varnishes to accelerate the curing speed, and reduce curing reaction and exothermic peak.

The Solvent is used to adjust the viscosity of the Insulation agent. The Insulation agent/ varnish should be viscous enough to be painted on a surface/ conductor.

Motor Varnish types.

There are 2 types of motor varnishes.

Impregnating Varnish:

Impregnating Varnish has 2 types. Solvent-free Varnish and Varnish with solvent. The physical description of Impregnating varnish is close to the paint.

It has good fluidity, fast curing, strong bonding force, can withstand high rpm of motors, excellent electro-thermal properties, and low viscosity properties. The use of this varnish type is further elaborated in the levels of the Varnish types.

Cover Varnish:

Cover varnishes are used to coat the immersed conductors such as coils and windings. These varnishes are used to coat the conductors to protect them from erosion, mechanical, and electrical damage.

Varnishes are chosen based on the motors’ class/temperature range. Just as we divided the motors into different classes, we have divided motor varnishes into the same classes. We call them levels.

Level Y:

As the maximum temperature tolerated by the class Y is 90-degrees Celsius, the same range is for Level Y. The material of this insulation varnish is Silk, Paper, and cotton yarn. None of these materials should be impregnable (the material from which fluids can pass through are impregnable).

Level A:

The maximum temperature tolerated by Class A is 105 degrees Celsius. The limit of level A varnish is also the same.

The composition of level A and level Y are almost the same material but the cotton yarn, silk, and paper used in this level varnish must be coated with transformer oil first.

Level E:

The temperature range of class E motor type and Level E type varnish is the same i.e., 120-degrees Celsius. The composition of Level E varnish consists of synthetic organic enamel and synthetic organic film.

Level B:

The class B motor type and level B varnish type share the same temperature limit i.e., 130 degrees Celsius. The composition of level B varnish has asbestos, mica, and glass fiber. Using asbestos is an old method for making Level B type varnish. 

Level F:

The class F motor type and level F varnish type share the same temperature limit i.e., 155 degrees Celsius. The composition of level F varnish has mica andpolyester epoxide.

Level H:

The temperature limit is 180 degrees Celsius. It is made of Asbestos, mica, and glass fiber and then these materials are impregnated in silicone rubber.

Level C:

The motors with a temperature limit of more than 180 degrees are varnished with level C varnish. The composition of this varnish is made of Ceramics, Quartz, Teflon and, mica.

Can I use wood varnish as motor varnish?

No. The wood varnish should not be used as the Insulation Varnish of the motor. The wood varnishes are a simple combination of Carbon and plastic pyrolyze with other organic compounds.

Wood varnish has a limited range of temperature toleration and then it either decomposes into its components or just evaporates. The Insulation varnishes used in the motors have different levels based on the temperature ranges of the motor classes. While the wood varnishes do not have such levels.

The composition of Insulation varnish is a proper quantified structure of different compounds. Some similar things both shares are that they can be painted and have colors. Conclusively, we should not use wood varnish as a motor varnish.

How do you Varnish Motor Windings?

generator winding varnishing
generator winding varnishing

There are 4 methods to varnish a motor’s windings.

Dip and Bake:

“Dip and bake” is the most commonly used varnishing method. Motors after winding are done are dipped into a tank of varnish.

Then put into an oven to cure. For better results, when the motor is cooled down after curing, it is dipped again and cured again for the second time.

The results are better after the second dip and cure because the varnish lasts longer than a single dip and bake. But if it is dipped and baked for the third time, it reduces the efficiency of the motor by adding weight to the motor.

Trickle Varnishing:

Trickle Varnishing is better than dip and bake. In trickle varnishing, the motor is linked to electric wires and a rotating machine. Then a stream of varnish is poured over it. 

The resistance is applied using wires. Once the windings are saturated, the current through the wires is increased for curing purposes.

Vacuum Pressure Impregnation:

In this method, the varnish is poured into a tank, then the tank is heated. Then the motor is placed inside the tank and the tank is vacuumed. This method does not satisfy the time constraints.

Ultra-Seal winding:

If the motor is to be used in extreme atmospheric conditions, engineers prefer the ultra-seal method of windings. This method is used for motors only. Coils are sealed with the high molecular mass thermoset resin of polymer.

Top Electrical Insulation varnish manufacturers

  • Master Bond.
  • Koala Insulation.
  • Anti-Seize Technology
  • Norland Product, Inc.

Does motor varnishing really increase insulation?

The only purpose of Varnishing is to insulate the motor windings. So, the answer is quite obvious that the motor varnish, improves the insulation of the motor windings.

We in my work use varnish to increase electrical motor insulation, as well as generators insulation. However the conductors of the winding are insulated, its a good practice to increase its insulation with insulating varnish.

Does motor varnish increase its temperature?

The motor Insulation may increase the temperature of the motor windings. But it is not the case when you insulate the motor windings for the first time with the varnish. The temperature increases in the motor winding when the same winding is dipped again and again in the varnish for insulation purposes.

The varnishing of the windings blocks the air vents and opening of stator and armature. The airflow through these vents reduces significantly. This reduced airflow leads to the rise in temperature of motor windings.

So, I recommend to check the winding before varnishing it. If the Insulation of windings is in poor condition, it must be varnished but if the varnish is intact then it is advised not to varnish it again.

For more details about, How does temperature rise affect electrical devices? Read my article.

Why is insulation have paper backing?

To separate the motor windings from each other, insulating paper must be inserted as a good insulation layer. This reduces the chances of the short circuiting. Conclusively, we can state that the paper avoids the Inter-turn Breakdown in the windings. You know that the insulation breakdown causes internal short circuit, which can destroy the motor winding.

In many cases in my work, short circuit destroy electrical generators and motors if the protection device failed for any reason. Read my detailed article about, What is short circuit? Why is it dangerous?

What Motor Insulation is the Inverter Duty Insulation?

Class F motor is the Inverter Duty Insulation. The Class F insulation is used for Inverter Duty motors because the Inverter Duty motor must be insulated with voltage spike-resistant insulation.

In industrial areas, the temperature level is too high. The inverter-duty motors run at a low frequency. These motors have a fan for cooling purposes that is mounted on the shaft for cooling purposes.

When they run at a lower frequency, the fan also moves with lesser radial frequency. This low fan speed slows down the cooling process that increases the temperature of the motor.

This will eventually cause the motor to either stop or catch fire, depending upon the safety system used. But if the class F insulation is used, the chances of these fatal accidents minimize.

Class F insulation has tendency to tolerate temperature up to 155-degrees Celsius. So, the insulation doesn’t let the windings to short-circuit even at the higher temperatures. Even if the temperature increases to 155 degrees Celsius, the insulation will stay intact for 10 more degrees.

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