The winding insulation on your electric motor has a major impact on its life expectancy and reliability, which means that using the wrong insulation class could be very costly. The best way to avoid this mistake is to be familiar with the basics of NEMA insulation classes.
NEMA Insulation Classes
The purpose of NEMA motor insulation classes is to describe the ability of the motor winding insulation to handle heat. There are currently four electric motor insulation classes in use: A, B, F, and H (although there are also N, R, and S classes). Of these four, B, F, and H are the most commonly used. These classes specify the allowable temperature rise from an ambient temperature of 40°C.
- Class A Insulation:
- Maximum Temperature Rise: 60°C
- Hot-spot Over Temperature Allowance: 5°C
- Maximum Winding Temperature: 105°C
- Class B Insulation:
- Maximum Temperature Rise: 80°C
- Hot-spot Over Temperature Allowance: 10°C
- Maximum Winding Temperature: 130°C
- Class F Insulation:
- Maximum Temperature Rise: 105°C
- Hot-spot Over Temperature Allowance: 10°C
- Maximum Winding Temperature: 155°C
- Class H Insulation:
- Maximum Temperature Rise: 125°C
- Hot-spot Over Temperature Allowance: 15°C
- Maximum Winding Temperature: 180°C
The maximum winding temperature is the sum of the ambient temperature (40°C) and the allowed temperature rise. The allowable temperature rise is made up of two parts: the maximum temperature rise for the insulation class plus a hot-spot over temperature allowance.
Why Winding Temperature is Important
When your electric motor operates at a temperature above its allowable winding temperature, the service life is always going to be reduced. In fact, a 10° C increase above the allowed maximum can cut your motor’s insulation life expectancy by half.
If you have a motor with Class A insulation, its maximum winding temperature is going to be 105°C. If it is operating at 125°C, that is 20°C over its limits and each 10° increment over that limit reduces life by 1/2. This operating temperature will reduce motor life to just 1/4 of its original life expectancy!
Is It Too Hot?
Keep in mind that the surface temperature of your motor may seem high but still be within range. Let's say you have a Class F insulated motor rated for a winding temperature of 155°C. You or one of your technicians inadvertently lays their hand on the surface of the motor and notes that the motor feels warm. Is the motor running too hot? Maybe but probably not -- a rule of thumb is that the surface temperature is typically just 30°C less than the winding temperature. So, with all that said, a motor that is extremely hot to the touch is not necessarily operating beyond its rated temperature.
Measuring Motor Temperature
There are a couple of different ways to measure motor temperature. You could get a quick estimate of the winding temperature by measuring the surface temperature and adding 30°C, but that is certainly not the best approach.
Embedded thermocouples or (RTDs) resistance elements (temperature and resistance are directly related) can be used on the windings to provide more accurate readings. These readings can be automated and sent to a condition monitoring system, which has the added advantage of demonstrating how temperatures change over time. Thermal imaging can also be a powerful tool for measuring motor temperature.
Temperature Rise Letter
You may see the use of a temperature rise letter used along with the insulation class, such as an F/B motor. The F refers to the insulation class for the windings, while the B refers to an 80°C rise (the maximum temperature rise for Class B is 80°C).
This type of notation is becoming more widespread because many motors use Class F insulation, and there is a reason for that. Let's go back to the F/B motor: it's rated for a maximum winding temperature of 155°C and a maximum temperature rise of 105°C. The actual expected temperature rise is 80°C, which leaves a a thermal margin of 25°C and the potential for a significantly longer winding life.
What Insulation Class Do I Need?
The right insulation class for a motor is based on two things: the temperature of the ambient air and the temperature rise in the motor. These form the starting points for selecting the right insulation class.
You probably noticed that the ambient temperature for the insulation classes discussed were all 40°C -- but what if the ambient temperature is going to be higher than that? In such cases, the motor is normally de-rated or a higher level class insulation material should be used.
When it comes time to purchase a replacement motor, be careful to not choose a motor with an incorrect insulation class or you could risk premature failure. To be safe, you should specify a motor with either an equal or higher insulation class.
Using the wrong insulation class for your electric motors will shorten the expected life of your motor, leading to expensive downtime, loss of productivity, and costly repairs. Take the time to make sure that you are using the right insulation class for your motors.
AUTHOR & CONTACT INFO: Steve Mazziotta ( firstname.lastname@example.org)