User Rating: / 1
PoorBest 

Keeping Motors Cool

The components of an electric motor are separated from one another by the use of insulation. This is done to prevent short circuiting of the windings, and resulting in an electrical failure within the motor (burn-out). There are many things that could cause a motor winding to burn out, things like:

• Temperature
• Moisture
• Vibration
• Chemicals and Abrasives that are suspended in the air
• Mechanical binding or alignment

However of all these it is temperature that can be the deadliest killer for motor winding insulation. Since temperature is the number one on the list lets concentrate on it.

The rule of thumb is that if a rise of 10 deg. C occurs the result is a 50% decrease in the life expectancy of the insulation. Yet if there is a 10 deg. C decrease in temperature the insulation's life span will increase by 2X. What this indicates is that if a motor is kept cool enough, the winding could last indefinitely. The other variable mentioned in the list also too come into play, but putting those aside it could be possible.

The real issue is at what temperature the motor windings are designed to operate for, and a predictable insulation life of 20,000 hours or more. NEMA, the National Electrical Manufacturers Association, has implemented temperature standards, these are based on thermal classes, the most common classes are A, B, F and H.

What is most commonly found in the industry today is the use of NEMA Class B or Class F rated motors, these motors usually have a “T frame”. It is common for manufacturers to design their motors to run cooler then what they are rated for. An example of this could be that of a Class F motor insulation rating but actually it has a Class B temperature rise, this will allow the motor to have a larger thermal margin. The Class H insulation systems are rarely found in general-purpose motors, but rather in special designs for very heavy-duty use, high ambient temperature or high altitudes.