Cycling A Fog System And The Use Of A 3 Phase Motor

December 5th, 2017 | Posted by Gary Wintering

Motors include a number of different ratings.  One of them is Insulation Class (which defines the acceptable temperature rise from ambient for the operation of the motor).  Most 3 Phase motors are rated ‘F’ which allows for a 105 degree C rise (189 F) or H which allows for 125 degree C rise ( 225F) while most single phase motors are rated B which allows for 80 degree C rise (144 F).

From the Baldor site:

The common insulation classes used in electric motors at the present time are Class B, Class F and Class H. The present frame size assignments are based on Class B insulation. It is the predominant class of insulation used in motor manufacturing today. Based on a 40 degrees C ambient temperature, the Class B insulation is suitable for 80 degrees C rise by resistance. Class F insulation is suitable for 105 degrees C rise by resistance. Class H insulation is suitable for 125 degrees C rise by resistance. Use of Class F insulation or Class H insulation can increase the service factor or the ability to withstand high ambient temperature conditions.

Since this rating is based on its ‘rise above ambient’ with ‘ambient’ being defined as 40 C (or 105 F), the maximum temperature rating for each class rating is as follows:

Class B: 40 C + 80 C = 120 C or 250 F

Class F: 40 C + 105 C = 145 C or 290 F

Class H: 40 C + 125 C = 165 C or 330 F

The issue with cycling systems is the heat that is created each time the motor is started (imagine a lightning bolt inside the motor each time it is started).  As the motor continues to operate, this heat is dissipated due to the cooling fan in the motor.

If the motor is ON for a relatively short period of time, the fan does not run long enough to dissipate the heat.  In this case, a longer OFF time before the next cycle begins will help provide sufficient time for the motor to cool down.

If the ON/OFF ON/OFF cycling occurs too often and the ON time is too short to allow the motor’s fan to cool down the motor and the OFF cycle is too short to allow the motor to cool down naturally, heat will continue to build in the motor as each successive ON/OFF cycle is implemented.  This is not necessarily a problem unless the motor temperature rises above its rated ‘temperature rise from ambient’.

In any case, if cycling results in the motor temperature rising above its rated ‘temperature rise from ambient’, it will cause the motor winding’s insulation to deteriorate over time eventually causing premature failure of the motor.

Ideally, a motor is not started until the temperature of the motor has dropped to ambient.  This operational characteristic will ensure the longest life for the motor and the insulation.  However, since this is not practical in many applications, motors are built to allow for a defined temperature rise without concern.  In these cases, it is best if cycling systems utilize the higher temperature ratings of a 3 phase motor.

All that being said, a cycling system is not necessarily a concern even with a 1 phase motor.  It will depend on the specifics of the application.  The specifics will include the ambient temperature where the motor is located; how often the motor is started and stopped; how long it is ON during the cycle; and how long it is OFF before the next cycle begins.

As good rule of thumb to use for a cycling system is no more than 5 ON/OFF cycles per hour for a single phase motor (6 minutes ON followed by 6 minutes OFF) and no more than 10 ON/OFF cycles per hour for a 3 phase motor (3 minutes ON followed by 3 minutes OFF).  Variance from these recommendations may impact the warranty offered on the motor.

Ultimately, site conditions and exact cycling characteristics will determine the temperature rise of any given motor.  It is recommended that the temperature of the out skin of the motor be checked throughout a cycling operation to ensure that the rated temperature of the motor is not exceeded.