What is a Motor Service Factor?

Friday - 02/08/2019 23:10

What is a Motor Service Factor?

Motor service factor (SF) is the percentage multiplier that a motor can handle for short periods of time when operating within its normal voltage and frequency tolerance. In other words, it is a fudge factor that give extra horsepower when it's occasionally needed.
For instance, this 1/2 horsepower shown in the photo has a service factor of 1.25 so it can actually output 25% more power required for short periods of time. This comes in handy if the density of the liquid increases or a higher than normal flow rate is required.

    Fractional horsepower motors usually have a higher service factor up to 1.5 since their power consumption does not lead to significantly higher winding temperatures. Motors of 10 hp and up usually have a service factor of 1.15.

The Canadian Electrical Code defines service factor as a multiplier that, when applied to:

    The rated horsepower of an AC motor,
    To the rated armature current of a DC motor, or
    To the rated output of a generator,

... indicates a permissible loading that may be carried continuously at rated voltage and frequency. Note how the CEC allows for continuous operation while we reserve it for only short periods of time to ensure reliability.

As electrical consultants, using the service factor gives us a margin of safety that allows our design to:

    Extend the life of the motor by lowering the temperature of the insulation winding.

    Compensate for low or unbalanced supply voltages.

    Accommodate the variability in horsepower. A 15% buffer is a nice margin to have especially for those occasional overload conditions.

Why Have a Service Factor

Operating a motor at its limit makes it more prone to overheating. A service factor allows the motor to operate below its theoretical maximum so it can run continuously with a cooler winding temperature at rated load. This leads to a longer life and better reliability.

Designing with the Service Factor

We never design systems to operate continuously at the maximum level (redundancy is a different topic). It is good practice to size a motor for continuously operation that is below the service factor percentage. An Electrical load monitoring test help to see how efficient a motor is running.

Operating in the service factor area may result in:

    Decreased efficiency (more energy usage).
    Decreased power factor (more reactive power usage).
    Overheating and damage to the wire insulation.
    Incorrect starter sizing which may lead to inadequate starting & pull-out torques.

A motor services factor is a margin of safety which increases the reliability of building systems.

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