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Motor Service Factor – What is it?

The service factor is a motor characteristic that usually goes unnoticed on the motor nameplate. It is mainly used in system designs and motor studies for larger commercial and industrial installations. When used properly, the motor service factor will allow for momentary bursts of power for uneven loads. Using the service factor negates the installation of an oversized motor that would mainly be under-utilized most of the time.

Quick Calculation of the Motor Service Factor

The service factor of an AC motor is a multiplier that electrical engineers use so a motor can handle an intermittent heavy load.

A motor nameplate documenting the motor service factor.

This motor in a BC ice-arena has a motor service factor is 1.15.

For instance, in an industrial setting, a fluid having varying viscosity may needsto be pumped to a higher elevation. Instead of oversizing the electric motor and conductor size to handle the rare instance of mud, sludge or a viscous plug in the flow, we can rely on the motor service factor to handle this momentary need for extra pumping power. It will momentarily consume more electricity, move outside of its torque specifications but once the blockage is cleared, the motor will return to its normal operating range.

This motor as a service factor of 15% as seen on its motor nameplate. We can operate it briefly at 115% of its rated load and horsepower. The motor’s windings, wire insulation, shaft, bearings and heat dissipation fins have been designed for this elevated use, but only temporarily.

Don’t Abuse the Motor’s Service Factor

The definition of ‘intermittent usage' is subjective to engineering interpretation. As electrical consultants, we respect the rated voltage, frequency and loading of the electric motor. When we go beyond the rated operating conditions, the service factor can only be relied upon for a temporary burst of power. Prolonged operation above a motor’s rating will damage the motor’s electrical windings or mechanical parts.

Motors Operating Above its Rated Power

An AC-motor is designed to operate continuously at or below its rated horsepower. When the motor operates above its normal operating ratings,

  • Its efficiency will decrease. This is the conversion of electrical energy into rotational mechanical motion.
  • Its power factor will deviate from normal and consume more electricity than normal (active & reactive power).
  • The branch conductors are only sized for the MCA ratings during normal operating conditions.
  • The rotational speed of the shaft may decrease.
  • Its locked-rotor, pull-up torque and breakdown torque will remain the same (this is the stuff electrical consultants deal with), but the reliability of the entire system is lowered. When production lines halt, so too does the line workers and the supply chains making a shutdown very costly.
  • The motor's lifespan will decrease and may fail prematurely.

If a motor is operated continuously above a service factor of 1.0, this means:

  • Your motor is too small and the system should be redesigned to match the speed, torque and horsepower appropriate for its load.
  • The heat will burn and melt the insulation on the wires which will lead to electrical shorts and explosion.
  • The added heat, presure and friction on the bearings will have a reduced their life.
  • The motor will fail. The bigger the motor, the bigger the explosion.

Motor Failings

As engineers that conduct motor studies, motor failures usually means:

  • Winding failures which result in lots of smoke and arcing,
  • Bearing failures which produce lots of vibrations, friction and then smoke and flames,
  • Shaft breakage. We’ve never seen this type of failure in our career... at least not yet!

What it means for our clients?

Motors are used in every home and building. They are used in HVAC systems, furnaces and plumbing systems.

  • If an HVAC system goes down, it could take weeks for a large chiller motor to be replaced or repaired.
  • If a motor in a production line goes down at an industrial site, production slows or stops. Most production facilities have redundancy but the backup motor is usually smaller than main motor.
  • If a pump’s motor is damaged, then water delivery will stop. Thankfully in BC and Alberta, water pumps for the municipal freshwater systems are readily available for quick replacement... as we personally discovered on our street in Coquitlam.
  • Why do Engineers use Motor Service Factors?

    • Have extra capacity on inconsistent system flows while not oversizing a motor for the application. Think of the vary load and consistency within the municipal sewage systems.
    • The insulation on the motor’s windings can withstand the generated heat.
    • Motors with service factors can usually work at elevated ambient temperatures.
    • These motors can compensate for the uneven rotational magnetic fields created by unbalanced supply voltages.

    When sizing motors for optimal performance for your building, only rely on the service factor for the occasional burst of power and to help ride-out unbalanced power supplies. Do not abuse the service factor or your motor will fail well before its full life expectancy.