Reject These Motor Myths
EP Editorial Staff | April 1, 2022
By Thomas H. Bishop, P.E, Senior Technical Support Specialist, EASA Inc., St. Louis, easa.com.
The tongue-in-cheek saying “If it’s in black and white, it must be right” is a helpful reminder that not everything we read (or hear) is accurate or complete. It’s always best to check sources and verify facts before accepting consequential statements as true. A similar adage underscores the importance of this advice in the digital age: “If it’s on the internet, it must be true.” With these things in mind, here’s a selection of common misconceptions about three-phase squirrel-cage motors and the facts that deny them.
Soft-starting motors reduce utility demand charges.
Soft starters typically ramp up the voltage applied to a motor over a few seconds at start-up, reducing winding heating and starting current. This may extend the life of the winding for motors that start frequently, but it doesn’t affect utility demand charges. That’s because the electric meter averages the kilowatts consumed over each 15-to-30-min. period, not just for the few seconds that the soft starter reduces input power to the motor.
Power factor correction capacitors can reduce motor energy consumption.
Applying power factor capacitors at the motor terminals increases the power factor on the supply cables but does not change the motor’s power factor. Increasing the power factor on the supply lines reduces current in them, causing a corresponding but typically insignificant reduction in I2R losses (energy) in the supply wiring. The primary reason for reducing supply circuit current is to add electrical loads without rewiring a facility.
A motor can be loaded up to its service factor current.
An example of this would be loading a 1.15 service factor motor up to its service factor current (typically ~1.15 x rated current). That would be a problem, according to clause 14.37.1 of NEMA Stds. MG 1-2016: Motors and Generators (MG 1): “A motor operating continuously at any service factor greater than 1 will have a reduced life expectancy compared to operating at its rated nameplate horsepower. Insulation life and bearing life are reduced by the service factor load.”
Further, the service factor only applies to Usual Service Conditions (MG 1, 14.2). These include operation at an ambient temperature of 5 F to 104 F (–15 C to 40 C) and at an altitude of less than 3,300 feet (1,000 meters) when rigidly mounted in areas or supplementary enclosures that do not seriously interfere with the machine’s ventilation.
Oversized motors, especially motors operating below 60% of rated load, are not efficient and should be replaced with appropriately sized premium efficiency (IE3) motors.
On the contrary, matching motor horsepower (kW) rating to the load will usually mean a slightly lower efficiency at that load than using the next larger sized motor. The reason is that motors tend to peak in efficiency between 75% and 80% of rated load. Motors that drive supply or return air fans in HVAC systems generally operate at 70% to 75% of rated load, making them candidates for use with oversized motors. Further, even at 60% of rated load (which more than one industrial motor study found to be the average load level), the next higher power rating motor could be more efficient at that load than the appropriately sized power rating. Some high-inertia loads also require more HP/kW to start than to run the load. Reducing the HP/kW to match the running load could result in the motor being unable to start the load. EP
This article is an excerpt from the original article by Thomas Bishop, Bust Nine Common Motor Myths. That original article and several other motor-maintenance publications can be downloaded at easa.com/erc.