Effects of reduced/low frequency (VFD) on a motor

[SparkOmatix]

I know the effects of increasing the frequency (above 60 Hz) essentially shortens a motors life (heating, etc.). I wish to know what, if any, detrimental effects occur from running a motor at a lower frequency…say 40 Hz, for a majority of the time.

 

[Brad1979]

Assuming you have an inverter duty motor, the manufacturer probably has on the datasheet somewhere a ratio (10:1, 1000:1, etc.) that indicates the constant torque region of the motor. A 10:1 motor should be able to go down to 6 Hz at the rated torque (NOT power) and a 1000:1 motor should be able to go down to .06 Hz at the rated torque. Motor life shouldn't be affected in this case.

If you don't have an inverter duty motor, than the motor could have a shortened life due to voltage spikes from the VFD.

 

[dpc]

As long as the motor is not overloaded, operating a 60 Hz motor at 40 Hz should not be a problem. At very low speeds motor cooling becomes a concern for motors that rely on a shaft-driven fan for cooling the motor. The voltage stresses will be a potential issue at any speed on a VFD.

 

[jraef]

You must first clarify your question. Are you referring to reducing the frequency AND the RMS voltage proportionately using a VFD? Or are you referring to reducing ONLY the frequency? The way you asked the question forces supposition in any responses.

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[LionelHutz]

Your first statement is not necessarily true. I have seen more than one motor that was running well over it's rated 60Hz speed, and rated 60Hz voltage for that matter, without having any issue.

The answer to your question can range from nothing bad to a quick failure depending on how you apply the motor and VFD. There will be no problems if you apply the motor and VFD properly.

As for the VFD voltage spike comments. It seems these days all general purpose motors are wound the same regardless of having a VFD rating or not unless you buy a very premium VFD rated motor (think Marathon Blue Max type of motor). A number of manufacturers just slap their "VFD Rated" tag onto the standard motors to make the VFD rated versions. So, voltage spikes can always be a concern even with a VFD rated motor.

 

[eeprom]

One other consideration is how the motor is cooled. If this motor has a shaft mounted fan, such as a TEFC motor, then you are reducing the cooling of the motor when you reduce the shaft speed. This of course increases motor heating and decreases motor life.

EE