motor deration for AFD

[mengbly]

hi there

i was told that any motor rated for 1.15 service factor when connects to AFD; its service factor will reduce to 1.0. It applies to all motor manufacturers regardless the motor brand.

can someone please explain the reason for this reduction

i am from a mechanical background and i schould accept that i have a very less knowledge on electrical side

 

[jraef]

Service Factor in a NEMA design motor is based on the motor's ability to absorb and dissipate the extra heat created from over loading it by the SF percentage for short periods of time, with and acknowledgement that performance and motor life may be sacrificed.

Running a motor from a VFD has two issues in that regard; reduced cooling capabilities if it is fan cooled, and increased heat in the windings because of the PWM pseudo-sine wave power delivered by the VFD. Since 99% of motors that have a 1.15 SF are fan cooled, you immediately run into that first issue. But even if a motor is DESIGNED to be run from a VFD and has a separately powered fan, you will STILL see the motor mfr tell you that the SF is 1.0 if used on a VFD, because the added heating of the PWM is going to consume that margin of extra capacity.


"Will work for salami"

 

[dpc]

Agree with jraef. The ratings for any motor are based on running on a sine wave. Running on a AFD output generates additional heating in the motor that reduces its maximum power capability. The limiting factor is the heat generated in the motor and the winding temperature. So in the end it does come back to the mechanical side - too much heat.

 

[mengbly]

Thanks For the Reply

Do you mean to say that all motors which run on VFD will be derated

we cannot use the 1SF motor for variable speed as the output of the motor reduces

please advice

 

[dpc]

Here is how we specify motor sizing:

Non-VFD: If SF = 1.15 - brake horsepower required cannot exceed the motor nameplate hp at any point. If SF = 1.0 - bhp cannot exceed 87% of motor nameplate horsepower.

VFD Motor: Must have SF = 1.15 and brake horsepower cannot exceed 87% of motor nameplate.

Any motor running on VFD will have reduced capacity compared to running on a sine wave.