multi-speed motors

[Grouch1980]

Hey all.
When the NEC mentions 'multi-speed' motors in article 430... do motors that are controlled by Variable frequency drives (VFD's) fall under this category?

 

[Smart $]

Hey all.
When the NEC mentions 'multi-speed' motors in article 430... do motors that are controlled by Variable frequency drives (VFD's) fall under this category?

IMO, no.

 

[Grouch1980]

IMO, no.

what would be the difference between them?

 

[iwire]

what would be the difference between them?

To me a multispeed motor is a motor that has separate leads for each speed. It would also have a label with more than one RPM listed. A common application of these has been HVAC blower motors.

On the other hand a VFD supplied motor is a one speed motor, with one RPM listed on the label but it is being supplied by a speed control.

 

[Smart $]

what would be the difference between them?

A multispeed motor is wound such that varying its electrical connection provides more than one predetermined rate of rotation.

A typical motor driven by vfd has but a single speed at rated voltage and Hertz.

 

[Grouch1980]

Aaah. I get the difference now. Thank you both for the help! It was easy to understand. I'll look into those HVAC blower motors.

 

[iwire]

Here is a small one.

Its a three speed, hi, med, lo. Connect 120 volts to white and one of the other leads.

 

[Grouch1980]

Here is a small one.

Its a three speed, hi, med, lo. Connect 120 volts to white and one of the other leads.

Nice. Thanks for the example and visual.

 

[kwired]

HVAC blower motors is a pretty common example. There are other motors that have completely separate sets of windings, that you can connect to obtain different speeds, even some three phase motors. With VFD being much more common and somewhat low cost compared to what they once were you don't find too many newer multispeed three phase motors though.

 

[Grouch1980]

HVAC blower motors is a pretty common example. There are other motors that have completely separate sets of windings, that you can connect to obtain different speeds, even some three phase motors. With VFD being much more common and somewhat low cost compared to what they once were you don't find too many newer multispeed three phase motors though.

Never had to deal with multi speed motors at work. Usually it's just motors with VFDs or regular starters. I came across 'multi speed motors' in article 430 and got curious if that included motors with VFDs. From everyone's responses, it's a no :happyno:

 

[Grouch1980]

I'll rephrase... I've never dealt with multi speed motors directly. Looks like they're integral to some HVAC units... In which case I just work with the FLA and MOCP that I receive from the HVAC engineer.

 

[kwired]

I'll rephrase... I've never dealt with multi speed motors directly. Looks like they're integral to some HVAC units... In which case I just work with the FLA and MOCP that I receive from the HVAC engineer.

Some shutter mounted fans or pedestal fans are other common multispeed motor applications.

A three phase application I recall was an agitator motor for a tank in a food processing plant. The thing had a drum switch on it to switch between the two speeds, but essentially had three input leads for one speed and three leads for the other speed - basically two motors wound in one case sharing the same rotor, you can only energize one set of windings at a time though.

 

[Grouch1980]

Some shutter mounted fans or pedestal fans are other common multispeed motor applications.

A three phase application I recall was an agitator motor for a tank in a food processing plant. The thing had a drum switch on it to switch between the two speeds, but essentially had three input leads for one speed and three leads for the other speed - basically two motors wound in one case sharing the same rotor, you can only energize one set of windings at a time though.

Thanks everyone for the help and all the examples.