VFD's - An Explanation Please

[goldstar]

Be aware the addition of the VFD may likely cause GFCI tripping. Might be in long term better to get a non VFD option as the VFD would get nuisance tripping issues.
GFCI is required in 210.8(B)(2) for the kitchen area of non dwelling units.

(B) Other Than Dwelling Units.
All 125-volt through 250-volt receptacles supplied by single-phase branch circuits rated 150 volts or less to ground, 50 amperes or less, and all receptacles supplied by three-phase branch circuits rated 150 volts or less to ground, 100 amperes or less, installed in the following locations shall be provided with GFCI protection:

Great. That would have been my next question.

 

[busman]

I think the VFD also needs to limit current thru the motor since lower frequency means lower impedance in the windings.

 

[winnie]

I think the VFD also needs to limit current thru the motor since lower frequency means lower impedance in the windings.

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

 

[busman]

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

Yes, answering the OP. And yes, what you describe is what I understand also.

Mark

 

[kwired]

If you are answering the original question of 'what does a VFD do' you are correct.

In general, when the VFD changes the _frequency_ of its synthesized output, it also changes the _voltage_ in a corresponding fashion, to maintain a constant V/Hz ratio and compensate for the changing winding impedance.

Slightly smarter drives add a factor which compensates for the winding resistance separately from the winding inductance.

There are much smarter approaches to controlling the motor, but in the end they all involve tweaks to the frequency or voltage used to operate the motor.

-Jonathan

Also remember if you have say a 10 HP,480 volt 60 Hz motor and are running at 30 Hz the motor voltage like you said will be 240 and current will be right about nameplate amps if motor load is right about 5 HP and basically is what the motor is rated for if supplied by 240 volts @ 30 Hz.