VFD harmonics effects on generator set

[Tawanda]

I would like to use VFDs for motor sizes ranging from 0.4kw to 15kw. My idea here is to reduce the generator foot print to the lowest possible value. My thinking is that, theoretically, I could use a 25kva generator set to drive a 20hp motor (80% operating capacity) and since the VFD eliminates the high starting current I should be okay. But my biggest concern is: how do I account for harmonics in this system. What harmonic factor can I use to correctly size the generator. Is there a filter solution for sizes 0.4kw to 15kw used in a similar set up as above.

 

[berkeman]

I would like to use VFDs for motor sizes ranging from 0.4kw to 15kw. My idea here is to reduce the generator foot print to the lowest possible value. My thinking is that, theoretically, I could use a 25kva generator set to drive a 20hp motor (80% operating capacity) and since the VFD eliminates the high starting current I should be okay. But my biggest concern is: how do I account for harmonics in this system. What harmonic factor can I use to correctly size the generator. Is there a filter solution for sizes 0.4kw to 15kw used in a similar set up as above.

Welcome to PF.

So you are using a VFD to drive a generator? Are you using this as a motor-generator setup to change AC Mains voltages/frequencies? What are the input and output frequencies? And why would the input AC Mains harmonics generated by driving the VFD affect the AC Mains output voltage waveform?

 

[Baluncore]

So you are using a VFD to drive a generator?

Or using a generator to drive an electric motor, and think a VFD for soft-start might make it easier for the generator.
That will depend on the VFD input circuit and control.
I expect there will be a power factor control circuit on the VFD input.

 

[berkeman]

Or using a generator to drive an electric motor, and think a VFD for soft-start might make it easier for the generator.

Hmm, interesing. Yeah, which drives which...? Guess we need the OP to return to give us more information.

 

[Tawanda]

Welcome to PF.

So you are using a VFD to drive a generator? Are you using this as a motor-generator setup to change AC Mains voltages/frequencies? What are the input and output frequencies? And why would the input AC Mains harmonics generated by driving the VFD affect the AC Mains output voltage waveform?

I'm using the generator as the power source to run a 20hp motor and I am thinking the VFD could help to suppress the high starting current whilst still giving high starting torque. The idea is to use the smallest generator possible for the 20hp motor whilst leaving a 20% capacity headroom.

 

[Tawanda]

Or using a generator to drive an electric motor, and think a VFD for soft-start might make it easier for the generator.
That will depend on the VFD input circuit and control.
I expect there will be a power factor control circuit on the VFD input.

That's right. The idea is to make the motor starting easier on the generator. But I'm worried about the effects of harmonics on the genset if I were to size the generator to run at 80% of it's full load capacity. The VFD has a 6 pulse rectifier input circuit.

 

[Baluncore]

The VFD has a 6 pulse rectifier input circuit.

Do you know that the 3PH VFD has a simple 6 diode full-wave rectifier to the capacitor bank?
An expensive VFD might have input power factor control circuits.

Is your motor connected as Y or ∇ ?
Maybe a star to delta starter would be sufficient.

 

[Tawanda]

Do you know that the 3PH VFD has a simple 6 diode full-wave rectifier to the capacitor bank?
An expensive VFD might have input power factor control circuits.

Is your motor connected as Y or ∇ ?
Maybe a star to delta starter would be sufficient.

The motor is delta connected

 

[Tawanda]

Do you know that the 3PH VFD has a simple 6 diode full-wave rectifier to the capacitor bank?
An expensive VFD might have input power factor control circuits.

Is your motor connected as Y or ∇ ?
Maybe a star to delta starter would be sufficient.

The motor is delta connected

In fact the motor has all six leads coming out so I can connect in the star or delta configuration. Problem with a starter - delta starter is that that starting current would still be very high still requiring an overly sized genset to allow for starting. That's why I was thinking along the lines of a VFD.

 

[Dullard]

If you really need high starting torque (I don't know anything about your load), a VFD is a good choice. If your load doesn't need high starting torque (many loads don't), a soft-starter is typically a lot cheaper and will provide the same benefit to the generator.