VFD or Rotary Phase Converter

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Little Bill

Little Bill

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I suppose my first question should be can you use a VFD if you don't have 3? power available?

Then if you can use a VFD without 3?, would it be better to use a phase converter or a VFD? From both a practical stand point and a cost stand point.

I haven't seen the loads but from what I've been told they are small 1/2-1 HP motors.
 
A rotary convertor can be any size you want, but if it is a motor it needs to be a factor larger than your largest motor load. And it must be running (up to full speed) before you start your motor.
Even then there can be starting current issues with motors that start loaded.
A VFD, OTOH, can be turned on when you want your motor to start and will greatly reduce any starting current peak on the motor.
The VFD also has to be oversized for your motor and you must not use a bypass switch.
VFD makers will be able to guide you on sizing for this application.
From my experience rotary convertors are used mostly when the user has a large three phase motor that they are not using. :) Or when the motor will be running a large portion of the shift and will start up unloaded.
 
Little Bill said:
I suppose my first question should be can you use a VFD if you don't have 3? power available?

Then if you can use a VFD without 3?, would it be better to use a phase converter or a VFD? From both a practical stand point and a cost stand point.

I haven't seen the loads but from what I've been told they are small 1/2-1 HP motors.
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For that size motor I would very much consider replacing with a single phase motor.

What for is it?
 
Any VFD is technically capable of accepting a single phase input, but not all mfrs allow you to do it, for various reasons. So to that point, Golddigger is right, ask your favorite supplier for assistance with that. As a grossly GENERAL rule however, 230V at and below 3HP is usually available for single phase input without de-rating the drives. All other sizes above that, and all 480V drives, must be de-rated by at least 50%. This is because all of the current going to the motor must come from 4 diodes instead of all 6 on the converter section, so the current increases by 1,732x, plus there is more DC bus ripple, so you need more capacitors to smooth it. The result is a 50% de-rate (or more). Other rules:
  1. Don't turn motors on and off DOWN stream of a VFD, so one VFD per motor, or if you use multiple motors, they all turn on and off together using the VFD.
  2. Use the digital inputs of the VFD for on-off and reversing functions, don't use reversing contactors (same issue).
  3. Pay attention to environmental issues. The #1 killer of VFDs is heat.
  4. Don't use an RPC to try to feed 3 phase to a VFD, the RPC output is high on the manufactured phase, which can damage the VFD.
  5. Don't power up anything else besides a 3 phase motor behind the VFD, including control power transformers in control panels.
  6. You get what you pay for. If the price looks too good to be true, it's junk.

Use an RPC for when you must power up many motors in a shop. That's it's only advantage.
 
ActionDave said:
For that size motor I would very much consider replacing with a single phase motor.

What for is it?
Click to expand...

I actually was just trying to answer questions from another electrician for a job he is looking at. I don't know all the loads but one was for an industrial sewing machine. I know plenty about those as I worked on, changed, and wired them for years. I told them to tell the customer they would be better off changing to a single ? motor as they're not that expensive.

But there is some other machine they want to power, not sure of the requirements there.

I have seen time & time again where people will buy machinery, especially industrial sewing machines, only to find out they are 3? and they only have single ? service.
 
ptonsparky said:
AD ...$117-$134. They may not be fancy, but you may not need fancy. I have had very good luck with the next step up from these. So economical a guy should have one on the shop bench just for learning. Read the directions.
Click to expand...

Not sure what you're saying here in red. Unless that was supposed to say "At" instead of "Ad". But are you referring to a VFD with those prices?
 
Little Bill said:
small 1/2-1 HP motors.
Click to expand...

Have had cheap $120 ebay VFD on 240V single phase with neutral running my 2 HP drill press and lathe for a few years, no problems. Variable speed at the twist of a know or input from laptop is a big plus for those applications.

since I had a few 120V muffin fans, screwed one of those to the heat sink for cooler running operation.

1-2HP vfd probably cheaper than a single phase motor that fits the sewing machine.
 
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