Rethinking My Approach with a VFD

[strantor]

If you figure that the startup draw of a motor is quite high like 8X the FLA, a VFD can provide probably 2-3X, and then factor in derating for single phase 2X. You are probably need 6-8X the the motor FLA for the VFD output

The starting current of a motor across the line is 8-10x the running current, because there is nothing to limit it except the resistance of the windings and the wires to the motor. It's that high because it can be. The VFD monitors output current and regulates its duty cycle to prevent blowing itself up, so it won't (shouldn't) let startup current go that high. So I think that half of the equation can be disregarded and we're back to the classic 1.7-2x derating factor. That said, I've never tried to start a motor off a VFD that was already running 60Hz. All the MFG guidance I've ever read, advised against doing this.

I agree with you that RPC ir phase perfect is the way to go.

 

[mksj]

You have derating for single phase input which is 2X, and then you have derating in that the VFD will trip an over current error because it can't supply the needed current. It has no way to regulate the current demand because it is not adjusting the acceleration in V/Hz to control the current. You have it set at 60Hz. There may also be additional safe guards in newer VFD's if they were to monitor the output, you may get an error with no motor attached, generally and issue with sensorless vector. I have had VFD's trip the over current error with sudden changes in speed. So I still think that you need a significantly larger VFD if you wanted to use it as a power source, I do not recall the numbers, but higher than a factor of 4.

 

[JimDawson]

When I start up my ADX30 (30HP) phase converter, it causes the lights to flicker in all the houses on my end of the street.

If you spin up the idler motor with a pony motor it won't do that. I spin my 15 HP up with a 1/2 HP motor that is powered by a small VFD. I spin it up to 1800 RPM over 10 seconds, and use the VFD onboard relay, programed to close ''At Speed'', to switch the main contactors in, and simultaneously drop out the VFD run signal. The VFD is programmed for ''Coast to Stop''. My shop lights don't even flicker.

 

[TNBen60]

Wow you guys are amazing! Thanks for the information. I spent some time looking at the options you brought up. While looking over the rotary converter option I called up the folks that helped me in the past with a rotary phase converter for a headsaw that needed a 75 HP idler motor with a really clean third leg due to its control system. Their customer service was excellent back then and was the same when I called them for this. In trying to balance everything I have decided to have them build a rotary converter for me.

I’m going to keep the VFD on my mill though.

I’m really impressed with the level of information. Thanks again.

Ben

EDIT: I came back to re-read this thread because there is some good information in it. In re-reading my last post it came across to me like an advertisement. That was not my intent.

 

[strantor]

If you spin up the idler motor with a pony motor it won't do that. I spin my 15 HP up with a 1/2 HP motor that is powered by a small VFD. I spin it up to 1800 RPM over 10 seconds, and use the VFD onboard relay, programed to close ''At Speed'', to switch the main contactors in, and simultaneously drop out the VFD run signal. The VFD is programmed for ''Coast to Stop''. My shop lights don't even flicker.

That's a pretty clever idea, and if I had built this RPC from a 2nd hand idler I'd be contemplating your suggestion pretty hard right now. But this RPC is conceived and born as a RPC, so it has no external shaft on which to couple something like that. Given its size and the fact put they controls in it with circuit boards, it seems a notable oversight that they didn't include any kind of soft start feature.

Although maybe I could use a VFD to spin up the idler iself... maybe a few series RL circuits in the phases so the little VFD doesn't complain (if needed)?

Edit: you can see the shaftless motor in this video I recorded a few years ago:

 

[JimDawson]

But this RPC is conceived and born as a RPC, so it has no external shaft on which to couple something like that.

No external shaft access at all? The purpose built RPC idlers that I have seen basically had the shaft cut flush at the end bell, but was still accessible. In my case I just turned the end of the armature down to fit the flex coupling I had, 12mm as I recall. Adding a stub shaft to yours shouldn't be a huge problem even if the end bell is closed off. That's what they make drill bits for.

 

[strantor]

No external shaft access at all? The purpose built RPC idlers that I have seen basically had the shaft cut flush at the end bell, but was still accessible. In my case I just turned the end of the armature down to fit the flex coupling I had, 12mm as I recall. Adding a stub shaft to yours shouldn't be a huge problem even if the end bell is closed off. That's what they make drill bits for.

Yep, no access at all. You can see 360 degrees around it in the videos on this page. Taking it apart and modifying it sounds fun, but there are a lot of fun things already on my to-do list and this would be sufficiently near the bottom as to ensure it almost certainly never gets done.

 

[JimDawson]

Yup, that time thing is always a problem.

 

[TNBen60]

Well the rotary phase converter showed up today. A trip to the local electrical supply house and a visit from the electrician and I should be able to make lathe noises soon.

 

[MikeInOr]

That's a pretty clever idea, and if I had built this RPC from a 2nd hand idler I'd be contemplating your suggestion pretty hard right now. But this RPC is conceived and born as a RPC, so it has no external shaft on which to couple something like that. Given its size and the fact put they controls in it with circuit boards, it seems a notable oversight that they didn't include any kind of soft start feature.

Although maybe I could use a VFD to spin up the idler iself... maybe a few series RL circuits in the phases so the little VFD doesn't complain (if needed)?

Edit: you can see the shaftless motor in this video I recorded a few years ago:

When I built my 10hp rotary phase convertor about 25 years ago I found that the ramp up time on the idler motor, hence its current draw, was directly proportional to the capacitance of the starting capacitors. I remember the idler starting up near instantly when I first turned it on after building it and my shop lights did flicker. I pulled a few of the starting capacitors out of the chain and the motor now takes about 1.5 ~ 2 seconds to start and it draws MUCH less current on startup. My RPC uses an adjustable timing relay to cut the starting capacitors out so I just added a little to the amount of time the timing relay is energized.

I would be hesitant to change the starting capacitance on a factory bought RPC so I am not really recommending doing so to you. I just thought I would mention it is possible. I would not be surprised if a factory built RPC has a single starting capacitor of the needed value instead of a chain of them to get to the right value, so you potentially might have to pull the original starting capacitor and replace it with one of a lower value.



The round capacitors are the starting capacitors and the oval ones are the run (balancing) capacitors.

P.S. Running a VFD with a contactor between the VFD and the motor being powered is contrary to everything that I have ever been told about VFD's... but I have never actually tried it so I don't have any first hand experience.