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Old 02-04-2015, 11:57 PM   #1
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Question VFD Vs. Rotary Conversion

So i am kind of torn between getting a vfd or rotary converter for 3 phase in my home shop. I need something in the 20-30 hp range. All Advice welcome.
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Old 02-05-2015, 12:29 AM   #2
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It sounds like you're wanting to operate more than one motor, maybe several at once.

If so, a VFD would be a very poor choice. They do not do well with motors coming on and off while it's running.

If a motor is switched on or off while the VFD is producing power, it's very likely to burn up.

If a VFD operates one motor, the starting and breakdown torque will be less than if it were on POCO power, but it'll produce its rated HP once it's running.

A rotary phase converter doesn't care though. It's perfectly happy to run just about any 3 load within its rating. Figure about 80% of rated HP for any motor. And less starting torque.
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Old 02-05-2015, 01:14 AM   #3
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Originally Posted by mr.photon View Post
So i am kind of torn between getting a vfd or rotary converter for 3 phase in my home shop. I need something in the 20-30 hp range. All Advice welcome.
There are other options, depending on your price range, and what you intend to run in your shop.

Digital phase converters, for example. They can run multiple motors if need be, in the range of hp you are giving.

Never used them, just found them doing a search a few days ago, when someone else asked about running a three phase compressor on single phase.

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Old 02-05-2015, 09:35 AM   #4
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or put a small vfd on each 3 ph motors
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Old 02-05-2015, 09:58 AM   #5
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So i am kind of torn between getting a vfd or rotary converter for 3 phase in my home shop. I need something in the 20-30 hp range. All Advice welcome.
Is your service sized to handle it? Most of the shops I've seen use a rotary phase converter. The biggest issue I've run into is you have to make sure none of the controls are connected to the manufactured leg. Some of the cheaper ones, or the homemade ones, seem to have a higher starting current than the high end ones. How many motors are you going to be running at the same time and what size?
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Old 02-05-2015, 12:52 PM   #6
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I would like to be running about 6 small motors, around 1-2 HP, on and off regularly, and two 10-15 Hp motors for the compressor and what not. My shop has 200 amp service.
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Old 02-05-2015, 12:55 PM   #7
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also should i build one or get a stock commercial?
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Old 02-05-2015, 01:15 PM   #8
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Morning mr.photon:

Your profile states your an Electrical Engineer.

Where is your shop, and what price range are you looking at to set up three phase?

Borgi
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Old 02-05-2015, 01:24 PM   #9
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I am an electrical engineer, just not in the complete sense. I never finished college for financial reasons so i could not be certified, but i do know alot.

Anyway, I can spend about 2g. It would be located in my home shop(a pole barn on my property). i have 200 amp service.
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Old 02-05-2015, 01:31 PM   #10
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What do you guys think of this one? is it a good price?

http://www.ebay.com/itm/75HP-3-PHASE...-/261752941603
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Old 02-05-2015, 02:16 PM   #11
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It might worth pricing out individual VFDs, but it sounds like you have quite a few motors. As others have said, VFDs don't like motors coming on and off on the load side of them. They are designed to start and stop the motor themself.
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Old 02-05-2015, 08:38 PM   #12
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What do you guys think of this one? is it a good price?

http://www.ebay.com/itm/75HP-3-PHASE...-/261752941603
Probably oversized for your application. The converter must be sized to the largest motors you're going operate and other factors. An oversized converter can cause problems. A number of manufacturer's web sites have sizing charts etc.
Be sure to study the appropriate code section to find out what the requirements are.

http://www.phaseconverter.com/rotary-converter.sizing/
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Old 02-05-2015, 08:52 PM   #13
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What type of operation do you have going in your shop?
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Old 02-05-2015, 09:10 PM   #14
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Well i have a couple drill presses, a large compressor about 15 hp, a garage opener, a lathe. I do repair work on some motors, just some small projects. I do odd jobs for people.
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Old 02-05-2015, 10:06 PM   #15
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What do you guys think of this one? is it a good price?

http://www.ebay.com/itm/75HP-3-PHASE...-/261752941603
I'm pretty sure the current on this one would be more than 200 amps. A 200 amp service might not even be able to start it.

This one is also a basic ∆ connected motor with the shaft cut off and some capacitors added to get it started. It'll work, but the phase balance won't be as good as a unit designed from the ground up to be a converter.

There's a good chance that you'll need to reduce the sheave size on the motor of the compressor. This will result in less air but the motor might not be able to start the compressor with the factory sheave. And even if it does, it'll very likely trip its overloads within a few minutes because of the voltage imbalance inherent to all rotary phase converters.

As noted, pick one that's closer to your actual loads. It's usually ok to oversize a little, but not a lot.
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Old 02-05-2015, 10:08 PM   #16
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Well i have a couple drill presses, a large compressor about 15 hp, a garage opener, a lathe. I do repair work on some motors, just some small projects. I do odd jobs for people.
Well maybe if you know how to rewind motors start a little motor rebuild ,motor rewind shop.Small fab shop manufacturing gig. Good luck to you.
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Old 02-05-2015, 10:09 PM   #17
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I'm pretty sure the current on this one would be more than 200 amps. A 200 amp service might not even be able to start it.

This one is also a basic ∆ connected motor with the shaft cut off and some capacitors added to get it started. It'll work, but the phase balance won't be as good as a unit designed from the ground up to be a converter.

There's a good chance that you'll need to reduce the sheave size on the motor of the compressor. This will result in less air but the motor might not be able to start the compressor with the factory sheave. And even if it does, it'll very likely trip its overloads within a few minutes because of the voltage imbalance inherent to all rotary phase converters.

As noted, pick one that's closer to your actual loads. It's usually ok to oversize a little, but not a lot.
Ok thanks that explains it well. So should i look for something more like 30 HP? Lower?
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Old 02-05-2015, 10:10 PM   #18
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Well maybe if you know how to rewind motors start a little motor rebuild ,motor rewind shop.Small fab shop manufacturing gig. Good luck to you.
Thanks. That is actually what i want to do. I have rewound a couple of motors so i think i can make it work.
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Old 02-05-2015, 10:21 PM   #19
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Think it would be cheaper to put vfd at each machine or change motors for single phase
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Old 02-05-2015, 10:34 PM   #20
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Think it would be cheaper to put vfd at each machine or change motors for single phase
So, using your logic, instead of paying around 1500 for a rotary converter, i should pay 500 each for like 6 vfds?

I would if i only had one big motor, but why should i get a vfd for each drill press(for example)?
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