Choosing a Phase Converter

      Here's advice on choosing a phase converter to bump up two-phase power to supply three-phase motors, with some explanation of how those gizmos actually work. January 31, 2012

The line boring machine that I bought is a Detel MV-23P. It is 3hp and 3 phase. I only have single phase power so I need a phase converter and don't know anything at all about them. I don't know what is involved with hooking one up or how big of one to buy. To my understanding you plug a 3 phase motor into the converter and the converter cord into the single phase outlet? I've been looking at some that Grizzly has, but I honestly don't have a clue what I'm looking at. Any help is appreciated.

Forum Responses
(Cabinetmaking Forum)
From contributor L:
I made a homemade rotary converter a long time ago. It worked fine for several years until I moved to a shop space with 3 phase. From what little I know about them I'd get a rotary as opposed to a static (electronic).

From contributor Y:
First get a qualified electrician. I have a Kay phase converter. I've had it for ten years. We feed it from the main panel and it feeds a sub-panel. The one I have will run up to a 10 horse motor and a total of 30 horses at one time. I have 4pieces of equipment that I run on it. I can run any 3 at one time. You also may have to sync your equipment to the phase converter and that would take an electrician too.

From contributor E:
I have a Phase Quest rotary converter installed and can run up to 22hp I think. If I am not mistaken it is a 7.5hp. It works beautifully but you will definitely need a good electrician to hook it up.

From contributor H:
There are two types of phase converters - static and rotary. A static converter is fed 220 single phase and uses it to shoot a temporary power shot to the 3rd three phase leg on your motor at start up. This, along with the 2 single phase legs you have in the building get the motor going. Then that third leg that was surged during startup goes dead and the motor runs on single phase 220. As such the motor runs at 2/3 of the power rating.

A rotary converter uses the single phase 220v input to spin a 3-phase motor that is incorporated into its construction. Just like with the static converter it runs on single phase 220v. However since it is a 3-phase motor the third leg is actually generating power, somewhat like when a generator is running. This third leg is sent to your machine motor along with the 220v that is in your building. Now you have 3 power legs going to your 3-phase machine motor so you have full power. The trick here is to get all three legs balanced in terms of voltage and current draw. In some situations you have to be careful which input terminal on your machine receives that "manufactured" third power leg or the machine won't work properly or at all.

Those are the basics. If you need the full power rating of your line boring machine then you need to go with a rotary converter. If the machine is overpowered for what you are doing then you can get by with the lower HP output you'll get with the static converter. Static converters are much less expensive than rotary converters.

There is one more possibility. I know nothing about line boring machines but if there is a variable speed option on the machine then it might have a variable speed motor. AC motors do not do variable speed (at least economically). Instead they use frequency drives (also called inverters) to adjust the motor speed. Some frequency drives will input single phase and output 3-phase variable speed to the motor. These in effect have static inverter circuitry incorporated into them. So, if your line boring machine has a variable speed option you should look into these specs. They should be listed in the machine manual.

From contributor V:
Just make sure itís a soft start unit, or everyone will hate you when you dim their lights. I have a 40 hp unit that's not soft start.

From contributor B:
All good stuff here especially Contributor Hís clear explanations. I've been operating on the same rotary converter for at least 20 years now and the best part is that I can own and operate real industrial 3 phase equipment wherever I choose to set-up shop.

Be sure to allow for eventual machinery exchanges as you will undoubtedly want to do and do locate your converter right next to the main panel rather than running start-up wires some distant away. These things all require a significant kick start.

From contributor H:
I have a good friend who is an industrial electrician. He is one of the most knowledgeable and smartest people I know. He sets up robotics in factories, designs the power and wiring systems for hydro plants and sets up factories including welding up all the required custom steel. This is one very versatile guy.

Coincidentally we were on the phone just this morning talking about frequency drives. I knew they generally operated as AC in, DC in the middle and AC out. However I thought they were like static converters and the output was 2/3 of the motor's rated horsepower if you were inputting single phase.

After five minutes of his explanations about the only things I followed were first that I was wrong about them outputting only 2/3 of a motor's power, and second that the AC to DC and back to AC was generally correct but not operating anything like I thought.

It turns out the power output has nothing to do with the single phase input. If you think of it in terms of wire size and current draw the single phase input wires need to be sized to the full motor power as if it were running on single phase and the 3-phase output is sized to the 3-phase motor requirements. That is your wire TO the frequency drive will be larger than the wires FROM the frequency drive to the motor. This of course applies to frequency drives that are run from a single phase input as vs. those that input from 3-phase.

On the current type I thought the DC in the center of the circuit was there to allow for a variable speed output controlled in the DC circuitry and then converted back to AC on the 3-phase output side. It would seem this is not what is happening and he totally lost me with his explanation of what is actually going on.

From contributor R:
Since it is only a 3 hp motor would it be easier to just replace the motor with a single phase motor?

From contributor H:
"Since it is only a 3 hp motor would it be easier to just replace the motor with a single phase motor?" That depends. If there are no electronics involved then it could be nothing more than a motor swap and using two of the three terminals on the contactor and adjusting or replacing the overload sensor so it matches the higher current of the single phase motor. However if there are other electrical/electronic components on the machine then it would take a thorough examination of the entire circuitry to make sure all those components were still properly powered.

From the original questioner:
I've thought about changing the motor but itís an odd shaped motor (long and skinny), plus it is direct drive - nothing normal about it. I checked locally today for a converter and it was going to be 1K, and I told the guy I thought I could do better than that. I'm looking at a Phoenix rotary phase converter for about $300. Does this sound about right? 1K seems crazy to me.

From contributor H:
I think $300 would be a pretty good deal. However if there is any load at all on the motor during start up then you need to be sure to get a converter that is at least 4 if not 5 hp due to the increased starting current draw.

From contributor B:
What may actually be crazy is buying a small rotary converter that can only handle a single machine or two. I believe I paid nearly $1,800 twenty years ago. My converter will handle up to a 10hp motor and 40hp combined. Almost everything in my shop is older 3 phase equipment and most of it has been around almost as long as I have. I can also run several machines at once if I need too. Another consideration is the rent on industrial property (without 3 phase service) which is typically less and easier to find.

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