View Full Version : Three phase convertor question

07-02-2004, 02:18 PM
I just purchased a Clausing 1500 with a 7.5 three phase motor. I am getting ready to purchase a 3 phase motor to build a single to three phase convertor.

Can I go with a Baldor 7.5 hp with a SF 1.15 or do I need to buy a 10 hp?

The 7.5 is new and cheaper than than the used 10 hp. Plus I am concerned about the amperage draw on the 10 hp.

Second question, has anyone tried a 3600 rpm instead of the standard 1750 rpm? One of the website I was looking at, indicated that the phases would be better matched with a higher rpm motor.

Thanks for the help,

[This message has been edited by JPR (edited 07-02-2004).]

07-02-2004, 02:28 PM
I'm no expert on this, but it's my understanding that the rpm doesn't matter. Either one will generate 60cycles. As for the size, I've often read that actually the better arrangement is to match the 3ph generator motor to the size of the driven load. ...Or at least don't select a grossly larger generator. I think that's for cost efficiency concerns. It's not saying it won't work to have a large mismatch.

Maybe someone else more fluent in such matters will educate us both here.

07-02-2004, 05:46 PM
If you are going to the expense of buying a new motor, you would be far better off to spring for a VFD and get the benefits of speed control, braking, instant reverse and all the other goodies.
A home made rotary convertor is really only a compromise solution to three phase.

Forrest Addy
07-02-2004, 05:52 PM
I endorse JC's suggestion about VFD's (variable frequency drive). I have VFD's on almost all my machine tools.

There's advantages to phase converters and advantages to VFD's but the variable speed, full load motor HP, electronic braking, quiet operation, electrical efficiency, and the other features makes (to my mind at least) selection of the VFD route a no-brainer.

07-02-2004, 06:05 PM
See, for example, www.dealerselectric.com (http://www.dealerselectric.com) and several other web sites that sell VFDs at pretty reasonable prices...although one big enough for a 7.5hp motor is going to be pricey, anyway. Maybe no more than a new 7.5hp motor though.

As for building your own, if you go that route, my understanding of the "conventional wisdom" is that the converter motor ought to be about 50% larger than the motor you're driving for best results, but it doesn't need to be.

Another thought is to just replace the motor on the machine with a single phase motor.

I'd give extremely serious consideration to a VFD though. It does a great job on the phase conversion problem, and the variable speed aspect of it is a very nice bonus.

[This message has been edited by SGW (edited 07-02-2004).]

07-02-2004, 10:38 PM
Thanks for the info guys.
I found a Genesis KBN2-2310-1 10 HP, 230VAC drive locally for $700. http://www.kbelectronics.com/catalog_kbn2.htm

I think lowered starting amps and better efficiency will out way the cost.

[This message has been edited by JPR (edited 07-02-2004).]

07-02-2004, 11:01 PM
The way I read their catalog that unit requires three phase input.

Alistair Hosie
07-02-2004, 11:32 PM
Careful what your doing !!!! some invertors are three phase to three phase and are merely speed controllers and brake etc.
You still need to start out having three phase already installed for these so you will waste your money make sure you buy a single to three phase one Alistair

07-03-2004, 09:52 AM
No you won't find a inverter with a single phase input that big.

As for homebrew converters,the way I do it is by using a motor slightly bigger than the motor being driven in this case 10 hp and mount a centrifugal switch and start capacitor to use the third leg for a starting coil.

The switch contacts and flywieghts can be had from Graingers and motor winding shops.The capacitor should be 80-120mfd per hp

The dooms dayers and nay sayers always knock rotary converters,but they DO WORK AND WORK WELL plus they are cheap.I get about 90% of the nameplate power out of mine which is fine since most tools have more than they need anyway.

I have a 3hp,and a 10hp running right now,plus I built a 20hp for a friend which we ran the output from to a three phase breaker panel,its been a little over 3 years now on his system and its going fine(he owns a production cabinet shop and runs 10hrs a day 6 days a week).

I have found that the older the motor (within reason) the better,reason being they have extra mass in the rotor,I have always wanted to use a flywheel on the converter shaft to smooth out hard starts(motors over the converters capacity)and some high load starts like air compressors but haven't had the time.

Oh and my buddy with the big converter,he noticed only a $10 increase in his light bill,this after running a 7.5 hp radial arm saw and a 15hp wide belt sander.

07-03-2004, 10:01 PM
Now that's an interesting idea ..... a flywheel on the idler !!! Wish I hadn't cut the shaft off now. Balancing could be fun ..... wonder how the load would change things. Don't mean to sound crazy .... just thinking out loud.

07-09-2004, 09:59 PM
rpm does matter!the higher rpm motors have higher starting loads which means larger start capacitors and wear,motors that have less than 1725 rpm can cause nasty harmonics, as far as idler motor size that depends on starting load of the tool. id go with a idler of 1.5 times that of the largest motor that youll be running ,this will allow for easier starting and less voltage fluctuations etc. check for wiring type on your baldor its probably wired "wye" which causes 2 problems ,a high voltage on one leg and higher start up amperage draw, best to get a motor thats wired "delta"

07-09-2004, 10:05 PM
weirdscience, the flywheel is a novel idea !the only problem i could see is the extra start up load this would add! could be cured using a clutch?? probably easier to go with a massive motor? same net result but with added benefit of larger capacity.