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#34802 02/23/04 09:02 PM
Joined: Feb 2004
Posts: 2
Joe H Offline OP
Junior Member
I’m trying to help a distant (location) friend get a vintage (1950’s) lathe running. It has 3phase 220v input and the motor is two speed DC. 115v field, 200v armature (motor & drive by Louis Allis Co). He does not have 3-phase power. We are hoping to convert the drive to single phase rather than scrapping the whole setup and using a 3ph motor & VFD. His pics show a bewildering array of transformers, heat sinks and slide wire resistors. From his description I’m guessing the 3 phase feed is transformed into 115v & 200v 3phase and then these are rectified to 115v & 200v dc output. The 115v dc would then go to the motor field. The 200v dc would be split to feed the 2 sets of armature leads by a Hi/Lo switch. Then through a two section stacked Rheostat for infinitely variable speed to the armature leads. I would think the 3phase rectification circuits could be replaced by single-phase full wave bridge rectifiers (plenty of heat sinks available). Could the three-phase step down transformers be wired for single phase? Any comments or anyone familiar with this type of drive?

#34803 02/23/04 09:29 PM
Joined: Dec 2003
Posts: 524
... I've run into the same thing once, and I picked up a phase's sized according to the horsepower of the lathe..It's basically a capacitor,and a relay..depending on the situation, you may need a 3-phase motor that's higher or at least equal to your lathe acts as a phase "conditioner"..once the 2 phases are introduced, the converter "manufactures" a "B" phase, and once the 3-phase motor starts spinning,it acts as a generator,using the windings to make and continue producing the "B" phase..the latching relay then disengages, and the lathe motor runs on her own power...The converter is quite cheap..under $200.00, and you can use any old 3-phase free-standing motor,as long as you size it accordingly..The "conditioning" motor is load-free,and just free-wheels, while the lathe is in operation..I'll look for the company name for you, it escapes me now...sorry..

[This message has been edited by Attic Rat (edited 02-23-2004).]

.."if it ain't fixed,don't break a Licensed Electrician"
#34804 02/23/04 11:04 PM
Joined: Apr 2002
Posts: 2,527
One article on ‘rolling your own’ is

#34805 02/24/04 07:00 AM
Joined: Feb 2004
Posts: 2
Joe H Offline OP
Junior Member
We've tried a rotary phase converter and it doesn't work. Either there is something wrong with the drive or motor or the phase converter just won't operate a 3 phase rectifier circuit. We are concerned that the phase converter, while it will operate a 3 phase induction motor, it may not properly operate the rectifier circuit. With this application the phase converter is not operating a motor. We are trying to figure out the wiring so we can bypass the 3 phase rectifiers and see if a single phase full wave bridge will operate the DC motor.

#34806 02/24/04 05:04 PM
Joined: Sep 2001
Posts: 806
A 3-phase FW bridge is the same as a single phase, with 2 more added diodes. Applying single phase to any 2 of the input legs will produce DC at the output. If the drive is a rectifier and separate inverter, it SHOULD be able to run (at reduced power) from single phase.

You need to make sure that you use the 2 legs that are connected to the control transformer, so you have control power for relays and the like. If the drive circuitry incorporates missing phase detection, or is a direct offline converter (6 or 12 SCRs in a full wave bridge), then you will need the third phase.

[This message has been edited by NJwirenut (edited 02-24-2004).]

#34807 02/24/04 09:46 PM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and
I was thinking the same thing as Njwirenutt, regarding the Rectifier's Diodes.
Kind of simple to make a 3Ø FW Rectifier's Diode array into a 1Ø FW Bridge setup.

For the Transformers, this may be tricky!
If they are separate 1Ø 2 wire type, then not such a problem. Setup as 1Ø input/output.
If they are common core 3Ø type... problem! Time to look for replacement 1Ø Transformers!

Either way, the Transformer sizes + the ampere rating of the Diodes will need to be adequate for the load drawn. Since the Rectifier is 'downsized" from the 3Ø setup, you will need to compensate for this by an increased 1Ø Bridge size.
Same goes for the KVA of the Transformers.

Got more questions? Feel free to post away!


p.s. If you need schematics, let us know.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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