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#71772 - 11/06/06 11:26 AM Two phase motors
Trainwire Offline
Registered: 03/15/02
Posts: 360
Loc: Strasburg,PA,USA
Does anybody know of a simple wiring diagram for hooking up a two phase motor to a three phase supply?

We have an old mortising machine with a two phase 240 volt motor on it.

Replacing the motor is cost prohibitive, as it's cast into the machine.

It was working, and I can hook it up the way it was, it's just the way it was wasn't any where close to code.

I would like to cord connect it too.


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#71773 - 11/06/06 12:09 PM Re: Two phase motors
PEdoubleNIZZLE Offline
Registered: 06/16/04
Posts: 176
Loc: McKeesport, PA, USA
I think I have a diagram here somewhere, but I won't be able to post it until later tonight.
#71774 - 11/06/06 01:02 PM Re: Two phase motors
jraef Offline
Registered: 07/21/04
Posts: 93
Loc: San francisco, CA, USA
Look up the diagram for a "scott tee" transformer connection. Here is one,

Here is a thread from Mike Holt's on the subject which has another diagram inside.;f=3;t=001104;p=
#71775 - 11/06/06 09:27 PM Re: Two phase motors
Scott35 Offline

Broom Pusher and
Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
There are a few ways of going at this, depending on what level of complexity you intend to apply.
* some complexity is OK,
* whatever we can do to run the Motor the best way possible, is the only approach we will take,
* basically having it run is OK,
* no one really knows how to address this one.

If the Motor is a 2Ø 3 Wire Motor, it might be something that could simply be directly connected to an appropriately rated (Voltage wise) 3Ø Power System, without the need of any "Phase Alteration" techniques.

If the Motor is a 2Ø 4 Wire type, &/or the Machine needs to be driven from an actual 2 Phase System, then choose one of the following Transformer Options described below:

Fig. 1: 3 Phase 3 Wire "Closed" Delta Primary to 2 Phase 4 Wire "Taylor" Secondary.

This arrangement would be classified "Higher Complexity", as there are 3 Transformers, and some "non-standard" tap points (86.6% taps).

It does offer the best stability and load balancing options, and is very much a "Steadier 2 Phase Output" from a 3 Phase Supply.

Actual Phase Offset - in degrees, may "Creep" around, resulting in something that might be 105 degrees in one case, 82 degrees in another, or set precicely at 90 degrees all the time.

I am not sure of any issues of how well this connection scheme performs, in regards to the two individual Phases being exactly at, or remaining steadilly at, 90 degree offset.
(btw, the "two individual Phases" are the separate 1 Phase 2 Wire "Sets", represented as "...'A' & 'B' ... Phase 1..." and "...'A' & 'B' ... Phase 2..."

Fig. 2: 3 Phase 3 Wire Open Delta "Tee" Primary to 2 Phase 4 Wire Open Delta "Tee" Secondary.

Here is a simpler approach to the issue, yet still involves a "non-standard" Transformer (the 86.6% tap again!).
The configuration is much simpler to install, but has some balancing issues, which lead to some level of instability.

It nonetheless is still a 2 Phase 4 Wire SDS, derived from a 3 Phase supply - and the degree of offset may be more "easier" to keep at 90 degrees in this arrangement.

Again, I am unaware of any issues regarding Phase offset in these 2 Phase configurations.

Fig. 2: 3 Phase 3 Wire Open Delta "Tee" Primary to 2 Phase 3 Wire Open Delta "Tee" Secondary.

This one is somewhat similar to the one shown above in "Figure 2" - only there is a "Common" Conductor derived from two coil ends, and results in a 3 Wire output.

Additional options may include Motor-Generator setups, VFDs with 2 Phase output design intentions, or a stand-alone Diesel Prime Mover driven Generator.

Something to mention:
Many low power polyphase dry-type transformers (upto 15 KVA), with a Secondary rated as 208Y/120V 3 Phase 4 Wire, are in fact an Open Delta Tee configuration.
This is only a mention, but something to ponder...

Many options available, others may have much better examples to choose from; your results may vary



Links to 2 Phase pages at ECN:

2 Phase Transformer Schematics

Information relating to the posted images in this reply.

2 Phase Motor Drawings

Some Schematics of 2 Phase Motors.




It was working, and I can hook it up the way it was, it's just the way it was wasn't any where close to code.

Could you expand on this a bit, such as:

* How was it hooked up before,

* Was it connected to a 3 Phase system _DIRECTLY_ or _INDIRECTLY_,

* The Code Issues encountered on the existing installation.

TIA (Thanks In Advance)
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#71776 - 11/07/06 06:21 AM Re: Two phase motors
Trainwire Offline
Registered: 03/15/02
Posts: 360
Loc: Strasburg,PA,USA
Sorry everyone, but I'm back.

I hope to get some pictures posted of this "installation" both before and after.

It was hooked up before by running cords from the machine to the panel box, protecting 12 gauge cord with 60 amp fuses. It was hooked to a three phase system, motor phase one, hooked to one and two, and motor phase two hooked to two and three. It worked, for the most part, sometimes it wouldn't start, and you had to turn it off and try again.

I found a diagram in the 1996 edition of Ugly's. I think that will work. From the looks of it it's how it was hooked up before. What I hope to do is cord connect it with a plug.

Scott I am emailing a pdf of the page in Ugly's, maybe you can post it.

The machine is rarely used. To go the transformer route is to spend waaay more money than the machine is worth. We can buy a second hand machine for less.

I will be back as I have time.


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