Switching for 3 phase motor converted to 1 phase

Thread Starter

tooltalk

Joined Jan 14, 2024
21
Hi everyone. New member here with my first question. I did search the forums first and found some similar discussions, but I think my situation is somewhat unique. The other threads I found were all more than a year old too and some of the product links were outdated or dead as well.

So here's the situation... I bought a Rockwell 14 inch radial arm saw for woodworking last year. The motor on it was originally 575v 3hp 3 phase, but the previous owner had the motor converted to 220v single phase. I think this was done many years ago before Variable Frequency Drives became so common and affordable. The previous owner's widow sold me the saw and said her husband paid a lot of money to have motor converted.

A start capacitor was added to the motor, but since there was no centrifugal switch a momentary toggle switch was added to manually send power to the start winding. A second toggle switch is used to turn saw on and off.

So to start the saw I have to hold the momentary switch on, flip the on/off switch, then after a couple seconds when the motor is up to speed I release the momentary switch.

This works, but I'd like to upgrade the on/off switch to a proper push button switch with a larger mushroom or paddle style stop switch. For safety reasons I'd also like to eliminate the momentary switch so I can keep one hand on the saw carriage while I turn saw on with the other hand.

I've been doing some research on this and have seen recommendations for potential relays, adjustable timer relays, solid state start switches, etc. However, most these items have many different variations and specs, and I'm really having trouble figuring out what would work best on this particular motor.

I've attached some pics of the motor tag, the existing switches (labels added by me), and a schematic I drew of the wiring.

Sorry for the long post, but I would really appreciate it if anyone can make a recommendation for a specific product that I could use to replace the momentary switch.

Thanks very much.

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Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,681
Have you shown the motor label/info to a local motor repair/re-winding shop?
They may help in finding the right current relay for automatic starting, in place of the centrifugal switch method.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
Have you shown the motor label/info to a local motor repair/re-winding shop?
They may help in finding the right current relay for automatic starting, in place of the centrifugal switch method.
Thanks for your response. I did take it to one local motor repair shop, but the guy was really grumpy and was no help at all. He said he had no idea what had been done to motor in the past, or who did it, so he wouldnt even look at it.

There is another shop in town, so maybe I'll try them if I can't figure it out.

Thanks.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
If there have been changes to the motor then we have no clues. I suggest using a variable speed drive that delivers enough current.
If it helps, the motor tag was restamped 220v, 1ph, 14amps as seen in the attached photo above.

What kind of "variable speed drive" are you thinking? Do you mean VFD? I didn't think they could be used with single phase motors.

Thanks.
 

MaxHeadRoom

Joined Jul 18, 2013
28,681
What kind of "variable speed drive" are you thinking? Do you mean VFD? I didn't think they could be used with single phase motors.
You are correct, VFD's fail miserably when used with 1ph motors, they tend to drop out of run if loaded, especially at lower RPM's.
The main reason that the 3ph are most popular with 1ph supply.
It is odd that the motor label shows it as a 3ph motor?
If in fact it is 3ph, then a VFD could be a possibility.
The connection options also indicate 3ph originally.
A previous owner may have used it with Capacitor for it to run on 1ph.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
It is odd that the motor label shows it as a 3ph motor?
If in fact it is 3ph, then a VFD could be a possibility.
It might be hard to see in my photo, but the motor label was restamped 1ph. You can still see the original "3" in the phase part of the label, but it has been lightly scratched out and a "1" is stamped next to it.

When I got the saw I was hoping the motor was still actually wound for 3ph and the capacitor was part of a "static phase converter" which uses motor start capacitors to start a 3 phase motor on single phase power.

However, I posted some pictures and questions on a woodworking forum and someone there who seemed pretty knowledgable helped me test the windings to determine that it is now actually single phase with start and run windings. He also suggested a potential relay to replace the momentary switch, but he couldn't help in determining which one to buy. He said it would be a matter of trial and error and I would probably have to buy several before getting it right. I don't have the budget for that, and when I discovered these forums I thought maybe someone here could help narrow it down.
 

LowQCab

Joined Nov 6, 2012
4,061
It's pretty much guaranteed that the Motor was not purposely Re-Wound,
but simply Re-Wired externally to "make-it-work" on Single-Phase-Power,
although the Torque and Horsepower would be noticeably reduced.

A simple timer along with a SSR will make it start automatically.
( this assumes that it actually performs adequately the way that it's set up right now )

Start / Stop Buttons are an additional, but easy project.

A Single-Phase-Input, 3-Phase-Output, VFD is still the preferred way to go if You can afford it.
This will also provide a smooth RPM ramp-up, and "Automatic-Braking" upon shut-down.
This will also reduce the massive Current-Spike on your Home's-Electrical-Panel when Starting the Saw.
This will also mean that your "new" Start-Stop-Buttons can be any type of Button
that You choose, as they will only be handling Low-Voltage / Low-Current, Signals to the VFD.
.
.
.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
It's pretty much guaranteed that the Motor was not purposely Re-Wound,
but simply Re-Wired externally to "make-it-work" on Single-Phase-Power,
although the Torque and Horsepower would be noticeably reduced....
A Single-Phase-Input, 3-Phase-Output, VFD is still the preferred way to.
Thanks very much for your response. I actually bought a VFD and the vendor said the same thing as you. Said he never heard of a 3 phase motor being rewound to single phase. Based on his assurances it would work I ordered the VFD, but when I got it we couldn't figure out how to connect it because there were only four leads number 1, 2, 5, & 8 coming out of the motor. The vendor said there should six leads and the extra two were likely buried inside, but I opened up the motor and could not find any extra leads, or any evidence that anything had been cut or removed. So I was back to square one, but I ended up installing the VFD on another machine.

I had a similar discussion on another forum and someone had me test the resistance of the windings. I measured 1.3 ohms between leads 1 & 2, and 1.6 ohms across 5 & 8. I was told that was "conventional for single phase, single voltage with leads 5 & 8 going to/from the start winding".

Do you still think there's a chance this could be a "Re-Wired" 3 phase motor? I would certainly prefer that for all the reasons you outlined. I'm also not certain that anything anyone has told me about this motor so far is accurate, lol.

If I can't use a VFD, could you recommend a "simple timer along with a SSR"? I'm not sure what SSR is.

Thanks again!
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,681
It is probably now unknown what was done to the motor, a possible examination of the windings MAY indicate the signs it was partly rewound.
For that size of motor, you most likely will be disappointed in the results trying to run it on variable speed, Triac/SSR etc.
SSR = Solid State Relay (Triac based) should function as a normal current relay would.
As @LowQCab states, easy to mock up.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
For that size of motor, you most likely will be disappointed in the results trying to run it on variable speed...
I don't need variable speed for this type of machine, so that's not a concern. I would just like to be able to turn it on without manipulating multiple switches. Any suggestions for a timer that might work? I've seen some online, but there are so many different variations it's confusing.

I've also been looking for a 3ph replacement motor, but they're hard to come by. I had a lead on one, but it was missing the motor label and I didn't really want another mystery motor.

Thanks.
 

MaxHeadRoom

Joined Jul 18, 2013
28,681
Do you want to put something together yourself, or just buy off the shelf, which there may not be obvious solution.?
There is a mains powered circuit in the old GE "Experimenters Handbook" that could be adapted.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
I don't mind building something myself, as long I can find the components. I'll look for that "Experimenters Handbook". Is it available online?
 

MisterBill2

Joined Jan 23, 2018
18,461
My suggestion to use a VSD was based on one that is made for three phase motors. At this point, if it really IS a three phase motor, then it should have three windings of very close to equal resistance, if it is connected in either a WYE or delta configuration. If you can get aly help from the manufacturer based on the model number that might be useful. With a tag speed of 3450 RPM, it would be a two-pole motor,
If the TS is able to open the motor that might allow seeing what it presently is.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
28,681
At this point, if it really IS a three phase motor, then it should have three windings of very close to equal resistance,
The OP states they are different res. values.

I don't mind building something myself, as long I can find the components. I'll look for that "Experimenters Handbook". Is it available online?
I believe it is, if not I could photo-copy it.
It may have to be modified slightly as the circuit as is, is SW initiated, but a small relay could be added to do this automatically.
The up side is, it can be built with 120v/240v components. No LV.
 

MisterBill2

Joined Jan 23, 2018
18,461
The OP states they are different res. values.


I believe it is, if not I could photo-copy it.
It may have to be modified slightly as the circuit as is, is SW initiated, but a small relay could be added to do this automatically.
The up side is, it can be built with 120v/240v components. No LV.
That is not a lot of difference. AND no mention of resistance between the two sets of connections. If there is no connection then it may not be a 3-phase motor at all.. And how good are those resistance readings?? My model 260 gives that variation just with probe wiggles.
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
That is not a lot of difference. AND no mention of resistance between the two sets of connections. If there is no connection then it may not be a 3-phase motor at all.. And how good are those resistance readings?? My model 260 gives that variation just with probe wiggles.
I went back and checked my notes when I did the tests. I measured 1.3 ohms between 1 & 2, and 1.6 ohms between 5 & 8. There was no continuity between any other combination of leads. The guy who asked me to do the tests said " 1.6 ohm for the start winding sounds low but whoever rewound it was probably guessing."

Below is a better pic of the motor label and a pic of the capacitor and leads, as well as the hand written note I found inside. It was definitely a 3 phase motor originally, but you can see the 3 has been obscured and a 1 has been stamped next to it in the phase section of the label. From everything I've heard it is very rare and would have been very expensive to actually rewind a 3 phase motor to 1 phase, but it's not impossible.

I believe it is, if not I could photo-copy it.
It may have to be modified slightly as the circuit as is, is SW initiated, but a small relay could be added to do this automatically.
The up side is, it can be built with 120v/240v components. No LV.
I was looking for the Experimenters Handbook and found many different editions and with dozens of different projects in each one. So it would be great if you can provide a copy of this project.

Thanks to everyone who has replied!
rockwell-motor-tag.jpeg

rockwell-motor-cap-and-leads.jpeg


rockwell-motor-note.jpeg
 

Thread Starter

tooltalk

Joined Jan 14, 2024
21
No rush at all. Does the project or circuit have a name that I can look up? Just curious to know more about it. Thanks very much for your help.
 
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