RPC High Leg and Three Phase Motor
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  1. #1
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    Default RPC High Leg and Three Phase Motor

    Gentlemen,

    I have a 3/4 HP three phase motor in WWII vintage south bend lathe that I am cleaning and re-wicking.

    This motor was manufactured by Wagner Electric Corp. It is a 220/440 volt motor. MY RPC is a 220 volt design.

    One of the three phases of this motor is a start winding. This start winding has a capacitor and a centrifugal switch in series. When the motor starts the centrifugal switch opens and this winding with capacitor is no longer powered.

    As the motor is being run by an RPC I am thinking that the RPC high leg should be tied to the start winding. I have no reason for thinking this other than gut instinct. Does this seem appropriate?

    Also the motor would not start so I replaced the capacitor which resolved the problem. The replacement capacitor I used is a 105 mfd, 240 volt. Does this seem an appropriate size? it does from the charts I found online.

    Thank you for your assistance.

    Vlad

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    Quote Originally Posted by Vladymere gr View Post
    Gentlemen,

    I have a 3/4 HP three phase motor in WWII vintage south bend lathe that I am cleaning and re-wicking.

    This motor was manufactured by Wagner Electric Corp. It is a 220/440 volt motor. MY RPC is a 220 volt design.

    One of the three phases of this motor is a start winding. This start winding has a capacitor and a centrifugal switch in series. When the motor starts the centrifugal switch opens and this winding with capacitor is no longer powered.

    As the motor is being run by an RPC I am thinking that the RPC high leg should be tied to the start winding. I have no reason for thinking this other than gut instinct. Does this seem appropriate?

    Also the motor would not start so I replaced the capacitor which resolved the problem. The replacement capacitor I used is a 105 mfd, 240 volt. Does this seem an appropriate size? it does from the charts I found online.

    Thank you for your assistance.

    Vlad
    You might want to post the dataplate of that motor.

    Presence of a "start winding" and a "centrifugal switch" SHOUT that it is a SINGLE phase motor.

    3-Phase motors do not need, nor have, either one of those features.

    It won't care at all what you have for an RPC.

    It will want powered directly from 2XX VAC single-phase AKA "split" phase". Same as a clothes dryer, water heater, baseboard heater, or an electric range/oven.

    That said? If you now HAVE a decent RPC, it could be a good time to go and FIND a 3/4 HP 3-Phase motor and replace your single-phase one.

    If nothing else, it is easier to reverse a 3-Phase motor.

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    From your description (cent. switch and a start capacitor) I would very much doubt this motor was designed to be run from polyphase service. As mentioned it is probably a single phase motor, and the design of those is such that the start winding would not work well as a regular run winding. Also you would probably have to convert your 3 phase supply into a 90 degree true 2 phase version to get close to this idea working.

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    So you have an rpc. It will be about 208 volts from the generated leg to thr neutral of your 120/240v single phase supply.

    If you use an auto transformer of the correct ratio (to be determined) you can run your single phase motor as if its a 2 phase motor, using the auto tx to buck or boost the voltage. The auto tx gets connected from the generated leg to the neutral of your supply.

    Depending on the mass of copper in the start winding, you will either have a motor that starts and runs under load just fine, but if you reverse the motor it will continue to run in the same direction amd burn out the start winding (in which case you need to wire the centrifugal switch back in)

    Or you could have a motor with pug reversing capacity.

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    thermite, jim rozen, johansen,

    Thank you for your replys.

    The motor data plate does not photograph well but the date on it is as follows;
    Wagner Electric Corp
    Frame: 75U
    Model: B15785 K2083
    H.P.: 3/4
    Volts: 220/440
    Phase:3
    Code: L
    60 Cycle RPM: 1725
    50 Cycle RPM: 1425
    60 Cycle Amps: 2.6/1.2
    50 Cycle Amps: 3/1.5
    Rating: Cont.
    *C: 40
    Sec: Volts: Amp: No. 3 (I don't know what this line means.)

    There are six leads exiting the motor housing. Three leads are soldered together. The other three leads are where my three phase power connects.

    Of the three leads for power enactment two of them measure 5.3 ohms to the soldered end. The third, if the cap is removed and the two cap leads are tied together also measure 5.3 ohms, this would be going through the centrifugal switch.

    I don't have a proper switch to the motor right now, I'm using the three phase breaker in my three phase breaker box to turn the motor on and off as the current wiring is temporary while I reassemble the lathe. My RPC feed the three phase breaker box with a branch circuit to each of my three phase devices, Bridgeport mill, Cincinnati Tray Top lathe, automobile lift and now the Southbend lathe.

    The RPC does "grunt" a little when starting this 3/4 HP motor and the legs are not balanced well for this motor.
    the RPC is built on a 5 HP motor.

    I have added a crude schematic of the motor wiring.

    Thank you for any advice.

    Vlad
    Attached Thumbnails Attached Thumbnails scan_pic0007-2-.jpg  

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    starting that motor, with a cap in series with the start winding from the 3 phase RPC is electrically worse than leaving the motor connected single phase the way it was intended. the capacitor provides the 90 degree phase shift the motor needs, the rpc provides a 60 degree phase shift, plus 90, equals 150 degrees.


    if you remove the start capacitor, and connect the start winding to the neutral of your 120/240v system and the other end through the centrifugal switch to the high leg.. you should see the motor start up quicker. or not, if the voltages are completely wrong.

    anyhow, if the motor starts and works fine on single phase and you are happy with waiting for the motor to slow down and stop before reversing it, there is no reason to experiment further.

    it may be possible as i said, to remove the start switch, buy a 3 phase drum switch, and gain plug reversing capacity without burning up the start winding.

    with the motor running on single phase 240, what is the voltage generated by the start winding?

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    The fact that there are three separate windings, each with the same dc resistance, implies this was not originally a single phase motor. The nameplate bolsters this idea.

    It is possible the cent. switch was added by a previous owner, along with the start capacitor. Photos of this motor would interesting to see.

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    Curious, That motor could easily start and run on single phase just as a rpc does. At some power loss no doubt.

    Supplied with 3ph, I would jump the cap. ......

    As has been written previously!

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    Fellows,

    I will follow through with your suggestions and report back. Unfortunately I must delay in following up with your suggestions as I have been working on this lathe in my automotive bay and have two cars that I need to repair/service without delay.

    Being old, slow and crippled this will take me a week or two but as soon as I have finished with the vehicle service I will move the lathe into the bay to implement your suggestions.

    I will report back.

    Thank you for your help.

    Vlad

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    Quote Originally Posted by jim rozen View Post
    The fact that there are three separate windings, each with the same dc resistance, implies this was not originally a single phase motor. The nameplate bolsters this idea.

    It is possible the cent. switch was added by a previous owner, along with the start capacitor. Photos of this motor would interesting to see.
    Agree.

    My bet? The PO basically built a form of bastardized static converter onto a 3-P motor.

    Given he has an RPC?

    I THINK all Vlad has to do is disconnect & remove all the crap, connect it conventional 3-P as it was built OEM, and done.

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    Quote Originally Posted by thermite View Post
    I THINK all Vlad has to do is disconnect & remove all the crap, connect it conventional 3-P as it was built OEM, and done.
    +1, although I've mis-diagnosed the situation once already. I'd like to see a photo or two to be sure of what's going on.

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    I don't have a photo of the complete motor but I do have one of the centrifugal switch in the end bell.

    The wire on the left side of the photo is from the start winding. It connects to the lone screw on the left side of the centrifugal switch. The wire on the right side of the centrifugal switch connects to the start cap.

    All wiring from the motor appears, to me to be original factory cloth insulated wire with a lacquer or varnish coating

    Vlad
    Attached Thumbnails Attached Thumbnails dsc02318.jpg  

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    It could have been rewound. Need to look at the relative size and spacing of the three coils.

    Metal strap holding switch on does not appear original.


    What are the no load amps when running? If two of three phases are connected in parallel its going to be a lot more than normal for a 3/4 hp motor (1 to 2amps expected)

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    Centrifugal switch looks prettty standard to me, wonder what the capacitor mounting looks like.

    It may be that the two windings not associated with the start capacitor are set up to be configured either 120 (parallel) or 240 (series). If this is the
    case then the motor won't work well when each one is put across a phase, with 240 volts on it. I'm betting this really is a dual voltage single phase motor,
    right from the git-go.

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    Quote Originally Posted by jim rozen View Post
    Centrifugal switch looks prettty standard to me, wonder what the capacitor mounting looks like.

    It may be that the two windings not associated with the start capacitor are set up to be configured either 120 (parallel) or 240 (series). If this is the
    case then the motor won't work well when each one is put across a phase, with 240 volts on it. I'm betting this really is a dual voltage single phase motor,
    right from the git-go.
    But then there is that Wagner namepate that says 3-Phase.

    So many old motors. So little time.

    Really Brethren?

    Deep-six the f**ker and go find another motor.

    Hacker's First Law:

    Life is too DAMNED short to drink bad wine, use slow computers, or share a blanked with a bitch.

    This mystery, solved, won't make a f**king bit of difference in life's bitch tapestry, regardless.

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    Odds are that it is a cobbed-up 3 phase motor with a single phase start circuit squeezed in.

    Wanna know for sure? Switch the cap and so forth to a different winding, and see if it behaves the same (other than direction of rotation).

    If so, same current draw, etc, it's virtually certain to be 3 phase and you can get a VFD or RPC and use it.

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    Didn't make any progress on getting the auto bay cleaned out. Friday or Saturday I will take additional photos of the motor as well as motor interior. I will also measure the no load amps requested. I will also try running it with the cap and centrifugal switch removed and with the cap and switch on a different winding.

    From recent memory all of the winding's are of the same gauge wire and appear to be equally spaced but i will get a photo of this for verification.

    This lathe does have forward/off/reverse drum switch.

    The previous owner(deceased so I can garner no information from him) was running this on static phase converter.

    I could, as thermite suggested, purchase a new three phase motor but living on a fixed social security income I would rather dance with what I brung.

    Fellows, I sincerely appreciate the help you are providing.

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    Gentlemen,
    I had an opportunity last night to do some of the testing on my motor that was requested and take component photos.

    Johanson asked what the no load amps where on each leg. I place my Fluke DVM in series don one of the legs, set to AC amps and the 10 amp unfused common connection of the meter. Threw the breaker to start the motor. The motor grunted and tried to start, I spun the pulley by hand but id did not come up to speed. By the time I got back to the breaker box there was a cloud of smoke at the motor. The start cap, not the leg I had the meter on, blew in a spectacular manner. It didn't just pop the rubber pressure plug in the top of the but blew the entire top off the cap and ejected contents of the plastic "can". I have not seen a cap blow that violently before.

    I may have misunderstood another suggestion from johanson. Since photos where requested and the start cap was gone I opened the motor bypassed the centrifugal switch and start cap wiring. At this point one lead from the three windings was external to hook my three phase power feed to and the other ends of the windings remained tied together. I attached my three phase power from my RPC to the motor, one phase to one of each windings. The motor started OK but in reverse of what I needed. A contact tachometer showed, I think (memory failure) 1725 RPM in what would be the reverse direction and 1798 ROM in the forward direction when wired one winding to phase. I'm thinking this is how I will hook this motor up unless you gentlemen can explain why I should not. I'm thinking that this will allow me to plug reverse the motor also but may not be recomended with my screw on chucks. I do think I will open the motor one more time and remove the centrifugal switch from the end bell housing but will leave the centrifugal assembly on the shaft.

    I have attached photos that may give more insight about the motor. It may have been rewound, I don't know but, to me, the centrifugal switch expanding shaft weights look factory to me. Photos include the motor in situ, for jim rozen, a photo of the capacitor mounting bracket. Also included are photos of the windings, the rotor with centrifugal weights.

    I would appreciate your thoughts on the history of this motor and if connecting it up to three phase via the three leads would be appropriate.

    Thank you fellows.
    Attached Thumbnails Attached Thumbnails 20200826_212124.jpg   20200826_201442.jpg   20200826_201419.jpg   20200827_175443.jpg   20200827_181645.jpg  


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    Default More photos

    More photos.

    Vlad
    Attached Thumbnails Attached Thumbnails 20200827_181524.jpg   20200827_181645.jpg   20200827_181621.jpg   20200827_181514.jpg  

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    Additional information. With the motor hooked up to my RPC, no start circuit or capacitor, the three phase voltages, with motor running, are 215VAC, 222VAC and 34VAC. I obviously need to balance it better.

    I found my clamp on AMProbe and two phase show 3.75 amps drawn with no load on the motor. The high leg is drawing 2.5 amps.

    I don't know if this is significant or not.

    Thanks fellows,

    Vlad


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