Building a RPC out of a Dahlander motor
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  1. #1
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    Default Building a RPC out of a Dahlander motor

    Hi folks.

    Right now I am using a 10 HP Dahlander motor as a RPC. I spin up the motor, flip on the circuit breaker powering L1 and L2, and it keeps spinning. I have L1, L2, L3 tapped to power my mill and surface grinder and all's good. (I have capacitors on order for spin start)

    My question is this. I know the Dahlander has 6 windings and six leads, power depends on wiring. Lower power utilizes 3 leads (with the other 3 left open), high power has 3 leads wired together and the other 3 leads closed. Is it possible, and if possible is there any virtue, to powering the motor through 3 leads and tapping off the other 3 leads? My mental model is that it would act like two separate motors, one acting as a driver and the other as a generator.

    If my mental model is incorrect I'd appreciate an education.

    Thanks,

    philip

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    Quote Originally Posted by hahnpv View Post
    Hi folks.

    Right now I am using a 10 HP Dahlander motor as a RPC. I spin up the motor, flip on the circuit breaker powering L1 and L2, and it keeps spinning. I have L1, L2, L3 tapped to power my mill and surface grinder and all's good. (I have capacitors on order for spin start)

    My question is this. I know the Dahlander has 6 windings and six leads, power depends on wiring. Lower power utilizes 3 leads (with the other 3 left open), high power has 3 leads wired together and the other 3 leads closed. Is it possible, and if possible is there any virtue, to powering the motor through 3 leads and tapping off the other 3 leads? My mental model is that it would act like two separate motors, one acting as a driver and the other as a generator.

    If my mental model is incorrect I'd appreciate an education.

    Thanks,

    philip
    All windings are used in both configurations. As a guess, I would use the double star configuration because it will be transferring more current to the manufactured leg, but that is just a WAG.

    Dahlander pole changing motor - Wikipedia

    Bill

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    My inclination would be to connect it in the parallel connection, with the idea of low impwdance.

    Impedance is the big issue with an idler, you want low impedance to keep the voltage drop on the generated leg to a minimum.

    it appears there would be a number of issues with the connection you propose.

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    Yes the windings have to be in series for low speed and you will have twice the copper losses for a given current, and 4 times the leakage inductance and the rotor is at half speed.

    My guess is on high speed your 10hp motor will be similar in impedance to a 7 hp rpc, and on low speed similar to a 3 hp idler.

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    Quote Originally Posted by 9100 View Post
    All windings are used in both configurations. As a guess, I would use the double star configuration because it will be transferring more current to the manufactured leg, but that is just a WAG.

    Dahlander pole changing motor - Wikipedia

    Bill
    Got it. My mental model was incorrect. Thank you!
    philip

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    First, the diagrams are fuzzy because I have to go through a convoluted series of forms to get them in a format PM will accept. The information is still there.

    A Dahlander motor should idle on one set of the star windings. The first diagram shows that with the second set in place but not connected. The second diagram shows them connected to the ends of the first set, which should produce something approaching 416 volts, somewhat less since the second windings will be generating about the same as the back EMF of the first set. Feeding that through a star/delta three phase transformer will give you 240 V three phase with each leg a composite of the original generated voltages, the idea being that mixing them together will give more uniform results and a wider performance range. The capacitor may be optional or only used for starting. I think it would be best if it was optimized for the normal load.


    dahlander-star-mod.jpgdahlander-phase-converter-comp.jpg

    Bill

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    Bill that would theoretically work, but you would have to open the motor and bring out all 12 leads for the 6 separate coils.

    I just don't think the motor will have any thermal or magnetic budget left to be of much use if half the windings are pulling power out of the motor.

    here is a typical winding diagram. notice how short pitched the coils are.
    inst2.jpg

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    Quote Originally Posted by johansen View Post
    Bill that would theoretically work, but you would have to open the motor and bring out all 12 leads for the 6 separate coils.

    I just don't think the motor will have any thermal or magnetic budget left to be of much use if half the windings are pulling power out of the motor.

    here is a typical winding diagram. notice how short pitched the coils are.
    inst2.jpg
    I considered that. Of course part of the power is coming straight from the line, but you would have to derate the motor. The leads should be resonebly accessible. I replaced the leads on a Dahlander motor with no trauma. The insulation had turned to goo.

    I tried something like this with transformers in a broken star configuration and ran into a completely unexpected problem. The capacitor formed tuned circuits and I got really odd voltages and phase angles. You would expect the load to lower the Q to where that wasn't an issue, but it was.

    Bill


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