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Thread: A Few Notes On Two Speed Induction Motors

  1. #1
    Join Date
    Sep 2017
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    Poulsbo, WA
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    Default A Few Notes On Two Speed Induction Motors

    The following is based on my experience implementing a VFD on a 220 VAC, 3 Phase, 2 Speed, Induction motor on my lathe.

    I recently upgraded to a lathe with a 5 HP, 3 phase, two speed motor (3390 & 1710 RPM).
    The lathe has a two speed motor and a six speed gearbox that gives me 12 spindle speeds.
    I bought a VFD so I could run from single phase 220 VAC.
    The issue I had was that I could only wire the motor for one set of speed windings at a time.
    I did not want to vary the frequency from 60 Hz to 30 Hz to achieve the two speeds as I would suffer a significant torque loss.
    Also that would require that I “fiddle” with the VFD (mounted on the wall) when I wanted change speeds.

    After a little research, I discovered how the motor was made to run at two speeds.
    The motor is a “Pole Changing Motor” or a “Dahlander Motor”.
    Check out https://en.wikipedia.org/wiki/Dahlan...changing_motor

    The speed on an induction motor is a function of Frequency and Number of Poles per phase.
    Dahlander devised a method of re-configuring motor windings from 2 poles per phase to 4 poles per phase.
    The lathe had two start buttons – one for Low Speed and one for High Speed.
    The buttons were wired to be mutually exclusive so only one speed could be selected at a time.

    I ended up retaining the pole switching contactors and simply made their power soured the 60 Hz output from the VFD.
    Now I have the best of all worlds for me.
    The motor ruins at two speeds at full torque and the lathe operates as normal.

    Because the VFD is feature rich, I added a remote selector switch to vary the speed +/- 15%.
    I don’t how useful or useless that will be.

    The photo below shows the Dahlander pole changing configurations for 2 and 4 poles per phase.



    where
    ns = Synchronous speed, in revolutions per minute
    f = AC power frequency
    p = Number of poles per phase winding


    Respectfully Submitted,
    Be_Zero_Be
    Poulsbo, WA

  2. #2
    Join Date
    Nov 2007
    Location
    Woodinville, WA
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    Not sure of the gain over variable speed and just connected in the high speed configuration. You may have misunderstood how the Dalhlander motor works. Although some dual speed motors are constant hp, most Dahlander are constant torque. Which do you have? What does the name plate say?

    In most Dalhlander implementations torque is constant for either high or low connections, and hp varies at 2:1 (roughly). If you just use the high speed configuration you get full hp and same torque at base frequency (say 60hz); at 30 hz you have 1/2 of the "high speed" hp and same torque which is likely close to what you would have had if you switched.
    Last edited by lakeside53; 06-10-2018 at 10:05 PM.

  3. #3
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    lakeside53,

    You are correct.
    I reviewed the motor data plate and it states clearly that the HP is 5HP / 3HP and the rated current is 13.2A / 10.2A.
    Thanks for helping me better understand my motor.

    Be_Zero_Be

  4. #4
    Join Date
    Mar 2010
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    If you don't have a local control box( start, stop, forward, reverse, speed pot) for your VFD mounted on the lathe headstock you are missing out on the best features of VFD control. Do not worry about loss of torque or motor heating at low freq. You should set your VFD to go from 10 Hz to 120 Hz. I would use the 4-pole motor set-up. Pick a head stock gear setting that gives your desired max spindle RPM at 120 Hz( 3420 RPM at the motor). From that point, all speed variation is best done with the speed pot. If you do run out of torque at some point( unlikely), it's easy to select a lower headstock gear and then set speed by increasing motor freq.

    Once you learn how to use your new machine, you will better appreciate VFD control and the great flexibility it provides.

    RWO

  5. #5
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    Sep 2017
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    Poulsbo, WA
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    RWO,

    Thanks for the advice.
    I agonized over the choice to "run as original" or "modernize".
    This lathe is a considerable upgrade from my 10" Atlas.
    I have much to learn.
    As I begin to do more with this machine, it is very likely I will modernize as you suggested.

    Thanks and Best regards to all who responded.

    Be_Zero_Be

  6. #6
    Join Date
    Feb 2011
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    657

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    Modernize,for sure. I converted my 1340 lathe to 5 hp 3 phase motor with a delta vfd about 10 years ago. I finally got around to installing a brake resister this year. I leave the gear speed selecter in the 755 speed. This gives me 70 to 2000 spindle rpm. You Can set parameters to read actual spindle rpm on the readout. If I put the gear in lowest speed ,I can almost fall asleep when tapping a blind hole. The instant braking is wonderfull when doing 50 or 100 parts in and out of collet quickly. Edwin Dirnbeck

  7. #7
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    Quote Originally Posted by Be_Zero_Be View Post
    RWO,

    Thanks for the advice.
    I agonized over the choice to "run as original" or "modernize".
    This lathe is a considerable upgrade from my 10" Atlas.
    I have much to learn.
    As I begin to do more with this machine, it is very likely I will modernize as you suggested.

    Thanks and Best regards to all who responded.

    Be_Zero_Be
    You don't need to change the motor though.... just connect in 2 pole (hi-speed mode) and vary 15 to say 90hz (with care at the upper end if using your upper speed gear/belt). You will be fine. You are starting with 5hp. Even at 15hz you will still have 1.25hp at the motor. Good enough for 1 cubic inch of steel removed per minute. As above, gear down if you need to.

  8. #8
    Join Date
    Oct 2013
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    Setubal, Portugal
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    Due to the nature of a Dahlander Motor winding, it's best to connect the motor to the VFD in the low speed connection. The pole distribution for the high speed is a compromise so that the motor can function in both modes.
    Helder Ferreira
    Setúbal, Portugal

  9. #9
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    But.. then you limit the hp to roughly 1/2 of the max. High speed connection is usually the best choice.

  10. #10
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    Oct 2013
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    Setubal, Portugal
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    Quote Originally Posted by lakeside53 View Post
    But.. then you limit the hp to roughly 1/2 of the max. High speed connection is usually the best choice.
    In his case, low speed 5hp high speed 3hp.
    Helder Ferreira
    Setúbal, Portugal

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