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Thread: OT: Starting a 350HP 2400 volt synchronous motor with rotating 'stator'

  1. #11
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    A while ago I used to maintain some 100+hp old synchronous motors driving 1 'lung' compressors, they stood about 6ft+ tall.
    The motors were ran up as induction motors, they also had a rotor winding with a pair of slip rings, the slip rings served double purpose by detecting the slip frequency, when it was up to around 5-7 cycles slip, then DC was injected into the slip rings and the motor came up to 60Hz sync.
    They sound similar to those of RWO!
    Max,
    Last edited by MaxHeadRoom; 05-15-2019 at 03:29 PM.

  2. #12
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    About 40 years ago when we were in South Africa, my father an I built a 75 KVA 3 phase 50Hz power stabilizer by coupling 2 brushless alternator front to front on a single shaft. One of the alternators worked as a synchronous motor and the other as a alternator. To start, we made a centrifugal switch that shorted the 4 pole rotor windings up to around 1300 RPM. After this speed, the short was removed and the alternator's AVR regulated both motor and alternator voltage. Before modern electronics this was a good method to power a instruments calibration shop.
    There was very little "hesitation" of the rotor at startup but there was no load untill the regulator was connected.
    Helder Ferreira
    Setúbal, Portugal

  3. #13
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    That is an interesting motor. I wonder if it was originally made for a "Rossman" drive, which is an old variable speed drive where the outer portion of the motor, the part that has the brake in the video, is driven with a DC motor that can be varied in speed by series armature and field resistors.

    With that old drive, the total power is considerably less than using a variable speed DC motor would have used. The output rpm is the difference of the synchronous rpm and the rpm of the outer portion.

    These days, it would probably be done with a VFD, but back then there was no such option. The drives were made up to 2500 HP or more.
    1601

    Keep eye on ball.
    Hashim Khan

  4. #14
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    Several youtube posters have identified the motor under discussion as a "supersynchronous" motor; here is an in-depth, satisfying discussion of same:

    https://forums.mikeholt.com/showthre...61#post1722161


    Full credit to poster Phil Corso of the Mike Holt website for the linked information.


    Here's a very short article in the publication "The Blast Furnace and Steel Plant", April 1923, when the supersynchronous motor was still considered a "recent" development:

    https://books.google.com/books?id=Ua...page&q&f=false
    Last edited by tlfamm; 05-16-2019 at 10:12 PM.

  5. #15
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    Very interesting. Technically, nothing much new, but an interesting application technique that gets around many of the normal issues, by moving the location of the clutch, so it is not between the load and the motor.

    Actually does use the concept of the rossman drive, but uses the variable speed only for starting. Puts a heck of load on that clutch, but so would the normal arrangement of a clutch between motor and load.
    Last edited by J Tiers; 05-16-2019 at 10:48 PM.
    1601

    Keep eye on ball.
    Hashim Khan

  6. #16
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    Quote Originally Posted by J Tiers View Post
    Very interesting. Technically, nothing much new, but an interesting application technique that gets around many of the normal issues, by moving the location of the clutch, so it is not between the load and the motor.

    ...
    If it is true, as I read, that synchronous motors have inherently lower speed than equivalent (and more common, at the time?*) induction motors, substituting the former for the latter in an industrial setting potentially removes any intervening mechanical speed reduction, bringing motor and load that much closer. However, the complexity, cost, and maintenance of the speed reducer is more or less transferred to the startup-related mechanisms of the supersynchronous motor.


    * more common: meaning commonly used in start-under-load applications
    Last edited by tlfamm; 05-17-2019 at 09:55 AM.

  7. #17
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    They do, since they can have a lot of poles. IIRC, a 50 Hz synch motor can have a normal speed of as low as about 75 rpm. Must have torque out the wazoo, since the diameter for a motor of significant power is large.
    1601

    Keep eye on ball.
    Hashim Khan

  8. #18
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    ...I just kept waiting for smoke to start coming off the brake band...

    that was cool.

    t
    rusting in Seattle

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