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Slow AC motor

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  • Slow AC motor

    Hi All,
    I,ve been thinking of upgrading my lathe to 3ph via an inverter and have been purusing fleabay and have seen several new motors which seemed to be very cheap and have no bids. Looking at the motor spec thay all are rated at 920 rpm which I think is a bit low - I thought most motors were either 1400 or 2800 depending on the number of poles - so what would a 920 rpm motor normally be used for !!

    I have tools I don't know how to use!!

  • #2
    In the good ol' USA we think in terms of 60hz with 1200, 1800 and 3600 being the common rpms. After the slip factor these values come down a bit.

    The lower speed motors like your 920rpm sounds like our 1200rpm run at 50hz. They're nice for applications where you need the lower rpm. The downside is they tend to be fairly large in relation to hp.

    I have a 1hp motor out of a Hardinge lathe with two speeds, 1750 and 525 rpms. At low speed it's only 1/2 hp. That motor is so big and heavy it takes two men and a boy to pickup.


    • #3
      Peter a 920 rev motor is what is know as a 6 pole.
      They are usually used where the output of the machine or whatever is required to be slow, it takes less gearing to get 920 down to say 250 than a 1425 4 pole or a 2,800 rev 2 pole.

      Shapers used to use 6 pole motors a lot.
      You can also get 8 pole motors rated at 720 revs. Problem with these motors with more than 2 or 4 poles is they tend to get larger as they have more iron and copper in them.

      Herberts and some others used to make drilling machines with multipole motors and a back gear.
      The back gear gave you two speeds and the multi pole motor could switch between 8, 6, 4 and 2 poles to give you 4 speeds you could change on the fly, together with the gearbox this then gave you 8 speeds and was low cost to make, no pullies or expensive shift gearboxes.


      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.