can this motor be slowed?

I have an electric motor driving the lube pump and table drive gear train on my B&S horizontal mill. It replaces the original, probably 3 phase, motor gone long before I got the mill. The motor is connected directly into the gear train and pump through a flex coupling. The table drive is disconnected w/ parts missing, but much of the gear train is in place (to drive the oil pump)and makes quite a racket when the motor is running. It appears that there is no easy way to bypass this gear train and still keep the oil pumping function that oils the tableways and leadscrew, but I'm thinking if I could slow things down I could still get adequate oil flow but less gear noise. The motor has no starting capacitor.Does that mean I can use something like a ceiling fan control to slow it down? Obviously I could just change to a different motor, but I don't really know what the best speed would be, but I'm thinking half speed might be OK. The motor data plate has the following information on it that probably means something to someone besides myself:

1/8 HP, 2.5 A, 1 phase, 110 v. 1725 RPM (all this I understand), Type SA, Frame 147, Form DB. BTW the Date is Dec. 22, 1925.

I used this motor because I had it on hand. Maybe the best way to go would be to use a different motor? Although that means making up a new mounting adapter plate .One of these surplus treadmill motors I see advertised? Other ideas?

Thanks in advance for any ideas .


Reply to
John Hall
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The motor is already a four pole (a two-pole would run close to 3600 RPM). At 1/8 HP it could very well be a shaded pole motor. I really don't know if you could find a 6-pole (to run close to 1200 RPM) or not, or if a ceiling fan control would do it (I suspect not -- induction machines really don't like running at speeds significantly different from their synchronous speed).

A universal-wound (sewing machine) motor with a speed control may work if the pump's resistance increases nicely with speed. A DC motor with a speed control _would_ work, but may be expensive without some inspired scrounging.

Reply to
Tim Wescott

"John Hall" wrote: (clip) 1/8 HP, 2.5 A, 1 phase, 110 v. 1725 RPM (clip) ^^^^^^^^^^^^^^^^^

1725 is the "slip speed" of an induction motor whose field is going 1800 RPM. If you try to slow the motor down by reducing the voltage it will slow down a little, maybe stall, overheat and probably burn out.
Reply to
Leo Lichtman

Explain this slip speed for us.

Reply to
Cydrome Leader

Basic motor theory*: One magnet wants to turn to align to another. Presto! A motor -- at least for slightly less than 90 degrees, anyway.

Slightly more advanced motor theory: A stator produces a spinning magnetic field which the magnet in the rotor locks to. AKA a "synchronous machine" (if it's big-ass and connected straight to 440V

3-phase) or "brushless DC motor" (if it's little and connected to some fancy electronics). Great machine, but how do you get it up to speed, and what happens when it falls out of sync (bad things).

Even more (slightly) advanced motor theory: Turn the thing inside out, have a stator with a fixed magnetic field, and switch the electricity to the rotor so that _it's_ magnetic field is always crossways to the stator's. If you do this with a commutator then you have a traditional DC motor (or generator).

More advanced motor theory: Go back to the stator with a spinning magnetic field. Stick a copper squirrel cage inside, attached to the rotor. The field rotating in relation to the squirrel cage induces current, which _just happens_, through the magic of physics, to generate a magnetic field at right angles to the stator. So a torque is induced, the rotor tries to follow the spinning magnetic field, and everyone is happy.

But if the rotor is stopped then the spinning magnetic field can't induced much current in the rotor because of the rotor's self inductance. And if the rotor is going as fast as the spinning magnetic field then it doesn't see a _changing_ magnetic field, which is necessary for inducing a current. The difference between the rotation rate of the spinning magnetic field and the rotor is called 'slip' (see, I didn't forget the question). There will be some magic value of slip that results in the most torque, and another one that results in the best efficiency. These numbers are usually a few 10s of RPM to a few hundreds.

  • I don't _care_ about those dang variable reluctance motors, go _away_.
Reply to
Tim Wescott

"Cydrome Leader" wrote: Explain this slip speed for us. ^^^^^^^^^^^^^^^^ I'll be happy to try. The motor has a set of stationary windings which produce a rotating magnetic field. Depending on the number of poles. this field will go around at 3600 or1800 RPM, usually*. The rotor is made of magnetic laminations and heavy copper windings. If the rotor were to turn at synchronous speed (3600 or 1800)* there would be no "slip." So no current would be induced in the rotor windings, and there would be no torque. So the rotor would slow down. This would cause the rotating magnetic field to cut the rotor windings, inducing current in the windings. This induced current creates a magnetic field in the rotor which makes it develop torque. The more torque the motor is asked to deliver, the more it slows down, increasing the "slip," and developing the required torque. _____________________

  • In the US and other countries that use 60 cycles. In Britain, the syncronous speeds are 3000 and 1500.

"Slip" is the difference in RPM between the rotating magnetic field and the rotor.

Reply to
Leo Lichtman

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