Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work.

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John Hall
 
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Default 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 .

John


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Tim Wescott
 
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Default can this motor be slowed?

John Hall wrote:

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 .

John


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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/
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Leo Lichtman
 
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Default can this motor be slowed?


"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.


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Cydrome Leader
 
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Default can this motor be slowed?

Leo Lichtman wrote:

"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.


Explain this slip speed for us.
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Tim Wescott
 
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Default can this motor be slowed?

Cydrome Leader wrote:

Leo Lichtman wrote:

"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.



Explain this slip speed for us.


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_.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/


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Leo Lichtman
 
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Default can this motor be slowed?


"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.


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