On 8 Oct 2016 04:09:34 GMT, "DoN. Nichols"
wrote:

On 2016-10-08, Terry Coombs wrote:
I picked up a rock tumbler power unit today at a yard sale with the
thought of using it to spin a ball mill for other compounds . Problem is
that I think it might spin too fast , and I may need to slow it down . I can
use smaller rollers on the shafts (2 , parallel and both turned by a chain
drive) to slow it some , but might need to lower the input RPM's . Currently
has a 1/3 hp split phase motor turning at 1725 - is there any way to speed
control that motor short of changing the input frequency ?

Speed control of such a motor by frequency is not likely to work
over much of a range. Yes, a 60 Hz motor on 50 Hz runs slower -- and is
closer to saturation.

And the functionality of the motor depends on matching of the
run capacitor's value to the frequency. If you had a VFD locked to a
specific frequency, you could likely adjust the run capacitor for
reasonable performance -- but don't expect to just reach out and adjust a
pot to change the speed over a wide range.

Note that 1725 is not the slowest induction motor on a 60 Hz
line. The next speed down is about 1150 RPM, then about 875 RPM
depending on the number of poles. 2-pole is somewhere short of 3600
RPM, 4-pole is short of 1800 RPM (your 1725), 6-pole is short of 1200
RPM, and 8-pole is short of 900 RPM (see examples above -- loss of
speed is due to slip).

Other choices may
be a smaller motor sprocket , or maybe an inline (planetary?) gear reduction
unit or v belt and jackshaft arrangement . Could also change out to a
universal or DC motor and appropriate control system , or use a 3 speed fan
motor I have on hand .

3-speed fan motors depend on the load of the fan blades or
squirrel cage fighting the input current modified by the number of turns
in the selected winding. In your application, it would likely run close
to full speed on all three windings.


The multispeed blower motor on my old furnace switched the number of
poles and the speed difference with no load was VERY noticeable. The
torque was almost the same at either speed - resulting in a large
difference in horsepower as well.
A universal motor, on a dimmer would likely work fairly well for
your application. And at a slower speed, you likely would not need the
full 1/3 HP. Instead of the dimmer, a Variac or Powerstat (variable
autotransformer) would work better -- but they are significantly more
expensive.

But speed control of the current motor would be the
easiest and cheapest ...

Cheapest, yes -- but unfortunately, not easy.

Are you sure that it is currently too fast to do what you want?
Try it first, to see how it does. That could be the cheapest way to do
this.

Good luck,
DoN.