Re: Seek Replacement Small Variable Speed AC Motor

Bob Pinsker wrote:

> I have increased the resistance across the diode, just as Tom Lear
> reported, to slow down the motor, but evidently the torque that it
> can supply decreases substantially at way below synchronous speed,
> I guess, because the roll got stuck, the motor started to smoke, and
> before I could shut it off (and before the circuit breaker opened)
> the motor blew up!

Bob, what was the original resistance and to what did you increase it?

On the Dynavoice control the original pot was 200 ohms, I added an
additional 100 ohms to create a more realistic range (100-300), for
a total of 300 ohms.

It is also designed as you described it: "I guess one of the advantages
of this type of motor is that when the coil is not powered, a spring
pushes the rotor out of the laminations, thus neatly disconnecting the
rotor from the gear train and freeing the roll take up from the motor
for rewinding."

It sounds identical as to control and function to the Dynavoice motor,
except that your unit could be stalled.  The roll must have really been
completely stuck fast, as the torque doesn't seem to be affected at all
by the slower speed on my motor.

I added in an additional 2500 ohm pot in the speed control circuit with
which I can effectively stop the roll, although I see the rotor of the
motor is still ticking along about 4 revolutions per sec.

Out of curiosity, I tried grabbing the take-up roll to see if I could
completely stall the motor, and maybe it's because of the gearing, but
I couldn't completely stop the motor without exerting a really huge
amount of pressure upon the roll.  My conclusion is that the torque
does not drop off with less speed.  This little unit has an amazing
amount of torque at all speeds.  I guess it's designed that way to be
able to stabilize the velocity of the paper being pulled over the
tracker bar, in order to counter vacuum and alignment changes that tend
to slow the roll.

However, the motor does generate a _lot_ of heat at stall speeds, so I
would expect it to burn up at super slow speeds, and use them only very
briefly.  It shouldn't fail at regular playing speeds though.

If the roll became stuck for a couple of minutes, I think it would
probably burn out the motor alright.  That's pretty amazing to me that
you could get it to stall.  Because the gearing seems to give it so
much torque, I thought it might actually tear a roll up before it would
stall.

> I took it apart this afternoon, and found that one of the connections
> to the coil is just plain gone -- the coil reads an open circuit --
> and when I unwrapped the tape, which was all burned and smelly, one
> of the two connections between the leads and the coil has disappeared.

It sounds like it may be the lead going to the inner part of the
winding must have been the electrically weaker of the two.  Unfortunate.

Because the whole assembly -- the gear box, etc. -- may be impossible to
duplicate, I would go ahead and rewind the coil.  If the whole coil
isn't completely fried, maybe you will be lucky and discover that it's
the end of the wire on the outside of the coil that's burned off, when
you try to unwind it.

Or, after you partially unwind the coil, you may be able to access the
burned off lead going to the inside of the coil and be able to solder
a lead onto it.

However you get this thing working again, I guess it would be a good
idea to place a small fuse in the circuit, sized to avert any future
disasters from accidental stalling of the motor.

Please let me know if you have any questions about this.  I have some
other ideas and options if you get stuck.

Good Luck!

Tom Lear
San Francisco, Ca.