Re: Starting a Teletype sync motor question

Hi John,

> Original Poster: FutureT-at-aol-dot-com
> I'm using a 50Hz, 3000rpm 1/12th HP Teletype sync motor on 60Hz,
> for 3600rpm in a rotary sync gap.  I tried connecting a 7uF to 30uF,
> cap across the blue and red wire for starting, but it won't start this
> way.  Yellow and blue are connected to the line.  I then touched the
> red and blue wire together with no cap for starting, this works
> better, but the motor still often needs a push to get going, and it
> makes clanking noises as the speed ramps up.  Once it reaches
> max speed, I disconnect the red wire, then it locks up fine. I've
> been using this rather awkward method for starting it.
> I realize that Teletype motors usually use an external relay block
> for starting, but different motors seem to be wired a little
> differently?
> I wonder if the 60Hz on a 50Hz motor makes the starting a little
> more difficult?
> Does anyone have any suggestions or knowledge of the correct
> way to start this type of motor?  Or maybe the motor has an internal
> defect?

Maybe I can be of help, here. I have seen several different sync motors
that will run on 50 and 60Hz. Usually, the only difference is the start
capacitor. However, the difference is minor. For example, for a 7.5Watt
(I know, awesome power ;o} rating)  sync motor the startup cap was
2ĩF vs. 3ĩF. So, I donīt think this your real problem, as you have tried
a variety of cap values. Also, my machine shop has several different
dual voltage/ frequency induction motors, as we brought all the
machines with us on our move to germany. All motors spin up readily
(under load) and I have noticed no difference in 50/60Hz operation.

However, your two hints gave me a clue as to what might be wrong.
The snaaaap-claaack sound and the need to push-start it. My (now)
sync motor had the exact same problem. The snap-clack, you hear,
is an internal centrifugal switch and the need to push start the motor
seems to suggest that the starter winding isnīt doing anything.

My syncīd motor doesnīt use a cap at all. Both run and start windings
are simply connected in parallel (the start winding has the NC(!)
centrifugal start switch in series). Once run speed is achieved, the
switch simply cuts out the start winding. Iīll be willing to bet your
teletype motor pulls lots of current (10-15A?) before it starts turning.
The current goes down as the speed goes up, normalizing as soon
as run speed is reached, right? The winding probably gets pretty
warm, too, if you donīt push-start it right away.

Check the current your motor is pulling (at start up) and then tear the
motor down. Have a look at the start-up contacts. More than likely,
these will be badly pitted and / or out of adjustment. I needed to make
a new set of contacts for mine. I spot welded a couple of big relay
contacts to two pieces of beryillium-copper (very springy material).
No, I didnīt build the spot welder, I just borrowed it ;o). The contacts
must be closed at startup and open at specified speed. The
adjustment is a little finicky (meaning you might have to tear down the
motor more than once). If the motor hasnīt run hot (in itīs original
service), I doubt that the windings have gone bad and aside from the
switch and the windings, there is nothing inside, that can die
(electrically, I mean). If a visual inspection shows nothing, measure
the startup winding directly (before it goes into the switch). If itīs
open (which I doubt), itīs probably cheaper to find another motor.

While your are at it, clean all parts with gasoline (I know) and soapy
water. Blow it out with compressed air and let it air dry a few hours.
(An old electric baking oven works wonders for speed drying and
hardening (i.e: painted objects) most anything). Fresh oil and a bit of
grease (in the right places) finishes it up. Your motor will love you
for it.

Good luck.

Coiler greets from germany,