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How to check a motor for sync.



I have been modifying motors for sync-gap use these last days, and have
found a nice way to verify, wether it is syncėng, or not, and would like
to share it with all.

Since I had no success with the "strobe" effect of neon tubes, this is
how I did:
The motor preferably has a thread in the end of the axle, because then
you can just screw a piece of PCB board, or plywood, whatever, right at
the end of the axle. Else use the hub for the rotor.
Now drill 2 holes, about 1/2 inch from the perimeter, 180 degrees apart,
and mount an iron screw with nut in each hole.
These screws will act as moving shunts for a magnet you will fix to
something, so that when the disc spinns, the screws will pass close to
the magnet.
 
                 Hold pickup coil here  | 
                                       \/

                                     +-----+
+------------------------------------+-+-+-++
+-+---+------------------------------+-|-|-++ <- Disk
  |   |                              +-|-|-+
  |   |                                | |
  |   | <- Motor axle                  | | <- bolt and nut
  |   |                                +-+
  |   |                              +------+
  |   |                              |      |
  |   |                              |      |
  +---+                              |      | <- Magnet
                                     |      |
                                     |      |
                                     |      |
                                     +------+

Now hook one channel of the scope to the mains, and trigger on that
channel. The other channel is connected to a small pickup coil you make,
a cliplead spun into 4-5 turns is fine.
Now start the motor, and move the pickupcoil close to the spinning disc,
where the screws are flying by, and on the scope you will se a fine
little peak, for each passing screw, and if the 2 curves are aligned,
the motor is in sync.
  


   +--------------------------------+
   |                                |
   |                                |
   |------/\  -----------/\  -----  |
   |        \/             \/       |
   |        **                      |
   |      *    *                    |
   |    *        *                  |
   |   *          *                 |
   |  *            *                |
   |               *            *   |
   |               *            *   |
   |                *          *    |
   |                 *        *     |
   |                   *    *       |
   |                     **         |
   +--------------------------------+

It is nice to see, how much friction you can apply, and how little the
rotor actually is displaced in relation to the rotating field, doing
this.
I also think you need to do this only once. In the process, you will
develop a feel for the sympthoms of the motor "hunting", and know from
this behavior, that it does indeed sync.

I would imagine, that applying DC to the (closely spaced) electrodes of
a finished gap, would allow some similar ( capacitive ) pickup to
occour, for calibrating the finished gap, but I have not tried this yet.
Anyone tried this?

Hoping this is usefull, 

Cheers, Finn