"Ian Iveson" wrote in message
...
Roger Thorpe wrote:

William Sommerwerck wrote:
Many turntables have a line resistor. It "does something" (that I don't
understand) to improve speed stability. Or torque. Or something. It's
not to
reduce the line voltage.


It's a while since I looked, and I could be wrong but:
The Rega planar motor is a pair of multi pole two phase synchronous
pancake motors sandwiched together with the poles interleaved.
The resistor and capacitor are there to change the phase (lag or lead?)
of one motor so that the assemlby turns in the right direction when it is
started.
Incidentally mine is a Rega 2 and the resistor burnt out too. A day after
a short friendly 'phone conversation with the company the postman handed
me an envelope containing a new resistor (with a higher power rating).
--

I read it that the resistor is in series with the whole motor. How could
that introduce a relative phase shift between the two halves?

I looked at the site someone linked to, at the circuit for a simple
synchronous motor. When a resistor is used to drop the supply voltage, the
value of the recommended cap remains the same. I didn't mention phase,
therefore, not because I know it's not significant, but that I have no
evidence to suggest that a resistor in series makes any difference.

Motors are something of a black art, AFAIK. Two motors in parallel sharing
a cap and resistor is about as black as art can get. How can a symmetrical
circuit favour one direction? In what way is the circuit not symmetrical?
Are you sure that direction isn't determined by geometry?

Actually, this isn't a good place to explain such a thing. If you have a
name for the type of motor arrangement, maybe I could look it up?

Ian



Ian. See my later post regarding the correct value, and how it all performed
when fitted. I think that I'm down the same road as Jim was when he did his
mate's one, and judging by what someone else said about Rega sending them a
shiny new resistor of increased power rating, that would seem to be the
right road, if their answer is also upping the rating from what they
originally designed in. Also, its primary function does seem to be that of a
simple dropper to get the 240v mains supply down to something closer to the
110v rating of the motor. My replacement R resulted in 240v in, 98v out
across the motor. I would say that the 0.22uF cap is there to introduce
phase shift between the two stacked windings, to ensure startup, and as
someone else suggested, correct rotational direction. I'm not sure that
stalling is a big current-draw issue for a synchronous motor like it would
be for AC or DC brushgear types. Anyone actually know ?

Arfa