Jinx wrote:
>
> > Have you tried running it from mains?
>
> Oh yeah, it works OK
>
> > I suppose the thing I *should* have asked is what it came out of!
>
> Bought cheaply as a new new part, place in town was having a clear
> out. But I have quite a few old appliances (taken from the side of the
> road during inorganic collections) that have these motors in such as
> fans and microwaves. If I can sort out a controller for them they'd be
> ideal as high-torque robot motors for eg lifting arms, particularly those
> with gearboxes. Which is why I wanted to know if they're bi-directional.
> I've tried making this one go backwards but it won't. Modifying the gear
> box doesn't appeal (not impossible) and it would be much simpler if
> it could be done electrically at the motor. Reversible synchronous
> motors are available but I've yet to see one that has an associated
> speed control

From the picture you showed me it looks exactly like a microwave
turntable motor. These are a cheaply made ac motor with a two-stage
gearbox with lots of slop. They can chew gears quite commonly, as
the gears are a cheap alloy metal. Most modern motors like this I see
start randomly in either direction. Some of the older microwaves
had larger motors that always start clockwise. If it is that type I
have no idea how to reverse it!



> What do you reckon about this attempt at a circuit ? The +ve side looks
> OK but I'm not sure about the -ve side, can't find a PNP opto. Because
> the o/p transistors have quite low hfe there needs to be both voltage
> gain and current gain. It's basically half of an H-bridge (for a non-
> reversible motor). I'll have to be very careful that the PIC o/ps are
> initialised properly before any mains is present, perhaps hold it off
> with a relay
>
>     ---------------------------------------------------------------

Your circuit scares me immensely!
1. you are feeding high voltage AC to the motor, is it wise
to have a diode across the motor winding?? ;o)
2. your opto secondaries are switching 320v constantly??
try putting them at the rail potential, so they turn the
transistor OFF B-E when the opto conducts. When the opto
secondary is O/C it will only have 0.7 volts max across
it (B-E of the transistor). Much safer.
3. Change the first transistor from NPN to PNP and put its
emitter at 320v. Same swap at the bottom rail too.
That way its collector is doing the big voltage swing and
the B-E is stable with the opto at the rail.
4. Likewise with the two main power transistors, keep the
top one PNP but put its emitter at the 320v rail.
5. Put one end of the motor to ground and the other end
goes to the two collectors of the output pair arranged
as half a h-bridge. (a "T bridge"???)
6. Only need one inductor, in series with the motor,
on either side!
7. Commercial circuits would use a snubber across the motor,
a highpower resistor and 630v poly cap about 0.1uF or bigger.
If the resistor is too low it will get very hot, if it is
too high resistance you will get a larger switching spike.
(basic snubber rule)

Hope some of the proper engineers can help out here, there is
always a lot of personal preference too in circuit design.
:o)
-Roman

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