Jinx wrote:
>
> This kinda relates to the mains frequency topic today -
>
> During a clean-up this week I came across an unfinished motor
> project. It was intended to be a speed control for a 250VAC
> 60W synchronous motor for a display turntable. I made a start on
> it a few years ago but the customer never got back to me. Originally
> a Mot 705K1 was the source of the drive frequency and I'd like to
> update it for a PIC. That part is easy enough but where I now recall
> getting stuck was the output for the motor. I got as far as rectifying
> the mains via 1N4004s into a 400V 470uF cap and then chopping
> that with a 2SJ1075 FET. I now realise that the motor was getting
> only about half the voltage it would see on mains and/or the motor
> didn't like the square wave as it hardly moved
>
> 1) does a synchronous motor need a good sine wave to run properly
> 2) can this be done "simply" with semis or would a low voltage high
> current sine amp feeding a step-up transformer be better
> 3) will a synchronous motor always rotate the same way
>
> Cost isn't too relevant, just looking for suggestions and opinions


Hi Jinx,

1. Dont use 1N4004, not enough PIV, always use 1N4007 for mains rects!
2. Some sync motors always run the same direction like in record
player turntables, others as in microwave turntables randomly pick a
direction when they start. exactly what type of motor is it?

3. I have seen a similar circuit that worked, instead of a full wave
rect giving 320vdc, keep the neutral as "earth" and use two 1N4007
diodes and two filter caps to give +320vdc and -320vdc. Half wave
rectifiers but you don't need a lot of current from the sounds of it.
Also use 2.2ohm 5w resistors in series with your diodes to make the
whole thing last longer with diode switching currents and mains spikes.

Once you have +/- supplies, you can use 2 switching semis, to switch
alternating + and - dc to the motor, like this:

25% duty off
25% duty on+
25% duty off
25% duty on-

If I could use ascii art I would put a pic but I think that makes sense.
I have repaired a couple of really cheap 12v to mains inverters that
used
this system, if the motor has enough inductance this will work as good
as from the mains. If the motor has very little inductance, you may
need to add a simple inductor in series with the motor, (and maybe a
resistor of a few ohms), I have used the low-voltage winding of a small
mains transformer for similar things, they make a pretty heavy duty
inductor and are cheap/free if you have a good parts bin.

If I knew exactly which motor you have I could suggest more?
-Roman

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads