Jinx wrote:
>
> > 1. always use 1N4007 for mains rects!
>
> Okey doke
>
> > 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?
>
> It's a Brevel 40W (not 60W, possibly a BT40, the ink is smudged),
> with "Model 715" on the gearbox, and the motor windings have a
> label with "715-982568" printed on it. The gearbox IIRC is >100:1.
> That would be a shaded pole motor ? Just wondered whether the
> direction is set in the windings or some how you can give it a kick
> to make it go a certain way. My original wood lathe motor (1/6th HP
> washing machine motor) could go backwards if you had the wontons
> to flick it that way when you put the juice on, but the last thing you need
> is the lathe going backwards and all the wood flying back on to the job
>
> I haven't checked prices out recently but large AC motors were
> always much cheaper (and commoner, eg fans, washing machines,
> driers, turntables etc) than large DC motors, and I doubt if that has
> changed. The only drawback is that anything you put them into isn't
> very mobile. In the past though I've picked up second-hand cordless
> drills and for $20 these are good value for a pretty gutsy DC motor
> and gearbox. Universal motors from mains tools like drills and
> circular saws are easy to control but, oh, the racket !! Would they
> be any quieter on DC or is it not worth the bother ?

Not sure from the model numbers. Going from wattage and 100:1 gearbox
it sounds similar to some of the motors I see in older microwaves
as turntable motors. These will start equally in either direction if
it is so. Have you tried running it from mains?

I suppose the thing I *should* have asked is what it came out of!


> Yes, I realised that today when I found a second big cap in the box.
> There were also a couple of 0R47 5W resistors too but I don't recall
> why. No provision for negative rectification on the board so have to
> assume I was not going to get it right the next time either ;-(

Yep, 240vac goes from 320v peak + to 320v peak - each cycle. Gives you
a nasty boot too if you are unlucky enough to get across it! :o)

> > 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.

>
> Each diode ? How about protecting the semis (FETs I presume). Poly
> switches ? Varistors ? Zeners ? The board so far had a diode and
> varistor across the FET and it lives on. I'll have to get into the archives
> and refresh myself on FET protection, they don't last long with a spike
> in the right/wrong place

I was referring to two diodes in a dual half-wave rectifier, giving
+/- 320vdc. The resistors are a TV thing, no matter how cheap and nasty
the TVs are lately they still have the bridge rectifier protected by one
of these. I can only assume it really helps protect the recitfier
semis from spikes on the mains and reduce startup current to charge
the main filter cap.

As for protecting the FETs, you are on your own. Again as a TV guy I
generally will pick something I have lying around, usually a TV horiz
output transistor good for 1500v, 8amps and has internal diodes and
resistor protection. Suitable for switching 320vdc into an inductive
load. :o) I'm not a big fan of FETs, and neither are the manufacturers.
Many of the FET driven switchers I work on are very unreliable,
and apart from the small fet craze a few years back the manufacturers
seem to be sticking with bipolar NPNs for most high-power high-voltage
stuff that I see. FETs are good on paper. In the real world it seems
you can usually get a bipolar that is cheaper and more reliable in
the same package.

>
> > 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-
>
> The 12VDC - 250VAC inverter I have uses a two-step wave (ie 0V
> 50%+ 100%+ 50%+ 0V 50%- 100%- 50%- 0V) to drive the Tx and
> I notice in the text it says that some motors and appliances don't
> care for this too much, even though it's closer to a sine wave than a
> square wave is

Yeah I have seen a lot of these too, and 4-step ones.

Back to your project, really would help to know what the motor came
out of, or physical description, and what you intend to drive
with it. You need to know the result to plan the plan. :o)
-Roman

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