Re: motor starting and reversing.

On or around Sun, 24 Aug 2008 02:47:21 -0700 (PDT), BobKellock
<bob@xxxxxxxxxxxxxxxxx> enlightened us thusly:

On 22 Aug, 09:24, Austin Shackles
<austinDITCHTHISFORBETTERRESU...@xxxxxxxxxxxx> wrote:
On or around Thu, 21 Aug 2008 22:59:03 +0100, Austin Shackles
<austinDITCHTHISFORBETTERRESU...@xxxxxxxxxxxx> enlightened us thusly:
...............

A bit more thought and several more bits of paper and I reckon this circuit
does it.

http://www.roman-road.co.uk/temp/DSC00200.JPG
.............

That circuit will probably work but, in the interval between pressing
the FWD or REV button and the relevant relay making contact (about
10ms), current flows through both the other relay coils connected in
series. Typically, the specified minimum coil operating voltage of a
relay is 75% of its nominal voltage but some will work at a much lower
voltage so you could have a race condition.

During that same interval the button is carrying all the current to
the start winding of the motor. Typically start/stop buttons are rated
at 3A. If you happen to press the button near the peak of the AC cycle
the current through the button will probably be considerably more than
its rating which could result in shortened contact life or, even,
contact welding.

There's a new one now, 201.JPG instead of 200.JPG - that circuit, as was
pointed out, won't stop again and has the motor permanent live, which I
didn't really intend.

The buttons don't supply motor current though, start or run. they only feed
the relay coils. Well, I suppose they supply current while the relay is
switching. That's about 15ms if I recall the spec *** right. If it
causes issues, then it could have a current-limiting resistor in series with
the buttons, or something.
--
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
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