Re: motor starting and reversing.

On Aug 25, 6:13 pm, David Powell <ddotpow...@xxxxxxxxxxxxx> wrote:
In article
<37938fb7-c68e-4173-9282-79acc4b98...@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,  
 houstonceng <and...@xxxxxxxxxxxxxxxxx>  in uk.rec.models.engineering
wrote:

Big SNIP

Sorry David.

It's the "English usage" in your explanation that threw me, viz :-

"They can be switched by spdt switch and separate main contactor.
Better to use dpdt with centre off position, then you can control the
contactor to give forward-off-reverse control."

The way it read was that the motor could be switched by the SPDT
switch (between forward and reverse) with only the need for a main
contactor.

You then went on to mention that a DPDT switch could control the
contactor to give Fwd-Stop-Rev, so I assumed that the contactor
referred to was the "main contactor", so I was interested to see how
you could do that with it. My mistake. Shouldn't have read it the
way it looked.

You then said :-
"Usually cheaper to do it this way than some relay switching to
reverse one winding."

Which implied that you used the DPDT switch to reverse one winding
since, if you were using two contactors (One Main, One Reversing),
that's exactly what the contactors would do - reverse one winding (ie
the same as using two relays). So it also seemed that you were using
a DPDT, switch to carry Start or Run winding current.

You reinforced the implication with :-
"Nothing special needed for the switch, I use toggle switches as the
contactor does all he power stuff."

See my confusion with your explanation ? "The Contactor" instead of
"Contactors" and the implication (incorrectly inferred) that a
contactor and DPDT switch were all that is needed.

Since you also said :-
"Same basic idea, consider the two starting coils to be a single,
centre tapped coil. Or feed a single start winding from one side of a
240/240-0-240 transformer. We can choose to reverse either the start
or the run windings. I'd choose the one with the lower current
demand every time. (Unless it's a DC motor.)"

I think you should re-read what I said about 220v, 3 wire, 1-phase
motors in USA on Jet and Griss lathes. 220v is supplied (via two
poles of a contactor) across the centre-tapped (110v - 0v - 110v) RUN
winding and the motor direction is dictated by connecting the free end
of the (110v) START winding to one end of the RUN winding - using the
3rd pole of the contactor (the other end is connected to the centre-
rap of the RUN winding. The reversing circuit uses a second 3-pole
contactor to connect the RUN winding to 220v as before and the free
end of the START winding to the other end of the RUN winding. To
ensure that the two contactors are not operated simultaneously, to
provide NVR and guard interlocking, additional relays are used to
provide the logic and hold functions.

In any case, suggesting that "ye olde forward/stop/reverse gear lever
is ok", isn't a solution to Austin's request for a "how-to" using his
Push-buttons, that Bob, Mark and I are trying to provide.

Andy
.