On Aug 25, 6:13 pm, David Powell  wrote:
> In article
> ,  
>  houstonceng   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

On or around Sat, 6 Sep 2008 03:49:14 -0700 (PDT), John S
 enlightened us thusly:

>On 6 Sep, 10:22, Austin Shackles <austinDITCHTHISFORBETTERRESU...@ddol-
>las.net> wrote:
>>
>> OK, not likely, but we are trying to deal with sod's law as well, to an
>> extent, and sod's law dictates that if someone's daft enough to press both
>> fwd and rev at the same time, the contacts will make sufficiently close
>> together as to cause a problem if the possibility exists.
>> --
>> Austin Shackles.  
>
>So cross wire the fwd / rev buttons thru the aux contacts on the
>opposite relay.
>Whilst fwd is running there is no feed to the rev button.
>Then put a big  stop button between the two so you need two hands.

that (the cross-wiring) is what the circuit we ended up with does.

>The advantages of this on single phase is that you can switch from fwd
>to rev whilst running ready to hit stop then start like at the end of
>a thread.

Not sure the motor would like that, more to the point, not all the machines
will stand that sort of instant reversing.  Some will, some won't.  In
particular, any lathe with a screwed-on chuck will undo it's chuck.  Which
could of course be handy :-)
-- 
Austin Shackles.  www.ddol-las.net  my opinions are just that
Travel The Galaxy!  Meet Fascinating Life Forms...
------------------------------------------------\   
   >>  http://www.schlockmercenary.com/  <<      \  ...and Kill them.
a webcartoon by Howard Tayler; I like it, maybe you will too!

On or around Sat, 6 Sep 2008 11:40:58 -0700 (PDT), John S
 enlightened us thusly:

>I was referring to the typical single phase motor with centrifugal
>start as fitted to the Myford.
>Until the motor stops and the switch is closed the changes can't take
>effect.

when does the switch close though?  It stays in circuit during start up
until the motor is up to speed, but does it actually come to a halt before
the switch re-makes?

and do ALL such motors do so?

The one that the circuit is intended for is a 2HP capacitor start/capacitor
run with 2 caps on it.  Without studying it in detail, I'd prefer to play
safe.

-- 
Austin Shackles.  www.ddol-las.net  my opinions are just that
Travel The Galaxy!  Meet Fascinating Life Forms...
------------------------------------------------\   
   >>  http://www.schlockmercenary.com/  <<      \  ...and Kill them.
a webcartoon by Howard Tayler; I like it, maybe you will too!

Austin Shackles wrote:

> On or around Sat, 6 Sep 2008 11:40:58 -0700 (PDT), John S
>  enlightened us thusly:
> 
> >I was referring to the typical single phase motor with centrifugal
> >start as fitted to the Myford.
> >Until the motor stops and the switch is closed the changes can't take
> >effect.
> 
> when does the switch close though?  It stays in circuit during start up
> until the motor is up to speed, but does it actually come to a halt before
> the switch re-makes?
> 
> and do ALL such motors do so?
> 

But the centrifugal switch doesn't close until the motor slows down -
and it doesn't do that until the power is off.  The start/reverse switch
changes the direction of the starting coil - the main switch still
controls the power - so it'll be fine unless you try and start it in the
other direction before it's stopped.

My previous lathe was wired like this.  It's a good system and is much
cheaper than Dewhurst type reversing switches.

Russell

On 7 Sep, 18:46, Russell  wrote:
>
> But the centrifugal switch doesn't close until the motor slows down -
> and it doesn't do that until the power is off.  The start/reverse switch
> changes the direction of the starting coil - the main switch still
> controls the power - so it'll be fine unless you try and start it in the
> other direction before it's stopped.
>
> My previous lathe was wired like this.  It's a good system and is much
> cheaper than Dewhurst type reversing switches.
>
> Russell

If you try and start it in the other direction before it's stopped
nothing will happen as it has forward momentum and will carry on, like
a bad motor you rope start.
It needs the centrifugal switch to be closed before it can start in
reverse and this happens at low speed anyway so if you are that quick
with the start button as soon as you hear it click then it will
instantly reverse but only from a few revs.

This won't harm the motor as there are many standard 3 phase motors in
industry that are designed to plug reverse and do so all day long.

John S.

In article ,  
 pentagrid@yahoo.com  in uk.rec.models.engineering wrote:

>On Sat, 06 Sep 2008 10:22:56 +0100, Austin Shackles
> wrote:
>
>>On or around Fri, 05 Sep 2008 19:39:12 +0100, pentagrid@yahoo.com
>>enlightened us thusly:
>>
>>>
>>>       When forward or reverse is selected the other button is
>>>disabled.  If both are accidentally pressed "simultaneously"
>>>whichever button actually makes contact first takes over and
>>>disables the other.
>>
>>provided, of course, that the relay action times allow this.  Relays are
>>quick but not instant, ditto contactors.
>>
>>OK, not likely, but we are trying to deal with sod's law as well, to an
>>extent, and sod's law dictates that if someone's daft enough to press both
>>fwd and rev at the same time, the contacts will make sufficiently close
>>together as to cause a problem if the possibility exists.
>
>
>
>  Fair comment but sod has to be particularly vindictive to to
>cause a problem. 
>
>        This is of course a setup that I have used for many years
>without problem but,since I'm not addicted to simultaneously
>pressing Forward and Reverse buttons, this is not useful
>evidence.
>
>          There's no doubt that the Break before Make sequencing
>logic works fine if there's a significant time difference in the
>"simultaneous" pressing of the two buttons,  What is less clear,
>as you have pointed out, is the behaviour if the relay closures
>are truly simultaneous. 
>

Use decent industrial p/bs, the ones that you would find in clunky old
relay control stuff.  Usually 1n/o, 1n/c contacts which can be
assembled as break before make or v.v.  Interlock the p/bs and it
doesn't matter what the relays do. 

>           To test this I disconnected the motor load (to avoid
>overheating from frequent stop starts) and proceeded to
>repeatedly press first Stop, then both Forward and Reverse
>buttons at the same time. Behaviour was as expected with correct
>closure by either of the two contactors in apparently random
>order. All went well for 50 successive attempts but at the 57th
>try the supply fuse blew!
>

That's all that should happen!  However, in a previous existance I
used to specify multi-motor control boards for power station stuff.
The contactors for a reversing drive were allways "electrically and
mechanically interlocked".  Before the elf and safety fools buggered
things up you could usually get 3ph reversing starters for a beer
token from your local scrappy.  Ok, at the worst all you have to do is
rewind the coil with 240/415 the number of turns in 415/240 csa wire
of the original 

>          This clearly justifies your comments but also gives
>some idea of the extent of the problem. 
>
>            Accidental simultaneous pressing of both buttons is,
>in itself, a pretty rare event but even if it does happen there's
>something like a 50:1 on chance that the setup will still behave
>correctly. If it does fail, the supply fuses blows. This is the
>result of the short circuit across the motor terminals - there is
>no possibilty that this will damage the motor.
>
>            It's a simple  and very convenient setup if you have
>have the right contactors and I will certainly continue to use
>mine. However it doesn't provide total protection against
>simultaneous selection of Forward and Reverse. 
>

If that's important, use a selector switch, not p/bs.  I don't like
p/bs for any application where nasties could happen if the wrong
selection is made.  All too often, reversing is done in auto pilot
mode and if you get it wrong you only find out after it's rotating.
A forward-off-reverse switch with 45 degree indexing and an arrow
shaped operator will give you tactile feedback before you goof.


Regards,

David P.

On or around Tue, 09 Sep 2008 16:49:37 +0100, David Powell
 enlightened us thusly:

>If that's important, use a selector switch, not p/bs.  I don't like
>p/bs for any application where nasties could happen if the wrong
>selection is made.  All too often, reversing is done in auto pilot
>mode and if you get it wrong you only find out after it's rotating.
>A forward-off-reverse switch with 45 degree indexing and an arrow
>shaped operator will give you tactile feedback before you goof.

that's what it might have already, if such a thing and at a suitable rating
were even remotely easy to find.  All I actually needed was a big DPDT
switch, ideally with centre off position.  But you try finding one.
-- 
Austin Shackles.  www.ddol-las.net  my opinions are just that
Travel The Galaxy!  Meet Fascinating Life Forms...
------------------------------------------------\   
   >>  http://www.schlockmercenary.com/  <<      \  ...and Kill them.
a webcartoon by Howard Tayler; I like it, maybe you will too!

On 7 Sep, 11:45, Austin Shackles <austinDITCHTHISFORBETTERRESU...@ddol-
las.net> wrote:
> On or around Sat, 6 Sep 2008 11:40:58 -0700 (PDT), John S
>  enlightened us thusly:
>
> >I was referring to the typical single phase motor with centrifugal
> >start as fitted to the Myford.
> >Until the motor stops and the switch is closed the changes can't take
> >effect.
>
> when does the switch close though?  It stays in circuit during start up
> until the motor is up to speed, but does it actually come to a halt before
> the switch re-makes?
>
It doesn't come to a halt before closing but , normally, it's not far
off. Just try running the motor without any load connected, switch it
off and listen for the switch to close. I think that there's a very
strong chance that, if you press the reverse direction button just at
the point the switch has closed then the motor will reverse OK.

> and do ALL such motors do so?
No. The secondary winding of Permanent Split Capacitor (PSC) and Cap
start/Cap run (CSCR) motors is normally called the Auxiliary winding,
instead of Start winding, because it is always energised at the same
time as the main winding and uses a heavier gauge wire.

If you reverse the connection to the Aux winding (plus its series
capacitor) it will probably try to decellerate it and eventually
reverse it but a lot of heat is generated and there will be a large
current spike, particularly if the reverse connection occurs at, or
near, a peak of the AC cycle. Single phase motors and controllers are
made using this principle to stop them (known as plugging) but I
wouldn't want to do it with a bog-standard motor.

Just because a motor has a centrifugal switch doesn't mean that you
can switch it into reverse without it having any effect when the motor
is running. CSCR motors have a centrifugal switch (although some have
a timer instead) but that simply switches out the start capacitor; it
still leaves the aux winding and its capacitor energised when the main
winding is.

>
> The one that the circuit is intended for is a 2HP capacitor start/capacitor
> run with 2 caps on it.  Without studying it in detail, I'd prefer to play
> safe.
>
As it's a CSCR I'm sure that that's the wise way to go,

Bob

On Tue, 09 Sep 2008 16:49:37 +0100, David Powell
 wrote:


  SNIP

    I used the "snip" is because the intro is getting a bit long.
It's not necessary because I believe we are in full agreement on
the facts - the issue is really a matter of philosophy which
perhaps merits wider discussion. 

     For professional installations and the more luxurious
amateur workshops your viewpoint is undoubtedly correct - but
this is a Usenet group for MODEL engineers. It is true that there
are welcome and extremely helpful inputs from professional
engineers but it still remains a group of MODEL engineers many of
whom have to do the best they can on a very limited budget.
Inevitably, corners are cut and few if any home workshops would
pass a professional Health and Safety survey.

        It helps to try to plug in a few numbers. They won't be
very accurate because of the limited data but give an idea of the
scale of the problem.

        I don't believe I've ever simultaneously pressed Forward
and Reverse buttons but lets assume that someone is sufficiently
ham fisted to do this twice a year. At this rate it will be
somewhere near year 2033 before an interlock failure is likely.
This "event" would require replacement of the supply fuse!


     We can compare this with another common home workshop
stratagem.

     Many of us are still using ancient three phase motors which
were designed long before VFDs were even thought of.  They were
never intended to be run significantly above or below their rated
speed and the insulation stress distribution was designed on the
assumption that the supply waveform would be sinusoidal.

       We now cheerfully run these machines from modern VFDs over
a very wide range of speeds and operating from a VFD output
waveform which induces insulation stresses in the windings far
beyond the original design limits.

           By and large this works because of the generous safety
factors built into these venerable machines. 100% overspeed is
rarely troublesome and the low speed overheating is usally
manageable.

          The voltage stress is more serious because not only are
VFD peak voltages higher than the motor rated voltage but the
strange nature of the fast switching of the voltage waveform
concentrates excessive insulation stress in the first few turns
of each winding.

        VFD rated motors take this into account by using higher
standards of insulation and with special attention to the winding
starts. 

         Insulation failure results in a burnt out motor and a
possible fire risk. Ideally we should all replace our old motors
but how many of us will do this? 

   I've not enough information to properly quantify failure rates
but I have no doubt that it's higher than once in twenty odd
years. It's also a LOT more serious than just replacing a supply
fuse! 

         Regards

           Jim

On Sep 7, 7:45 pm, John S  wrote:
> On 7 Sep, 18:46, Russell  wrote:
>
> If you try and start it in the other direction before it's stopped
> nothing will happen as it has forward momentum and will carry on, like
> a bad motor you rope start.

So long as the centrifugal switch hasn't remade.

> It needs the centrifugal switch to be closed before it can start in
> reverse and this happens at low speed anyway so if you are that quick
> with the start button as soon as you hear it click then it will
> instantly reverse but only from a few revs.

But it won't do the motor much good if you do this too often.  Besides
which, if it's a Capacitor Start & Run motor it may not have a
centrifugal switch !
>
> This won't harm the motor as there are many standard 3 phase motors in
> industry that are designed to plug reverse and do so all day long.

But three-phase motors start and run using an entirely different
principle to single phase motors - so you're not comparing like-with-
like.
>
> John S.

Andy