motor starting and reversing.

snipped-for-privacy@yahoo.com in uk.rec.models.engineering wrote:
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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.
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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
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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
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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.

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

Jim mwrote :-
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One of the circuits I shared with Austin and Bob was a two-contactor interlocked system with Aux contacts. Both 3-pole (Switching Line only) and 4-pole (Switching Line & Neutral).
Obviously, switching both Line and Neutral on a single-phase system is not absolutely necessary - but is the safest thing to do on a machine tool - Neutral is rarely 0v wrt Earth (sometimes as high as 50v) and a Lathe or Milling Machine is a large lump of earthed metal.
BTW. Your circuit is a "No-No" because the stop button is in the Neutral and you should only switch the Line or both Line & Neutral - never Neutral on it's own..
John S wrote :-
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Not a good idea. In a capacitor Start & Run motor there'd probably be a "bang" and a damaged Cap or Motor - and in an Induction Run Motor (either Split phase or Capacitor Start) the centrifugal switch re- makes before the motor stops, so it'd try to restart in reverse whilst still running - all be it slowly - forward.
Andy

On 22 Aug, 09:24, Austin Shackles
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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.
Bob

On or around Sun, 24 Aug 2008 02:47:21 -0700 (PDT), BobKellock
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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 sheet right. If it causes issues, then it could have a current-limiting resistor in series with the buttons, or something.

My previous posting was incorrect when I referred to DSC00200.JPG. It was, in fact, regarding DSC00201.
I've also spotted another "feature". If the fwd or rev button is held down at the same time as stop then the motor will run in the relevant direction. To an optimist this is the jog function but, to a pessimist, it's a no-no as stop should always override go.
You shouldn't discount possible damage to the switches because of the short duration; it's the act of making or breaking contact that does the damage. The holding current of most switches is normally many times greater than their switching capacity.
Bob

On or around Sun, 24 Aug 2008 06:35:02 -0700 (PDT), BobKellock
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There's another circuit I may use instead, supplied by yon Houstonceng chappie. I shall study it.
I don't know the details of the button contacts, but they look quite solid.
As for saftey features, it's got to be better than the current setup which has no NVR and dodgy reverse switching.

In article
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The type of motor that does what he describes is a sp induction motor with separate forward and reverse start windings. 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. Usually cheaper to do it this way than some relay switching to reverse one winding. Nothing special needed for the switch, I use toggle switches as the contactor does all the power stuff.
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Regards,
David P.

On 19 Aug, 19:09, Austin Shackles
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SNIP
Austin
No. That isn't correct.
The neutrals should NOT be switched.
OK. Assuming you have obtained two mains coil relays with contacts capable of taking the motor current - remembering that the "start current" will be higher than the run current - and that you've got some form of Overload cut-out/Fuse in the mains supply.
For the Start function. Use a double-pole/single-throw (DPST) or doble- pole/double-throw (DPDT) relay. Wire one end of the relay coil and one end of the single-phase motor "Run" winding to Mains Neutral. Wire one side of the NC Stop switch to Mains live and the other side to one side of the Start switch. Wire the other side of the start switch to the - as yet - unconnected side of the Relay coil. Connect one set of the relay NO contacts across the Start button. Test. Relay should pull on and hold on operation of the Start button and drop-out when the Stop button is pressed.
Wire one side of a Tripple Pole/Double-Throw (TPDT) or Quadruple pole/ Double-Throw (QPDT) Relay coil to Mains N. You could use a contactor with Main Contacts DPDT and a separate light current "hold contact". Let's assume we have a TPDT relay. Wire one side of the Reverse switch to the junction of the Stop switch and Start switch (previously wired as above) and the other - as yet - unconnected side to the TPDT relay coil. Connect one set of NO contacts of this relay across the Reverse switch. Test as above. Pressing the Reverse button should energise the TPDT relay which will then hold until the Stop button is pressed.
OK So far ?
Start Relay to Motor. Wire one side of the - as yet - unconnected NO Relay contacts to Main Live. Wire the other to the - as yet - unconnected side of the motor "Run Winding"
Reverse Relay to Motor. Let's call the common of the remaining rwo sets of contacts C1 and C2 and the NO/NC Contacts as NO1, NO2 and ditto NC1 & NC2. Dirstly, wire C1 to one side of the Motor "Start winding" and C2 to the other side of same. Connect NO1 ro NC2 and NO2 to NC1. Connect NO1/NC2 junction to Mains Live and NO2/NC1 junction to mains neutral.
Operation. For forward rotation, press Start Button only. Stop motor with Stop button. For reverse rotation, press both Reverse and Start button together. Stop as above.
Wiring in this way does not require any special circuitry to :- a) Prevent both Start and Reverse buttons being operated simultaneously b) Prevent trying to start the motor in opposite directions simultaneously (Which would probably mean connecting Mains Live to Main N - which not a good thing to attempt with a relay contact !)
Downside ? You need to press two buttons sumultaneously to get reverse. So what ? You'll probably only use reverse very occasionally.
I hope you can follow this. If not, I can send you a wiring diagram if you can ley me know how to send you a PM with attachments (I'm new to this group and they all use different mechanisms)
Andy