When I wired up my CNC mill the contactors had switches added to the sides which are used to prevent both forward and reverse contactors from being energised at the same time. The coil from one contactor is wired through the NC switch on the other contactor. When a contactor is energised the switch turns off so that even if the control tries to turn it on it can't because no power will flow. Why can't you do this with relays with moltiple contacts? To make the scheme work the motor will need to be turned off first which is fine for your situation because your single phase motor will not plug reverse. If you want I can make a sketch, scan it, and post it to metalworking.com. Or e-mail it to you. Maybe this has already been discussed and rejected. If so then of course just ignore this post. Eric
Yes. That circuit would work. I'm just a bit worried by the fact that applying mains to the motor terminals 1&4 when the circuit, as you posted it last, is in the "at rest" (ie Off state). By this means you could cook the motor as it will not rotate until either the FWD or REV button is pushed. Similarly, you'll have to be quick to hit the main switch as pushing the STOP button will have no effect if the motor is running.
Not, IMHO, a very safe way of controlling the motor.
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.
On or around Fri, 22 Aug 2008 05:44:44 -0700 (PDT), houstonceng enlightened us thusly:
hmmm. that's a point. The reversing switch on it now does the same as the setup in the latest drawing but is downstream of the main supply switch (with the overload cut-out), so there are no volts anywhere when the supply is off. Ideally, I need to get the start buttons to control the main switch as well, then.
Or perhaps I need another relay to make the L and N lines into the motor 1 and 4 pins.
yeah, needs more work... It'd be a damned sight easier if I could just buy the necessary box.
2HP high starting torque. I've got 30A relays on order...
David P wrote:- "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."
Yes. I'm aware of the type of motor that Austin has. I was thrown by his remark in an earlier posting viz:- "The forward and reverse on this motor is just a case of connecting it the other way around, and it can be done with a single DPDT switch."
That appeared to say that the complete feed to the motor was connected the other way giving reverse whereas, after I checked by asking, Austin clarified matters with a circuit diagram and words.
As his motor is 2hp (1.5kW), I doubt you would find a DPDT, centre off toggle-switch capable of taking the starting current. Austin was also asking how to use the Fwd-Stop-Rev pish-button set he'd sourced from e- bay and the only way of doing that is to use relays or contactors. Besides which, I cannot see how you'd control a single contactor with a DPDT, centre off toggle-switch to give Fwd-Off-Rev as you suggest, so I'd be interested to see the wiring diagram.
BTW. Many of the Taiwanese lathes sold in Anerica (and some sold here with control transformers) use 110v relays to do the interlocking, NVR and hold functions which then switch 110v coil contactors switching mains to the 220v (240v) motor. Jet and Grissly (sold in USA) lathes have 3-wire single-phase motors switched by two three-pole contactors. This type of motor has a 110v start winding connected to the centre-tap of the 220v run winding. The run winding connexions are, usually, U1 & U2 and the motor is reversed by connecting the free end of the start winding (Z1) to either U1 ot U2. With a three pole contactor you can connect the run-winding to 220v and Z1 to, say, U1. Then, with the other 3-pole contactor, connect run-winding to 220v and Z1 to U2.
For the motor that Austin has - usual separate run and start winding (ie 4 connexions) - you'd need two 4-pole contactors (assuming you interlocked, etc, using relays as per Jet/Gris).
I proposed a design using 3off DPDT relays, which provides interlocking, NVR and reversing from Austin's push-buttons. You could use a DPST, NVR/Overload contactor - with C/O Aux contacts - to do the main switching, but you'd still need two DPDT relays for the reversing (one to do the motor switching, one for the interlock) OR 1-off QPDT relay.
Yes. That would work as well. Ideally, you'd need two 4-pole ST contactors each with Aux C/O contacts. You could get away with two 3- pole ST contactors with Aux C/O contacts if you wired one side of the motor run-winding permanently to Main N.
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.
On or around Sun, 24 Aug 2008 02:47:21 -0700 (PDT), BobKellock enlightened us thusly:
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.
It doesn't need to take the starting current, that flows thro' the main winding (and is switched by the contactor). All the switch is doing is, on one pole, selecting which starting field is to be used, and on the other pole, taking the coil current of the contactor. If it's for a battleship, RS 316-642 is 15A, @240VAC A. (Making capacity not specified) . Or, as Austin almost says, a conventional motor p/b starter with all the trimmings and a separate dpdt switch to reverse the start winding. I like KISS!
Must admit, it was somewhat confusing as to exactly what he had and what he was doing.
Yes, you need to memorise the states of the p/bs.
Any clearer now? Obviously some misunderstanding when you refer to the switch taking starting current.
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.)
Depends on what you want. Provided it's a machine under operator control, emulation of ye olde forward/stop/reverse gear lever is ok by me. I like to see what's selected. Don't see much need for interlocking forward and reverse, except via the switch. Worst that can happen is that the switch is flipped too quickly and it carries on in the same direction. No NVR release either. Not suitable for (say) a remote suck/blow fan in ductwork. Oh! Almost forgot,and just to placate the elf'n'safety stazi, a stayput emergency stop p/b in series with the contactor overload.
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.
Austin, Andy (houstonceng) and I have been pooling our thoughts, off- group, on a the design for a 3 button reversing starter which has been finalised (until somebody reports an error).
The circuits all look OK but you've chosen to to switch BOTH the live and the neutral lines of the single phase input (in the off position neutral is no longer connected to the motor or the relay sequence).
In single phase systems it's only necessary to isolate the live line - the neutral line can always remain connected .
Using live line switching and permanently connected neutral the control complement can be simplified and reduced to two, three phase motor control relays fitted with 240V coils.
These are fitted with three heavy duty MAKE contacts and two light duty contacts. One light duty contact is normally OFF the second is normally ON. The construction guarantees "BREAK before MAKE"
One relay is used to switch the motor to forward, the second relay is used for reverse. The light duty contacts are interconneced to provide no volt release and to interlock the forward and reverse functions.
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.
The snag is that I don't know of a retail supplier of the necessary relays - my own are all rescued from scrap machinery!. These are fairly easy to find but may not have the right combination of light duty MAKE and BREAK (twin MAKE is common). However,in the ones I have played with, MAKE or BREAK operation is determined by the way the contacts are assembled into the bakelite housing. It's pretty easy to open the housing and reassemble with the contacts moved to the desired locations.
Alternatively, any pair of three phase contactors can be augmented with a separate pair of light duty relays. The light duty MAKE and BREAK function must be on the same relay and the contact arrangement should be of BREAK before MAKE design (most small relays fitted with a pair of changeover contacts work this way) .
The system will still function correctly if the incorrect MAKE before BREAK relay type are used but the both button pressed protection is lost. If Forward and Reverse are are pressed simultaneously there is a small chance (dependent on relative time delays) of a catastrophic supply short circuit.
However it's still an OK system if the Forward and Reverse buttons are arranged so that it is physically impossible to press both at once.
I've posted the circuit and also JPG of a typical three phase contactor in the xs.to image hosting site :-
On or around Fri, 05 Sep 2008 19:39:12 +0100, snipped-for-privacy@yahoo.com enlightened us thusly:
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.
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.
Failing that have a two position switch to select fwd / rev and a start stop button. 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.
John S. I had this system on my Myford for years with just one mains relay swapping wires.
On or around Sat, 6 Sep 2008 03:49:14 -0700 (PDT), John S enlightened us thusly:
that (the cross-wiring) is what the circuit we ended up with does.
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 :-)
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.
Will ?? you read too many horror stories, but if it worries you that much stuff a centre in the end of the work and it can't unscrew as it has nowhere to go.
Incidentally in the 24 odd years I owned two Myfords I only ever had the chuck come loose twice, note loose not unscrewed. both times I was pushing the machine more than it was capable of, in fact in retrospect that was for the whole of the 24 years. Yes in theory they can come unscrewed and someone always has heard of a brother / co-worker / maiden aunt / escaped prisoner [ delete as required ] who had this happen to them.
After mixing with people who uses machine tools seriously for a living none have ever had this happen to them.
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