motor starting and reversing.

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

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.

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.

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.

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!

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.

Jim

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.

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

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:

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.

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.

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.

As it's a CSCR I'm sure that that's the wise way to go,

Bob

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

So long as the centrifugal switch hasn't remade.

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 !

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

Andy