Electric motor reversing question

All,

I am adding a motor to a mini lathe and have a wiring question. The motor I have is reversible. I can add a switch to the circuit to switch polarity and change rotation direction. Here is the question:

Can I switch the motor from forward to backward when the motor is already turning? Or do I need to cut power and stop the rotation first, before switching direction and reapplying power?

Thanks, Joe Fleming - San Diego

Reply to
Joe Fleming
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"Joe Fleming" wrote: (clip) Or do I need to cut power and stop the rotation first, before switching direction and reapplying power? ^^^^^^^^^^^^^^^^ Cut power first. The safest thing would be to use a three-position switch that will not pass through center without stopping.

Reply to
Leo Lichtman

yes

most likely if you switch while running nothing at all will happen, but to be on the safe side, let the motor stop before starting it in the reverse direction

Reply to
William Noble

A polarity reversing switch will instantly change the direction of the motor, regardless of whether it is in motion or not. This is incredibly dangerous. Think of how much strain you put in the motor spinning at XXXX rpms and you instantaneously make it turn the other direction. The jolt to the setup would be unnerving.

Now think of having something chucked up, and you switch directions at the flip of a switch. The inertia that a spinning piece of wood might have spinning at a modest rate (say 2000 rpms) that is in effect being slammed into a dead stop (and at the same instant - reversed!) would throw just about anything off the chuck/faceplate/centers. And with a

10 - 20 pound piece of wood on the spindle, how much force would be retained by the launched wood, and how much would it take to bend the lathe spindle shaft or break your tailstock?

There is a guy in our turning club that has a motor assembly he bought somewhere that has a variable speed reverse. He said that when he doesn't remember to stop 100%, it sounds like a pistol shot when he just hits reverse. I can't how that is a good thing...

Robert

Reply to
nailshooter41

Joe When I put a reversing switch on a motor I use one that has a "dead" position in the center so that I can not forget to turn the motor off before changing direction. On the other hand I am a devote coward who hates pain to himself or the lathe :-)

Reply to
Darrell Feltmate

Well.... sort of. That is only true when reversing a DC motor or a three-phase motor. Reversing a single-phase motor will do nothing since the 'reversing' is done on the starter circuitry to get it initially spinning in the direction you want.

Somewhat true for motors that can reverse while turning. The danger is if your stock is mounted with a screwed on faceplate or chuck. The sudden, opposite torque can unscrew it. Of course, it may also tighten it even further depending on the threading and rotation direction.

If a lathe can handle starting torque then it can handle the same torque that is applied when reversing.

Dan

Reply to
Dan Bollinger

wrote: A polarity reversing switch will instantly change the direction of the motor, (clip) Then Dan Bollinger wrote: Reversing a single-phase motor will do nothing since the 'reversing' is done on the starter circuitry to get it initially spinning in the direction you want. ^^^^^^^^^^^^^^^^^ First, Nailshooter, regardless of what type of motor it is, it will never instantly reverse. It may start to decelerate rapidly--but I think Dan is right: once a single phase motor is up to speed, the direction of rotation is not controlled by the wiring.

The thing that could be harmful in this situation would be this: You turn the motor off, and as it slows down, the starting windings kick in. If you chose to reenergize the windings for reverse rotation before it stops completely, the motor would draw higher than normal starting current. If this did not kick the breaker, it could damage the motor, or shorten the life of the starting switch.

My main concern is not the mechanical lathe parts--it is the effect of higher than normal current during reversal.

Plan on stopping the motor, for all of the above reasons.

Reply to
Leo Lichtman

Leo, I don't see how this is so. Even if it did draw more to overcome stopping and then reversing (which I doubt), it would be a small increment because by the time a 1725 rpm motor's starting circuitry could kick in it's rpm would be down to about 2-300 rpm. That's not a lot of torque to overcome.

And, my experience with reversing a motor that uses starter circuitry is that it must be at 0 rpm otherwise the motor just spins back up in the same old direction, making it a moot point anyway.

Finally, I have yet to have a problem with reversing such a motor including one connected to a very massive 18" sanding wheel. The 1.5 Hp motor takes about 5 seconds to get it up to speed. That's a large starting torque by anyone's measure. And I've flipped that reversing switch at all rpms until I learned that it needed to coast to a stop first. Not once did the thermal switch open or the 'magic smoke' leave the motor.

I've worked in factories and shops all my life and I have yet to replace a motor because it was reversed improperly or too often.

Unless someone can come up with a significant history or evidence of what you are talking about I'm just going to write this off as something for Mythbusters to call 'busted!'

Dan

Reply to
Dan Bollinger

Thanks everyone for the information. I can tell by the questions that I didn't provide enough information, and I have OK - I want to be extra clear.

A. My Stubby lathe has a 3-phase motor on a controller that allows me to switch the motor direction while the lathe is spinning. the controller, I presume brakes the rotation down to zero rpm, and then starts rotation in the opposite direction. I do not stop the spinning before switching directions. Once in a great while the braking torque does cause a chuck to unscrew, but I generally tighten the chuck so this does not happen. The slow down and speed up are rather gradual. there is no "hard slam" reversing going on. So, this is my experience with reversing on a lathe.

B. The motor for my mini lathe is a single phase 110 motor that has wiring to allow rotation in both directions. I will not have a controller. It will be wired directly to 110 AC through whatever switch(es) I install.

So, based upon all the great responses, here are my more precise questions:

  1. If I flip the polarity on the switch to instantly reverse motor direction, does the motor gradually slow down, then gradually speed up in the other direction, or does the motor attempt to slam direction changes? In other words, to avoid slamming direction change, I must allow the rotation to stop before re-energizing the motor in the opposite direction?

  1. If I flip the polarity on the switch to instantly reverse motor direction, does this damage the motor? In other words, to avoid motor damage, I must allow the rotation to stop before re-energizing the motor in the opposite direction?

Assuming the slamming effect is what happens and/or that motor damage happens, then I will use some sort of three position set-up where I turn off rotation in one direction. Wait until the rotation stops, then re-energize the motor for reverse rotation.

Thanks again. Joe Fleming

Darrell Feltmate wrote:

Reply to
Joe Fleming

joe - for your mini, if you flip the switch, one of three things will happen:

  1. nothing - the motor will continue to run in the same direction - this is what happens with my reviersible grinder, for example - if the motor is turning more than a few RPM, it will just keep going. Many motors are bi-directional on the run-windings and use the start winding to make them go in the desired direction.

  1. it will go BANG-POOF!! and will hurt itself, probably by smoking one of the windings or the capacitor when it attempts to revese instantly. You can do this instant reverse with a 3 phase motor (my mill does this to actuate the rapids), but most 110V single phase motors won't do this. You just MIGHT get lucky, but personally, I wouldn't push my luck.

  2. it might actually reverse instantly. If this happens on a wood lathe, you will also nearly instantly unscrew the chuck or whatever is on the spindle, much to your annoyance. One can get away wtih this on metal lathes because when you do it (like when tapping) you are going much much slower (10 RPM or 20 RPM, not 2,000)

I use a center off DPDT switch to control the motor on my mini lathe, I strongly suggest you consider the same.

Reply to
William Noble

"Dan Bollinger" wrote: (clip) Unless someone can come up with a significant history or evidence of what you are talking about I'm just going to write this off as something for Mythbusters to call 'busted!' ^^^^^^^^^^^^^^^^^ Dan, based on your experience, and the fact that your logic is better than my logic, I'm BUSTED.

Reply to
Leo Lichtman

No, that requires a VFD controller.

It can't slam if it is a reversible capacitor start induction motor commonly used on shop equipment like Grainger item 4k144.. The reversing switch changes the polarity of the starting capacitor. When the motor is running, the starting capacity is not energized, hence reversing it has no effect on the running motor.

See above. In order to achieve reverse rotation the motor must be turned off and allowed to stop then turned back on. When you reverse the capacitor makes no difference.

How can it? See above.

No, to avoid motor damage don't hit it with an 8 pound sledge hammer. To reverse the motor, see above.

What slamming effect? There is no slamming effect with this motor unless of course you are using the MOTOR as an 8 pound sledge hammer!

You can use a center-off three position switch if you like but it will be a waste of money. A regular old DPDT switch will work just fine to revers the capacitor. To turn the lathe on and off (regardless of rotation) just use the switch that came with the lathe.

What I do is mount a toggle reversing switch in a little housing attached to the motor wiring cover when the motor is accessible. See Grainger item 2x660, Motor Switch Mounting Box

Dan

Reply to
Dan Bollinger

Write to the guys at Mythbusters, maybe they'll purchase your idea! They like to blow things up. :)

Reply to
Dan Bollinger

That is good to know. I never said that the fellow's machine I was referencing that heard the loud bang was single phase, it may or may not be. So if you are right and motors react completely the same "regardless" of what type of motor it is, if ALL act the way you say under all set up conditions, it sounds like I may have heard some baloney over here.

I will see him at the next meeting and ask him if he was pulling my leg. You know how some can't resist an exaggeration to tell a good story. He doesn't seem the type, but...

Joe - I hope you let us know what you decide and why.

Robert

Reply to
nailshooter41

That's it! All it sees is a power interruption until it's coasted down enough to kick in the centrifugal switch. Sees 120 of those every minute anyway. Once the switch is in you enter the zone of uncertainty where issues are sometimes decided after an exchange of smoke signals....

Now how about one rigged with a start/run capacitor.

Reply to
George

Well, I have, reversing a motor that has either a improperly rated (or no) overload and the wrong accel/decel settings - then you get "arcing and sparking and (still) popping the breaker" and sometimes motor smoke; do it often enough (or with enough load) and the motor will be toasted - 37 years aerospace industrial electronics. Unlike many single phase motors, a 3-phase AC motor CAN be reversed instantly but is VERY likely to pop the overload and/or burn the motor. I've seen the after affects of a motor reversed at speed and load - 2" thick shafts twisted, couplings bent and, somtimes, motors toasted. There is even a type of welding that uses stored inertia, try reversing that systems motor at speed (less than 100 RPM). Any system that lets you reverse a motor at speed and load, without a decel/accel curve, is poorly designed.

Also, brushed (AC or DC) motors (think router) can be reversed instantly, but, again, you're likely to either pop the overload or smoke the motor; mostly the brushes/commutator. I've got a blender that uses a router motor that has a reversing switch - the whole blender jumps and twists and you get major arcing when it is used.

BrianC

Reply to
Brian C

All,

Thanks for all the great information. Here is what I have decided to do:

Two switches: one to turn power on/off. One to reverse the direction. I thought about a three position switch, but I don't want the chance that I flip the switch right past the "off" position to the other "on" position and cause problems.

Joe

============================ Brian C wrote:

Reply to
Joe Fleming

joe - you are in a location with good surplus stores - look around them - there is a type of three position switch that forces you to stop in the center position, don't remember the name of them, but if you find one surplus (generally mil surplus) they won't cost much more (if any) than any other switch.

Reply to
William Noble

Thanks Bill

William Noble wrote:

Reply to
Joe Fleming

Whether or not it can be reversed at speed or not depends on the type of motor it is: single phase induction motors canNOT be reversed at speed (these are the most common type on a single phase system), brushed (AC or DC) motors (think router) CAN be reversed instantly, at speed, but then you're likely to either pop the overload or smoke the motor; mostly the brushes/commutator. I've got a blender that uses a router motor that has a reversing switch - the whole blender jumps and twists and you get major arcing when it is used.

How to tell if your motor has brushes? Look at the near end the wires go in, does it have two knobs or screw slots of non-metallic stuff, then it is probably brushed and can be reversed at speed.

Reply to
Brian C

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