Three phase reversing

The Smart & Brown model A lathe has a 1 1/2 hp three phase motor and in addition to the on off buttons it has a lever switch with forward-stop-reverse. What I want to know is whether throwing the reversing lever from one direction to the other without stopping the motor will be advisable. The lathe is over 70 years old and I have never tried this before and don't want to do any damage.

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Emphatically NO!

DO NOT reverse the the motor switch with the spindle moving.
Doing so is likely to cause mechanical and electrical damage.

Robert.

I was worried because of the machines age mostly, as the manual states that it can be done when metric threading using the back gear, so as to keep the leadscrew nuts engaged, but not at high speeds. Then it goes on to say that the lever switch can be used to slow the spindle quickly from high speeds, no doubt with the intention of switching to off as the spindle stops, and with a screwed on chuck, to boot.

Others have said it and so will I - DON'T DO IT ! Noel.

3 phase motors can plug reverse but it puts a lot of stress on the machine both mechanically and electrically if done at speed so only do it at faily low RPM and not if it's turning a large load.

My lathe has instant-reverse (It's DC) but it has an anti-plugging relay that won't allow it above 200rpm (I think). It brakes the motor and then reverses it when it gets slow enough.

Depends on the motor, as Pete says plug braking is a recognised way of slowing an induction motor. I rarely screwcut metric threads on my imperial lathe, but when I do the motor is reversed as per the manual quoted by Old Mart.

On my repetition lathe the manual recommends slowing the motor by flicking the direction control lever briefly into reverse. The headstock has two electrical switches with levers. One for changing direction and one for full and half speed. In normal operation one, or both, switches can be moved to change direction and/or speed while the motor is running. When churning out parts I change direction and speed without a second thought, as you would expect for a lathe designed for production.

If the Smart and Brown manual says it's ok to reverse at low speeds then I wouldn't worry about doing it.

Andrew

Could be pretty effective way of releasing a screwed on chuck.

Regards Ian.

As I was thinking in terms of threading 2.5mm pitch, and using the lowest back gear speed, 39rpm, and also cutting left and right hand threads, the power reverse is tempting. The motor runs at 1440rpm and I would definitely have to try some dummy runs first. The danger of the chuck unscrewing is not present, as I can lock any size chuck, 3 or 4 jaw. I was hoping to get feedback from somebody who has regularly used a three phase instant reverse without causing damage. Andrew, your post is very reassuring.

Edited By old mart on 11/01/2022 19:38:20

Today, I tried out the reversing lever. Without a chuck and in backgear. The lathe was in the fourth backgear speed at 102 rpm, it will go down to 39rpm. The instant reverse was painless, no strange noises which was encouraging. I noticed that just switching the lever to off from forward or reverse stopped the spindle in only 1 1/2 seconds. Having a chuck on would increase that slightly, but the extra drag from having the leadscrew engaged would probably compensate. It looks like that I will be using the lever to stop first and then change direction, this will give me plenty of time to back off the cut on the return run.

I would think the mechanical stresses are more likely to cause damage to the machine rather than the motor. I worked on a 20 ton crane that had a 40hp hoist motor with electronic speed control. The control was simply a thyristor control but if the load was heavy enough it could cause the motor to over speed when lowering so a reversing contactor would swap to hoist and the motor would be driven in reverse to control the descent speed. I used to feel for the poor motor but I only remember it being rewound once in about 20 years.

Mike

Old mart. Take a look at this youtube video. Indian i think reversing a huge lathe while threading.

Steve.

Reversing a large lathe while threading.

I revers my lathe by throwing the switch at full speed all the time without any problems. The Herbert preopt turret lathes has a "plugging" switch mounted at the rear of the motor (many people confuses it with a tacho for speed control). This device is driven by the shaft and is arranged so that the motor when running sets up a opposite direction as soon as the motor is stopped through a set of "plugging relays" enabling a quick stop of the motor.This work in both directions. Check schematic drawing for no. 5 and 7 preoptive lathes. The big Herbert drilling machines has a manual reversing switch along with a 4 speed switch and is reversed at any speed without any ill effects. i shall not attempt this with screw on chucks on a lathe, I will post part of the wiring diagram later

As far as the machine is concerned there's no difference between accellerating the spindle to x rpm and decellerating it from x to a stop in the same timeframe. The mechanical loads are the same.

Egyptian Steviegtr. He is speaking some vernacular dialect of Arabic

regards

What Pete said. Plug reversing a machine puts exactly the same loads on it as two starts in quick succession.

The only time it is a major issue is with extremely large machines (in the MW range), which will have the maxiumu number of starts per hour as part of the spec.

For there to be no difference mechanically the reverse absolutely has to be 'in the same timeframe'. Does the machine guarantee that? Maybe, maybe not.

If the machine is running slowly, as when threading, it's not unreasonable for a reverse to be approximately in the same time-frame, hurrah. But if the same machine is spinning a heavy lump of metal quickly, the reverse definitely isn't in the same time-frame and the amount of kinetic energy stored in the system is sky high: chances are damage will be done.

There's an electrical problem too. Electric motors starting from a stand pull much more than normal current until the rotating armature generates a back-EMF. When the power is reversed on a spinning motor, the back-EMF becomes a negative resistance and an exceptionally large surge current flows. Unless the motor and electrics are rated appropriately, they will get hot, perhaps too hot. Again, the amount of energy in the system and how long it takes to dissipate makes a big difference.

One of the advantages of ex-industrial machines is they are built to take abuse. Hobby machines are more delicate, and their operators have drive them carefully. Unwise for Myford owners to power reverse a lathe fitted with a single-phase motor, screw-on chuck, and weedy Dewhurst. Myford lathes have many virtues, but they're clearly not designed for that operation. Nor are Chinese mini-lathes!

Dave

Herbert PreOptive!

That brought back memories of being a first year Apprentice in the Training school.

One of the only two new machines in there! (21" Dean Smith and Grace was the other. Both a delight to use )

Howard

I wonder if the operator in that video uses the calipers as a spanner as well as hooking the swarf out from under the chuck when it's running!