Another motor question about reversal

[Arthur.Marks]

Prompted by http://bbs.homeshopmachinist.net/thr...iring-question
Can anyone explain in relatively layman's terms a 3-phase, instant reversing motor? It is my understanding that not all 3-phase motors can instant reverse. They must slow (albeit, quite rapidly) and then re-speed in the opposite direction. I had a drill press once that was "instant reversing." It really did reverse instantly! When I was checking all the switch wiring off the machine, the reverse spit the motor over on its side. It was faster than I can blink. So what about the motor construction allows that to happen?

[atomarc]

IMHO, any 3 phase motor can instantly reverse, and by that I mean they can reverse as quickly as they can stop and change direction. This would be predicated on the inertia of the motor guts (size) plus the inertia of the load they are powering. I can't imagine any internal motor construction that would allow them to reverse any faster than 'they could reverse'...does that make sense? Isn't this 'plug reversing'..the incoming phase is reversed which electrically reverses the rotor/motor. That's about as 'instant' as you can get I think.

Stuart

[PStechPaul]

The rapidity of reversal will depend on the rotational inertia of the motor itself along with whatever load is connected to it, and its maximum torque. As you have seen, there is also a gyroscopic effect that flipped the motor as its rotor slowed and changed direction. Induction motors have a maximum torque of about 2-4 times rated, depending on design type (A, B, C, D...). It may be quicker to use a clutch and reverse gear drive and reverse the machine that way.

[Arthur.Marks]

What I am reading from these responses is that the difference must be in the switch. I have other three phase machines -- some run on a VFD, others with for./rev. switches -- and none reverse like that drill press motor. All my switches must pass an "off" position, though, to reverse direction. The drill press had completely independent switches: one forward/reverse, the other on/off. If there's no motor difference, I can only imagine the difference is in the control components.

[J Tiers]

What I am reading from these responses is that the difference must be in the switch. I have other three phase machines -- some run on a VFD, others with for./rev. switches -- and none reverse like that drill press motor. All my switches must pass an "off" position, though, to reverse direction. The drill press had completely independent switches: one forward/reverse, the other on/off. If there's no motor difference, I can only imagine the difference is in the control components.

Only to an extent...

the single requirement to reverse a 3 phase motor is to reverse the direction of the rotating field. Reversing any two wires will do that, so a switch that independently does that will reverse the motor as instantly as the motor can reverse itself plus the connected drive components.

A motor by itself will reverse VERY quickly, because the rotating mass is small. After other stuff is connected, the motor cannot reverse as quickly, because its torque is limited. Reversing time may be about 2x the start time, since the motor has to do a negative acceleration from full rpm to zero speed, and another from zero speed to full rpm.

A drill press may not have a lot of rotating mass, so it may be made to reverse quickly in order to do specific tasks such as tapping.

The regular reversing drum switch will do the same thing... there is no magic reason why it must pass thru stop... it could just as easily be stop, fwd, rev, rather than rev stop,fwd. But the rev, stop, fwd is more convenient, because you can go directly to either fwd or rev from stop, so pretty much all are made that way. I have never seen one with a special detent to force a delay at the stop position, but to my understanding, they do exist.

If you flip the switch directly from fwd to rev, it will reverse the motor "instantly", known as "plug reversing".

A VFD is another story. They are normally set up with a decel and accel time, so that there is a minimum time that is required between fwd and rev. The VFD also usually cannot pass the amount of current required to reverse the motor instantly, which is generally considerably more than the current to start from a stopped condition.

For those reasons, the VFD won't plug reverse.

[mikey553]

A few years ago I have designed and built a plug reversal test stand for my employer. We were in need to qualify a new supplier of 250 and 350 HP hermetic compressor motors. Such motors are rather small in size for their horsepower rating and they do not have their own casing or shaft. They use compressor components instead and employ liquid refrigerant cooling to keep heat in check.

Plug reversal test is a standard procedure for motor manufacturers (at least for hermetic motors). This is the only way I know to do the accelerated motor test so you don't have to wait years to find out if this particular motor is any good. After a certain number of plug reversal cycles any motor will fail, it is just a matter of time. Such test stresses motor components severely, both mechanical and electrical. The current peak exceeds motor nominal current many times over. Special heavy duty contactors are used for this test and contacts do not last a long time. So yes, switching gear is very important for instant reversing.

At the beginning of my career I have seen people using drill press to do power tapping with instant motor reversing. At that time I thought about how smart those guys were. I have changed my opinion - I would never do it on my machine. I have seen the kind of damage it does to the motors and there is no doubt it would damage the other components of the drive train as well.

At this point I am not interested to learn just how many plug reversal cycles will my machine survive - 1000 or 100000. I would rather take my time and do the machining a proper way.

Mike

[mikey553]

A few years ago I have designed and built a plug reversal test stand for my employer. We were in need to qualify a new supplier of 250 and 350 HP hermetic compressor motors. Such motors are rather small in size for their horsepower rating and they do not have their own casing or shaft. They use compressor components instead and employ liquid refrigerant cooling to keep heat in check.

Plug reversal test is a standard procedure for motor manufacturers (at least for hermetic motors). This is the only way I know to do the accelerated motor test so you don't have to wait years to find out if this particular motor is any good. After a certain number of plug reversal cycles any motor will fail, it is just a matter of time. Such test stresses motor components severely, both mechanical and electrical. The current peak exceeds motor nominal current many times over. Special heavy duty contactors are used for this test and contacts do not last a long time. So yes, switching gear is very important for instant reversing.

At the beginning of my career I have seen people using drill press to do power tapping with instant motor reversing. At that time I thought about how smart those guys were. I have changed my opinion - I would never do it on my machine. I have seen the kind of damage it does to the motors and there is no doubt it would damage the other components of the drive train as well.

At this point I am not interested to learn just how many plug reversal cycles will my machine survive - 1000 or 100000. I would rather take my time and do the machining a proper way.

Mike

[lakeside53]

+1

When I accidently do it on my lathe it's BRUTAL on the gear train. I'm very careful to NOT do it as a matter of course. 1250rpm (2500rpm max) and a 8 inch chuck. Frigg, the lathe bounces...

[Arthur.Marks]

Many thanks, Mike, for volunteering your informed knowledge and experience. You give solid reasoning to back up your stance on "plug" reversing. I had a hunch there were differences. The question was to satisfy my curiosity and not for any practical application. I've noticed certain, specific machines are identified by their manufacturers as "instant reverse" capable while others are not. Details like that lead me to start wondering.

[J Tiers]

Plug reversing is a hard stress. But if the motor cools down between times, it isn't so bad. As a test, it can heat up a motor nicely, in a short time.

I don't plug reverse any of my 3 phase motors... the RPC won't allow it, for one thing, and for another I'm not in that much of a hurry.

[Forrest Addy]

I've always thought induction motors as being pretty robust; hard to make fail in normal service.

When I was an apprentice I worked a rotation in the turret lathes where they had a an old drill press (Walker- Turner I think) set up for tapping. There was a stool, a foot switch, and a honkin' big cabinet full of drill press tapping gedgetry. The drill press was started in reverse and switched to forward when you stomped the switch. IIRC it had a two speed motor and the electrics were doctored so it reversed in full speed but tapped in half speed thus shortening the cycle an important trifle.

Erring aqpprentices were put on it as penance for breaches of The Code of the Turret Lathe: screwing up work, lame excuses, back late from lunch, splashing coolant on the floor, wising off to the boss or the instructor, etc. Just being young, healthy, and in high spirits on Monday morning would do it.

We tapped ring nuts for switches, weld sockets, machine nuts of all descriptions up to 5/8", you name it. Get a rhythm up and you could tap 1/4-20 monel nuts at about 800 an hour. I was too fumble fingered to hit that pace but Bonnie Trask, a Rosie the Riveter hold-over from WWII, could do it all day. The drill press was bullet-proof. It never needed service beyond cleaning and oiling. It had to be built in the '30's and if it ran 24/6 in WW II and 5/5 from WWII to 1962 (when I ran it.) Assume 100 cycles per service hour; about 800,000 plug reversals and that figure may be on the low side.

There were hundreds of motors in that same facility where plug reversal was a normal part of daily operation: Gisholt turret lates, plate rollers, cranes, elevators, conveyors in sand reclaiming conveyors, drill presses, and a dozen other applications. I worked for a while in maintenence and went on PM rounds servicing motors with Paul White, a savvy old electrician. He checked contactors etc while I greased bearigs and looked for developing mechanical problems. I remember we had the plug reversal discussion. He was of the opinion that if not done to excess a plug reversal was no different from two across-the-line normal starts.

Given that, I can't see how plug reversing is detrimental to a three phase induction motor so long as it's not overheated. If we got a controversy brewing about motor life Vs plug reversal, there's a ton of material in the technical literature to offer fact based guidelines. This offering sums it up:

http://ecee.colorado.edu/~ecen5737/Files/induction%20machines%20paper%203.pdf

Look at para 2.1.7 for S7 duty. This authorative application note doesnt seem to get exercised over plug reversal. It assigns it a duty class number, offers a regime of suggested operation where the net average current doesn't exceed FLA, etc, and lets it go at that.

Maybe we're jumping at shadows.

[PStechPaul]

Good article. I think the conditions of full voltage starting are about the same as for plug braking and plug reversal. It matters little when the slip speed exceeds that which produces maximum torque, and the current drawn will peak due to the inductance and resistance of the primary (stator) and secondary (rotor). As long as the windings are thermally protected, and overload devices are properly applied, there should be no damage, especially if the motor is designed for such duty. It should be possible to provide faster reversal by using a VFD which can supply (and sink) full current at maximum positive and negative torque, since breakaway torque is usually greater than locked rotor starting torque.

[mikey553]

The motor may not experience bigger torque during plug reversal than during normal full voltage startup. I have never verified that, so it would be better to say - I do not know. But what is more important is that torque changes its direction on a regular basis during plug reversal test and sooner or later some of its components will get loose and will start to move. Eventually it will kill the motor.

I have tested only 2 pole motors, the ones with synchronous speed of 3600 RPM here in USA. Maybe 4 pole or 6 pole motors will reverse the rotation in a softer way. I wish you had seen what is happening during big motor reversal. The concrete floor is shaking, 4-0 power cables jump apart from each other due to the tremendous amount of current going through them. The whole reversal from full forward speed to full reverse speed takes about half a second.

Did I mention that every motor will eventually fail? This is not from a book or an article. This is a personal experience from dozens of tested motors. The typical failure mode is not an overheating since temperature is monitored and controlled. It is in most cases a ground fault, where stator windings insulation fails from constant rubbing and gets shorted to the ground. The mechanical integrity of stator and rotor is also verified during this severe test - it is not uncommon to find rotor or stator falling apart after a number of cycles.

Every across the line start stresses the motor. The bigger the motor gets, the more limited it becomes in regards to starting frequency. Big induction motor (3000 HP and higher) manufacturers limit the starting frequency to a few starts per day. I do not know why they do it, but suspect it is not due to motor overheating.

[J Tiers]

A motor will stand most anything except overspeeding and overheating.

The maximum torque is limited with any motor, and any motor worth building in the first place will stand the torque it can itself produce until the cows come home and milk themselves.

Asmall motor quickly heats up, and as quickly cools down, even if not running, although running at lower load cools it quicker. What kills motors is time at temperature. The insulation, if of any organic material, chars slowly at ANY reasonably elevated temperature... eventually causing insulation breakdown.

A large motor does heat slower, but it also stays hot for a longer time, running or not. And being larger, the mass per HP is not that far different from a smaller motor, so the heat energy per kg per start is not so very different from a small motor.

But the surface area per kg is very different, meaning that the heat, once in the material, is slow to escape, and the motor stays hotter longer.

Implied in these ratings is the idea that you will start the motor and run it at something approaching full load, thus heating it to normal temps without considering the extra starts. The large motor, being slow to lose heat, will not tolerate so many starts on top of full load. The smaller motor quickly loses extra heat, and can tolerate more.

If you start a motor more times per hour than rated, but do not run it at full load between times, that may be more tolerable, since the heating of full load is not superimposed on the heating from starts.