How long will a three phase motor last constantly reversing rotation?

rj1939 · Feb 9, 2016

I picked up a Reid 618 surface grinder a while back and while checking it out, found the table feed not working. Was very surprised to see how they were doing it, electric motor and gearbox with a reversing switch setup. Seems pretty crude way to do it. How long would this motor last if I replaced it and made it functional again?

Ziggy2 · Feb 9, 2016

Apparently, in this application, a long time. The big issue of continuously reversing a 3ph motor is motor heating, mainly the stator coils.

Being that this is a rather small grinder, it is unlikely that it would normally see a very large run time cycle. If the motor was properly sized for the application it should work okay. Not ideal but straight forward simplicity.

TDegenhart · Feb 9, 2016

Ziggy is correct, motor heating is a problem unless the motor is designed for that application. Also severe is the duty cycle of the contactors. American design (NEMA) contactors are designed for this heavy duty application. European designs require special sizing for reliable application.

Tom

9100 · Feb 9, 2016

Some sort of soft start system would be beneficial here. If you had a controller that limited the current to the normal running current of the motor, you would get away from excessive heating by starting surges. You don't give the hp but it is like small, so a VFD would not be expensive.

Bill

billzweig · Feb 9, 2016

It will also depend on the load. A heavy reversing load will be harder on the motor.

Matt_Maguire · Feb 9, 2016

It depends, plugging motors are more expensive (built tougher), depending on the drive it can get really expensive depending on how it starts, brakes and reverse starts. Done right it lasts as long as any motor as long as you don't stall & pull locked rotor-ish amps.

A bridgeport has a plugging motor & drum switch - if you run it for power tapping day in & day out it won't last very long.

Good luck
Matt

Mark Rand · Feb 9, 2016

TDegenhart said:
Ziggy is correct, motor heating is a problem unless the motor is designed for that application. Also severe is the duty cycle of the contactors. American design (NEMA) contactors are designed for this heavy duty application. European designs require special sizing for reliable application.

Tom

Cites ????

TDegenhart · Feb 9, 2016

Mark Rand said:
Cites ????

Cites as in where did come up with this? Cites as in I spent 15 years designing industrial motor control for General Electric including the complete 300 line size 0-5, and 7 years with Furnas Electric designing the size 4.

One of the characteristics of European (IEC) control design are the applications manuals that need to be studied to get the correct contactor for the application. An advantage of the IEC approach is that the users gets just what is needed for the application. This saves money and space.

The American approach was (is?) to designate a size to a device that is rated for the most rugged of applications but which is overkill for most uses. Why? Simplicity. Got a 10 hp motor? Size 1, regardless of application. Simple starting a light load, the contactor will last forever. The same size 1 is rated to plugging, jogging duty on a constant basis. One characteristics of JIC is that the minimum starter of be used is a size 1 regardless of application.

Tom

1 Jaw Chuck (Chuck Evans) · Feb 9, 2016

Guys,

Would a VFD that 9100 Bill mentioned be a better option than a heavy
reversing contactor?

Just wondering.

Chuck
Burbank, CA

JST · Feb 9, 2016

Mark Rand said:
Cites ????

Citation would be any table of IEC ratings, and equivalent NEMA ratings. There are a large number of slightly different IEC ratings. AC-1 thru AC- I don't know what, for noninductive, slightly inductive, more inductive, magnet duty, etc etc. For switching wound rotor vs squirrel cage motors, for switching running motors, vs starting of motors, for capacitor banks, etc, etc, etc. A bewildering number of fractionally different duty types. And similar for DC.

The NEMA approach is to make it simple, so that a competent electrician can determine what to do, and do it. No need for a "Dipl. Ing" (A P.E. in US parlance) to decide what exact type of switch to use that will be just good enough..

sfriedberg · Feb 9, 2016

Just a reminder that we started talking about a 6x18 surface grinder. I don't know if the Reid has plain ways or ball/roller bearings, but if the latter, you can accelerate the table with a light finger's touch. And it would be unlikely that a 6x18 would have a table load of even 100 pounds. So we are not talking about reversing a two-high rolling mill moving a 4 ton slab back and forth!

If the table drive motor was properly selected/engineered for frequent plug reversing, there would be no issues with motor life. And if it's not a custom motor, replacement with another standard motor of the same nameplate ratings would work just fine. However, in this application, you would have to pay attention to more than just HP and RPM on the nameplate. The nameplace service factor (SF) and insulation class would also be very important when making a replacement.

Added in edit: Here's a catalog page for Leeson's fractional HP motors specifically designed for plug reversing.

SteveBausch · Feb 9, 2016

Regarding the wimpy nature of IEC stuff, I was on a large commune/farm that decided to get a 75 pound Wascomat washer.

Perhaps it is a wonderful device, but it didn't like running on a rotary converter.

It ended up sitting in a barn.

The OP is wise to stick with three phase; single phase motors are usually rated to no more than ten starts per hours.

fastone53w · Feb 9, 2016

I have a cylindrical grinder that operates the same way. Motor never gets hot and it is older then me and still running.

TDegenhart · Feb 9, 2016

A VFD is probably a very good application here, so long as it is properly sized with braking resistors and what not. The problem with the contactor is that each time the table is reversed, the contactor is making locked rotor current. With the contact bounce that is inherent in mechanical switches, there will be a fair amount of wear of the contacts do to the make arc.

Tom

Ziggy2 · Feb 9, 2016

TDegenhart said:
A VFD is probably a very good application here, so long as it is properly sized with braking resistors and what not. The problem with the contactor is that each time the table is reversed, the contactor is making locked rotor current. With the contact bounce that is inherent in mechanical switches, there will be a fair amount of wear of the contacts do to the make arc.

Tom

Good reason to use arc suppressors in this application with the contactor.

One downside to using a VFD on a grinder is that the reverse with the ramp down, braking, and then acceleration is not as snappy as just plugging but considering the size of the grinder in iscussion, it is a rather moot point.

CarbideBob · Feb 9, 2016

How long would an AC servo motor last doing G00 X10.0....G00 X0.00....?
Not the same but close to it.
Is the motor dead?
I'd expect the contacts to arc/burn out with enough cycles. Common in my experience to see automation from WW2 still running using this method.
It seems all wrong now that we have nicer electronics, but 70+ years and still running is better than most new cncs can hope for.
Will your Hass or Mori cycle in 2085?
Bob

9100 · Feb 9, 2016

Chuck Evans said:
Guys,

Would a VFD that 9100 Bill mentioned be a better option than a heavy
reversing contactor?

Just wondering.

Chuck
Burbank, CA

If I was doing it on my own machine, I would use a magamp, but that is just me.

Bill

rj1939 · Feb 9, 2016

The grinder in question has plain ways under the table. The motor that was on the table drive was a 1/4 horse IIRC, I have a 1/3 hp that I was going to try to substitute. It really isn't a big deal to run by hand, very easy to crank and so I haven't been in a big hurry to fix it.

TDegenhart · Feb 9, 2016

9100 said:
If I was doing it on my own machine, I would use a magamp, but that is just me.

Bill

I'am not sure if you can get components now except for stuff on Flea Bay

Tom

JST · Feb 9, 2016

Bill MAKES them, and has stock of core material, I believe.

How long will a three phase motor last constantly reversing rotation?

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