Reversing a three phase electric motor.

[Electricclass]

We have a three phase electric motor that we want to run in reverse, but we dont know how. We need to know what to do. If anyone has a website or just general knowledge of this plese let us know.
Thanks,
The Electronics class of BHS

 

[wbd]

Just swap 2 of the 3 leads. This can be done at the motor starter for a permanent reversal or you can install another contactor and 3 position switch for running it in either direction when you want.

 

[gordonl]

Some manufacturers produce reversible starters which integrate 2 contactors as mentioned by wbd in one package. You the just energize the forward or reverse coil.

One example is the Allen-Bradley Bulletin 505 starters.

 

[Marke]

Reversing two of the incoming phases will reverse the motor provided that it is not running off the output of a variable speed drive. If the motor is three wire connected, ie star or delta connected with only three wire going from the motor to the starter, then this reversal of two phases can be carried outanywhere. ie. at the motor, at the starter or at the point of supply. If the motor is 6 wire connected, as in a star - delta starter or an inside delta connected soft starter then life can be more interesting. With the star delta starter, reverse two of the phases applied to the starter. With an inside delta connected soft wtarter, refer to the soft starter manufacturer as some are phase sequence sensitive. Mark Empson

http://www.lmphotonics.com

 

[Odair]

I won't repeat the information already posted. Only one tip: in case of using 2 contactors, be carefull to provide an electrical or mechanical construction that guarantees that the 2 contactor will operate always one at a time.

 

[NEWFIEBOY]

All of the above postings contain accurate advise. Maybe you could elaborate on your application to get more a precise answer to your question. Do you want to reverse the motor permanently? Will the motor be reversed manually at certain times or will it reverse automatically at some point in an automated process?

 

[electricpete]

Some motors (particularly large motors) have directional cooling characteristics and will overheat if run at rated load in the the reverse direction.

Motor manufacturer can get better cooling/higher efficiency by designing a motor with directional cooling. Efficiency is generally more important for large motors than small.

You can figure it out by examining the rotor fan blades and the cooling slots to see if they give preference to one direction of rotation or not. Also manufacturer may provide this info on a label plate.

 

[EKTORAS]

I'd like to add to the above postings that it is important to reverse the direction of rotation by the method proposed
(swapping two of the phases) after the motor has stopped. If you do it while it is running, it will be braked (condition named plugging) drawing a very high current equal to or greater than the starting current. It will reach zero speed quickly and start rotating in the opposite direction. You may overheat the motor this way causing damage to it. You can do this only with slip ring motor conecting external resistors and using special contactors of AC-4 type. This must not be your case, so take care that the motor is stopped before you reverse the direction of rotation.

 

The info above is good just make sure to use an interlock for the contactors if you are using the two contactor method- this ensures that you can't have both contactors on together.

 

[RavenJoe]

All of the info above is accurate. But as mentioned above more is needed about your aplication. If it is a 3 phase squirrel cage motor or most 3 phase motors. All you need to do is swap 2 of the 3 phases. But if you want permanent control to change from forward to reverse at will, you need a forward reversing starter with mechanical and electrical interlocks to prevent a fault. Also a stop,forward,reverse control station.(To do it right)
Push buttons can also be interlocked mechanically or buy stacking contact blocks and electrically interlocking them.You also can have a zero speed switch added to your control to assure the motor has fully stopped before you can go in the other direction. A little food for thought.
Have fun, Hope this helps.

 

[SteveKW]

Those are all the easy ways to reverse the motor. The hard way is to disassemble the motor and swap the stator or rotor around in relation to each other.

 

[Odair]

A 3-phase motor doesn’t need to be stopped before reversing (of course it’s desirable, but not necessary). The magnetic field is able to stop the rotor and start the reversing procedure. Some cares must be taken.
Certainly it demands an extra current drawing, that is not higher than the peak current of a regular start (there is no higher current than the regular start current). The difference is that reversing a moving rotor demands more time (and not more current) than reversing a stopped rotor. Don’t forget that a 3-phase motor will operate at the rated current only when the rotor reaches the rated RPM.
To operate 3-phase motors (no frequent starts) you need to use na AC-3 type contactor. But reversing operations in rotatig sets is a condition involved by AC-4 category. Few manufacturers really produces AC-4 type contactor. Actually, AC-4 contactors are oversized AC-3 ones. Chose an AC-3 contactor that is 2 or 3 times the rated contactor for the same motor in regular conditions of starting. Reversing running motors is not so critical for the contacts of the contactor, since the start current in a reversing procedure will be basicaly the same of a regular start. The motor’s stator is the one that suffers most the risk of being damaged, followed by the contactor’s coil (and not the contacts, when oversized). A forced refrigeration is interesting, stators with better insulation windings, as well.
Using external resistors may be good or bad: they will reduce the current, but will do the same with motor’s torque, thus avoiding the rotor to reach its intended new rotation, making the “peak current condition” longer. Sizing all that stuff is not that easy.