Reversing a three phase electric motor.
Reversing a three phase electric motor.
(OP)
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
Thanks,
The Electronics class of BHS





RE: Reversing a three phase electric motor.
RE: Reversing a three phase electric motor.
One example is the Allen-Bradley Bulletin 505 starters.
RE: Reversing a three phase electric motor.
Mark Empson
http://www.lmphotonics.com
RE: Reversing a three phase electric motor.
RE: Reversing a three phase electric motor.
RE: Reversing a three phase electric motor.
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.
RE: Reversing a three phase electric motor.
(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.
RE: Reversing a three phase electric motor.
RE: Reversing a three phase electric motor.
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.
RE: Reversing a three phase electric motor.
RE: Reversing a three phase electric motor.
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.