We built a portable machine using a 25HP 3 phase motor. This machine is hooked up in different facilities and we need it to rotate in the same direction no matter how the cord is hooked up. Our competition's machine is capable of doing this. How is it done?
bruce at bwmanufacturing. com
A phase sequence relay/device which can indicate correct phase sequence and two contactors would be required. The three phase wiring would be straight to one contactor and one phase interchanged to second contactor. If phase sequence is correct first contactor switches on and if incorrect other one will switch on. Out put will always be correct. Both contactors will need to be interlocked to each other through aux contacts to prevent them switching on simultaneously.
Responding to Bruce's 18-Nov-08 (15:17) query... the circuit you seek is used all over the world:
It can be found anywhere a ship is connected to shore-power. Typically, it consists of a sequence or rotation detector, and a reversing controller or contactor. On some of the more frugal installations it is done manually by reversing two phases.
Regards, Phil Corso (firstname.lastname@example.org)
I suspect the answer is in the VF drive. Are you using a VF drive or a contactor to run the motor? A closed loop vector drive gets its velocity and position info from a motor mounted encoder, but the encoder adds cost. I would pose the question to some companies who sell VF drives.
In any VFD for that matter, the line simply powers a rectifier. Unless the drive manufacturer monitors input phasing for some reason, line phasing shouldn't make any difference. In fact, some drives can even be run from single phase supplies. There's a funny story that illustrates this: I got a call from an electrician who had reversed wires at the disconnect to a VFD driven motor many times and the #@$%^
motor _still_ ran backwards. :^)
1. If you were operating your motors on a VFD, they would always go the same way as the AC is rectified to DC then converted back again.
2. Elevators used to be fitted with a phase rotation relay to disable the elevator if the phases were accidentally reversed, e.g. out on the street.
There may be some device that will automatically swap two phases.
Hope this helps,
Motors powered from a variable frequency drive go the same way even with the input wires swapped. I'll bet that is how your competition does it.
I hope this helps,
Three phase motor rotation can be done by reversing any two of the three wires. What you want to do is have the motor rotation be the same for any sequence of input power connection. You could just reverse any two power input wires, but it would be preferable to have any two power input wires reversed automatically if the motor rotation is backward. To do this a velocity transducer, something like a tachometer, could be used to measure velocity. When the transducer voltage represents the correct rotation, nothing happens. When the reverse transducer voltage represents an incorrect motor rotation, then any two input power lines can be reversed. How can this be made automatic? What you need is a three pole contactor identified as a 3 pole double throw contactor. The contactor will have a double set of 3 pole contacts.
One set will connect the 3 phase power to the motor as normal. The second set of 3 pole contacts will also connect to the 3 phase power, but the connection to the motor will have any 2 of the power wires reversed.
Thus, the double throw contactor has one set of contacts that operate for one direction and the second set operates to reverse the motor. The contactor can only close one set of contacts at a time, that is why it is called a double throw contactor. The voltage polarity from the motor speed transducer will be used to operate the contactor in one connection and a reverse transducer voltage will cause the contactor to close in the other direction. You could call one set of 3 phase contacts F for forward, and call the other set of 3 phase contacts R for reverse. This scheme for motor reversal has been in use for years. If my memory is correct, it may be how washing machine rotation is controlled. The velocity transducer voltage will most likely not have enough current to operate large contactors, thus you need to raise the power level from the speed transducer. You can design any control circuit you wish to use, but a simple relay could be used to provide single phase ac power to the contactor input. The relay should be DC. For example, if the motor rotates correctly, the DC relay will not energize the contactor because the speed transducer will have a reverse dc to the relay and it will not operate. If the motor speed is incorrect the velocity transducer voltage will be opposite and the relay will operate, providing ac power to the contactor.
There are a number of ways to provide a speed transducer. A tachometer is one of simplest. A digital speed transducer is another technique and could be more reliable than a tach.
Thus in operation, if the motor rotation is incorrect, the contactor will automatically reverse two of the power wires and reverse the motor. If the motor rotation is correct, the relay does not operate and the contactor does not operate, and the 3 power wires to the motor do not change.
George Younkin, P.E.
Or if speed control would be of any value in your application, you'll get this intrinsically in a VFD also, since the drive rectifies the incoming AC to DC, then inverts back to AC, so motor rotation is determined by motor lead connection to the drive and drive control settings.
Change the input phase rotation and the motor will still rotate the same direction.
A VFD or Variable frequency drive is the option of choice, although a bit expensive. It will ensure that the motor never runs in the wrong direction. There are parameters that allow counter-clock/clockwise rotation and don't allow other.
Cheaper option can be a forward/reverse starter (I think it would be difficult to use direct online one at all places??) with a phase sequence indicator. Upon powering up, you will get phase sequence and then reverse through a selector switch whenever needed.