3 phase motor rotation detection.

[SummitDawg]

Is there any hardware out there that will correct the rotation of 3-phase power input to a specified rotation? I have an application for a 3-phase motor (3 to 10 hp) that is on a portable conveyor, and can be moved from one power receptical to another. The portability of this device is such that it will operate at one site, and then be moved to another location. The 3-phase power supply may be clockwise, or counterclockwise, but I must guarantee that the motor will start the same direction each time. I'm not looking at phase correction, I'm looking for rotation correction. It would detect CW or CCW in the power supply, and output a corrected phase rotation. A reversing drum switch will not work for this application because it requires human input. The rotation correction must occur automatically.

 

[jraef]

They exist, do a Google search on "phase monitor relays" and you will get a plethora of choices. I would look for one that also checks line voltage level and phase loss as well, it won't cost much more. They will have a contact that changes state if the phase rotation is not as you set it. i.e. A-B-C, so just wire that into the main control circuit so that it it trips, they have no control power for the machine and it lights up a pilot light on the door that says "SWAP WIRES, FOOL!"

Good inexpensive brands:
Time Mark
SSAC (now ABB I think),
Diversified /ATC
Symcom
Syrelec / Crouzet


Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Barry1961]

A VFD does not care which way the phases are connected. A standard 3hp VFD will cost around $250 and a 10hp around $600. If there is going to be some volume you might be able to use an open frame OEM type and save some money.

Barry1961

 

[AFab]

A phase monitor relay can be used to control a reversing starter.
When the relay detects the correct phase then the fwd. will enable when the phase is incorrect then the rev. will enable.

 

[jraef]

That's not a bad option if you can afford it, because if the VFDs are properly sized it also allows the user to give it 1 phase power if that's all that is available on-site.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

Hi AFab
You probably know this but for those who don't;
The circuit that you describe is standard on many container mounted refrigeration units. For reasons that have been described in another post, reversing an older refrigeration compressor may lead to lubrication failure, hence the automatic phase correction.
yours

 

[OperaHouse]

Getting back to the circuit, I would use two phase relays for completely automatic operation. The first phase relay would operate a reversing relay and the second phase relay would operate the contactor (in conjunction with existing control circuits) that control the motor.

The first phase relay could be fooled by a loss of phase so the second phase relay must be used. The second phase relay would provide a delay (so the reversing relay isn't switching under load) with checking for a lost phase or low voltage as well as correct phasing.

 

[davidbeach]

Could all be done with one good voltage relay. Positive sequence voltage near nominal voltage indicates connection provides proper rotation. Negative sequence voltage near nominal voltage indicates connection provides opposite rotation. Anything else indicates other problems.

 

[waross]

Thermo King has done this for decades. Tens of thousands of units in service.
I just checked a typical scymatic. They use a reversing contactor. A phase rotation relay (Or protector) closes one or the other of the contactors depending on the phase rotation. A third motor starter is used for motor starting.
I think you can find them at

http://www.thermoking.com

Search drawings - Product Thermo King
yours

 

[jraef]

3HP reversing contactor ~ $180
3HP regular motor starter ~ $120
Cheap phase monitor relay ~ $120
----
$420

3HP 230V inexpensive VFD ~ $300

Even if you don't use the second starter to maintain normal control, I'd still use the VFD. Less wiring and more flexible.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

thanks for the pricing jraef
At those prices, I have to agree with you.
Many of the container units were dual voltage 240-480. They have an internal transformer to match the voltage when needed. The pricing for 480 volt equipment probably skews the economics in favor of the starters.
yours

 

[OperaHouse]

Certainly at 3HP a VFD is viable, but if you get up to 10HP that won't be true. On second thought, you can do it with two phase relays and two motor starters in parallel. Those prices seem way high, well under $300 seems more reasonable.

 

[sreid]

Th VFD would also provide some nice features for a conveyor such as variable speed control and soft acceleration/decelerations. Plus some built in motor protection.