Phase sequence detection methods

[nkn5]

Hello,

I have case to troubleshoot. We have two identical motors following attached schematic. The controller has an indication of 'phase reversal' probably comparing with what is defined in factory as correct. This indication makes the motor / pump unavailable for a remote start.

We are testing the controller with main power contactor disabled as we do not want to on/off the motor many times due to limited generator capacity. So when the incoming supply is available, it displays a phase rotation. One controller shows 0,120,240 whereas the other shows 0,240,120 when fed from same source. The power wiring is identical and correct in both. I doubt the secondary wiring connected to the controller which we are still checking.

This raised me a question, how its is possible for the controller to detect the phase rotation, given only two VTs as shown in the schematic and CTs out of the picture as the main power contactor is open. I'm in opinion we need all three phases to be able detect the phase rotation / angles. Is it possible to detect phase rotation based on two of three phases?

Could you please share your thoughts and experience on this?

 

[Skogsgurra]

Yes, that should be possible. The F4/F5 are connected to L1/L2 and the F6/F7 are connected to L2/L3. That makes angle between PT 1 secondary and PT 2 secondary either 120 degrees (if L1, L2, L3) or 240 degrees (if L2, L1, L3). A simple phase sequency relay or a dedicated piece of code in the controller (if freely programmable and fast enough) can evaluate the difference.

I assume, of course, that the IS is closed and that the MC is open.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

First find out if the phase rotation is reversed or if the phase rotation is correct but the controller is not reporting it properly.
In the absence of a phase rotation meter or a motor rotation indicator, locate a small three phase motor.
Disconnect both large motors.
It will eliminate the possibility of crossed wiring between the controller and the motor if this is done at the motor terminal boxes rather than at the controller.
(The voice of Hubris;
"It will be easier to do this at the controller. It can't make any difference."
Days and dollars later, The voice of humiliation;
"Well I didn't think it would make a difference. How was I to know that the wires were crossed between the controller and the motor on unit #2."
Voice of experience;
"Been there, seen that.")
Connect the small motor in place of first one and then the other large motor, using the same connections for the small motor as was used for the large motor.
Start the small motor and observe the direction of rotation.
This should show whether the phases are actually reversed or if the phase rotation indication is faulty.
Inadvertent phase reversal can occur when two motors are fed from opposite sides of a "Back to Back" MCC with common vertical bus bars.
Somewhere between the supply and the controller a connection may have been crossed.
Faulty indication:
This is probably a programming issue with the controller. When an open delta arrangement of PTs is used for phase rotation indication, the indication is often reversed from the true rotation.
This happens when the relay or controller does not have isolated input channels.
When a device has one terminal that is common to all input channels the phase rotation indication will be reversed.
When an open delta with phases A-B and B-C is used the "B" terminal is common to both phases.
You want to know the relationship of A-B to B-C. with B as a common terminal, the relay sees the second phase as C-B rather than as B-C. This must be reversed either by programming or changed connections for a true indication of the phase rotation.
(The voice of experience;
"To see the proper phase rotation of this selsyn motor we must invert one channel of the "scope."
The voice of hubris and authority;
"No we don't."
BANG
Days wasted and tens of thousands of dollars damage.)
By the way, page two of the drawing may have been interesting>

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[nkn5]

Thankyou Skogsgurra and Waross!! You just taught me phase sequence detection using open delta PT. I will study more about it to get my own understanding.
We are now testing the motor uncoupled from the pump for bump testing. So I wouldn't worry much even if the motor run reverse for a few moments. However before concluding that the phase sequence is indicated wrong, I made sure that the motor runs in correct direction.

We have a phase sequence tester(MFT1735), but isn't good for 6600V. I will check at the open delta VT secondary and see how they compare. Also we dont have 6600V small motor to test.

The controller and motor-pump are located on the same skid and cabling between the motor and controller is done by the vendor at their factory. Based on what I understood, vendor should have defined a phase rotation say 0,120,240(CCW) is correct for the motor direction to be correct. The switchgear feeding to this is ~400mtrs away whose connections are done by us. For this reason, I would prefer to correct the connections at the controller rather at the motor. However, in my case the motor runs in correct direction so I think problem is with secondary wiring. I still have to check this part.

The controller is for a NFPA 20 firewater pump. So to keep the certifications, I may not be allowed to reprogram. So conclude that the problem is with the program, I have to contact the vendor.

I attached the second page,- may not very interesting