Soft start fault

[Thedroid]

Interesting problem today. We had a big mill go down several times last night due to a phase loss fault monitored by a Motortronics MVC Plus 4160v soft start. Pulling the data from the event recorder in the MVC shows that the amperage at the time of the fault was 242, 154, 151, and the the voltage was 4220, 4219, 4214, on phase A, B, C. Interesting thing also was the recorded power factor was 1.21.

We also monitor this motor with a Multilin 469 and SPM combo. Neither one of these units recorded any type of problem. The 469 and MVC don't share the same CT's so I'm thinking the there is a possible intermittent problem with the Temp/CT board in the SCR stacks in the medium voltage compartment.

Previously we were getting all of our readings in the control room off of the 469 and the SPM through a modbus connection. We hooked up the MVC today so we could monitor it's metering data also. I'm hoping that we can isolate the problem by comparing the readings. We opened the medium voltage compartments and checked the wiring and the connections, but couldn't find any obvious problems, and there was a rush to run the machine. It's running OK right know, but I'd like to find the problem as soon as they're down.

 

[davidbeach]

I'm not sure that I'd trust anything out of a device that can tell you the power factor is 1.21.

 

[waross]

We don't allow over unity discussions on this site.
LOL
Do you have an offset neutral?? We had a problem with a soft start tripping on voltage phase shift. The utility had a very long (maybe 20 miles) of two phase plus neutral distribution line. Our plant was the last on the three phase line. The heavy neutral currents and the reactance of the lines gave us quite a bit of current on the neutral.
Can you safely check your phase to neutral voltages? In my experience, I see more voltage difference phase to neutral than phase to phase. Your soft start may be responding to a low phase to neutral voltage or annunciating a voltage phase shift as loss of phase. In this condition, the phase to neutral voltages are no longer at 120 degrees to each other.

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

 

[jraef]

When the connection from the Temp/CT board is lost, the CT signal to the mP goes high from that circuit. It's their indirect form of wire break detection. Could be a bad component on the board, a bad fiber optic connection or even a bad CT. But if it were the CT, I doubt it would work now. Older Temp/CT boards had squirrelly power supplies. If it's a relatively new starter, could be a new problem I'm not aware of.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Thedroid]

Whats tough is that it's an intermittent problem. I agree the 1.21 power factor is definately weird. I don't think there is a neutral on the supply it's 4160 delta secondary whith only 3 leads going out to the equipment. There are 2 PT's providing our 120V control power. I also think that our power supply is good, because we have a second set of CT's hooked up to a 469 that didn't pick up any inconsistencies. These starters are about 2 yrs old, and havn't given us any problems at all. I'm still suspecting the temp/ct board has a problem. It would be easier if it just failed all together.

What could cause a device to read a power factor of 1.21 anyway. I believe that MVC reads a power factor of .020 or something like that while it's running also. The 469 shows the expected .96 - .98.

 

[jraef]

Hmmm... thought I had answered that. Maybe you didn't see it or understand. If even one of the current signals goes essentially to infinity and stays there regardless of the measuring point of the sine wave, it will skew the phase angle differential that is being measured. The results are going to be unpredictable, but showing an extreme leading pf would not be unlikely.

The Temp/CT board is just an interface that accepts the Temperature Sensor from the stack heat sink and the CT secondary signal as analog values, then digitizes them through an A/D chip. Then it also does the fiber optic conversion for sending the values back up to the mP in the control unit, isolating the mP from any noise that could be introduced if wires were used. No other MV RVSS manufacturer does it this way so nobody else is going to understand what a "Temp/CT board" is by the way.

As I think about it more, the problem couldn't be the entire Temp/CT board or the fiber optic connection however, because you would get the same problem on the temperature sensor; it would tell you that the heat sink was 65,536 degrees C (the max value). So it is likely either a bad connection from the CT to the Temp CT board or a bad CT. Motortronics makes their own CTs, they have had internal quality problems in the past so it's not unheard of. It could also be a bad A/D chip on that board as well, that has happened too.

Those boards are not terribly expensive and are very easy to swap out, have you tried contacting Motortronics?




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Thedroid]

I didn't understant that it would also affect the power factor reading. I did notice however a temperature alarm after turning on the power. It reset right away and the machune started. I have several whole SCR stacks with CT/temp boards on them. I talked to the manufacturer and they sent me the DC offset procedure. If the problem happens again I will replace the board. Any other ideas?

 

[Thedroid]

Also, there is a bigger CT around the the phase lead and it's lead goes through the board CT. Would it be recommended to replace that one also? I would think it would either work or not, and from jraef's descriptions the problem seems to be in a component on the board.

 

[Thedroid]

Checked the currend trend from both CT's today, and phase A from the soft start had numerous spikes up to about 230A. One of these spikes took the mill down. I'll be installing a new CT board asap. Until then I disabled the phase loss trip.