Motor Trips on Current Imbalance During Startup
Motor Trips on Current Imbalance During Startup
(OP)
We have three phase induction 160kW each motors driving pumps to pump water. One of the motors is tripping on current imbalance during startup. The motor is supplied from the switchboard by the transformer. We have changed the motor have changed the CT's we have changed the relay, we have changed the supply contactor .... what could be causing current inbalance at startup???





RE: Motor Trips on Current Imbalance During Startup
Consider adding a time delay to override imbalance trip during the start up. If it runs OK after the dealy, then all wiring and CT are OK. If not, check the wiring.
The tranformer may have unequal impedance between the phases. What type of transformer is it?
Check for source of voltage or current imbalance, inclduing tranformer taps etc. See if all parallel condudtors in all phases are good and sharing the current equally.
RE: Motor Trips on Current Imbalance During Startup
What level of ub is set to trip? 5% is a typical setting for this. This can be changed depending on how loaded the motor is.
I think the time delay is key here. Generally, UB trips occur in no less than 2 seconds but I normally set them to ten seconds. Verify the delay is something reasonable and measure your supply voltage as close to the tripping motor as possible to watch for voltage dips at start-up and, ideally, read all three voltages.
If the installation is an existing one that has ran in the past then the time delay should be okay but still verify the supply voltage is acceptable on all three phases at startup.
If the supply is ok, look at the contacts on the contactor for pitting, wear. I have measured the voltage drop across the line and load side of these sometimes just to see the voltage drop across them. Sometimes this can help troubleshoot worn contacts other than a visual inspection. Loose connections may also cause this. Just think, any added resistance will reduce the voltage at the motor terminals. Hopefully this gets you started.
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
I cant imagine this affecting anything unless leads were shortened or lengthened, if so, better look at burdens on the CT. Seems marginal, I know, but still possible.
What is your current unbalance set at? What about the time delay?
Is this the last motor on the feed? If so, I strongly believe that it is just a nuisance trip due to the reduced voltage at start-up. There is nothing that guarantees that all three phases will drop equally. Voltage imbalance causes a current unbalance. Say VUB is 5% then CUB could be 6-10 times this or up too 50%.
Your last sentence is not clear but if the motor is up to speed in less than 2 seconds, I still think your trip time/curve for CUB is set to low. No motor is going to damaged in less than two seconds as a result of current unbalance (I do not consider loss of phase to be unbalance).
RE: Motor Trips on Current Imbalance During Startup
I am suspicious of high resistance points in your motor feeding line. Make an infrared checking looking for hot spots on all connections and contactors, this problem is more critical under the high starting currents since the losses are proportional to the current squared.
RE: Motor Trips on Current Imbalance During Startup
Assuming DOL, I'm thinking along the lines of aolalde. Since you say that "some components" were changed in the switchboard, I wopuld start there. A slightly higher resistance at a bus bar connection point may not show much at normal currents, but at high starting currents it becomes rapidly worse. That can take place anywhere in the circuit, but since you mentioned that some changes had taken place in the switchboard prior to this occurring, that would be the likely starting place. Aolalde is right, a good IR thermal scan should show it.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
There could be any one or moer of numerous causes but I dont think this is a serious issue. One probability may be some deformity in rotor?
RE: Motor Trips on Current Imbalance During Startup
1. Checking the percentage voltage unbalance. should be between 1-1.5 %. Limit is perhaps 2 %. At startup, it could be more pronounced with some inherent phase resistance unbalance.
2. Check the motors phase resistances with a DLRO. Unbalance between resistances should be less than 0.5 % , i think i remembered the figure correctly.
3. Note the motor acceleration time and check it against that of other similar application motor. It should give you some idea for the curve setting.
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
Every supply has an inherent imbalance in impedance and in voltage vectors. Every motor has an inhgerent imbalance due to the grain orientation of the steel used for the laminations.
Often the imbalance can be reduced by rotating the three phases around so that the motor and supply imbalances complement each other. This may help in this instance, however it does sound as though you have an intermittant source impedance somewhere.
I am curious about your comment that two phases become 180 degress for a few milliseconds. This would be very hard to determine when you are dealing with fractions of a cycle. If you mean tens of milliseconds, then fair enough, I would be looking at a supply issue. What does the current on the third phase do during this period? If you are referring to fractions of a cycle, then this could be an indication of additional current going somewhere that it should not, and I would be looking at an intermittent shorted turn in the motor stator.
I have had situations before with such intermittants that would not show up under any normal testing, but did under controlled heating of the motor.
One suggestion, a difficult one I know, is to swap motors between a working installation and the faulty one and see if the problem moves or remains. That is one way to split the field.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
Check the wiring of your metering equipment that you used to make this determination.
If your worried about the transformer, do a doble or similar test on it. Any issues will be quickly identified especially if it is causing the strange phase relationships.
I still think the thing is nuisance tripping but the 180 deg shift has me puzzled if it the equipment was hooked up right.
I am not familiar with nuke specs but outside of that, I would never set unbalance trip to be instantaneous.
RE: Motor Trips on Current Imbalance During Startup
RE: Motor Trips on Current Imbalance During Startup
That 180 degree phase sounds to me like a similar problem on the primary side of the transformer. Metal oxides and sulfides make for tremendously nonlinear semiconductors especially because of the high impurities in naturally occurring corrosion. You probably have a connection that turns into an effective open circuit at high current and then recloses at lower current. Essentially a Mickey Mouse self resetting circuit breaker.
RE: Motor Trips on Current Imbalance During Startup
I had a problem some years ago when Iwas chief engineer in a paper mill,I worked my way back to the switch room removed the rear of the cubicle and gues what, one copper bar of the red phase in to the back of the isolater had come loose,it was pitted around the washer.I did a resistance check on it,it was ok but when a large current was taken you could see a slight arking. we made a new copper conducter fitted it and the system ran ok.