motors

[mark480]

We have motors that are drawing uneven currents.Parallel feeds from soft start motor starters. The problem is that I cant figure out why each leg draws different current readings.They are 480 volt 3 phase 100hp induction motors that drive a gear box the current is on# 1 of the parallel wires A phase is 89amps the #2 wire A phase is 100 amps B phase #1 wire is 98amps #2 wire B phase is 110amps and C phase #1 wire is 110 #2 wire is 90 amps . I know the wire size is the same,length material,cir mill area,terminated in the same maner as per code. Why would the current be different.By the way the voltage is a-b 489v b-c 494v a-c 491v any info would greaty be appreaciated. I am thinking about the voltage relation to current

 

[bphgravity]

Re: motors

Manufacturing discrepancies?

 

[charlie tuna]

Re: motors

you know what they say about oppinions!!!
first of all, just consider what happens when yoy balance the loads in a panel. you only have to shift half (or less) of the amount of unbalance to bring the load in balance. so look at the amount of unbalanced current in your case to half in each case--which isn't alot of current. parallel feeder lengths are critical to balanced current carrying of the shared phase leg. remember that amperage is a direct number used in the voltage drop formula. six or seven feet is all it takes to shift the current to the other parallel leg. my working buddy and employee used to caution me about parallel feeder lengths -- and we ran into a situation where we had to run eight 500 mcm per phase 110 feet long to re-feed a 4000 amp switchgear after it exploded. it was an emergency deal and we were rushed to get the building back "on line". we didn't check each feeder length from the supply house --- just rolled the cables out and started connecting them up. i was suprised to find out what a two foot difference had on the current carrying cable. some were up around 400 amps and another on the same phase was at 250 amps! as the loads got higher during peak load periods the unbalance difference reduced. since the cables were run on the floor from the power company's vault into the switchgear room, we built a plywood cover over the top of the cables and used fans at one end to force ventilation around the cables to reduce the heat. since that time i was always very critical of parallel cable lengths. we have installed jumpers on large bus ducts and still find a sizable difference in parallel feeder's amperages --- knowing the cables are the same - i could only think that it had something to due with the copper inside the cable???

 

[mark480]

Re: motors

Thanks alot charlie tuna & bryan for your comments I think that the lentgh seams pretty close but the material is another concern.The motors are new and supposable terminated in the pecker head the same.I was just trying to see if I missed something maybe internally in the motors.

 

[mark480]

Re: motors

Thanks alot charlie tuna & bryan for your comments I think that the lentgh seams pretty close but the material is another concern.The motors are new and supposable terminated in the pecker head the same.I was just trying to see if I missed something maybe internally in the motors.

 

[peteo]

Re: motors

Mark, I think your original write up is not clear. I understood that you had two identical starter/cabling/motor setups, and that your question was about 1)'why would the motors have unbalanced currents.' Bryan seemed to understand this as well. Then I reread it, and you talk about 2)"parallel conductors" A #1/#2, etc., and it seems that Charlie had something very interesting to share which I'd certainly never encountered.
Now we're back to 'same length,' and why would motors behave this way, right? Are the parallel you've described, two identical setups next to each other? If so, found a writeup about this at http://www.cinergypsi.com/Exploring_Energy/Power_Quality/tech_tip_seventeen.asp ?a 1% voltage unbalance can easily translate to 5% to 9% current unbalance on induction motors.? It?s my experience that you?ll get a much larger difference than what you describe, if you try different meters (RMS/ averaging/ response time) for the same measurements.

 

[charlie tuna]

Re: motors

again, just my .02 cents... consider how an induction motor works --- only opposition to current flow is due to induction reactance. then consider the difference in a winding's resistance between a 1/4 h.p. motor and that of a 100 h.p. motor. due to the large wire size used in a 100 h.p. motor, it's resistance will be very low. anything that effects the voltage to the 100 h.p. motor will cause a large shift in current draw to another parallel conductor. and it has been my experience that parallel conductors that are overloaded seem to shift this "difference" more than when loaded within their rated current. we had an emergency jumper arrangement feeding the upper 1/3 of a 55 story building. we used parallel 500 mcm and it was cut with a two foot difference due to make-up room --- now the jumper was only 15 foot long. we also underestimated the load. end result was the shorter parallel run was drawing at peak demand times 740 amps, while the longer parallel phase was drawing less than 300 amps. i thought the heat generated by the high current would tend to shift the current due to resistance--no way! these cables were run in free air between floors.

we learnt the importance of exact lengths of parallel conductors even if we had to make a loop inside a switchboard. looks funny, but it balances the load between parallel feeders!

 

[don_resqcapt19]

Re: motors

charlie,

and we ran into a situation where we had to run eight 500 mcm per phase 110 feet long to re-feed a 4000 amp switchgear after it exploded. it was an emergency deal and we were rushed to get the building back "on line". we didn't check each feeder length from the supply house --- just rolled the cables out and started connecting them up. i was suprised to find out what a two foot difference had on the current carrying cable. some were up around 400 amps and another on the same phase was at 250 amps!

There was something else going on to change the impedance of the parallel cables other than a two foot length difference. The current division in parallel cables where everything other than the length is the same is a function of the conductor length. If there is a 2 foot difference in each of the 8 cables (116, 114, 112, 110, 108, 106, 104, and 102') and the total load is 4000 amps, the currents will be ~468, 477, 486, 495, 505, 514, 523 and 532 amps (longest to shortest) assuming that the only difference is the length. Cable configuration, terminations and other things also can change the division of current, and I don't doubt that you had that much difference, I just don't think that the reason was a 2' difference in length.
Don

 

[tom baker]

Re: motors

What I have read on motors with unbalanced currents is to roll the leads, A>B, B>C, C>A.
This is in the NEMA information on motors. I've never tried it, though.

 

[mark480]

Re: motors

Thanks alot!!!!!!!!!!! for the comments. Peteo that web site was helpful.I do appreciate all the info from the members.I had a meeting with 2 engineers and they had no clue.I think I got into more bull then I could take. The conclusion still sits, but I had fun making a fool out of the county engineers standing there scratching there heads.One guy had a thermal gun the other had all kinds of testing equipment at the end of the survey nothing was answered. Thanks alot for the great information ...