Three phase motor current. 1

[QUAVIET]

I am having trouble understanding the current of a three phase motor. Say I have a 500HP Y/D refrigeration motor with overload protection. The overload protection is sensed by T1. T1 pulls 520A, T2 pulls 550A and T3 pulls 570A at full load. FLA=570A, 460V with a service factor of 1.5. It seems to me that one should monitor T3 since it pulls the most amps but there is nothing which directs me to find the tap which pulls the highest current to monitor for an overload condition. The motor and screw are operating fine. With the preceeding amps I mentioned what is the current pull of the motor, logic tells me to take the average.

 

[motorsdirect]

Hi Quaviet
The variation in current may be caused by a difference in the supply voltages..to confirm this you could move the supply phases around and monitor the current draw then!!
Then, i would call the current draw of the motor at the maximum....but that would just be my preference....and would move the supplies around until the max current draw was through the o/l.
Jeff

http://www.motors-direct.co.uk

 

[bklauba]

You are right, monitoring just one is only making the assumption that T1 is representative of the other legs. Best to monitor them all. What would happen if you lost T1? How long do you think single phasing thru T2 and T3 could be tolerated?

In such situations I recommend using a sophisticated electronic motor protector, such as the Sprecher & Shuh CEF1-52 ( 160 - 630 FLA ), that provides thermal O/L on all three legs, single phase/phase unbal protection, electronic O/L protection and thermistor overtemp protection. These cost a pittance, compared to the repair costs of such a motor.

BK

 

[QUAVIET]

I think that you all missed my original question. If a motor is pulling 520A on phase A and 550A on phase B and 570A on phase C. What is the average current?

 

[edison123]

Overload condition, you mentioned. Then, why would you take the average if you want to protect the motor ? As Einstein said, if I put one foot in boiling water and the other in ice, I wouldn't be still comfortable since the average is only 50 deg c (not that 50 deg C is by any means comfortable).

To be serious, your O/L should monitor the maximum current since that is the one that will burn your motor if it exceeds the rated current. But then, you should seriously investigate why you have such unbalanced currents (as suggested by others) before playing around with averages.

 

[buzzp]

An overload is not an overload unless it monitors all three phases (NEC requirement). In the old days they tried monitoring only two phases with single phase conditions burning up motors.
The overload is designed to protect your motor. If you only monitor one leg then your not protecting the motor.

If you have to monitor only one phase, temporarily until you get a replacement, then put it on the high leg for now but there is no guarantee that this high leg will remain on the same phase.

Get a three phase overload relay and protect the motor correctly.

 

[jraef]

I think bklauba had a very good point. The UNBALANCE in this application is of more serious concern than averages, which leg etc. A 50A delta on a 570A motor is nearly 9%, an unbalance that will cause a seriously disproportionate heating efect in the motor. Many solid state motor protectors will plot that unbalance into the thermal model and bias the OL tripping curve to correct for that extra heating effect. Standard bimetal or melting alloy OL relays cannot do that, and motor damage may result by the time the OLR trips.

Just for clarities sake however, all modern OL relays DO monitor the current in all 3 legs and trip based upon the highest current in any one of them. They do not "average" anything. As Buzzp mentioned, 2 leg OL protection went by the way of dinosaurs some time ago, and not just in the US. QUAVIET, if you only have an OL relay on one leg, you are in trouble no matter where you are.

"Venditori de oleum-vipera non vigere excordis populi"

 

[rbulsara]

Buzz P is correct.

As for the original question, the current pull is what you measured for each phase.

The 'average' is meaningful for limited use.

The 'average' is a meaningless value for O/L as each phase need to be monitored not to exceeed rated current.

'Average" may be helpful, if you are trying to come up with kVA drawn, using the formula 1.732*kV*A.

Or in otherwords, if the accuracy with which you need to know the current in each phase depends on what you want to do with it.

You need to differentiate between the need to have precision vs. accuracy.

 

[QUAVIET]

Thanks for the input. I was looking for theoretical answers which you provided. To the meat of the problem. The motor is protected by monitoring all three phases with its seperate circuit. The motor drives a screw compressor. The ammonia screw compressor electronics only monitors one phase though and in this case it is the phase which pulls the least current. The screw compressor electronics determines how much loading to place on itself from the single phase monitoring of the motor current which in this case is the leg which pulls the least current. Nothing in the literature of the screw compressor tells me to seek the leg with the highest current to place its CT transformer to determine motor loading so that it knows how much to open the slide valve. Logic tells me that it should be on the leg with the highest current. I was asking about average current because I thought that it may give me a more accurate representation of motor work. In re to the inbalance of motor current, idealy all should be equal but in larger motors as long as the inbalance is below 10% it is not a problem.

 

[tinfoil]

Quaviet:

I am curious which reference you are citing that suggests up to 10% current imbalance is 'acceptable'?

I have never seen one that suggested more than 4-5% sustained imbalance would be acceptable. Is this a special-built motor of some sort?

If it were me, I would be trying to find out where the big imbalance is originating. Do you know what your three different phase voltages are?

 

[electricpete]

In response to tinfoil's question about acceptable current unbalance- There is a curve for derating as a function of VOLTAGE unbalance:

http://www.joliet-equipment.com/voltage_unbalance.htm

Roughly speaking we can interpret that the current unbalance will be approx 5x the voltage unbalance.

So 10% current unbalance roughly corresponds to 2% volt unbalance at which operation is acceptable with 0.95 derating factor. I suspect many applications are below 0.95 times full load and this derating is not a huge concern.

With that said, in my particular plant if I saw 10% current unbalance on a 4kv or 13.2kv motor near full load I would be very concerned - not from a heating standpoint but because it is simply abnormal for us. We are at a power plant and have very stable balanced voltage and never expect to see 2% voltage imbalance and 10% current unbalance. Therefore we would investigate if we did see it.

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[electricpete]

I guess there is also the issue of negative sequence component of unbalanced voltage causing reverse-rotating field which causes heating of the rotor. It is more a concern for large motors than small motors due to skin effect concentrating this current on surface of the rotor. I hear this mentioned as a basis for unbalanced protection of large motors. I have to say I have never heard of a rotor being damaged by unbalanced voltage during steady state (although it may be an aggravating factor during start).

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[buzzp]

So your not talking about an overload then? This current is monitored to change the process?
Likely, the reason they don't tell you which leg to put the CT on (low or high leg)is because they assume they are balanced. Not knowing more about what is going on with this control circuit, it is hard to determine if you should be on the high or low leg. Seems in this case, the average current may be better but as I said I don't know the system.
Voltage unbalance requires derating motors (NEMA)with even a 1% unbalance (although the derating is rather small at this point). The current unbalance can be 6-10 times this voltage unbalance (some only say up to 6 but I have seen 10 as well as some other sources).
Have you ever tried rolling the leads to see if the high leg stays on the same source leg? There are three possibilities of wiring while still maintaining the same rotation. One of these will give you the smallest current unbalance. This is often used to determine if the source of the unbalance is because of the load or source. In your case, I am sure it is your source (given your voltage readings) but you may be able to make the current unbalance better by rolling the leads.
I would call the mfg of this product and tell them your situation and see which leg would be best to monitor.
Not knowing anything else, I would put the CT on the middle leg (550A).

 

[QUAVIET]

I thought I would critique the ex-problem I described. The solution was accomplished due to present and past forum responses and it was something I did not anticipate. On larger motors i.e. 400HP and above which are high speed i.e. 3700RPM and the bearings and the stator have been replaced, then the problem is that with the rotor uncoupled it should pull 25% of FLA. My motor was pulling 50% FLA uncoupled. Centripical forces vs centrifual forces caused the rotor to deform. Too bad it's a $50K motor. Thanks again, Bob