Motor amperage

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patpappas

patpappas

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Name plate rating on a 30HP 208v 3PH motor 39.8A, the motor is used on a water well, it is been pumping since 1996, the amperge draw is reading 45A, 41A, and 40A. No one knows what the amperage started at in '96, could these numbers show the motor aging or just maybe to much load on the motor from the start.
 
patpappas said:
Name plate rating on a 30HP 208v 3PH motor 39.8A. . .
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I am confused, but that is not hard for a motor to do to me. ;)

Table 430.250 tells me that a 3 phase, 208 volt, 30HP motor should be calculated as comprising a load of 88 amps. That is nowhere near what you are describing. Are you sure you have the numbers right?

As a motor and the associated mechanical components age, an increase in friction can certainly result in an increase in current draw. I would tend to judge the health of a motor first by whether it is abnormally hot to the touch, when running under its usual load. Can you speak to that issue?
 
A good method of tracking a motor is to megger regularly.

I cannot address the disparity in readings between 430.250 and your nameplate. BUT there will always be some discrepancy in a hand held readings due to location of conductor in the clamp, placement of the clamp in relation to being off perpendicular to the conductor, and acceptable errors in field measurements.

Add to this differences between the motor readings normally (seldom is are the readings exact) and then there is the difference in line voltage between the phases which result in amperage varying.

1. Meg the motor.
2. Measure Phase to Phase voltages
3. Take conductor amperage's with clamp on perpendicular to the conductors and the conductor's dead center.
 
080819-1308 EST

patpappas:

An easy quick check follows:

Assume a pure resistive load and this will require the least input current for a given output power. Line current is in phase with resistive voltage if you assume a Y connection. Thus, use 120 V and 39.8 A and the calculated total power is 120*39.8*3 = 14328 W or 14328/746 = 19.2 HP. There is power factor and losses and at 39.8 A input you are not close to 30 HP output. Probably real output is closer to 19.2*0.8*0.9 = 13.8 HP. 30 HP = 22380 W output and this calculates to a line current of 62.2 A for a resistive load. Power factor and motor efficiency will raise this current. Charlie gave a value of 88 and that is probably a good estimate. This is a ratio of 0.707 for both power factor and motor efficiency. Or if equally divided 0.84 for PF and 0.84 for efficiency.

How is this motor wired? Y or delta? To what does the nameplate voltage refer? Same for current? What is your line to line voltage? Is your source Y or delta? When running continuously can you hold your hand on the motor?

.
 
charlie b said:
I am confused, but that is not hard for a motor to do to me. ;)

Table 430.250 tells me that a 3 phase, 208 volt, 30HP motor should be calculated as comprising a load of 88 amps. That is nowhere near what you are describing. Are you sure you have the numbers right?

As a motor and the associated mechanical components age, an increase in friction can certainly result in an increase in current draw. I would tend to judge the health of a motor first by whether it is abnormally hot to the touch, when running under its usual load. Can you speak to that issue?
Click to expand...

If this motor is a special motor - which it is - then table 430.250 is out the window.

It sounds like an oversized motor to me. Similarly a motor at 10000' feet would be derated from it's nameplate rating, but if the motro is designed for that operation then the nameplate would automatically list the lower amperage, nu?
 
My bad, I was talking with an associate about 208v and typed that in, the motor is 480v 3PH. Current draw is 41A, 44A, and 38A, the name plate is 39.5A. The well is 233' deep and 64 lbs present at line pressure. Sorry for the confusion.
 
patpappas said:
My bad, I was talking with an associate about 208v and typed that in, the motor is 480v 3PH. Current draw is 41A, 44A, and 38A, the name plate is 39.5A. The well is 233' deep and 64 lbs present at line pressure. Sorry for the confusion.
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That's better:smile: .

Seems like you may have a voltage imbalance. That could be the cause of overloading. With this 'overload' ~10% and proper cooling the motor will last for a long time if only this is the problem. Correct the voltage unbalance - if any - and you will see the currents lowering and the imbalance is improving. Your cable and winding resistance imbalance could also contribute to the imbalance and by rotating the phases around you can also notice improvement.
 
Much better. Amperage is unbalanced, but still may be within SF. Rotate the leads as suggested, that does seem to help on occassion.


I have never heard of just rotating the leads to possibly correct this problem, that simple enough, I take it XYZ to become ZXY.
 
brian john said:
A good method of tracking a motor is to megger regularly.

I cannot address the disparity in readings between 430.250 and your nameplate. BUT there will always be some discrepancy in a hand held readings due to location of conductor in the clamp, placement of the clamp in relation to being off perpendicular to the conductor, and acceptable errors in field measurements.

Add to this differences between the motor readings normally (seldom is are the readings exact) and then there is the difference in line voltage between the phases which result in amperage varying.

1. Meg the motor.
2. Measure Phase to Phase voltages
3. Take conductor amperage's with clamp on perpendicular to the conductors and the conductor's dead center.
Click to expand...
Just out of curiosity, when you meg a motor, should the values be the same as when you meg a wire? In other words, should you be able to get almots infinite readings? Is it any different for AC motors and DC motors?
 
DanZ said:
Just out of curiosity, when you meg a motor, should the values be the same as when you meg a wire? In other words, should you be able to get almots infinite readings? Is it any different for AC motors and DC motors?
Click to expand...


If the motor is in excellent shape, yes.

Remember your infinity 2000 or 4000 or 8000 or a gigaohm not mine, as there are meggers out there that read into the teraohms.

I always put in my reports if meter maximum reading is 2000 megohms.

2000 megohms maximum reading form this instrument at 1000 VDC
 
I've done a lot of water well pumps/motors. The motor load on a centrifugal pump is controlled by pump. If the pump is not sized correctly, the motor will be overloaded. If the pump has no head, or discharge pressure, the pump is way off of its curve and the motor will be overloaded.
Typically you are allowed a service factor of 10% for well pump motors. meaning you can run over the rated nameplate amps by 10%, but this will result in additional motor heat and shortened life.
Try taking shut off amps - shut the discharge valve momentarily and see what the amps are, they should drop as you close the valve. don't leave it shut longer than necessary as you will overheat the motor -it needs the water flowing by it to cool it.

Also what type of overloads do you have? Typically need quick trip ambinet compensated.

Has anything changed on the load? more sprinklers, connections etc?

A one time megger reading means little. What you are concerned with is the trend. And I like to meg wells 5 minutes after running.

The best way to meg motors is to do a 10 minute dielectrical absorbition ratio, plot the readings at 30 seconds and then every minute for ten minutes and see what the curve is doing.

Biddle or who ever they are now has some great information on megger
testing.

Anyway check the pump load. Its not always an electrical problem...
 
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