Submersible induction motor surface voltage calculations 1

[MOSTAFA86]

dear gents

i have a question about ESP considering esp submersible motors
which is more practical and correct way in calculating motor surface voltage,do i use the motor name plate data rated voltage and ampere or i use motor rated voltage and use motor running amp in calculating voltage drop or not.i want to know if there is a recommendation from API OR IEEE as i searched alot and found nothing helpful.

best regards.

 

[d23]

MOSTAFA86
It usually doesn’t make much difference, so I would suggest using nameplate amps. If you look at a volt loss chart you will find it will only be 1 to 2 percent difference unless you have a very lightly loaded motor.

If you have a lightly loaded motor, say a 100 HP motor with a 50 HP load, you need to work with the manufacture to de-rate the motor. They can give you a new HP, Volt, Amp rating for the motor being used that matches the operating load. Then use the new nameplate amps for the calculation.

Depending on the motor, a submersible induction motor operating at 50% of its nameplate rating will only be about 75% efficient. That means that 25% of the power input is wasted energy that you must pay for every month on your power bill. Of the 25% wasted energy most of it becomes heat that could shorten the run lift of the motor. De-Rating will lower the power cost and cool the motor.

 

[1gibson]

Are you suggesting that new paperwork/nameplate will improve efficiency of an untouched motor running at part load?

 

[d23]

1gibson

That is an over simplified question to a very complex problem. I will try to give you a generalized answer without writing a book. Submersible motor basics:

1. Submersible induction motors used in the petroleum industry typically range from 1 kV to 4 kV nameplate volts at 60 Hertz.

2. Standard motor design used will be insulated between 4 kV and 5 kV regardless of nameplate voltage required.

3. The transformers typically used are multi-tap that allows 60 volt increments throughout the entire range. Basically any supply voltage is available.

4. Submersible motors are space limited; they must fit in a well. Typical sizes are 3.75, 4.5, 5.62 inch OD. The maximum length for a single housing can be up to 31 feet. Due to the OD size limitations we may manufacture a 450 HP, 5.62 inch OD, and 31 foot long motor.

5. Winding 20 to 30 foot long stators is a nightmare. Almost all stators must be hand wound. Submersible motor manufacturing is a labor intense process.

6. Most manufactures have a motor testing lab complete with dyno capabilities. You would think a onetime test is enough, but motor labs operate continuously year after year.

7. NEMA standards do not apply to submersible motors used in the petroleum industry. They are two different worlds.

During testing any motor manufacturer will make an efficiency curve by maintaining a constant voltage and varying the load. Submersible manufactures take this one off test a little further. We know the best efficiency with nameplate conditions. We can then vary the load and supply voltage to find the same best efficiency point at various loads. This sounds a little confusing, so I will use an example for a motor that I believe is no longer manufactured by any supplier.

Actual Nameplate Conditions:
70 HP,
1313 V
36.3 A
80.7 Efficient

If I want a lower rating for the same motor it would operate as:
56 HP
1218 V
32 A
80.5 Eff

I could go the other way with the rating.
87.5 HP
1412 V
43.1 A
80.8 Eff

The industrial world is limited to nameplate values simply because variable volts typically are not available. Realistically a motor does not know the nameplate value; it simply tries to drive the load attached to it. By adjusting the input we can match the motor to the load attached to it. We are not “simply supplying new paperwork” we are applying test data.

One thing I would caution you on as the load increases so does the internal heat rise. Any recommended adjustment should come from the motor manufacture. Manufactures know what their heat rise is, the effect of velocity past the motor, effect of specific heat of the produced liquid etc…

 

[1gibson]

Gotcha, wasn't aware that the voltage could be adjusted so readily. I assume this only has a minimal (different slip) change in speed?

 

[d23]

1gibson

There is a change in slip. Most submersible motors in the petroleum industry operate between 3450 and 3500 RPM (two pole) at 60 Hertz full load. Slip for a typical TEFC induction motor includes windage drag which is usually a minimum value. For a submersible motor, filled with mineral oil, windage becomes viscous drag and causes a noticeable increase in slip.

For the motor listed above full load speeds would be:
56 HP Rating = 3461
70 HP = 3450
87.5 HP = 3441

Quick note about efficiency. Compared to a TEFC motor submersible motors seem to have a low efficiency. The biggest part of that is due to slot fill in the stator. When winding long skinny motors we do good if we get 80% slot fill, usually less. Most TEFC motors will have 98+ percent slot fill.

 

[1gibson]

I guess we got a little off topic and may have scared off the OP, but great info, thanks.