Speed of Submersible motor 1

[Grover194]

When full load test and running Industrial size submersible motors (up to 250hp,water filled type from 6" to 12" diameter) and pumps,it is often required to know the accurate speed of the motor.The shaft is not acceptable, as it is coupled to the pump and it is submersed (Up to 100 meters or in a 3 meter test tank).The voltage ,full load current , p.f. is able to be recorded.The pump out- put is dependent on the speed,therefore any variation (slower) will decrease the flow.The rpm will vary from one manufacturer to another.
At an older test plant many years ago I saw a Coil of winding wire,at slightly bigger diameter, placed over the test motor, with Two leads going to a milli amp or milli volt meter the meter was centrer zeroed with plus on the left and minus on the right.The personal had changed and no one knew any details(turns, gauge,)ratio etc.or how to convert to rpm.I can imagine this would be like a secondary winding of a transformer.
Does any one know or able to offer any advice or technical support or alternative method of speed recording?
Can any member offer any links to technical information to submersible motors (not pumps) Design , efficieny ,laminations ,winding data etc.

 

[DougMSOE]

The method that you describe is "magnetic slip frequency".
This method is based of the slip freq of the rotor compaired to the synchronous speed of the stator.
Any good book on induction motors will describe the slip frequency. Older books will have this info in it. Calibration is straight forward.

 

[Shortstub]

Mechanical:
Try measuring the vibration on the downhole pipe flange!

Electrical:
Although I've not tried it nor do I have concusive evidence of its efficacy, I did hear of Fourier Analysis of stator current to pick up slip speed! Perhaps one of the motor design folks can help!

 

[Grover194]

Thank you Doug, can anyone refer me to a title and/or publisher of any reference book on slip frequemcy.

 

[d23]

Grover194:

I'm not sure how bad you want to know RPM, but there are products that do this for sub-pump / motor systems. Try this link just as an idea:

http://www.slb.com/Hub/Docs/connect/completion/Progressing_Cavity_Pumps/Phoenix.pdf

Phoenix is a petroleum type product so an adapter and special consideration would be required. They have been doing motor winding temp, well fluid temp, vibration, intake pressure, discharge pressure even RPM over 20 years. This would not be a cheap solution for your problem. My only point here is that products and technology already exist.

You could design and build an adapter device to go between the motor and pump that will read true RPM. It's just a couple more wires on the finished system.

Considering possible voltage at the motor terminals verses surface voltage, viscous drag of the rotor verses windage etc. calculations may be off by some fractional amount.

I'm not sure how accurate you need to be or the value of the information that your looking for.

Good luck!

 

[d23]

Grover194

I got a little lengthy, sorry! Is something like a proximity switch and counter circuit a reasonable consideration for your problem?

 

[jbartos]

Suggestion: Visit

http://www.zyn.com/flcfw/fwtproj/Contactl.html

etc. for more info

 

[buzzp]

When I was testing downhole equipment, we had a little device that would go on top of the well head with no electrical connections which would display the RPM's. It used vibration has shortstub indicated. Sorry, I can not remember the mfg.

 

[Grover194]

Thank you Jbartos this site is the type of device I require.
My application is every day testing in the workshop test tank under no load conditions ,(no pump attached) and with pump attached,in field operation.
In Australia 50 Hz is only available ,this results in a decrease ( compared with 60Hz ) of speed hence a decrease in pump output compared with the same size pump for 60 Hz. Calculation of speed is critical (or knowledge of accurate speed)Typical RPM is 2850 very approx.
Reasons in brief : the authoraties are very cost cons.,actual cost of gall per min V's kwatt,effiency, p.f.,etc.

Rpm varies a lot from motor to motor.If a user inter changes from one faulty motor to another brand of motor , a change in flow out put will occure,
The end user is always asking for the RPM,with the rest of the workshop test results.
Manufacture spec sheets are not always accurate ( or Truthfull)for a given Motor.Not all motors are Full load Dyno tested in the manufactures production plant.

D23,no, the shaft is not accessable.
buzzp,this could be practicable, any clues from members welcome.