Operating VFD above 60 Hertz on pumps

[mechconst]

Has anyone been involved in a project where the limits on a VFD were set above 60 Hz in order to get the pump to increase RPM's and maximum flowrate? Pumps in question are submersible 125 horsepower, 460 volt and operate on VFD's. I've never seen this done and it doesn't seem to be an industry standard - any pros or cons? Also, spinning the pump up means pumping more and pulling more amps, can a pump be operated at or above it's full load amps for continuous duty?

 

[BigInch]

The pump or motor cannot be safely operated above its power rating for continuous duty. Most all types of equipment can tolerate intermittant variations during start and stopping, but that is very much a different thing than continuous duty loads. For submersible pumps it is especially common to even see a suggested maximum number of starts per day, specifically to limit the amount of overloading that is known to occur during starting. You can always push limits, but never without eventually paying the cost of doing so. Higher speeds mean more unbalanced pressures and wear on the bearings to contend with in a pump and more heat in the power supply, wiring and motors to dissipate. The more you push the limits, the sooner all those turkeys will come home to roost. And they will come. Well... if there's still a home left for them to come home to.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[itsmoked]

VFD which tries to keep the V/Hz ratio steady cannot do so as you go above the motor's design frequency. As you speed the motor up the torque does not increase.


If the pumps have a service factor it can be permissible to run a bit faster but you will make things warmer and the chemistry rule of reactions doubling comes into play.

Running 10% fast may also be somewhat counteracted if the well water is substantially cooler than expected.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Provided the motor stator current stays within limits, then yes it can be oversped.

Below base speed (nameplate speed) the motor will behave in a constant torque, variable power mode with power proportional to speed. Above base speed it will behave in a constant power variable torque mode, with torque inversely proportional to speed so the torque will drop away pretty quickly as speed rises. That's usually bad news for a pump drive, but provided the motor power output (at the reduced torque) meets the load requirement at the higher speed then it will be ok within reason. Don't expect to run the motor at 10k RPM without some problems though!

You can sometimes play a trick with the winding connections which effectively moves base speed of a 50Hz motor up to 87.5 Hz so you can have rated torque all the way up to 73% overspeed, which is about 5000 rpm for a two pole machine. This probably ain't much use on a pump either, but allows a certain frame size motor to produce much more power than it could on a standard 50Hz supply. It is hard on bearings and the mechanical aspects really need thinking about.


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If we learn from our mistakes I'm getting a great education!

 

[d23]

mechconst

Operating submersible pumps above 60 hertz is a common practice in the petroleum industry however; you must plan for that type operation before buying or installing the product. As a somewhat generic practice we operate up to 75 hertz. There are a lot of units operating as high as 90 hertz.

If you do not use a transformer between the VFD and downhole motor then you cannot maintain a constant volts to hertz ratio. In this case the motor will operate in an extended speed range or field-weakening mode. The motor sizing (oversized motor) must be adequate to account for the higher speed.

If you have a 125 HP, 460 V submersible motor I would expect it to be about 160 amps. The quick way to determine F max is to operate the VFD in a field-weakening mode with the current limit set at the motor nameplate current. F Max can be calculated, but that is a lengthy process.

You will need to check with the pump and motor manufacture before exceeding 60 hertz. The motor should have insolating bearings to prevent pitting. The pump will need adequate radial support bearings for the higher speed.

Assuming an industrial type centrifugal motor it will have a pad type axial thrust bearing. For a pad bearing the thrust limit will increase linearly with speed. The thrust generated by the pump will increase at the square of the speed change. At some point the axial thrust bearing will be overloaded.

You should also consider shaft torque limits. Increasing rotating speed will increase the torque required at the cube of the speed change.

D23