Speed increase by 6 pole to 4 pole

[ad90]

We have a pump drive that is currently a 6 pole motor, to increase pump capacity it has been suggested that we change to a 4 pole motor(1200 -> 1800 rpm).
Is it really that simple?
What increase in current (starting and full load) can we expect?
Will the motor be the same frame size?

Thanks

AD90

 

[ScottyUK]

Yes, it is simple that the pump will rotate roughly 50% faster than it did with the 6-pole motor. As for whether the pump will withstand this overload, good question. Is the suction and discharge piping sized to cope with additional flow? You will need to look at the pump curves to determine the new motor rating. Instinctively the new motor will be significantly larger in rating and likely at least one frame size larger.


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

 

[ad90]

Thanks Scotty,

It is a quite a small pd pump and we have confirmed that the piping is OK and that we will still have enough NPSH on the pump, and it was the pump OEM that suggested the motor change.

As for rating, does the rule for Centrif/Axial Pumps (BHP2=BHP1x(N2/N1)3)[That should be to the power of 3]apply? That makes the power requirement 3.375 times greater!

 

[electricpete]

As for rating, does the rule for Centrif/Axial Pumps (BHP2=BHP1x(N2/N1)3)[That should be to the power of 3]apply? That makes the power requirement 3.375 times greater!

For PD pump: Flow rate ~ N. A given system characteristic will obey DP ~ Flow-rate squared ~ N^2 for high velocity turbulent flow. Fluid horsepower ~ Flow rate * DP ~ N * N^2 = N3. That suggests the motor power draw is roughly proportional to speed squared ASSUMING:
1 - efficiency of pump does not change significanltly accross the range
2 - the fluid system does not change. * Often times the system has throttle valves or parallel paths that are switched in/out - changing these will have a BIG effect.


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[itsmoked]

What hp motor presently?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

ad90
depending on the HP/kW then the frame size could change, meaning a different flange and shaft size.

 

[MotorVib]

By the way it is not 50% change in speed

4 pole @ 60 Hz is about 1800 RPM's & 50 Hz is about 1500 RPM's

6 ploe @ 60 Hz is about 1200 RPM's and 50 Hz is about 1000 RPM's

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[davidbeach]

Last I checked 1800 rpm is 150% of 1200 rpm and 1500 rpm is 150% of 1000 rpm. If you start at 100% and go to 150% that is most certainly a 50% change in speed.

 

[ad90]

System DP will remain 'unchanged' The purpose of the mods is to increase flow - to 'as much as we can get' (without major mods)

The current motor is 3.6kW (4.8hp) and we run a 60Hz system

Thanks

ad90

 

[rbulsara]

ad90:

Yes, if the piping and pump has no mechanical restrictions, flow will vary proportional to cube of the rpm and power will also vary accordingly. You can contol some power by controlling the flow through valves.

Consider using a higher speed motor with a VFD so you can adjust speed with much more flexibility.

 

[Compositepro]

Pump power is proprtional to flow rate times differential pressure. For a positive displacement pump flow rate is propartional to speed. Pressure is controlled by system downstream from the pump, which has been stated to be constant. Therefore, in this case, motor horsepower will increase linearly with speed. A centrifugal pump would behave as rbulsara said.

 

[ozmosis]

However, nothing will happen if you can't fit the motor onto the pump. I would expect a motor increasing in one or maybe two frame sizes to achieve the same rated torque at the higher speed required.