Motor selection for 20kW pump. 1

[michaelLeeKH]

I have a 20kW pump. What is the best motor size to be used for the 20kW pump. Can I choose a bigger size motor or slightly smaller size motor. What are the advantages and disadvantages of both selection? Does the type of starter or drive of the motor make a difference?

 

[ScottyUK]

You could use an oversize motor, but it will cost more and take more reactive current from the line. If your pump requires 20kW, why would you use a motor which delivers less? If your pump requires less than 20kW under the conditions you are using it, then 'maybe'. The nearest IEC size capable of meeting your load is 22kW.


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Sometimes I only open my mouth to swap feet...

 

[michaelLeeKH]

If I use a 30kW motor, how much current will be drawn? Will it be very inefficient? Does the type of starter or drive for example Star/delta or VSD matters?

 

[michaelLeeKH]

ScottUK,

Could I jsut ask why does it take more reactive current from the line? Does power factor plays apart in this? Could you please elaborate? Thanks.

 

[ScottyUK]

Essentially the reactive current is the current required to establish the magnetic field in the motor. You have more iron in a bigger motor, so it takes more current to establish the magnetic flux.

The bigger motor won't much less efficient unless it is grossly oversized, but the reactive power it draws will be higher, so you need to increase the size of your conductors and switchgear to carry it. As you draw more reactive power, assuming a constant kW load, the power factor goes down, so you have more current flowing to do the same work. You won't see much difference on the kWH meter (real power), but if you are an industrial facility you will see an increase on the kVAr meter (reactive power). It will cost you more to run if your utility charges for reactive consumption. Obviously a bigger motor costs more to buy and overhaul too.

It's difficult to say what the current draw will be - the motor speed has a big part to play, with low speed motors with high pole counts tending to draw high currents on account of their poor power factors. Assuming you are in North America the following might help:

http://www.klocknermoeller.com/dilm/hp2.htm

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Sometimes I only open my mouth to swap feet...

 

[waross]

I agree with ScottyUK and would like to add:
Where did you derive the figure of 20 KW for the pump? The power required for a centrifugal pump increases with a drop in back pressure. Such pumps have the motor sized by refering to the performance curves of the pumps. An undersized motor will overheat and may fail quickly.
Positive displacement pumps require more power as the discharge pressure increases.
The proper size motor is the best. The next choice is an oversized motor. Yes, it will draw more VARHrs but if VARHr consumtion is incurring penalties there are schemes to correct the plant power factor and avoid the penalties.
and in any event paying power factor penalties is usually cheaper than repairing or replacing burned out, undersized motors.
respectfully

 

[michaelLeeKH]

The drawn power as mentioned in the pump curve is 18kW. The original motor fixed to the pump is 20kW and the maintenance personnel wants to change the motor to 30kW. I am trying to collect data to pursuade him otherwise.

 

[ScottyUK]

If you have the 30kW motor already and it physically fits there's not much to be lost except possibly some extra cost over the motor lifetime if you are billed for reactive load or if you need a repair. You might want to consider compensating some of the reactive power demand with capacitors, but don't try to compensate all of it.

Why are you trying to find reasons not to use the bigger motor - don't you want the pump fixing? The pump will only take the power it needs - the 30kW motor will deliver the same power to the load as the 20kW one, assuming that they run at the same speed. The pump determines the load, not the motor.


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Sometimes I only open my mouth to swap feet...

 

[waross]

In an ideal world, the design of the power supply system would be re-evaluated to determine if the system could supply the greater starting current of the 30 KW motor vs. the 20 KW motor.
At the least, the disconnect switch, the motor starter, the overload relays, the motor supply conductors and possibly the grounding conductor should be upgraded to supply the larger motor.
The starting surge will be about 50% greater.
As ScottyUK pointed out, the cost of energy will be about the same and may even be a little less. A 30 KW motor delivering 18 KW may be more efficient and run slightly cheaper than a 20 KW motor supplying an 18 KW load.
Your energy consumption and what you pay for is determined by the sum of the energy to do the work of the pump and the energy to supply the losses. Work energy should be the same if the motor speed remains the same, but the losses in the larger motor may be less when the motor is operated at less than full load. The losses in the larger supply conductors will probably be less also, but the power factor and the location of corrective capacitors have an influence here.
The reactive current will be higher in most cases and may increase your penalties slightly. However if your plant has automatic VAR compensation you will probably not incur penalties.
I would evaluate at least four factors.
1> Will the 30 KW motor be purchased or do you have one on hand?
2> What will be the cost of upgrading the wiring to supply the larger motor?
3> Power factor. Will there be additional power factor/VARHr consumption penalties or will you have to purchase capacitors to avoid penalties?
4> Will there be additional installation costs with a larger motor such as changing base-plates, or mounting adapters, and/or special couplings or machining?
respectfully