Oversized driver 1

[DerekLJ]

What happens if the centrifugal pump driver is much greater than the required HP.

For example ,the rated(required) power for the centrifigal pump is 25HP, and the actual driver has 50 HP. What is the result?

 

[electricpete]

You are absolutely right that bigger inertia slows down the acceleration. I never said otherwise. I said bigger accelerating torque means faster acceleration (change one thing at a time).

Under normal power supply conditions, a larger motor is generally able to accelerate its own inerta of course. The smaller is the attached pump inertia and torque load, the faster the machine will accelerate and the less of a challenge the start is for the motor.

I agree with your comment that faster acceleration is better because the motor spends less time drawing the high startup current. That startup current creates a lot of heating, especially on the rotors of large motors.

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[electricpete]

One thing you are right, a bigger motor draws a larger locked rotor current (regardless of it's load). There is a limit to what size motor you can put in on a given power system.

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[electricpete]

And you could certainly get in trouble putting an oversized motor in on a power system that was not designed to tolerate it. The voltage would droop excessively at the motor terminals during start and the motor would take longer to accelerate and would trip (or if not properly protected with the right relay settings, could damage itself)

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[electricpete]

A check for suitability of the motor for the power system is part of the design for installing the new motor.

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[dickon17]

We might all benefit from applying the "Occam's razor" principal.

One should not increase, beyond what is necessary, the number of entities required to explain anything.

This is one of the fundamental building blocks of the scientific principal and is generally accepted to mean "All things being equal, the simplest solution tends to be the best one." This has certainly been my experience. Why search for complication where none exits?

 

[BigInch]

E pete,

I didn't change to a larger inertia. I considered that inertias remain essentially equal, ignoring the slightly increased new motor Ir. Yes I know that a larger motor has a proportional increase in torque to compensate for its own acceleration needs, so I didn't feel the need to mention the slightly increased Ir.

I said the higher available torque from the larger motor (acting on the same mass) will consume more power and that power demand must be higher than what it was for the smaller motor (with less available torque). So, I think I said the same, the higher available torque/power delivered by the larger motor causes the faster acceleration in the face of equal inertias.

Hydraulically, the faster fluid accelerations will cause velocities and pressures to increase more quickly than before. What happens as the result of those faster increasing velocities and pressures depends on how the rest of the system and its controls respond.

http://virtualpipeline.spaces.msn.com

 

[ClicketyClack]

I have put 50 HP motors on 25 HP pumps and have NEVER had a problem.

I pray this kind of thought and energy goes into cancer research.

 

[pumpking]

Coming from the Eco warrior brigade (not really !!) and energy costs soaring, a motor is most efficient when operating at 70-85% of rated power - that is a big issue now when selecting motors to go with our pumps.

Now theres another angle to consider !!

Yours,

Global cooler

Ash Fenn

http://www.cdrpumps.co.uk

 

[TenPenny]

The vast majority of our customers use motors sized to cover runout, but also have standard motor sizes (usually, no 30, 40, 60, or 125 hp units), so that means a pump that may take 21 hp at running load, but, say, 27 at runout, will be typically installed with a 50 hp motor.

Never seen any problems whatsoever.

Yes, the motor will be less efficient, but incrementally not very much.

Yes, the starter/MCC has to be sized for the motor used, but that wasn't part of the original question.