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Oversized driver

Oversized driver

Oversized driver

(OP)
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?

RE: Oversized driver

Nothing.  The pump will draw the HP needed from the motor.  You may pay a little extra for a larger baseplate and coupling.

RE: Oversized driver

You might tend to overheat more easily at minimum flows and there may be a possibility that you can develop higher pressures and higher flow velocities, provided the horsepower rating is not greater than the drive shaft of the pump, in which case you might do some shearing at high power loads.  Your acceleration during starting may be much faster and higher transient (water hammer) pressures, or intermitant cavitation may be a possibility.  It is also possible you will burn up the power supply in start-up or runaway conditions.

http://virtualpipeline.spaces.msn.com

RE: Oversized driver

Hopefully you are starting up with a throttled discharge valve and are not running at min flow and runout conditions. I shouldn't have assumed you were not, which brings on other problems.

Explain the higher pressures and velocities. I have not heard that one Big Inch.  Is it due to reduced slip so the pump is operating at higher RPMS's?

RE: Oversized driver

(OP)
BigInch, you said that alot of things MIGHT happen. What do they depend on?

I'm just courious, but do you have personal experience with this happening?

RE: Oversized driver

I agree with ClicketyClack, nothing significant should happen.  

RE: Oversized driver

Yes I have personal experience with this.  Check out my MSN space.

Torque and horsepower can and often do go to max when starting, therefore fluid accelerations are high. A typical electric motor will develop starting torques and power draws 50% higher than at running torque.  If you have a 2X  HP motor, then you could have 3 x the rated pump power capacity during starts.  Most electrical systems are sized at 2 x start-up draw.  Now you have 3x.  I don't recall having seen a circuit designed for 3x start currents.  

When pumps start, all fluid in the system is being accelerated and power draws are high.  That power going into the fluid during start of (correctly sized) motors is a typical cause of transient water hammer pressure waves.

Heating at low flows primarily depends on motor efficiency at that low flow, not the actual power required by that flowrate.  Since the flow is low anyway, overheating can occur more easily.

All shafts have a shearing load capacity, at least check that.  

Also possible that short circuit torque loads to the skid and foundation or motor mounts go to 3 times what what was originally designed for.

With a 50 HP motor, maybe nothing happens, but try it with a 5000 HP.

http://virtualpipeline.spaces.msn.com

RE: Oversized driver

(OP)
Thanks BigInch for your discription. that was very helpful.

RE: Oversized driver

At low flows, the major portion of the power being consumed goes to overcoming static and dynamic friction at stuffing boxes, etc. and not so much of the total power demand is from the hydraulic power requirements alone.  At 20% BEP flow, you can develop heat problems, at 10% its pretty much assured some heat-up will occur.  Its best to only use low rpms for start-up/warm-up purposes that won't last more than 15 minutes, but that depends on the head development rate /rpm that needs to be developed.  Low head starts are not so severe, but starting against a high head can give you some heat troubles as flow is recirculating inernally and doesn't start to develop a net outflow from the pump until you reach entry head/pressures into the discharge header.

http://virtualpipeline.spaces.msn.com

RE: Oversized driver

You need to be more specific, can I assume that you are referring to electric motor drive and that the 25hp is the power being used at the normal operating condition. However,what happens if the pump runs out on its curve for some reason - does it need the 50hp.

If the maximum power that can be absorbed is 25hp, having a 50hp motor just means that the motor is running at lower efficiency and power factor which translates into higher running costs, there are no other impediments to having an oversized driver, unless you are talking about an internal combustion engine and then you have other considerations.

RE: Oversized driver

You guys seem to be seriously over complicating this imaginary problem.

RE: Oversized driver

Dickon,  What are you trying to say?  Do you not agree that these problems could be possible with a 2X motor?  If not, please explain.

http://virtualpipeline.spaces.msn.com

RE: Oversized driver

(OP)
In my case, the question has been answered. We aren't operating with a 2X motor, so based on what EVERYONE here has said, i feel that there shouldn't be a problem.

I have another question though, if you all could be so kind as to help me. I have 3 IDENTICAL centrifugal pumps running in parallel. The only difference is in the driver; 2 have 30HP electric motors, the other has a 40 HP electric motor.The rated(required) power is 25.3 HP.

I don't see a problem in this configuration during normal flow/start-up/and low flow.

Does anyone disagree with me? if so why.

RE: Oversized driver

No disagreement.

RE: Oversized driver

BigInch.  I agree, any or all of these problems could be possible.  However, it has been my experience that most simple questions usually have simple answers.  If a lawn mower suddenly stops running, the most likely reason is running out of gas.

RE: Oversized driver

Dickon, I still don't understand... especially the lawn mower running out of gas ..analogy.  The reasons I gave seem logical and simple enough to me.

http://virtualpipeline.spaces.msn.com

RE: Oversized driver

"Most electrical systems are sized at 2 x start-up draw.  Now you have 3x.  I don't recall having seen a circuit designed for 3x start currents."

I have never heard of the problems discussed by BigInch for electric motors.

The pump has a torque speed curve.  The motor has a torque speed curve.  The difference between them is the accelerating torque.  Bigger difference does not mean that more power is provided to the pump the machine accelerates to full speed faster.  As far as the motor and power system go, accelerating faster is better.

A pretty good model is that the motor will draw locked rotor current (typically 5 x full load current) until the motor is up to 2/3 speed, at which point current starts to drop towards running current.

It doesn't matter what kind of load you put on it, the motor will never get above locked rotor current (excluding any decaying dc offset transient component).  Putting a higher pump load means the machine accelerates slower... putting a lower pump load means the machine accelerates faster.  As far as the motor and power system go, they would love to have the least  load possible so the motor gets up to speed fastest (better for the motor) and the duration of the higher current draw on the power system is less.

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RE: Oversized driver

BigItch..If you have to ask the question then you have already made my point much better than I ever could.  

RE: Oversized driver

electricpete,

"Bigger difference does not mean that more power is provided to the pump the machine accelerates to full speed faster."  Then could you please explain how the faster acceleration is accomplished, especially since the rotational moment of inertia would also be larger on a larger motor, right?


ω = T/Ir

ω=Angular acceleration
T= Torque
Ir=Rotational Moment of Inertia

If acceleration is faster (as we both agree), would not that equation indicate that more power is consumed?  

What is good for the motor is not necessarily good for the hydraulics.  

Additionally, why is it good for the motor?  Is it because starting loads are high and if any excess heat is generated at all, it is from the motor windings which was created when starting?  Therefore faster acceleration means less time spent drawing high current and heating up the windings.

http://virtualpipeline.spaces.msn.com

RE: Oversized driver

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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RE: Oversized driver

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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RE: Oversized driver

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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RE: Oversized driver

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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Eng-tips forums: The best place on the web for engineering discussions.

RE: Oversized driver

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

Quote:

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?

RE: Oversized driver

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

RE: Oversized driver

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.

RE: Oversized driver

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

www.cdrpumps.co.uk

RE: Oversized driver

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.

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