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?
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
RE: Oversized driver
http://virtualpipeline.spaces.msn.com
RE: Oversized driver
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
I'm just courious, but do you have personal experience with this happening?
RE: Oversized driver
RE: Oversized driver
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
RE: Oversized driver
http://virtualpipeline.spaces.msn.com
RE: Oversized driver
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
RE: Oversized driver
http://virtualpipeline.spaces.msn.com
RE: Oversized driver
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
RE: Oversized driver
RE: Oversized driver
http://virtualpipeline.spaces.msn.com
RE: Oversized driver
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.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oversized driver
RE: Oversized driver
http
RE: Oversized driver
"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
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.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oversized driver
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oversized driver
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oversized driver
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Oversized driver
RE: Oversized driver
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 pray this kind of thought and energy goes into cancer research.
RE: Oversized driver
Now theres another angle to consider !!
Yours,
Global cooler
Ash Fenn
www.cdrpumps.co.uk
RE: Oversized driver
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.