Reducing VFD Power output without reducing speed?

[Istakoza]

Hi,

My client wants to run a 500HP Pump with a 1000HP motor run on a VFD. Is it possible for the VFD to reduce the power output of the motor without reducing the motor speed?

I think if we end up reducing the motor speed, we will compromise pumping capacity as well.

Let me know what your thoughts are.

Thanks,

 

[jraef]

I think if we end up reducing the motor speed, we will compromise pumping capacity as well.

Yes, the VFD is incapable of FM*.

If the pump requires 1000HP to pump what needs to be pumped at the rate and/or pressure it needs to be pumped at, then the pump will pump roughly 1/2 of that if you only give it 500HP woth of power. Why would he think otherwise?

However, if the pump only REQUIRES 500HP to pump what needs to be pumped at the rate and/or pressure it needs to be pumped at, then having the VFD will in effect make the 1000HP motor into a 500HP motor, albeit at a reduced speed, because that is part and parcel to what a VFD does.

So without someone doing an in depth study of what is truly NEEDED from the pump system, there is no viable answer to the question yet.

FM is a coloquialism meaning F###ing Magic.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

If the mechanical load on the motor at a given speed is 500 HP then the motor will produce 500 HP at that speed whether driven from a VFD or directly from the supply. (Direct On Line)


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The motor doesn't determine the power requirement - the load does. The motor will do its best to deliver the power required by the load, within the limits of what it is capable of.


----------------------------------

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

 

[jraef]

Yeah, in re-reading my post I don't like thevway I said it, thanks for the clarifications.

So here is what a VFD really does for you. If the load only requires half of the mechanical power that the motor is capable of, the motor itself will only draw enough electrical power to do that job, plus any losses in the system, with or without the VFD. But if you have a 1000HP motor driving a 500HP pump requirement, you undoubtedly have some method of reducing output of the system. If that output reduction is restriction, there is energy loss in that process. Using a VFD to instead reduce the flow by reducing speed, you get the same results with much fewer losses. Plus you reduce the losses inside of the motor as well, because a portion of the motor losses are in the magnetics which is associated with the voltage. With a VFD, voltage ton the motor gets reduced to be only what it needs to be for the speed required. There are new types of losses introduced by having the VFD, but contrary to the belief of some, they are much less than what having the VFD reduces, so there is going to be a net increase in system efficiency. That is where they pay for themselves.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[PersonalProfile]

Yes - with a suitable VSD / motor etc - maybe upto a "couple of percent" improvement through reduction in iron losses may be possible.

Google search as a start, if you are interested:

http://www.google.com.au/#sclient=p....,cf.osb&fp=e6cee81619a0b78f&biw=1311&bih=627

Regards,

Lyle

 

[LionelHutz]

As already posted, a 1000hp motor will only produce 500hp if the load requires 500hp. You don't need a VFD and you shouldn't even spec a VFD if you will run the motor at rated speed. Using a VFD to run a motor at rated speed wastes energy due to the extra losses in the VFD. I would expect the extra losses to be in the 20kW to 40kW range depending on the VFD.

 

[Istakoza]

Thanks for the valuable input.

In conclusion, I don't need a VFD, There is no problem with running a 500HP pump with a 1000HP motor other than some efficiency losses, and a VFD will not help the perceived problem anyway.

 

[waross]

A 1000 HP motor with a 500 HP load may be more efficient (waste less energy) than a 500 HP motor with a 500 HP load. The magnetizing current will be higher but the real current component will be less. The resultant I²R loses may be less.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Take a look at the efficiency of the 1000hp motor at 50% load and the efficiency of a suitable 500hp motor at 100% load. You may find that the 1000hp motor is more efficient.

 

[Brad1979]

However, if the pump is ever running below 500 hp, things could change quite a bit. The efficiency of a 1000 motor running at, say, 250 hp could be quite low compared to 500 hp.

 

[ozmosis]

Why did your client want to run a 500Hp motor with a 1000Hp VFD? did they have one spare?

 

[MAGTiger]

It's kinda like putting a 60w light bulb in a socket where previously there had been a 100watt bulb. The new bulb only draws 60watt regardless of the circuit and socket's ability to deliver 100w or more.

Neil

 

[Marke]

Such a common misconception that an induction motor produces it's rated power irrespective of shaft load. This is the basis of so many fabricated test results for those NASA style energy savers!!
The other common misconception is that a VFD will always save energy on pumps and fans irrespective of the operating speed.

Logic suggests that oversized motors waste energy, so install a VFD to save energy.
Logic is wrong.

Mark Empson
Advanced Motor Control Ltd