VFD Operated motor power consumption. 1

[farhanempower]

Dear Members,

We have 522 Kw 440 volts motor operated through VFD. Normally we operated 2 nos of motor with 44 Hz through VFD.My question is if i distribute same load from 2 nos of motor to 3 nos at 26 Hz each will it reduce the electric consumption (KW) for the motor or increase the consumption or the consumption will remain the same.

As of now what i have observed is our electric consumption have reduced by operating an extra motor

Kindly help

 

[davidbeach]

How does your load respond to a reduction in frequency?

 

[jraef]

VFDs are not magic boxes. They "save" energy by eliminating waste in some other form of speed / flow reduction. In other words if your process or task needs to vary the amount of work done, and that can be related to rotational speed, then a VFD might be a better way to change speed without introducing as much waste in the overall throughput efficiency.

But once you have already made that speed change conversion with a VFD, adding more VFDs and motors to the same load will ONLY serve to INCREASE the wasted energy, because the VFDs themselves are not 100% efficient, neither are the motors.

You do not describe the machine, but no matter what it is, adding another motor and another VFD will absolutely INCREASE the wasted energy in your process. If anything, the only thing worth considering is REDUCING it to one motor and one VFD. But that is a totally different discussion since we do not know what the decision process was to justify 2 + 2 as you have now.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[electricpete]

There are three places that losses can change when you adjust the system under some assumption of "constant load" (which I interpret as constant total fluid horsepower of driven machines).

1 - Motor - adding an identical motor with the total bhp of 3 motors remaining the same has somewhat unknown effect. The total I^2*R losses will likely go down (because each motor carries less current, and the I^2 part tends to dominate... if all 3 motors had for example 3/4 of original curent, each motor would have 9/19 of original I^2*R, and the ratio of new/old total I^2*R would be [3*(9/16)] / [2*1] = 84% (a reduction). What would increase is the total core and friction/windage losses. Which one wins of these two opposite effects would require some careful study.
2 - VFD. We have similar situation: the total load-related I^2*R losses will decrease, but there are other losses where the total increases. I'll take Jeff's word there are likely more losses in three vfd's operated at 66.6% than two vfd's operated at 100%
3 - Load - completely unknown to us. Often driven loads operate at much lower efficiencies than the motor that drives them and changes in efficiency of load under different operating conditions can be huge.

Whether #3 will counter #1 and #2 seems to me difficult to predict. I do certainly agree with jraef and davidbeach that a lot more data would be required to draw any useful conclusions.

Adding a 3rd machine and associated vfd is an expense. Aside from any change in efficiency, I'd think it generally tends to increase your reliability and flexibility (for example if 2 machines carry 100% load, then we can stay at 100% load even if one machine stops or is taken down for maintenance).

So, more info about the loads. What type are they and how are they interconnected. If possible what would be the driven machine efficiencies when operating at 44hz and 26hz and carrying the respective expected load (efficiency depends on fluid load and speed). And how did you determine that 3 machines at 26 hz would "carry the same load" as 2 machines at 44hz for this system?

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(2B)+(2B)' ?

 

[LionelHutz]

Input power = work done + losses.

If you need to do the same work then the only way to save power is to reduce your losses. Adding more motors and VFD's is not very likely to reduce your losses.

I personally see little for applications where a VFD would save more energy compared to using other means. Other means can include things like on-off control, multiple fans or pumps to match each demand, a flow control valve, using a water tower for storage, etc. However, the VFD is often the simpler and cheapest solution and as such it's employed. The VFD is also being blindly touted by every goverment and elecrical utility as being the salvation to all the worlds enery usage problems. There have even been lots of incentives handed out to buy a VFD regardless of any energy savings achieved.