VFD POWER CONSUMPTION

[dglaab]

I have been told that using a VFD on a partly loaded motor will reduce my power bill. Is this valid and why. I am getting unclear answers from my power/control guys. Where I am located I do not have a penalty for power factor.

thanks

Dan

 

[davidbeach]

It depends; maybe, maybe not. Maybe if you provided some information somebody might be able to answer the question.

 

[jraef]

It's a very very common misbelief that just putting a VFD on any motor will save energy. It might save energy, but the devil is in the details.

IF you have a centrifugal load such as a some types of pump or fan and you are running it at full speed and restricting the output to regulate flow, then modulating the motor speed can save on the losses in that flow restriction method.

Often times people will quote what is called the "Affinity Laws" to point out that power varies by the cube of the speed of the motor. While this is true, what they often fail to realize is that power use varies with flow anyway, albeit maybe not as much. The formula for determining that is more complex because it involves more issues such as head and specific gravity, but keep in mind that these issue are the same regardless of how you vary the flow. So as I said, the VFD can save you energy based on the difference, if any. Only a well thought out evaluation of all factors can determine that in advance.

That said, generally it works out that a VFD does save energy on most types of centrifugal pump and fan applications, usually enough to pay for the added cost in a reasonable amount of time. But if you put it on a conveyor or a machine tool, it will not save you anything and in fact may end up costing you more money because of losses in the VFD.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[dglaab]

OK, perhaps a little background will help my question. I have 350 hp motors on refrigerant compressors. Due to the fact that the colder you are gas density is lowered, the calculated horsepower requirement in some of our operation stages is about 170 hp. I need to stay with same rpm and need the HP for pulldown stage, so I cannot put a smaller motor on. Someone had suggested that since a frequency drive chops up and recombines its output, the real power input would be lowered in a low hp requirement, thus reducing my power bill. Any validity to this??? I pay 19.1/kw, so savings add quickly

I am not planning on using the vfd's for actual frequency control, just potential power savings (if its valid) otherwise I stay with the softstarts.

thanks

Dan

 

[davidbeach]

The only possible means of energy savings is by running the motor slower and refrigerant compressors are not a good candidate unless designed for that application; in which case they would already have drives. There is no way for the drive to save you energy while running the motor at rated speed. Far worse, running the motor at rated speed you would have all your present losses plus all the losses associated with the drive.

If your compressor only needs 170hp then the motor is only putting out 170hp to the shaft. Add in the internal motor losses and your motor is drawing less than 200hp. Amps probably won't go down by the ratio of 200/350 but that is because the power factor goes down, but if all you are paying for is kWhr you would be seeing something on the order of 200/350 as much power into the motor compared to full load already.

 

[LionelHutz]

It was a bad suggestion. If the motor has to run at full speed then that system is not a good candidate for a VFD energy savings project.

You must improve the efficiency of the motor to have a more efficient running motor. You can not just connect some other piece of equipment onto the motor leads and get a more efficient motor.

If you need to correct some fairly serious power system issues then you might see some improvement. Harmonics or a voltage imbalance could affect the motor efficiency. If you are running with a high line voltage you could also see an motor efficiency improvement by dropping the voltage slightly via the transformer taps.

A 350hp motor will typically be very efficient down to at least 175hp. The efficiency curve of a typical induction motor stays quite flat until you get down below 50% speed.

 

[rockman7892]

Jraef

So with a centirfugal fan or pump are you saying that the main cost savings when using a VFD is a result of the system losses due to the friction of whatever means if restricting flow without a VFD? Would this be losses through a valve, damper etc...?

Can you explain your third paragraph regarding the affinity laws and how flow has to be taken into consideration? I am not fully grasping what you are trying to explain.

Thanks

 

[jraef]

Here's a rework of that ramble, sorry.

If you reduce flow in a centrifugal pump, the power consumed reduces as well, regardless of how you get there. If you reduce flow with a valve, there are mechanical losses across the valve in the form of friction, turbulence etc. If you reduce the flow by modulating the motor speed, AND REMOVE THE VALVE, you will eliminate those losses and save energy. In addition, the rate at which energy is reduced with flow restriction is not quite as great as when you reduce it by lowering motor speed because of the affinity laws. That is where most of the VFD energy savings is coming from. The point I was making is that sometimes VFD suppliers overstate the issue by comparing VFD energy consumption against full speed full flow energy consumption without considering that energy will reduce with flow restriction anyway.

Here is a graphic depicting what I am talking about. I happen to think this is a little exaggerated; the throttling valve curve is a little more linear in my experience. But the VFD curve is relatively accurate.

(It was apparently created by a VFD manufacturer, hence the probable data skewing)

There are additional benefits of using a VFD in reducing the energy consumption because you can also reduce some of the electrical losses in the motor itself, what is called the "energy saver mode" because a small proportion of the motor losses are related to the applied voltage and as the speed is reduced, so is the voltage and therefore the losses are lower.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[rockman7892]

Jreaf

Thanks for the explanation and the curves it helps visualize. So is the more linear shape (as you state) of the throttling curve more related to the the fact that there are losses associated with this method of flow restriction, or is it liner due to the fact that flow is simply linear when throlling, and has more of a logrithmic curve when flow is controlled by propeller speed as stated by the affinity laws.

I guess the energy savings comes simply from that at any given flow the VFD curve will always be below the throttling valve curve and thus require less input power. Seeing this I would it would appear that this additional HP for throlling would have to be strictly related to losses?

 

[Compositepro]

The big problem with the analysis, so far, of potential energy savings is that for most pump systems one requires a certain minimum pressure as well as a variable flow rate. Slowing a centrifugal pump will reduce its pressure output as well as its flow rate.

The OP already said that he must stay with the same rpm (is it a centrifugal compressor?). In that case there is zero potential savings with a vfd.

 

[jraef]

Compositepro,
You should know by now we are a tangential group around here, the OP's initial issue is buried in history now...

Good points though.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[LionelHutz]

Here is one simple way to look at it.

Power is proportional to flow x pressure

If you throttle, then the pump still makes full-speed pressure but the flow is reduced.

If you slow down the pump, then the flow and pressure are both reduced.

Now, the flow accounts for most of the pump input power so lowering the pressure helps lower the power used but it's typically not a very large gain.

As a general rule of thumb, you want a low head application and the need to lower the flow 15% or more a lot of the time if you expect to save energy. I believe it was Biginch here that posted about the 85% speed or lower rule of thumb.

 

[rockman7892]

LionelHutz

Looking at a pump curve it looks like as flow is increased head pressure is decreased so they appear to be inversely propotional. So how does the pump sill need or make full speed pressure if the head pressure would be reduced for a given increased flow?

In other words wouldn't flow strictly be a function head pressure for any given speed?

 

[LionelHutz]

Like I posted, that is a simplified way of looking at it.

In other words wouldn't flow strictly be a function head pressure for any given speed?

Not with a valve in the system.