# VFD Questions



## BBQ (Nov 16, 2010)

Skip the VFDs altogether, they are expensive and in my opinion much less durable than a standard starter.

I would set up belt drives or gear boxes in the correct ratio to dial the speed down.


What I am getting at is that VFDs are great when you have to change the speed all the time but if you need to just adjust the speed down and keep it there a VFD is an expensive way to go.


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## Angus (Jan 27, 2011)

BBQ, That is what I suggested all along...but a salesman told them they would save a bunch by running the motor or motors off a drive at 80%. Now I am working with this idea in their head.

Angus


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## dronai (Apr 11, 2011)

Angus said:


> BBQ, That is what I suggested all along...but a salesman told them they would save a bunch by running the motor or motors off a drive at 80%. Now I am working with this idea in their head.
> 
> Angus


The good news is you get experience with VFD's on their ticket


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## BBQ (Nov 16, 2010)

You might send JRaef a PM, he is an expert at this stuff and can help out greatly.


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## JRaef (Mar 23, 2009)

BBQ said:


> Skip the VFDs altogether, they are expensive and in my opinion much less durable than a standard starter.
> 
> I would set up belt drives or gear boxes in the correct ratio to dial the speed down.
> 
> ...


Absolutely correct. If the machine needs to run at 1000RPM continupusly, get there mechanically. The VFD will add losses. 

Plus, you have to address torque _at speed_. With a mechanical reducer, as you drop speed you INCREASE torque. With a VFD, you MAINTAIN the same torque. So if someone based the HP requirements by ASSUMING a 1750RPM motor and a gear/belt reducer, then using the same HP and turning the speed down with a VFD may leave you with LESS torque at the motor shaft than anticipated. You may have tp start out with a LARGER motor so that you end up with the right torque at the desired speed.


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## JRaef (Mar 23, 2009)

Angus said:


> BBQ, That is what I suggested all along...but a salesman told them they would save a bunch by running the motor or motors off a drive at 80%. Now I am working with this idea in their head.
> 
> Angus


A very common misconception, often used by ignorant salesmen and scam artists. It's hard to know the difference, but assume they are just ignorant in order to avoid a libel lawsuit...

This is true ONLY of very specific types of loads, called Centrifugal machines, like centrifugal pumps and fans. But not all pumps and fans are centrifugal, so be careful there too.

The WAY the VFD saves energy is where most people get screwed up. The scam artists / ignoramus salesmen try to say that "If you turn down the speed, the motor only uses as much energy as it needs to." THAT is the fallacy, because that is ALWAYS the case with an AC motor. If you have a 100HP motor and it is running a load that only requires 50HP, the motor is only going to consume the energy it requires for producing 50HP. It doesn't matter if it is running full speed or half speed, it is INHERENT in AC motor design. So you do NOT need a VFD for that to be true. And that's the scam... the statement is true for VFDs, the scam is not telling you that *the statement is also true WITHOUT the VFD*.

But if you MUST reduce the speed, then the WAY you reduce the speed makes a difference. In a Variable Torque load, such as a centrifugal pump, you typically run the motor at full speed and reduce the OUTPUT of the pump with a valve of some sort. But still, the "affinity laws" that say that "_*The energy used reduces at the cube of the flow*_" still applies, even with a flow restriction device. But... that valve then creates a pressure drop and turbulence in whatever you are pumping, that THAT represents an energy loss in the system. With a VFD, you get rid of the restriction device AND you still reduce the flow, so the affinity law applies, but WITHOUT the losses in the valve. That can represent significant energy over time and it is therefore true that a VFD can then save enough energy to pay for itself in a relatively short time. But again, IN A CENTRIFUGAL LOAD.

None of this applies to any other type of load. If you have a positive displacement vacuum pump, as 90% of them are, then the power consumed does not change whether you change speed with a VFD or if you did it with belts and pulleys. Yes, there are losses in the belts and pulleys, but there are losses in the VFD too, so it's a wash. If there is some advantage, as BBQ said, to constantly changing speeds, then the VFD makes sense. But from an energy standpoint, no difference or at least not enough to see a reasonable ROI.


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## Angus (Jan 27, 2011)

JRaef Thank you for the very detailed explanation...it is what I thought I would hear. 

I have talked them into adjusting with a belt drive system. 

Thank you all again - Angus


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## mbednarik (Oct 10, 2011)

keep in mind utility requirements for line starting large motors. Here Alliant energy will not let us start anything larger than a 50 hp without a vfd, soft start, or wye-delta setup.


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