# Saving Money With Vfd's

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Looking for information concerning converting all our starter application to VFD's. All motors are 480V/3P, they are all currently in fan applications for dust collection. I've been researching if just converting the motors in a range of 75HP to 250HP will save money. I've seen a few charts indicating that at 100% load for a line fed motor (approx %80 eff) versus a VFD fed motor holding all condiditons constant, the VFD uses approx %40 less energy. Most articles go on to talk about reducing the speed to reduce energy consumption. Later I will do more calculations for varying the speed, but right now I am looking for a 1-1 comparison without the speed change. Please and thank you.
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Hello wzrdstrm

Welcome to the forum

If you use a VSD to operate a motor at line frequency, you will not save energy, infact you will use more.

• The efficiency of the motor will fall slightly due to harmonics in the output waveform.
• The VSD has losses of up to 5%
Energy can be saved with a VSD only in situations where there is a mechanical loss in the system that can be reduced by reducing the speed of the machine. If the machine needs to operate at full speed in order to produce full output, you will need to operate it at full speed and the total efficiency will be less than without the VSD.

The best applications are those where the output requirement varies, such as a pump with a variable flow requirement and the pump can be operated at reduced speed for considerable periods of time.

Best regards,

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Hello wzrdstrm

Welcome to the forum

If you use a VSD to operate a motor at line frequency, you will not save energy, infact you will use more.

• The efficiency of the motor will fall slightly due to harmonics in the output waveform.
• The VSD has losses of up to 5%
Energy can be saved with a VSD only in situations where there is a mechanical loss in the system that can be reduced by reducing the speed of the machine. If the machine needs to operate at full speed in order to produce full output, you will need to operate it at full speed and the total efficiency will be less than without the VSD.

The best applications are those where the output requirement varies, such as a pump with a variable flow requirement and the pump can be operated at reduced speed for considerable periods of time.

Best regards,

Thanks Mark, wasn't hoping to hear that and it appears I may of misunderstood the chart..I can still vary the speed, for these applications. Approx 100 to 300 times a day, a damper closes and the dust collection must "Pulse" blowing air into what amounts to big vauum bags. During this time the motor isn't really performing a lot of work and it generally lasts for 15 seconds, which as it stands now is about 30 hours a year. I was thinking that during this 15 seconds of actually dropping the motor Hz to 0 and then ramping the motor back up when the pulsing was finished. Sound viable?

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If you use dampers to reduce the airflow, then you would be better to reduce the speed to reduce the airflow.

The use of dampers and valves to control flow, causes the fan or pump to cavitate or spin the air or fluid. This is lossy and the overall efficiency drops in doing so. Reducing the speed prevents the losses in the fan or pump.

If you operate at reduced speed for only 30 hours per year, and the rest of the time at full speed, I do not think that you will get an acceptable payback period.

If you operate for a reasonable period of time with the dampers partially closed, then the speed could be dropped and savings achieved.

Best regards,

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Hi, I don't think that this is a viable application I am sorry to say. If we imagine a 100kW motor (~130hp) running at full load with VSD. I will keep the numbers as simple as I can simply for ease of use, these may not reflect your actual system.

Motor input = 100kW/80%(motor eff) = 125kW

Drive input = 125kW/95%(drive eff) = 131kW

Drive losses = 6kW

6000 hrs per year operation (assuming industrial site)

6000-30hrs (reduced running time as per your post) = 5970

5970 x 6kW = 35,820 kWh

These are the additional losses added into the system by the VSD running at 100% speed.

Savings = 30hrs x 131kW = 3930 kW.

Not good numbers.

You need to look if there are any dampers used in the system to mechanically reduce airflow which are always in use. If you can remove these and therefore slow the fan downyou will have some very real potential savings.

If you cannot do this you are much better off replacing your mtors with high efficiency models.

I must say that your estimate of 80% efficient sound very low. I would expect large motors like that to be around the 90-92% mark.

I think that you should carry out a calculation on the true efficiency of the motors and then see if they are worth replacing, if not at least plan for high efficiency models when they fail.

Kens

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

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