
Saving Money With Vfd's
#1
Posted 02 September 2007 - 04:17 AM
#2
Posted 02 September 2007 - 04:37 AM
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%
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,
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#3
Posted 02 September 2007 - 04:47 AM
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%
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?
#4
Posted 02 September 2007 - 05:00 AM
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,
Mark Empson | administrator
Skype Contact = markempson | phone +64 274 363 067
LMPForum | Power Factor | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | Pressure Transducers | Smart Relay | GSM Control | Mark Empson Website | AuCom | Soft Starters
#5
Posted 02 September 2007 - 09:11 PM
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
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