VFD Saves Energy?

[ataslaki]

Did a search on this subject here on the site, and the closest I came to was this thread, but not the answer I'm looking for:

http://www.eng-tips.com/viewthread.cfm?qid=39095

My question is simple. A 3HP 240VAC motor with appropriate VFD drive running at 60Hz continuously 8 hours a day, without intermittent stops and starts. Does the addition of the VFD (versus connecting the motor wihout a VFD) have any power savings ($$$)?

I've heard that using a VFD at constant 60 HZ has savings, but where does that saving come from? What features does a VFD have to save power? I understand how you get savings if you slow down the system to less than 60Hz, but how about at 60Hz?

 

[Skogsgurra]

It is only if you have a lightly loaded motor that you have any saving potential at all. The saving comes from reducing the magentizing current - which can be done it your motor is lightly loaded. Lower magnetizing current may result in lower overall losses and that *could* lead to some net saving. You have to set the V/f relation to have any saving and when the load increases, you will have a problem. Some vector drives adjust torque current AND magbetizing current automatically so you do not have to.

But, the VFD has additional losses in itself and I do not think that a 3 HP motor can benefit from being fed from a VFD. Not probable.

Gunnar Englund

http://www.gke.org

 

[aolalde]

At fix 60 HZ the total losses on a VFD-AC motor system will increase and the cost of operation is higher as compared to 60 HZ supply from the utility.
For certain type of loads where reductions of speed optimize the process, you could have an improved efficiency and then energy cost savings. Expert deep analysis of the complete cycle of each specific type of load is required to find if energy savings are feasible.

 

[ataslaki]

The motor is running a regenerative blower, to create vacuum to pick up parts off of a conveyor. So basically the motor is running a fan, which I'm assuming will be a constant load.

 

[jraef]

No savings. Losses in the VFD will more than make up for any small amount of magnetizing current savings. At full speed full load even the best VFds are around 97% efficient, maybe 98% at best. The other 2% are throughput losses. Running across-the-line has virtually no throughput losses.

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