Soft Starts vs Varable Frequency Drives

[Rob S.]

Good Morning ,

We have a few 125 HP motors that have Soft Starts on them
and I would like to replace them with VFD's. What energy cost
savings would be expected by replacing the soft starts with VFD's. Our power company is giving rebates for drive applications ,and I would like to begin to investigate the
possibility of doing this. Would having a VFD alleviate the
peak demand for these motors or stay the same.

Thanks for your thoughts.

 

[cornbread]

I think its going to help you with your power factor.

 

[rdrast]

/sigh.

What are the motors on?
Will they still run at full speed?
What are you concerned with? Starts? Continuous Run?

Power is power. 125HP DOL and 125HP on VFD are both 125HP. You might get a better (or worse) power factor with a VFD, depending on the VFD.

 

[OZEE]

Where you get savings going to a VFD is if you can slow the motor down. Yes, it will help a little on PF, but that typically doesn't have a tangible result to the power bill.

 

[leitmotif]

Good Morning ,

We have a few 125 HP motors that have Soft Starts on them
and I would like to replace them with VFD's. What energy cost
savings would be expected by replacing the soft starts with VFD's. Our power company is giving rebates for drive applications ,and I would like to begin to investigate the
possibility of doing this. Would having a VFD alleviate the
peak demand for these motors or stay the same.

Thanks for your thoughts.

The question is much more complicated than laid out here.

IF the issue is reducing demand charge the easiest is to stagger start your motors such that you start only one in each time frame set by the utility ie one every 15 minute for example. As with other issues you should be talking to the utility for assistance.

Replacing soft start with VFD will not probably have much effect on motor starting power by itself. Nor will it have much efffect on power needed to start the load. Some - if not most - of this will depend on how you control the load.

Where VFDs can and do pay off is with loads that are subject to the affinity laws such as centrifugal pumps blowers etc. WIth these power is a cube function of speed so if you can run pump at half speed you are consuming only 1/8 the power than if run at full speed and doing discharge bypass.

EMPHASIZE this is controlling the LOAD not the MOTOR.

The motor draws 25% of full load amp to run itself the other 75% is what the load demands - this is what you must know to make this idea pay or not pay.

The main advantage of VFD is speed control. Speed ramps can go a long ways in reducing wear n tear on gearboxes drive belts etc especially so with loads that are frequently stopped started or reversed.

I think to summarize
1. IF utility is covering entire cost then go for it.
2. IF utility is covering 1/2 cost look seriously into it
3. IF you are hoping to recover costs from long term energy cost savings be very careful in analyzing your operating patterns and loads driven by motors.
4. Talk to the utility -- their engineers are supposed to know about this stuff and you are paying their wages anyway.

Dan Bentler

 

[gkk1223]

There is one other item to consider with VFDs that caught us by surprise - the type of conductors and length of run. At 125hp, the soft starters are likely to be very close to the motors, but it can really pay off in some circumstances to use special cabling to the motors. We had an issue with insulation breakdown in a 150' run of THHN wire to a 3hp motor driven by a VFD.

Also, make sure your motors are inverter duty (see thread below for more details). The VFD waveforms can cause issues in older style motors.

Here's the previous thread:

http://www.plctalk.net/qanda/archive/index.php/t-38594.html

And a link to another white paper on the subject:

http://www.belden.com/pdfs/Techpprs/VFD_WP.pdf

 

[The Plc Kid]

When switching to a VFD there may be additional costs such as vfd cabling, line/load side filtering, Shaft grounding, consider these things as they may eat up any savings you may warrant from using a vfd.

In general if the motors will still run at the same speed or speed control is not needed by the application a vfd can be more problems than a soft start. Again it all depends on the load and the application.

Vfd may creat more harmonics probles also which will have a negative effect on power factor.

 

[DickDV]

The key factor here, just as PLC Kid and Leitmotif stated, is speed reduction potential. If there is none, a drive is not benificial. If there is only a little, you will have to figure pretty closely. If a lot, go for it!

Drives don't save energy. They allow the load energy to be reduced thru speed reduction.