DC to AC Motor Conversion

Not much I think, your main loss here as I see it is magnetizing current for the oversize motor...

"While the DC-motor, without regard to the drive, is more efficient than an AC-motor, the AC-drive is far superior to a DC-drive. When considering drive system efficiency, the AC drive system can offer an efficiency improvement in the range of ~3% when operating at near full load, where the DC drive efficiency is at its highest."

...an enhanced drive might lead to additional savings by minimizing magnetizing current under low load....

"Drive technology continues to tend to energy savings. A recent drive feature that is available in some drives helps the drive system in energy saving by reducing the AC-motor’s magnetizing current under no or light-load conditions. As discussed earlier, an asynchronous motor’s magnetizing current can approach half of the full load motor current. This means that drives that remain enabled under no or light loads can realize significant energy savings from the drive system."

http://www.aimcal.org/uploads/4/6/6/9/46695933/messer_abstract.pdf

How do you reckon electricity savings whilst ignoring efficiency?

Does the DC motor at 50% of rated speed use more electricity inherently than an AC motor with a VFD running at the same rpm? This is what I'm wondering.

yes, reactive power consumption is noticeably higher when compared with modern PWM drives.

Uhhh...bottom line. Sure efficiency has to play into this. Maybe the AC is more efficient and maybe not, depending on load setup now and load setup with the new motor in place. Surely they could try it for a month and find out the hard way.

Efficiency is the only thing you have going for you.

Your motor will be doing the same amount of work either way.

Thanks for the help guys. Much appreciated!

I think you'll find the front end of the ac drive will run at a much higher power factor than the dc drive, especially if you are phased back to half speed on your dc drive, for instance.

As Solar Eagle notes, not much in the way of energy savings. Your savings in reactive demand may account for some cash flow you can measure, if you are billed for it. The other savings is I squared R (heat losses) for the reduced current in your power distribution system down to your drive connection, transformers, cables, switches, bus bars, also very small magnitude to pay for your investment.

Since you're driving an extruder, I would think that the torque requirement would be a major issue. You might want to consider looking at ... http://www.vsdrive.com/blog/ac-motor-vs-dc-motor-for-high-starting-torque-362691.html

An extruder drive system is typically an easy application.

If you have no plans to go faster than 900rpm - then I would suggest considering changing the gearbox to run the motor closer to 1750RPM. The DC drive power factor will go from about .5 to .9, and the motor will run cooler, etc.

You are only using about 50% or less of the motor NP HP - consider a smaller DC machine if you have one lying around.

If you preheat the barrel then starting torque should not be an issue.

In my experience - unless there is a motor repair issue - the DC drive at this HP is a fraction of the cost of an AC drive. If you need a new motor then the costs skew a bit. The efficiency issue has to be looked at carefully - there is usually only a point or two +/-. In the odd case the AC has been less efficient.