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DC to AC Motor Conversion

02/14/2017 10:48 AM

We are currently running an extruder with a 400 HP DC motor. The motor is running at about 900 rpm, rated for 1750. Ignoring efficiencies based on new motor vs old, if I do a straight up conversion of this system to an AC motor and drive combination, what kind of electricity savings should I expect?

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#1

Re: DC to AC motor conversion

02/14/2017 11:19 AM

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

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#2

Re: DC to AC motor conversion

02/14/2017 11:34 AM

How do you reckon electricity savings whilst ignoring efficiency?

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#3
In reply to #2

Re: DC to AC motor conversion

02/14/2017 12:29 PM

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.

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#10
In reply to #3

Re: DC to AC motor conversion

02/15/2017 8:30 AM

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

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#4
In reply to #2

Re: DC to AC motor conversion

02/14/2017 12:32 PM

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.

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#5

Re: DC to AC motor conversion

02/14/2017 1:34 PM

Efficiency is the only thing you have going for you.

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

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#6

Re: DC to AC motor conversion

02/14/2017 2:23 PM

"... Ignoring efficiencies based on new motor vs old..."

You shot yourself in the foot before even drawing your gun...

But even if you DID consider efficiencies, you cannot get anywhere in a vacuum of detailed information, and all you gave was the speed. There are many mitigating factors such as what TYPE of DC motor you are discussing, what type of load is involved and the torque requirements at that speed, load profiling, ambient conditions, etc. etc. etc.

As a gross general rule, AC is slightly more efficient in many cases, but only by a few percent. As an example, Tesla Motors decided on AC over DC in their cars because in THEIR APPLICATION, they determined they would get about a 2-3% efficiency premium for going with AC. That was after some EXHAUSTIVE research funded by very deep pockets.

Performance, maintenance, longevity, initial investment, those are all much more valid considerations for going one way or the other, most of which have been favoring AC over DC and resulting in the market for DC motors and drives hitting rock bottom now. In the past 5 years, I have done exactly ONE project with DC drives, that that was because the end user already owned the 600HP DC motors, so they didn't want the expense of replacing them. I have not done one single new project where DC was considered superior to AC, but energy savings was never a consideration.

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#7
In reply to #6

Re: DC to AC motor conversion

02/14/2017 11:40 PM

thanks for that very useful experiential advice, a GA. This is what I truly love about CR4, I don't say thanks enough.

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#8

Re: DC to AC Motor Conversion

02/15/2017 7:57 AM

Thanks for the help guys. Much appreciated!

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#9

Re: DC to AC Motor Conversion

02/15/2017 8:27 AM

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.

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#11

Re: DC to AC Motor Conversion

02/16/2017 8:36 AM

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

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#12

Re: DC to AC Motor Conversion

02/16/2017 4:57 PM

Though there is a lot to be said for a DC drive the ethos of today is to use AC VSFD technology.

The cost of replacing the DC drive to AC may incur a problem I have encountered a couple of times where a standard frame AC Machine was used and the actual torque/speed of the driven unit was not properly understood resulting in premature mechanical failure of the AC Machine i.e snapped shaft.

The DC motor inherently is more maintenance intensive (brush and commutator maintenance) versus an AC Machine.

Though the cost of a new AC Machine may be a deciding factor is the drive requires a none standard design.

In terms of just efficiency then one would need a lot more information to say that a new AC drive would be more efficient.

I am from an era where DC drives were the norm so if your drive is performing adequately and you have experienced maintenance staff who understand the intricacies of DC motor maintenance the why change it.

Is there anything to be gained from looking at the AC to DC conversion process to see if the latest AC/DC drives may assist you?

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#13

Re: DC to AC Motor Conversion

02/16/2017 5:11 PM

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.

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Users who posted comments:

Andrew Westman (1); commutator (1); GW (1); James Stewart (1); JRaef (1); lyn (1); PFR (1); Poison (2); rwilliams (2); SolarEagle (1); Spinco (1)

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