Cost of operating AC vs. DC 2

[StarTribune]

I am considering changing out DC motors and drives on printing presses with AC motors and drives. Is the hourly cost of operation lower with AC components compared to DC components?

Thanks,

Greg

 

[electricpete]

One thing to mention is dc motors can require more maintenance than ac motors (brushes and slip rings).

As far as efficiency of the motors and their drives, I'm not sure.... I'm sure others will chime in.

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[DickDV]

As a general rule, if you use a modern high or premium efficiency AC motor, the drive/motor system will be about 6% more efficient in AC than in DC. Of course, there are exceptions but in most cases it works out to about 6%.

 

[itsmoked]

The other is that low speed torque of the AC motors are a fraction of the DC motor torque at the same low speeds. This can result in a comparing the full speed parameters like "it's 10HP DC motor so lets get a 10HP AC motor", then you end up with AC motors that are only 50% the torque required at the low speeds.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jasonf987]

From a maintenace point of view an AC installation is far cheaper, especially if you have a failure of some sort.

 

[edison123]

Aren't printing presses a high torque load ? May be the drives gurus can chip in here on the applicability of VFD driven motors on such loads.

Muthu

http://www.edison.co.in

 

[StarTribune]

These are large printing presses (newspaper) that put out 25,000 to 60,000 papers per hour.
Each press has eleven 75 hp DC drive motors.

To simplify the question: What is the kW required to operate my press drives using AC vs. DC at full load, and at partial load?

The existing DC motors are 500 volt and rated at 123 FLA.
The proposed AC motors are 75 hp, 460 volt rated at 90 FLA.
Thanks,

Greg

 

[PaulBr]

About 15 years ago I was working at newspaper printing facility, Cincinnati Enquirer. Printing presses were similar to what you have described. There were 10 75HP DC motors, which were working on common shaft. Each motor was controlled by SCR drive by Fincor. Two years ago I have attended drive seminar by one of our suppliers. It was about Rockwell Automation drives. One of the presenters at that seminar has mentioned this printing facility as one which has switched from DC motors and drives to AC motors and drives, of course by Rockwell. Try to get in touch with Cincinnati Enquirer, so you may find more info.

Paulbr.

 

[itsmoked]

I'd expect the proposed motors to be tooooo small.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

1HP = 746 watts (.746kW)

AC or DC has nothing to do with that. The amps / volts go into the equation, but the kW remains the same. Mechanical "HP" is a short hand expression of torque and speed, so if your press needs 75HP mechanically, then how you get there is irrelevant, except as noted below.

What DOES make a difference is the efficiency of delivering that power from the utility source to the load. As DickDV said, there are differences in the total throughput efficiency (electrical line to mechanical load) between AC and DC, with the newest AC motors and drives having a slight edge over DC. Add to that the fact that there is less maintenance on the AC motors compared to DC and the overall operating and ownership cost is going to be lower for AC.

If you were starting from scratch however, the AC systems would likely cost you slightly more up front. But for you to make a reasonable ROI decision, you must weigh in the fact that you already have the DC system in place. So your change-out cost is going to be very high, even compared to upgrading your DC drives (if they are giving you trouble), so that must be weighed against the operating cost savings.

In the 3 systems I have worked on proposing a DC-to-AC change out all ended up having an ROI that exceeded what the bean counters considered acceptable (3+ years), even factoring in the DC motor maintenance. In one case they did it anyway because they had been told by an AC drive vendor that "there is no future for DC drives, in 5 years you won't be able to get them serviced". That was 15+ years ago...


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[itsmoked]

Excuse me Jeff but it appears you're completely ignoring the actual torque curves which can be very different.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Not ignoring them, but I have seen some very impressive torque comparisons of modern high-end Vector Drives and motors vs DC drives and motors, I'm fairly convinced that the differences are approaching negligible now in most applications. All of these comparisons were done by people who make both products, i.e. ABB, Siemens, Reliance (Rockwell era). The big difference was always in braking because DC could do that easily, but the latest Line Regenerative AC drives from ABB and Siemens I have seen solved that problem very nicely. More expensive up front for sure, but the performance is impressive. In things like printing machines though, the "common DC Bus" arrangement feature for multiple AC drives has always competed well against DC, line regen may not be necessary.

Example: there is a chart on page 2 that sums it up.

http://www.reliance.com/prodserv/standriv/appnotes/d7725.pdf

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[DickDV]

I have always found that the "torque" advantage that DC supposedly held over AC was largely meaningless. I say that because, in DC drive/motor systems, while the motor may have as much as 400% overload torque capability, unless you oversize the drive to take advantage of it, it is largely unusable. Typically, the drive is sized at around 150% of motor FLA which puts the system well into the range of AC drive/motor overload capability.

Actually, the same thing happens in the AC realm. A NEMA B motor is capable of 220% short-term overload torque but it is rare indeed to find the drive sized for more than 150% overload. Even 110% is common so, what benefit is there to have a motor capable of 220% when it is limited by a 110% rated drive.

Similarly, what good is it to have a 400% motor when the drive is sized for 150%.

That's why I usually place the "high torque" comments about DC motors in the "snake oil" catagory.

 

[itsmoked]

StarTribune; You might find this an interesting thread.

thread237-224589

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Nice reference Keith.

Aside from torque, EDM in bearings, EMI and Power quality issues with VFD, what about the efficiencies of drives themselves ? For the same HP, torque-speed requirement, is a DC drive more efficient than a VFD ?

Muthu

http://www.edison.co.in

 

[StarTribune]

Gentleman,

Thanks for your input. It was helpful.

Greg

 

[LionelHutz]

Well, you didn't tell us the rated rpm of the DC and proposed AC motors so I can't say if the proposed AC motor is suitable or not.

If you are in the USA I would suggest you contact Parker SSD Drives division in Charlotte, NC. They cater to system integration type jobs such as this.