Cost of operating AC vs. DC
Cost of operating AC vs. DC
(OP)
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
Thanks,
Greg





RE: Cost of operating AC vs. DC
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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RE: Cost of operating AC vs. DC
RE: Cost of operating AC vs. DC
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cost of operating AC vs. DC
RE: Cost of operating AC vs. DC
Muthu
www.edison.co.in
RE: Cost of operating AC vs. DC
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
RE: Cost of operating AC vs. DC
Paulbr.
RE: Cost of operating AC vs. DC
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cost of operating AC vs. DC
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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RE: Cost of operating AC vs. DC
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cost of operating AC vs. DC
Example: there is a chart on page 2 that sums it up.
http://www
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RE: Cost of operating AC vs. DC
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.
RE: Cost of operating AC vs. DC
thread237-224589: DC to AC migration
Keith Cress
kcress - http://www.flaminsystems.com
RE: Cost of operating AC vs. DC
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
www.edison.co.in
RE: Cost of operating AC vs. DC
Thanks for your input. It was helpful.
Greg
RE: Cost of operating AC vs. DC
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.