Motors; Leave them on or start/stop to save energy consumption.

[highleg]

I am working on a energy consumption analysis for a truss manufacturing plant where I am employed. Here are the bullet points so to speak:
We have truss plate presses that contain two 480 volt 3 ph. 3hp motors. They run 20 hours per day 5 days per week. I want to know how to calculate whether it is more economical to start/stop these motors. They only need to run for approx. 20 seconds every 15 minutes. I am going to install soft starts to negate the mechanical disadvantages of installing these timers that will subsequently stop the presses when not in use. I know this is abstract, but if someone could teach the formulas for me I will do the math, Thanks for your help.

 

[jraef]

Simple formula when referring to energy consumption: There is no better energy saver that the Off switch! Simoply convert your HP to kW (1HP = .746kW), then multiply by the running time (in hours) to get kWh, the amount that your utility bills you for.

Starting and stopping the motors does not consume "extra" energy in the long run. That is a myth that seems to have come from the earliest fluorescent light fixture manufacturers (which is even no longer true for them either).

It is true that an AC motor will draw high amps in the instant it is turned on. But unless you are using high inertia machines such as centrifuges that take 10 minutes to wind up to speed, or have an extremely high duty cycle, that power is an insignificant blip in your overall energy bill. 20 seconds run time every 15 minutes is a very LOW duty cycle.

AC motor starting consumes more kVA than running kW because the power factor is low at first, but utilities charge for kW. kVA is paid for through Power Factor charges, however I seriuosly doubt that this low of a duty cycle will even make a difference there.

The soft starters are a good idea because the real cost of starting and stopping a lot is the long term mechanical strain on drive train components. Good move.

"Venditori de oleum-vipera non vigere excordis populi"

 

[electricpete]

I think jraef covered it well.

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