4 replies to this topic

[marke]

That google link has finally got the better of me.

I keep seeing a google link

Soft Start to Save Demand
Reduce demand charges with soft start controllers (0.5hp - 1100hp)

and it has finally made me jump into print.

In my book, a soft starter will not reduce the demand charges when they are based on a quarter hour or half hour integral, in fact I believe that the demand may go up because the area under the time/current curve will increase.
If we halve the start current, the torque will be a quarter and so the start time will be four times. - The energy will be the same but the average current will be higher.

Any comments?

Best regards,

[GGOSS]

Yep, that's my interpretation also. However, I receive many customer calls about this and the latest one was just yesterday morning.

Regards,
GGOSS

[jraef]

That guy was a customer of mine! I attempted several times to teach him the error of his ways, he was uninterested in the truth. What a surprise....

There are a handful of utilities in the US who have a ratcheting instantaneous demand charge system. I know of only one, but I have heard anecdotally of a few others. The one I know of is a small 2nd tier electric co-op and irrigation district who buys bulk power from other larger utilities. They have maybe 10,000 customers, mostly residential and farms using 1 phase power, so every other 3 phase load they connect represents a power delivery demand problem since their primary mission is reliable irrigation water delivery. Needless to say, this is the extreme exception to the norm.

I pointed out to our "friend" that using a rare exception such as this constitutes a weak argument for demand charge savings with soft starters, especially since he makes absolutely NO qualifiers in his advertisement. He just said something like "Well, it isn't an outright lie then is it?"

One guess as to which "energy saver" he is selling!

[kens]

Hi all, as most recording systems are based on half hourly intervals there is no benefit to the demand charges with any reduced voltage starting method. ( apart of course from the very rare case such as jraef mentions) however there can be a reduction in the required site capacity. In some cases this can be fairly significant, if a higher capacity supply is required simply due to the starting requirements of motors it is pretty easy to make svings in this regard with SS or similar as many lines companys (in this country at least) include a capacity component in their daily charges.

Ken

[jraef]

That's correct kens.

I have bored many people with this story before and I apologize in advance if you've heard it, but it does make sense to repeat it in this context.

I once was asked to take over a project where a sawmill was retrofitting soft starters onto every motor 50HP and up. When I took over, they were 90% complete or so, I just finished the project and commissioning of the remaining 8 soft starters. After doing the last one, I was discussing the overall project with the plant engineer and asked him why he had undertaken such a task in the first place (hoping to capitalize with other sawmills on his reasoning). His answer shocked me. Turns out the first salesman to get involved sold him on the idea that this would reduce his peak demand charges. Like a fool, I immediately corrected him on that concept, explaining that they were not going to help. He proceeded to show me his utility bills and in fact, his demand charges WERE lower! I had to investigate and discovered that because his operators were no longer afraid of restarting machinery, they actually turned things Off when not in use, i.e. breaks, lunch, shift changes etc. More Off time, less energy, less demand dollars! The demand charge rate was still just as high because his peaks were just as high as ever. But because his overall electricity use was down, the associated demand charge dollar reduction was real, it just didn't come from the direction he thought it would.