how do i calculate electric motor start up costs

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PANASONIC

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cambridge
i have a 75hp motor i run for a dust collection system, i run it 8 hrs daily. i want to shut down 4 times daily to shake the filter bags inside the unit, i was wondering how much it would cost me to keep shutting down and starting up as i do not have a soft start. i heard that it would save me alot of money installing a soft start, how much money does it cost to start up a motor compaired to just letting it run all day?
 

Besoeker

Senior Member
Location
UK
i have a 75hp motor i run for a dust collection system, i run it 8 hrs daily. i want to shut down 4 times daily to shake the filter bags inside the unit, i was wondering how much it would cost me to keep shutting down and starting up as i do not have a soft start. i heard that it would save me alot of money installing a soft start, how much money does it cost to start up a motor compaired to just letting it run all day?
Starting is usually a matter of seconds. Running it all day is a matter of hours. What do you think?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
You only pay for the actual energy used. when it is not running it is not using any energy. the amount of extra energy it takes to get it up to speed is very small.

however, if you start and stop a large motor like this, it can have negative impacts on its lifespan. Four times a day is probably not a big deal. Four times an hour probably is.

you are looking at starting and stopping it every two hours while it is in use. probably not a huge problem for the motor.

it takes the same amount of energy to get a motor started using a soft start, vfd, or across the line starting. the vfd or soft start just reduces the peak amount of current required. it also reduces the mechanical stresses on the equipment if a vfd or soft start is used.

most bag filter systems i have worked with use compressed air blasts to shake the bag while the blower is still running.
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You are not alone, even though you are mistaken. The concept of "A soft starter will save me a lot of money on startup costs" is a very common myth, often propagated by salesmen who are unable to articulate the REAL benefits of soft starters (as so eloquently laid out by Bob above). What's behind that myth is the misunderstood idea of savings on "Peak Demand Charges". People see a charge on their utility bill that reads something to that effect, so they think that if they lower the peak with a soft starter, the demand charge will go down. It just is not that simple. Peak demand charges are calculated based on what's called a "sliding window" of time, usually 15, 20 or 30 minutes. Any individual peak within that window is virtually ignored, other than the overall minuscule effect it has on the power consumption integrated over that time frame. So adding a soft starter has no tangible effect on changing the Demand Charge multiplier from your utility. If you call and ask them, they will explain it to you. Many people have been shocked to discover this after having spent the money on soft starters thinking that was the cure for their Peak Demand Charge ills.

What it DOES do is satisfy a SEPARATE requirement from the utility to limit your motor starting power, usually expressed as kVA/HP. The numbers vary from utility to utility so there is no universal number that will be the same across the country. But in GENERAL, it's usually somewhere around 50HP @ 480V (25HP @ 240V) and up requires SOME FORM of reduced voltage starting. A soft starter is one of the better ways to satisfy that, which adds the benefits Bob explained above. Those benefits all add up to money, but there is no advantage to leaving a motor running if it is not doing any useful work. No matter how you cut it, that is ZERO efficiency. When people have implemented soft starters on large machines and realized tangible energy savings, in my experiences they have always been the result of a behavior change that was barely noticed. Because of the myth of it costing too much to restart a machine, people would leave them running, just as you have. Then when they get the soft starter, they feel it is OK to shut them off when not used and restart them when needed. THAT saves the energy, but in reality it could also have saved the same amount without the soft starter had they turned it off. The soft starter just makes it "more OK" to do it without feeling like it's a problem. And in reality, they ARE preventing extra wear and tear on the machinery, so it a way, that is correct. It's just not really about the starting energy difference.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Good point. a number of years ago i worked on a vacuum palletizer that had a fan motor on it. I think it was about 20HP. They started it up in the morning and it ran all day and was turned off at the end of 2nd shift. usually about 20 hours a day 5 or 6 days a week. Since 1940 something.

When we replaced the control system because parts could no longer be acquired for it the shipping clerk asked me if the fan could be turned off when not in use. He did not like the noise. So I made a few tweaks to the code. Now it runs for <5 minutes 50-100 times a day.
 
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