Inrush limiting of across the line starts. Flywheel, etc??
Inrush limiting of across the line starts. Flywheel, etc??
(OP)
In a nutshell, friend needs to building something to reduce inrush to the power company for motor starts. The power company has not installed a demand meter but indicates they can "see" it by the sag in the lines. Even if they install a demand meter, this might be a worth while effort to make the PC happy and reduce the bill.
Lets keep in mind that adding VFDs to everything is not sensible. I was thinking about something like a flywheel system but have never built one and not even sure how to calculate it's value in circuit. Obviously they will cost something to just sit and run due to the large mass.
Does anyone have an idea here regarding inrush limiting by means of a separate device? basically if the PC stops seeing these 800A transients for motor starts, they might just leave him alone.
Lets keep in mind that adding VFDs to everything is not sensible. I was thinking about something like a flywheel system but have never built one and not even sure how to calculate it's value in circuit. Obviously they will cost something to just sit and run due to the large mass.
Does anyone have an idea here regarding inrush limiting by means of a separate device? basically if the PC stops seeing these 800A transients for motor starts, they might just leave him alone.






RE: Inrush limiting of across the line starts. Flywheel, etc??
There are a variety of soft start methods which might help... you haven't told us anything about the application. You might google soft start and then think about getting an engineer familiar with these topics involved.
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(2B)+(2B)' ?
RE: Inrush limiting of across the line starts. Flywheel, etc??
Motor starting seldom registers on a demand meter and seldom affects the power bill. "Sags" affect other customers on the system and are a concern to the utility.
Bill
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"Why not the best?"
Jimmy Carter
RE: Inrush limiting of across the line starts. Flywheel, etc??
RE: Inrush limiting of across the line starts. Flywheel, etc??
The thought in mind was simply to consider some method of storing up some energy to help with this inrush problem. Battery/inverter system? On crack?
I apologize for the inertial mass misunderstanding. In my experience, unloaded motors do pull a measurable amount of power to overcome friction. I was not sure how that might even fix the problem since everything would be synchronous. I know they use them as a UPS in certain systems but was not sure if they could work in this application.
RE: Inrush limiting of across the line starts. Flywheel, etc??
We've been over this point more times than enough.
Very few people have actually ever measured the power that an unloaded motor draws. It is much less than you think. Google "power factor".
Yes there are systems using large flywheels connected to generators that are used as UPS systems. They typically run in a vacuum to reduce windage and are sized in the MegaWatt range. If you can't afford a VFD you can't even afford a quotation for one of these systems.
Bill
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"Why not the best?"
Jimmy Carter
RE: Inrush limiting of across the line starts. Flywheel, etc??
Star for that. Regardless of the futility of the original idea.
Rafiq Bulsara
http://www.srengineersct.com
RE: Inrush limiting of across the line starts. Flywheel, etc??
If a VFD was hanging on the wall and set to 60hz and loads were then connected to it, it would, in effect, be an energy storage device but again the caps were not really intended to do this and decrease inrush. I supposed you could add some MASSIVE caps to a VFD for this purpose but it would take some calcs to determine what improvements you would actually get
We have used very large VFDs as phase converters. One was a 150HP system but has to be derated by 1/3 for the missing leg on the incoming rectifier system. Something to think about I guess...
RE: Inrush limiting of across the line starts. Flywheel, etc??
Or the old way, add impedance to the circuit.
RE: Inrush limiting of across the line starts. Flywheel, etc??
RE: Inrush limiting of across the line starts. Flywheel, etc??
OK, but be aware you may be disqualifying a viable solution with that statement.
You have a "couple of bigger motors" that cause the sag. Do they start at the same time? If so, do they have to? A few more specifics about what you are doing might lead to more specific suggestionsd. What are the motor ratings, and what is the driven load? What starting scheme is in use now?
RE: Inrush limiting of across the line starts. Flywheel, etc??
Wayne, I am not sure about the OP but we have dealt with machine tool spindles on many occasions where the current draw is up and down all day long. I have no idea if that could really be corrected. Most all the ones I would deal with are already on a VFD and limited to about 200% of FLA for motor starts.
RE: Inrush limiting of across the line starts. Flywheel, etc??
I have been aware of that motor starting technique for decades but this is the first time I have even heard about an actual application. Can you give us anymore information?
Thanks
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Inrush limiting of across the line starts. Flywheel, etc??
An induction motor with a flywheel spinning won't help with the inrush of other motors. A synchronous motor could help but would likely have to be much larger than the motor you are starting and then it would be difficult to start.
It sounds like this is a couple of motors so I would recommend you first investigate using soft-starters for these motors. You would then investigate using a VFD if the soft-starter will not work.
I have found this is usually not true. I have looked at a number of different utility company bills and they all seem to be about the same. The utility measured both the kW demand and kVA demand. The utility then typically multiplies the kVA demand by 0.9 and uses the larger of this number or the kW number. In other works, you pay for kVA demand unless your power factor is greater than 90%.
RE: Inrush limiting of across the line starts. Flywheel, etc??
RE: Inrush limiting of across the line starts. Flywheel, etc??
That would probably give the same bottom line on the power bill as charging for KVAHrs when the PF was below 0.9
Why? In the days when only electro-mechanical meters were available, there were small phase shifting transformers available which made it easy to measure KVARHrs with a standard KWHr meter.
The electro-mechanical demand meters responded to KW demand. I suppose that they could be phase shifted to measure KVAR demand but I have never seen it done. To be meaningful a KVAR demand would have to be used with the KW demand at the time of the peak KVARs and that was not an economical option with electro-mechanical meters.
Bill
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"Why not the best?"
Jimmy Carter
RE: Inrush limiting of across the line starts. Flywheel, etc??
My feeling is that you do not really understand the options that other members have described here.
The pony motor, a soft start with reduced voltage a solid state soft starter or even a KUSA starter or starting motors at intervals are all good and applicable solutions. So is the temporary addition of kvars (capacitors) and the reason is mainly because the induction motor's PF is very low during start and causes voltage drop in the inductive parts of the feeder (mostly in the transformer). The resistive voltage drop is usually not disturbing.
If you describe your plant: Number of large motors and their sizes, if they are started simultaneously or not the size of your main fuse or breaker and perhaps a few other details - then we may sort this out in a more productive way.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Inrush limiting of across the line starts. Flywheel, etc??
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