Inrush limiting of across the line starts. Flywheel, etc?? 2

[bob102939]

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.

 

[electricpete]

Attaching a flywheel to a motor will not reduce the inrush. Also it is backward that you single out the cost "to just sit and run with large mass" (rotating inertia)... there is no power required to keep an inertia running at constant speed... the only increased power requirement would be associated with acceleration and friction.

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)' ?

 

[waross]

In the absence of information to the contrary I will assume that you may just accelerate the large motor up to running speed with a small pony motor and then engage the clutch between the main motor and the driven machine.
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

 

[stevenal]

A soft start is unlikely to reduce demand charges. Demand is averaged over a period like 15m or an hour, so a motor start of a few seconds will have a negible effect. Demand is usually based on active power, and your inrush is mainly reactive. Don't expect any bill reduction even if the demand meter is installed.

 

[bob102939]

The word "sag" is what the PC is whining about. There are a couple bigger motors that pull the small service lines down and I guess knock people offline. Indication was 20% reduction in voltage. They do not want demand metering because that obviously never "improves" an electric bill.

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.

 

[waross]

"In my experience, unloaded motors do pull a measurable amount of power to overcome friction."
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
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

If you can't afford a VFD you can't even afford a quotation for one of these systems.

Star for that. Regardless of the futility of the original idea.

Rafiq Bulsara

http://www.srengineersct.com

 

[fastline12]

There really is not too many ways to store up AC power due to the oscillating effect of the power. Power can be stored as DC but all AC power would then need rectified. A VFD does this but would rarely have enough capacitance to have a measurable affect on inrush other than they are designed to soft start motors as I am sure you know.

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...

 

[cranky108]

A VFD isen't the only way to reduce the inrush of motor starting. Like said above a pony motor, or the use of reduced voltage starting, or switch in capacitors at the same time you switch on the motor.

Or the old way, add impedance to the circuit.

 

[ASimmons23]

We use capacitor banks when starting large (14000HP+) motors in order to keep system voltage above 80% nominal. Injecting VARS to the system could decrease the "sag" the PC is seeing when the motors are starting.

 

[wayne440]

Lets keep in mind that adding VFDs to everything is not sensible.

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?

 

[fastline12]

ASimmons - I might be a little outside my comfort level but curious, does the large cap bank switched in with the a motor start work to just improve the poor PF during starting to reduce line amperage draw or how does that work? We have NEVER started a motor near that size....


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.

 

[waross]

Hello ASimmons23;
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
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

So, let me guess. Your friend doesn't want to pay for a solution and you are here more or less as a hobbiest hoping to instead cobble something cheap together. I hate to break it to you but the popular methods for starting a motor are popular for a reason.

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.

Demand is usually based on active power

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%.

 

[jghrist]

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.

This must be a regional thing. I've dealt with many utilities and have never seen kVA demand metering, only kW demand, sometimes with a power factor penalty.

 

[waross]

I have seen bills from more than one authority that measured KWHrs and KVRHrs. From that they derived the average monthly power factor and applied a penalty when the average monthly power factor was below 0.9
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
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

bob

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Just a word of warning on using capacitor assisted starting. Most of those that I have seen are improperly applied. Once the motor hits about 75 or 80% speed, the power factor in the motor is now a LOT better than when you started, so the starting assistance caps become not only unnecessary, but you run a serious risk of over correcting and causing an over voltage event, one that often damages other systems. The starting method needs to switch the caps out, but people often get cheap and just wait until the motor is at full speed, which is too late. They need to be switched out at 80% speed, which means adding a speed sensor of some sort. Hardly anyone bothers and I have been to several plants where SCRs and diodes are getting killed in all kinds of other equipment from the repeated OV events created by these starting methods. We have discussed this in here in the past.

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