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Soft start for 2000hp 4160v air blower motor


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#1 Brian2903

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Posted 21 June 2005 - 08:47 PM

Below is the report from engineering consulting firm that investigated on the application of soft start for the 2000hp 4160v air blower motor in our facility.

"Demand was not a peak reading, but a calculated total usage for the period averaged for the 15 minutes to indicate the peak in the manner of an integral function or RMS measure common in electrical instruments. Because of this type of measure, a peak from the multiples of full load current at motor start is “averaged” with other readings during the 15-minute period to calculate peak. Thus, the peak has an effect on the final “calculation” as the energy is a multiple for the several seconds of blower acceleration and this is averaged with a steady state but does not, by itself, determine peak demand.
The amount of potential savings is considered so low that soft starts are not part of the rebate/financing program. Any reduction in demand, due to reduced current at starting, is offset by having an extended starting time."

The didn't provide any maths on the issue above. Please post any thoughts/comments you have.

#2 marke

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Posted 21 June 2005 - 09:32 PM

Hello Brian

I would agree that the use of the soft starter is not going to impact on the power usage and will have very little impact on an integrated demand tarrif. The major advantages of the soft starter are to reduce electrical disturbances during start and to reduce mechanical transients and resulting wear and tear on the equipment.
Some soft starter salesmen promote that soft starter as a means of saving energy. In most cases, this is not a true representation. The energy that can be saved is best done by turning the machine OFF. The soft start reduces the starting disturbances and torques and enables machines to be started more regularly than would be the case with star delta or DOL and this is where energy can be saved.

Best regards,

#3 jraef

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Posted 21 June 2005 - 11:20 PM

Brian,
I agree with Marke and the consultant that you posted as well, and I work for a manufacturer of Medium Voltage soft starters. In addition to bogus energy savings claims, it is also quite common to hear salesmen preach "savings on peak demand charges" as well. I would love it if it was true, but it isn't.

In their defense, some of that information comes directly from the marketing departments of the manufacturers they represent, but they don't know enough to contradict it. Where this comes from is that in a very very small percentage of utilities, they use what is called a "ratcheting instantaneous demand charge", meaning that they charge a small penalty for EVERY starting current peak, not a larger penalty for demands integrated over a 15-30 minute moving time window as most do. The rare occurrences of these systems can definitely benefit from reduced voltage starting, so the marketing people jump on that and put it in the literature. Usually they have some sort of disclaimer that explains something to the effect of "results may vary", but some do not.

A general rule of thumb is that if a salesman pushes this idea on you, he is ignorant of the subject (but when has that ever stopped a salesman?). If you still think there is merit in the product involved, ask to speak to their tech support department, most of them should be savvy to the real issues. There are plenty of great reasons to use soft starters. In most cases with large motors, the utility will require reduced voltage stating of some sort anyway so making it a solid state version comes with many additional benefits worthy of consideration. Peak demand reduction however is not usually one of them.
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#4 Brian2903

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Posted 22 June 2005 - 06:44 PM

Thank you very much for your help.

So, soft start basically use for keeping the life of the mechines, but no/very little energy cost saving. Or did I miss any advantages of soft start?

Since the air blower motor in our facility only run 50% load in average, I'm still think of the way to save the energy cost. What I researched on so far included medium voltage VFD (payback is 15+ yr), soft start and power factor correction.

Down to the road, I think power factor correction would be the most efficient way to save energy cost. As the displacement power factor of the motor only 88-89% all the time, and the nameplate pf also 89%. I was just wondering if it is possible to push the pf to 95% or even high percentage by doing either static or bulk power factor correct?

I can't estimate the payback of the the pf correction sicne I dont' have any info/price on that. Please give me soem idea if any one have experience on that.

#5 marke

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Posted 22 June 2005 - 07:33 PM

Hi Brian

If the plant operates in exactly the same way with and without the soft starter, there is no energy gain.
What the soft starter does do, is to offer opportunities to change the way things are run and enable more starts on a given machine in a given time. That way, the machine can be shut down more often and operate more efficiently.

Power factor corection will also save very little energy except in the power reticulation system. It will not have any real impact on your KWHr usage. If you pay a power factor penalty, then it will have an impact on that. You need to ensure that you have "extras" charged on your energy bill that will benefit from pf correction. Beware the salesman that promises energy saving fom the use of power factor correction!

Best regards,

#6 Brian2903

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Posted 22 June 2005 - 08:33 PM

Marke, could you explain to me in details why pfc could only save very little energy? From my calculation the amp would go down from 196A to 185A if the pf chagne from 88 to 95%. Our electricity company doesn't charge any fees for low pf yet, and I'm sure 88% is not taht bad if they charge it later. However, increasing pf should be better use of electricity in other word loss less in inductive load. Please correct me.

#7 Brian2903

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Posted 22 June 2005 - 08:38 PM

BTW, that motor is running 24/7 but rotating with other 3 same size motors once a month.

#8 marke

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Posted 22 June 2005 - 08:44 PM

Hello Brian

Yes, the improvment in power factor wil certainly reduce the current drawn from the supply. If we consider the motor, the addition of pf correction to the motor terminals will not change the current into the motor, merely the current drawn in the supply, so the current and KW consumed by the motor will not change. The current drawn from the supply will reduce, and this will reduce the I^2R losses in the supply.
You are almost definitely paying for KWHr. In this case, the KWHr copnsumed by the motors does not change. The copper losses in the cables between the power factor correction and the metering will be reduced, but this will be very small relative to the power consumed by the motors. If it is significant, then you need to increase the cable size for higher savings.
The most significant energy saving is in the suppliers grid where the lenght of transmission lines and cable is significant and the losses are also more relevent. The saving is for the suppler of the electricity, not the consumer.

If you pay a maximum demand (KVA) charge or some form of pf or KVAR penalty, the addition of pf correction will reduce your bill. If you pay for KWHr only, the impact will only be due to the reduction of copper loss and this will be highest if the correction is applied at each motor, and there are long cable runs to each motor, and the cable is undersized.

Best regards,

#9 jraef

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Posted 23 June 2005 - 04:15 AM

Marke,
I think what he is thinking of is the kind of sales pitch outlined in this tech note.

http://www.myronzuck...ACITALK 101.pdf

It starts off by making a big claim of saving kWH using capacitors, then eventually contradicts itself in it's cheif example. If you look on page 2, they show an anecdote of a plant that reduced the Amps used by 12% (2516A to 2223A) after adding capacitors at the motors. But if you pay attention, the KW was reduced by only 2 1/2% (1643kW to 1601kW). Care to comment on their claim?
"He's not dead, he's just pinin' for the fjords!"

#10 Brian2903

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Posted 23 June 2005 - 01:23 PM

Thanks guys. I'm new to this field and have a lot of concepts and comments that I couldn't understand.

Why current draw reduced from the supply won't reduce the current draw from the motor as the pfc done on the motor terminal? Does it imply that pfc is only using for reduce the cable loss,but not the inductive loss in te motor?

If no pf penalty in our region, the cost to run a low pf motor would be similar to high pf motor, right? However, motor efficient is associate with pf........ with higher pf dont we have higher efficient?

So, pfc shouldn't be done but thicken teh cable is preferrable?

For jraef's pdf file, the annual savings is 21,420 and the investment cost is 5801. Isn't the payback should be 3.25 year but 3.25months?

#11 marke

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Posted 23 June 2005 - 07:04 PM

Hello Brian

Firstly, adding power factor correction to the motor terminals does not in any way change the current in the motor windings. The current drawn by the motor is a combination of resistive current and inductive current. This gives us an inductive power factor. If we add a capacitive current to and inductive current, they cancel and we get left with the resistive current only. This is how power factor correction works. The current into the motor remains inductive and the curent into the capacitor is capacitive. Current in the cable upstream of the capacitor is resistive and so lower.

Power factor correction does not alter the power factor of the motor, neither does it affect the efficiency of the motor, only the current drawn from the supply.

If you have significant KW losses in your cables and a low power factor, then power factor correction will reduce the KW losses by reducing the current. Increasing the cable size will prbably do even better.

Best regards,

#12 Brian2903

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Posted 27 June 2005 - 09:19 PM

One more thing. Our facility could generate 1800kw electricity and the air blower motor, again, need around 1500-1600kw to run. I was just wondering by putting in the soft starter would that be possible to start our motor up by our own generator?

#13 marke

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Posted 27 June 2005 - 09:31 PM

Hello Brian

It depends on the overload capacity of your generator. There will be a high overload during start and I expect that your generator would not be able to carry that load. This would need to be fully engineered before an attempt was made. I suggest that you talk to the generator suppliers first and find out what the short term capacity of the engine and the alternator are.

Best regards,

#14 Brian2903

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Posted 27 June 2005 - 09:41 PM

Thank Mark,
How many times would soft starter reduce the current? If the motors are design with 243A.

#15 marke

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Posted 28 June 2005 - 07:41 PM

Typically, to start a large fan in a reasonable period of time, you would need 3 - 4.5 times the rating of the motor. DOL is 6 - 9 times the rating of the motor. The actual start current is dependient on the torque and inertia of the fan and on the motor starting characteristics.
Download Electrical Calculations form http://www.LMPhotoni...om/busbar32.zip and you can calculate the start current and run up times.

Best regards,

#16 jraef

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Posted 29 June 2005 - 08:31 PM

QUOTE
Originally posted by Brian2903
One more thing. Our facility could generate 1800kw electricity and the air blower motor, again, need around 1500-1600kw to run. I was just wondering by putting in the soft starter would that be possible to start our motor up by our own generator?


Brian,
Generally, a quick-and-dirty rule of thumb on generator sizing for using soft starters is that your generator can be as low as 1.5X the motor size. As Marke said, it really needs to be engineered properly to be sure, but at 1800kW and a 1600kW motor, I seriously doubt it will work with any sort of RV starting.
"He's not dead, he's just pinin' for the fjords!"




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