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Join Date: Sep 2013
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Energy Savings

11/24/2017 1:12 AM

Dear sir,

We are having 3 nos. 220 KW motors connected to VFDs for our clinker cooler cooling fans.They are 1500RPM motors and we are running the fans at less than 1000RPM.These 3 fans are running at hardly at 90KW power. I am having 3 nos.200 KW motors .Is there any advantage by replacing 220 KW motors with 200 KW motors?Can I visualize any energy savings?I am having 2 nos.250 KW vfd drives also,If there is any benefit,I will replace them with lower rating motors.Can any person help me by giving energy savings calculations.

2,Please explain me if the sbove motors are not VFDs ,what is energy savings if the above 22oKW motors replaced with 200 KW motors.220 KWmotors are drawing 90 KW power.

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#1

Re: energy svaings

11/24/2017 1:36 AM

Then why not replace them with 100 kW motors?

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#2
In reply to #1

Re: energy svaings

11/24/2017 1:53 AM

At present, I am having 200 KW motors.For 100 KW motor,I have to buy new one.If iI can get clear cost savings calculation, I will work in that direction.Thanks.

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#3
In reply to #2

Re: energy svaings

11/24/2017 2:14 AM

How do you change from 220 kW to 200 kW without buying new motors?

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#5
In reply to #3

Re: energy svaings

11/24/2017 2:51 AM

<...am having 3 nos.200 KW motors...>

It appears that the Original Poster already has these to hand.

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#4

Re: energy svaings

11/24/2017 2:30 AM

You have to know what the efficiency rating of your equipment is, what is your power factor with present setup? Ideally you should be at about 96% with your present setup...so you may be wasting your time for very little gain...

https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/motor_tip_sheet11.pdf

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#6

Re: energy svaings

11/24/2017 3:01 AM

There are strategic matters to consider as well as the energy saving potential:

  • The 220 and 200kW motors to hand may well have applications elsewhere within the facility, and may even have a value on the "previously enjoyed equipment" market if presented for divestment.
  • Whether a business case for change can be generated depends also upon the utility tariff in force at the facility and any need for some headroom on the utility supply, neither of which can be seen from here.

These factors also have a cash value, which is commercially sensitive and not for sharing here, and therefore need to be worked into the calculations.

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#7

Re: energy svaings

11/24/2017 3:37 AM

Typically, VFD could be wasting about 10% of power it is handling (experts on the forum may confirm). If replacing the motor could result in bypassing the VFD during steady state, for extended hours, that is a saving.

If your system is going to run only at 90kW for many days in a year, then it is worth considering I guess.

Then, you may have power factor issues to worry about as the VFD also helps in maintaining close to unity power factor.

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#8
In reply to #7

Re: energy svaings

11/24/2017 9:57 AM

I would say more like 5%, or even less, depending on the input waveform.

However, I think bypassing the VFD is unlikely. The output of a VFD is AC, and the input is DC. AC motors cannot be run on DC, as universal motors do not fit in this thread.

The question here is the efficiency calculations regarding replacement of one motor with another, which cannot be answered here due to lack of information.

I certainly would not be surprised to hear that the high power motor has smaller winding resistance which would lead to lower winding losses.

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#9

Re: Energy Savings

11/24/2017 8:32 PM

Whenever you connect a motor (or motors) to a load, the power required by the load equals the power delivered by the motor(s). The higher the motor RPM (higher flow rate) the higher the power demanded by the load. It sounds like you have throttled your motors back with the VFD to deliver 90KW (total or per motor?).

1. The efficiency of an induction motor flattens out beyond about 20% load, and the percentage load would be not much between 200 KW and 220 KW motors and the larger motor flattens out at a lower percentage. So I don't think that it would make much difference between 200 and 220 KW motors.

The diagram below shows typical efficiencies of different size motors.

2. If you did not throttle back your motors with the VFD, you would operate at a higher flow rate corresponding to 1500 RPM versus 1000 RPM. The power demanded by the load would be greater than 90 KW. You would use more power without the VFD.

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#10

Re: Energy Savings

11/25/2017 12:50 AM

In this partticular system, are three distinct fans required?

How had would it be to operate using only two motors and fans? Canit be done without degrading performance/safety?

If so, consider removing the VFDs from the motors and instead just run 2 motors and fans at closer to rating, leaving one in standby or as a spare?

This could allow use of the motor fan and vfd other places or sale thereof. Additionally, no power lost running the VFDs.

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#11

Re: Energy Savings

11/25/2017 2:26 AM

Hi,

It would be better to understand the load pattern, as with clinker cooling operation, load varies by 50% here and there,

Also, We need to measure Air flow, various temperatures etc at various operations , to arrive at resonable calculation. Hopefully you are already harnessing waste heat recovery.

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#12

Re: Energy Savings

11/25/2017 12:53 PM

Energy savings can ONLY be realized by finding and removing energy WASTED. If you cannot identify wasted energy in a system, you cannot save energy. So where can there be waste here?

The dives likely lose no more than 5% throughput themselves, but the speed reduction is saving you a LOT more than that. 1000RPM out of 1450RPM is 69% speed, so the power consumed by those fans at that speed is the cube of .69 (per the Afinity Law) so 32% of full rated power (which we actually don't know here). So the 5% losses in the VFD become a smaller fraction of the total energy saved here compared to running full speed. Not worth worrying about.

The energy being saved must assume that speed /flow control is essential to the process, so to accurately compare it, you must compare it to throttling control, so in the case of fans, that's either inlet or outlet vane controls. They too will decrease the energy consumed, but at a lower rate throught most of the flow curve. The flow you are running at is basically the point at which the VFD solution operates the best in comparison. Bottom line, don't mess with that.

The only other factor is the efficiency of the motor, as pointed out in and earlier posting. And per that post, the difference in efficiency between a 220kW motor and a 200kW motor are going to be so negligible that in my opinion it would not recoup even the labor and down time cost to make the switch for maybe a decade or more. What you MIGHT consider doing however is to wait until each 220 kW motor has to be replaced for some other reason ANYWAY, such as a bearing failure or winding issue.

But before you do that, make sure that in your process, you NEVER EVER need more than about 165kW. The general rule of thumb on this is that good mechanical engineers pick motors that will operate at their best efficiency point, which is typically considered to be around 75% of rated load, with an eye toward peak demands. 75% of 230 is 165kW, but 165kW is 83% of 200kW, so although the difference would result in only a fraction of a fraction in added losses, what you really might lose is that ability to occasionally run continuously above the best operating point of the motor or occasionally peak above 200kW.

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#13

Re: Energy Savings

11/27/2017 1:22 AM

Polyphase motors this size has an extremely high efficiency(98-99%) at 50-60Hz. VFDs usually operate from 0-400Hz to control the motors speed(rpm). Small motors, 7.5KW or less, a VFD works great, but when you get above that size a motor designed to work at 50/60Hz rating quickly becomes less efficient the farther you get from its design frequency,50/60Hz in the form of heat. Your second question about reducing motor size from 220Kw to 200Kw will be fine if you're using 90Kw, and by using half the motors and VFDs size is probably why nothing has burnt out yet. You asked about savings by switching motors, ideally they should use the same amount of power(90Kw) because it takes that much power to run the fans at their required speed. However this is not including the losses of power in the VFDs, motors, mechanical and electrical resistances, ect. Since this application is fans, it doesn't require large amounts of torque such as pumps, rotational dryers, crushers and any equipment such as this, I suggest a 120Kw or 150h.p. with a mechanical means of slowing down the fans to 1000rpm, such as pulleys and belts or a gearbox. Then and only then, you can use a VFD to fine tune the speed of the fan. But be sure to gear down the motor as close as possible to 50/60Hz, whichever is your countries line frequency. Now everything will be operating at peak efficiency, and with smaller motors your power factor will increase so your peak demand will be lower(I cannot give any amount because I have no information of the amount of power is being consumed), if these 3 motors represent 1/4 or more of power usage it should be noticed immediately in your power bill.

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