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Energy Conservation By Star Connection

11/18/2012 1:03 PM

In a lightly loaded AC induction motor ( less than 50% ) it is general practice to connect the delta connected motor to permanently on Star to reduce energy by improving efficiency. That is , star current will be less than delta current . But in one of the fan motors in our plant the star current is more than delta current.

Why is it so ? Pl. share the reason for this phenomenon.

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#1

Re: Energy Conservation By Star Connection

11/18/2012 1:16 PM

You will loose money, the money you have spent on the equipment to do the switching between delta star.

Energy out = energy in + losses

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#2

Re: Energy Conservation By Star Connection

11/18/2012 2:33 PM

"it is general practice to connect the delta connected motor to permanently on Star to reduce energy by improving efficiency."

Where did you come across this info.? What is your source?

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#13
In reply to #2

Re: Energy Conservation By Star Connection

11/19/2012 12:05 PM

Dear Joshi
Thanks for your mail.When an induction motor is lightly loaded the most effective method to conserve is by connecting in star, there by improving the efficiency , reduced losses.The output of the motor gets reduced by 1/3rd.For case study you can the below link.I have practically implemented this many motors few years back, but surprisingly I have observed a different phenomenon of star current being more than the delta current.

www.mahagenco.in/ENERGY-CONSERVATION.../CSTPS_29sept.p..

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#14
In reply to #13

Re: Energy Conservation By Star Connection

11/19/2012 12:14 PM

Time you read #12, and stopped kidding yourself.

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#15
In reply to #13

Re: Energy Conservation By Star Connection

11/19/2012 12:17 PM

"but surprisingly I have observed a different phenomenon of star current being more than the delta current."

Then, what is the flaw?

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#3

Re: Energy Conservation By Star Connection

11/18/2012 2:55 PM

In star, the motor is overloaded. If you were to measure rpm, there would be considerable slip.

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#4

Re: Energy Conservation By Star Connection

11/18/2012 3:13 PM

Buy properly sized motors.

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#5

Re: Energy Conservation By Star Connection

11/18/2012 7:45 PM

There is only one way to connect the motor, is by the motor nameplate. Most nameplates will be 720 star connected or 415 delta connected.Either way the phase voltage is the same.Motors less than 4kw is usually 415 star or 240v delta.

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#6
In reply to #5

Re: Energy Conservation By Star Connection

11/18/2012 9:02 PM

What happens if a motor is connected in Star at the voltage at which it is being designed to be coonected in Delta

See the Link

http://cr4.globalspec.com/comment/898400

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#7
In reply to #6

Re: Energy Conservation By Star Connection

11/18/2012 10:03 PM

your phase voltage is going to Vl/root3, thus your torque is going to be reduced by the same ratio.now you motor torque is going to be less than you load torque.You will see high amps before it trips on overload.It may run for a few minutes or weeks depending on the differences between motor and load torques.

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#8
In reply to #5

Re: Energy Conservation By Star Connection

11/18/2012 11:35 PM

There may be only one way in Oz, but in the USA there is more than one way, and that may well be true elsewhere...

Assuming your 720 and 415 values represent voltages, they are not familiar values to me.

In our current plant, we have 480 V, 277 V, 240 V, 220V, and 208V three phase, and at least three single phase voltages.

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#10
In reply to #8

Re: Energy Conservation By Star Connection

11/19/2012 4:49 AM

I used 720v and 415v to show the root3 factor between star and delta.My point is that depending on what voltage you have will depend on how you select the connection type.I guess in USA 480v star or 277v delta connection will produce the same torque.

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#9

Re: Energy Conservation By Star Connection

11/19/2012 3:14 AM

That particular motor is overloaded in star, therefore to leave it in star is a false economy.

The best way to reduce energy consumption is to switch things off.

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#11

Re: Energy Conservation By Star Connection

11/19/2012 7:00 AM

At low load the motor might run in star. In theory for a given load the line current is 1/√3 x the delta value but the voltage is x √3 so I doubt there is much if any energy saving, and it might damage the motor. It's well outside voltage tolerance so you've no comeback if it does.

I don't know why "in one of the fan motors in our plant the star current is more than delta current." but I would reconnect it in delta. Better still install the right sized motor for the job, as somebody else said.

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#12

Re: Energy Conservation By Star Connection

11/19/2012 11:28 AM

Quote:

    • "In a lightly loaded AC induction motor ( less than 50% ) it is general practice to connect the delta connected motor to permanently on Star to reduce energy by improving efficiency. That is , star current will be less than delta current . But in one of the fan motors in our plant the star current is more than delta current."

This is NOT a "general practice" as far as the knowledgeable electrical engineering community goes. This is at best an internet myth. I would dearly like to know who is promoting this concept, it comes up in this forum about 3 or 4 times per year, always from India or the Middle East somewhere. There must be some Professor at a University who thinks this is a good idea. He is mistaken.

The concept arises (from what has transpired in other postings) from the notion that current = energy. That is of course false at the outset. If also comes from the notion that motors "waste" energy if they are lightly loaded. This concept is promoted heavily by the "energy saver" module companies who are looking to part the unknowing from their hard earned money. It is a common trick to exploit a TINY kernel of truth into being a bigger deal than it really is.

The tiny kernel of truth is that electric motor power consumption is a combination of of the power required to move the load, plus any losses in the motor. So if you have a load that is ALREADY requireing only 50% of the power rating of the motor, that motor is ONLY GOING TO USE 50% OF ITS NAMEPLATE RATING TO RUN IT, plust any losses! So in ALL cases of "energy savings" techniques that anyone discusses, you are NEVER discussing anything more than ways to reduce the LOSSES in the motor. So knowing that, you then have to disect the losses in the motor to see what you can affect.

Fist off, if you are using a modern energy efficient motor, your losses are going to be, AT WORST, maybe 8% of the RUNNING load when a motor is at 50% load. So to give that perspective, if you have a 10kW motor running on a load that only requires 5kW, then the LOSSES are going to be at worst, 8% of 5kW (not of 10kW), so 400W.

Of those 400W of losses, 60% of them are I2R losses in the motor windings and conductors, what are called "copper losses". Those are totally dependent upon the actual current going through them, so since the current is already lower at lower loads, those losses remain consistent regardless of any energy savings techniques and can therefore be taken out of consideration.

So now we are left with 160W of losses on this 10kW motor. Of those, 15% (of the original) were from friction and windage, referred to collectively as "mechanical losses". Those too are going to remain consistent regardless of techniques applied, so are also removed from consideration. So now we are down to about 25% of the LOSSES in the motor, or 100W.

This final 25% of the original total losses are called the "magnetic losses" in the motor, the energy consumed in making the motor into a motor (as opposed to a boat anchor) when you apply voltage to it. These are directly related to the applied voltage potential given to the motor. THESE are the only losses that can be affected by changing the motor voltage. In that endeavor, only about 40% of those losses can be affected by reducing the voltage to a level that does not allow the motor to stall. Your total losses went from 400W to 360W.

So to review our 10kW application, we had only 400W of losses and by reducing the voltage to the maximum amount safely possible, we can save 40% of 25% of those losses. That means on that 10kW motor, you can save 40W of energy by running it in Star instead of Delta, assuming it does not stall as a result of that. If you are going to run that motor 24/7/365 in this manner, that 40W of energy savings can look to become somewhat significant. But if you are running this motor at 50% load 24/7/365, why do you have a 10kW motor? If you had a 5kW motor running at full load, the motor efficiency alone would be better than this. The total losses would only be 2-3% or 150W total, as opposed to 360W if it was a 10kW run in Star.

False economy, increased risk of failure. I don't see the point.

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#16
In reply to #12

Re: Energy Conservation By Star Connection

11/19/2012 1:22 PM

Yes. Indeed it comes up from India.The statutory body under ministry of power,GOI namely Bureau of Energy Efficiency (BEE) one of the most respected body responsible for energy conservation in India has to say this " for motors,which consistently operate at loads below 40% of rated capacity, an inexpensive measure might be to operate in star mode"

As everybody knows induction motor at part load is (less than 50% load) not only less efficient but also operate at less power factor which in turn increases the load current and hence copper loss.By connecting in star mode motor capacity is electrically down sized and thus made to operate near full load and hence improved efficiency and improved power factor .At least couple of companies in India have very successfully demonstrated the energy saving as per actual usage and doing good business with introduction of what is known as delstar converter.To go in for a new energy efficient motor or new properly sized motor has to be left to the consideration of OP instead of going for inexpensive way.

I would have gone in for a speed reduction by pully change which could reduce the power by a cube as in any case connecting to to star reduces the speed.The load current increase may be attributable to overloading in star connection as pointed out earlier

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#17
In reply to #16

Re: Energy Conservation By Star Connection

11/20/2012 9:41 AM

Yes. Indeed it comes up from India.The statutory body under ministry of power,GOI namely Bureau of Energy Efficiency (BEE) one of the most respected body responsible for energy conservation in India has to say this " for motors,which consistently operate at loads below 40% of rated capacity, an inexpensive measure might be to operate in star mode"

Huh. Folly if you ask me, but I suppose if you cannot afford to replace a motor consistently operating at under 40% load with one more suited and more efficient, I suppose it is the next best thing. But again, the risks of failure vs the lesser improvement of efficiency makes it a poor choice as a primary strategy. And yet it appears to have become so.

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#19
In reply to #16

Re: Energy Conservation By Star Connection

11/20/2012 12:44 PM

Dear Ramasamy
Thanks for your reply . pl. clarify the reason behind how the load current is star is more than in Delta connection. I have earlier on many occasion seen Star current being lesser than delta in the lightly loaded motor.Pl. see the typical values recorded recently on 10 KW motor with 21 amp FLA.
Delta run : Rph=8.6A, Yph=9.0A & Bph=8.9A. Star run : Rph=11.4A, Yph=11.3A & Bph=11.4A.
Thanks once again .

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#25
In reply to #19

Re: Energy Conservation By Star Connection

11/21/2012 7:21 AM

Dear Mr.Srivatsa

you have not indicated the kw loading on the motor when connected in delta as also voltage and pf.Assuming 415 V and .85 pf the loading will be about 5.37 Kw But when connected in star, to deliver the same kw the motor is supposed to take about 15 amps as the voltage is reduced to 240.In your case it is taking only 11.4 amps meaning it is delivering lower output.As the actual lading of 5.37 is about 54 % of the motor rating it is NOT a fit case for star connection.If you want to save energy you may control the inlet van if any (no cost) as suggested by JMUELLER or reduce the fan speed by change of pully (low cost) or go for VFD which is costlier option but more flexible and adjustable to process requirement.It is for you to calculate the ROI/pay back period if you go for VFD

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#18
In reply to #12

Re: Energy Conservation By Star Connection

11/20/2012 12:37 PM

Dear JRaef

Thanks for your exhaustive reply. The fact is , this method does save energy by reducing the loss and improving the power factor with NO investment as long as we are confident about the motor loading. This comes handy especially in some process plants many motors are under loaded and replacing all them will call for capital investment and can be taken up only in phased manner.

But my question is this lightly loaded motor will take less current than is Delta. I have come across a case where the current drawn in the star is more than delta, pl. see below the typical values of 10 kw motor with full load current 21 amps.

Delta run : Rph=8.6A, Yph=9.0A & Bph=8.9A. Star run : Rph=11.4A, Yph=11.3A & Bph=11.4A.
Curious to know how ?

- srivatsava

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#20
In reply to #18

Re: Energy Conservation By Star Connection

11/20/2012 12:57 PM

Something is surely amiss. Put another motor having same specs. & check. It may not be under loaded. There must be something that has escaped observation.

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#21
In reply to #18

Re: Energy Conservation By Star Connection

11/20/2012 1:51 PM

3↑

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#23
In reply to #18

Re: Energy Conservation By Star Connection

11/20/2012 5:27 PM
    • But my question is this lightly loaded motor will take less current than is Delta. I have come across a case where the current drawn in the star is more than delta, pl. see below the typical values of 10 kw motor with full load current 21 amps.
    • Delta run : Rph=8.6A, Yph=9.0A & Bph=8.9A. Star run : Rph=11.4A, Yph=11.3A & Bph=11.4A.
      Curious to know how ?

Most likely you have under estimated the actual load applied to the motor. When run in Star, the loss of torque will make it far more easy for a slight increase in load to increase the slip and thereby the current. That is the inherent risk I was referring to with this scheme, hence the folly.

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#22
In reply to #12

Re: Energy Conservation By Star Connection

11/20/2012 2:40 PM

Friends,

There are 2 limits to the conclusions that JRaef's post presents.

First, his discussion is only about the losses within the motor--within this limit, I have no argument against his comments. On the entire distribution system there will be I2R losses. The underloaded motor has a power factor that can be skewed heavily towards the leading side. Although a motor may have a significant reduction in its actual power draw when underloaded, its current draw from the line usually shows only a little change. Therefore the I2R losses in the rest of the circuit are a larger percent of the total circuit losses when a motor is underloaded as compared to when it is fully loaded. Reducing the voltage in the motor's windings by changing its connection from star from delta will provide a significantly larger savings of total I2R losses than the post implies, because the power factor is improved.

Second, although the original post only discusses reducing the motor voltage with the speed constant, this limited approach ignores the very real subject of the energy efficiency of the system. If the system is a variable-air-volume unit with dampers to control the volume of air flow, then a control change to use fan speed to control the air volume will allow energy savings that can be much higher than those calculated on a fixed-speed motor only. Pumps operate with similar energy savings. Look at the affinity "laws" for fans and pumps.

I realize that the reduction of the motor's effective operating voltage is an energy conserving approach with infrequent use in many areas, because other ways are available to achieve equal and greater conservation. However, in the case of the original post, it should not be tossed aside. If the available torque produced from the motor qt its reduced voltage is sufficient to supply the load's required torque, and the current is lower than the nameplate current for the motor, then I believe that no reduction in motor life is likely to occur.

--JMM

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#24
In reply to #22

Re: Energy Conservation By Star Connection

11/20/2012 6:15 PM

Oops, ...motor qt its reduced... should have been "...motor at its reduced..."

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