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Induction Motor Running in Star Connection

10/05/2009 6:42 AM

We are a boiler manufacturer for process and power plants. Along with boiler we are supplying number of smaller capacity (3hp to 15hp) three phase induction motors. Being small capcity (3hp to 15hp) we are providing only DOL starting. In some cases we have connected motor windings in Star connection. In one of our installation one motor was burnt (before the guantee period of the motor supplier). The supplier is arquing the motor has failed due to star connection only. We feel that motor should not fail as the current is the same and the voltage will be less by 1.732 times (only the torque will be reduced). We request the forums' view on this subject.

R.Thiyagarajan

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Pathfinder Tags: Motor operation in STAR
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Power-User

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#1

Re: Induction Motor Running in Star Connection

10/05/2009 8:52 AM

rajant_2001; you over loaded the motor. perry

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#2

Re: Induction Motor Running in Star Connection

10/05/2009 11:25 AM
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Guru
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#3

Re: Induction Motor Running in Star Connection

10/05/2009 11:09 PM

Maybe the motor was designed to deliver its rated power in delta only ? if so, in star mode, it may have stalled. that would surely burn it out. Are you not using any overload relay ? if there was one, it would have seen the stalled motor current as twice the rated current, and would have taken about a minute to trip. Maybe long enough for the motor to burn ?

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Participant

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#4

Re: Induction Motor Running in Star Connection

10/06/2009 4:22 AM

Generally the smaller capacity 3phase induction motors designed according to IEC specification will work 230 v DELTA connection & 400 Volt @ STAR connection. Ie At any point of time the voltage applied against each phase is always 230 volt . This motors will deliver the rated torque at star connection . if this motor in delta connected configuration feed with 400v supply, it will burn out.

But any motor designed for 415 v delta connection , if it is run at star connection then it will deliver less torque then required , then there is a possibility of burnt out of motor due to rotor stalling.

So any motor designed for 400 v star connection shouldn't be connected in delta 400 volt and motor designed for 415 delta shouldn't be connect on star 415 volt.

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Guru

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#5

Re: Induction Motor Running in Star Connection

10/06/2009 4:45 AM

Implication of your post is that the motor was connected in star even though it should have been in delta. Is this correct? The nameplate will say which is correct connection. If so, was this a mistake on the part of the fitter, or did somebody think it would reduce power bills?

The former is unusual - it's more common to wrongly connect delta when it should be star, particularly by older fitters as delta used to be the norm. If the latter, presumably the same somebody had checked the power needed by the load, and it was << motor rated power.

If it is wrongly connected for whatever reason, this could quite easily have caused the failure.

Cheers.........Codey

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#6

Re: Induction Motor Running in Star Connection

10/06/2009 7:52 PM

I assume that all 6 leads were brought out to the motot terminals. If this is so the motor can be connected in wye or delta and the nameplate should indicate the voltage requirement for each connection. The manufacturer has a duty to indicate the proper connection.

Hope this helps

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#7

Re: Induction Motor Running in Star Connection

10/06/2009 11:05 PM

Induction motor are designed for two type of connection. One is Delta Connection and the other type is Star Connection. If the motor name plate indicated that, the motor is a Delta Connection then you have to connect your motor in Delta Connection and If the motor name plate is stated as a Star Connection, then you have to connect your motor in Star Connection.

You cannot change the motor connection from Delta to Star connection or from Star to Delta connection. Either which ways, you will burnt your motor. One will lose the torque and sequentially, the motor will be over heated and burnt and the other will be over voltage and the motor will also be over heated and burnt.

It a simple rules to follow.

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#8
In reply to #7

Re: Induction Motor Running in Star Connection

10/07/2009 4:10 AM

Agreed, I'd just add that it's the supply voltage that determines whether star or delta is correct. Voltage for each should be on the nameplate. Star voltage = √3 x delta voltage.

Cheers.............Codey

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Guru

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#9

Re: Induction Motor Running in Star Connection

10/07/2009 11:15 AM

The motor manufacturer is right. You see, the line currents (i.e.) the currents you measure at the connecting cables, may be the same in both star and delta connections. But, in a delta connected motor, the winding would carry only the phase current (i.e.) about 57% of the line current and hence the heat produced would be lesser. Whereas in a star connected motor, the windings also will carry the line current, hence the heat generation by each winding will be 3 times more when compared to a delta connected motor. When a motor manufacturer designs the frame size of the motor, he calculates the surface area for the heat generation with the connection intended. When you violate this, he shall not stand guarantee.

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Anonymous Poster
#10
In reply to #9

Re: Induction Motor Running in Star Connection

10/07/2009 11:05 PM

In India, the 3Phase system is 415V and the motors are usually designed delta-connected, giving the option of using star-delta starters where the utility demands it. The motors can also be started DOL in delta mode. So, it is certain that the motor windings can take the line current continuously. When such a motor is run in star, it can only burn if it stalls or if there is single-phasing. A thermal overload relay with single-phasing protection, costing about $5 would have protected it from burining.

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Anonymous Poster
#11
In reply to #10

Re: Induction Motor Running in Star Connection

10/08/2009 1:22 AM

Agreed! But, a delta connected motor, with a star delta starter, runs in star connection only for a very short period of the starting time. You cannot continue to opearte it in star.

If, as per your arguement, a delta connected motor can be continuously operated in star, why have a star delta starter? You may as well go for a star-star starter.

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Anonymous Poster
#12
In reply to #11

Re: Induction Motor Running in Star Connection

10/08/2009 2:00 AM

my comment was in response to comment #9 which implies that in star connection, each winding draws more current than its windings can handle. Not so. This user used a delta-designed motor in star and burnt it out. My opinion is that it can only burn out if it stalls. The right thing would have been for the user to (a) either use a star-delta starter or (b) wire it up in delta and switch it on direct-on-line. In any case, a relay is always a good idea.

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Guru

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#13
In reply to #12

Re: Induction Motor Running in Star Connection

10/08/2009 6:08 AM

I stand by my statement that sustained operation of a motor designed for delta connection, in star connection definitely would kill the motor even without stalling. The motor capacities mentioned are too small to warrant a stra-delta starter.

You kindly recall that in Star connection Line voltage is 1.732 times the pahse voltage and line current is equal to phase current. In delta connection, line voltage is equal to phase voltage and line current is 1.732 times the phase current.

In a motor, line current is what is measured at the terminal connecting cables and phase current is that carried by the windings.

Obviously, in a delta connected motor, the winding current or the phase current is only 1/1.732 times or only about 56% of the line current (i.e.) the current that would flow in the windings in a star connected motor.

So, for the given HP rating, definitely, in a star connected motor the heat generated in the winding will be 3 times that in a delta connected winding.

Only because of this advantage of less heat produced in the windings, do all LV motors are preferred to have a delta connected winding.

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Anonymous Poster
#14
In reply to #13

Re: Induction Motor Running in Star Connection

10/08/2009 8:00 AM

Let us take specific values - Line voltage = 415 V therefore Phase voltage is 240V. The motor is designed to deliver some hp when delta connected and 415 V 3 phase is applied. If this SAME motor is now connected in star and 415 V is applied, 240V appears across each winding. The current in the star mode is now only 1/1.732 of what it is capable of continuosly carrying. How can it burn? The power delivered by the motor is 1/3 only in star mode, since it is designed for delta connection.

In India, small motors designed for star connection have only 3 leads brought out and the star point is permanently made inside. Motors with six leads brought out are designed for running in delta mode, since any motor of >5hp has to be started star-delta in most locations which depend on utility power.

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Guru

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#16
In reply to #14

Re: Induction Motor Running in Star Connection

10/08/2009 9:40 AM

Friend! Are you ready for a long, long discussion?

The most readily recalled formula for three phase power is:

P = 3 x VL x IL x Cos ø

Isn't it?

But, do you know how this formula is derived?

In fact, the basic formula for three phase power is:

P3-ph = 3 x (P1-ph)

= 3 x (Vph x IPh x Cos ø) …. (1)

But, since one can only measure line quantities, the above formula has to be converted in terms of line quantities.

With a star connection, Vph = VL/3 & Iph = IL

Substituting in (1),

P3-ph = 3 x VL/3 x IL

= 3 x VL x IL x Cos ø

And, with a delta connection, Vph = VL & Iph = IL/3

Substituting in (1),

P3-ph = 3 x VL x IL/3

= 3 x VL x IL x Cos ø

So, in terms of line quantities, the formula for 3-phase power remains the same, whether it is star connected or delta connected.

Now, supposing a motor is rated for 15HP, say,

15 HP = 3 x 5HP …. vide (1) above

So, in a 15HP 3-phase motor, each phase winding would be loaded to about 5 HP.

This 5HP is the single phase power to be delivered by each of the windings in the motor.

What would be the current then?

IPh = P / (Vph x Cos ø)

Assuming a power factor of 0.8 and a line voltage of 415V,

If the motor is delta connected, then the phase current (the current in each winding) would be:

= [(3.7 x 1000) / (415 x 0.8)] (5HP = 3.7kW)

= 11.14A

If the same motor is star connected, this would be:

= [(3.7 x 1000) / (240 x 0.8)]

= 19.27A

= 3 times more than that in delta connection.

Heat is I2; so, the heat in star winding would be (3)2 (i.e.) 3 times more.

Hope it is clear now.

Moreover, the restriction for the maximum HP motor that can be started Direct-on-Line is 5HP in some states of India, 7.5HP in some other states and even 10HP in yet other states. Also, please note that this restriction is applicable only if the customer is drawing power in Low Voltage. Because, if he starts a higher HP motor direct on line, then the line voltage drop due to the increased starting current causes his neighbours' loads to stall/stop, thus creating a social issue.

If the customer is drawing HT Power, then there is no such restriction by the authorities for DOL Starting. During my 22 years of field experience, I have even commissioned many 250 HP, 300HP Motors Direct on Line.

I do hope that the misconceptions are cleared now.

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Anonymous Poster
#19
In reply to #16

Re: Induction Motor Running in Star Connection

10/08/2009 9:55 AM

Sorry, but .. aren't you making a small wrong assumption? that the motor will be able to deliver 15 HP irrespective of how it is connected ? i was taught that the motor, if designed to run in delta to deliver certain HP, will deliver only 1/3 power if it is connected in star. With the windings having a fixed impedance, i cannot see how 240V across a winding can cause MORE current in a winding than 415V (which it will see in delta).

Moreover, if the star current can be high enough to burn the winding why does one start in star mode ? only to reduce the inrush current isn't it ? If DOL starting can cause an inrush of 6X rated current, the star connection causes only 2X ..

Anyway, the guy who had the problem doesn't seem to be interested, why should we bother ? Let us agree to disagree ..and close this

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Guru

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#20
In reply to #19

Re: Induction Motor Running in Star Connection

10/08/2009 10:04 AM

I can reply this too. But, since you wnated to close the discussion, I wouldn't elaborate further. Just in case you are interested to probe further, only let me know. Thanks for the interactions.

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Participant

Join Date: Jul 2010
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#32
In reply to #20

Re: Induction Motor Running in Star Connection

07/06/2010 2:06 PM

Dear ElectricalExpert65,

Your explaination about power calculation of 3 phase motor was awesome. Thank you very much. I have 2 question. please answer them:

1) is the FLA on motor's name plate an RMS value (which i sure hope is)

2) you said that one should apply connections to the motor as per configuration, i mean, like if it is designed for star connection then open up the connection box and configure the wires according to the drawing on nameplate of motor for star connection but my question is, how can one know if the motor is designed for star or for delta connection, since the motor nameplate's shows nothing. it only tells 415v star / 230 v delta. which shows that it can run on both configurations. how can one know if the motor is designed for star or for delta connection by readin only nameplate

your quick responce will be much apprecaited

regards,

maverick83pk

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#36
In reply to #32

Re: Induction Motor Running in Star Connection

04/20/2012 9:56 AM

Apologies for late reply to this, but ElectricalExpert65 hasn't responded, or come back on my post #35.

The answer to your question is simple. If your supply voltage (by the usual definition, phase-phase) is 415V, connect it in star. If supply voltage is 230V, connect it in delta.

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#34
In reply to #20

Re: Induction Motor Running in Star Connection

04/12/2012 2:40 PM

dear electricxpert and anonymous

I agree with anonymous ideas. so plz keep this discussion. actually i have a problem at plant.help me plz regarding to ds. problem briefly describes as

We have pump running on 160KW induction motor. We have two types of motors, one is of 160kw star conected motor and another 160kw of delta connected motor but the problem with delta connected motor is that after running it 20 to 25 times it burns on start up. so what may b the reason;

plz help me

Thanks in advance

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#37
In reply to #20

Re: Induction Motor Running in Star Connection

04/07/2014 8:16 AM

Dear Sir,

Request please elaborate further.

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Anonymous Poster
#21
In reply to #14

Re: Induction Motor Running in Star Connection

10/08/2009 10:37 AM

I agree with what the electricalexpert says. What the Guest is not getting is the load that the motor is supplying remaines the same unless it is reduced by some means . In wye if the load is too great for the motor it will eventually stall and most likely burn out because of the overload. If the motor carry the load it will also burn out because of the High current at 1.732 X delta current. You can operate the motor in wye but the load must be reduced i.e motor derated and load reduced. Hope this will clarify the situation.

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Anonymous Poster
#22
In reply to #21

Re: Induction Motor Running in Star Connection

10/08/2009 11:48 AM

Reluctantly coming back .. you are saying what i said after all...the 15HP motor (delta connected) will only deliver 5HP when used in star mode. If the mechanical load remains the same, it will most likely stall. Then it will burn in a few minutes if there is no protective relay. Please recall that the motor in question was designed to work in delta.

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#23
In reply to #22

Re: Induction Motor Running in Star Connection

10/09/2009 4:32 AM

Hello Guest - replying to #22 and #21 - do you mean running at the same (supply phase - phase) voltage in star and delta, or running at the correct voltage in each mode, star = √3 x delta?

The difference is crucial to this discussion

Cheers............Codey

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Anonymous Poster
#24
In reply to #23

Re: Induction Motor Running in Star Connection

10/09/2009 4:43 AM

Absolutely .. i mean running at the SAME voltage in star when the motor is actually designed for running in delta. This is what the originator of this thread did, and that is why i have been saying that the motor CANNOT burn out unless it stalls. The originator said he is ok with lower power and torque...

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#26
In reply to #24

Re: Induction Motor Running in Star Connection

10/09/2009 5:03 AM

OK thanks. Please ignore #25, I was replying to the wrong post!

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#15
In reply to #13

Re: Induction Motor Running in Star Connection

10/08/2009 9:38 AM

When you say............sustained operation of a motor designed for delta connection, in star connection definitely would kill the motor even without stalling.... do you mean running it in star at the supply voltage appropriate for delta? If so I'm sure that's correct, also if it were the other way round.

But I don't agree with your #9, repeated in the latter part of #13, that star connection produces more heat than delta when each is at the correct voltage. The internal windings don't know whether connection is star or delta, provided voltage is correct. You're right that the voltage is √3 x higher in star, but in effect it's across √3 x as many windings. Another way of looking at it is that in delta the line current is higher (as the voltage is √3 x smaller) but it splits between 2 windings at the terminal.

Cheers..........Codey

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#17
In reply to #15

Re: Induction Motor Running in Star Connection

10/08/2009 9:41 AM

Please refer to #16 above.

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#25
In reply to #17

Re: Induction Motor Running in Star Connection

10/09/2009 4:45 AM

I'm going to reply to that shortly

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#35
In reply to #17

Re: Induction Motor Running in Star Connection

04/12/2012 4:46 PM

I don't know whether you still visit CR4, but recent post has resurrected this old thread. Must have got sidetracked from replying way back in 2009.

My comments in #15 are relevant to #16.

You consider a 15hp motor designed for 415V star. Line current ~ 19.27A as you say (this ignores motor efficiency, but that doesn't affect the comparison).

Where you're going wrong is that you then work out the current for the same motor connected in delta at 415V. But 415V is not correct in delta. Correct voltage in delta is 415/√3 = 240V. Using this voltage the phase current is 19.27A same as in star, and the heating effect is the same.

Oh, and please spare us the lofty tone

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#18

Re: Induction Motor Running in Star Connection

10/08/2009 9:44 AM

Only aspects to affect motor heat stressing is the actual current, achieved RPM speed (that affects blower cooling capacity) and of course ambient temperature. If your actual application had serious impact on any of these factors (which I doubt) then yes your supplier is right. Difficult to tell though without knowing application specifics.

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#27
In reply to #18

Re: Induction Motor Running in Star Connection

10/12/2009 1:45 PM

http://cr4.globalspec.com/thread/44171/Help

So which is correct.....that the motor will burn out if connected in star from delta if we dont reduce th e load OR the losses will be more in star due to higher winding current & motor will meet the load same as that in delta???? confused................pls.

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Guru

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#28
In reply to #27

Re: Induction Motor Running in Star Connection

10/12/2009 2:06 PM

The answer is on the speed your motor achieved on your load. If your load did not force your motor to seriously underrev all was OK and failure had nothing to do with connection . For the same load even the current would be a little lower But be noted that warranty matters have to do with sellers good faith. I would try to proove only two things a)current b)RPMout were within specifications

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#29

Re: Induction Motor Running in Star Connection

10/12/2009 8:22 PM

To all the gentleman.

It seems that, there will be no end to this forum. Let put it in to conclusion with a simple statement.

If the motor is design to be run in Delta Connection, to drive the corresponded total designed load, therefore the total power delivery by the induction motor will be equal to the load.

By changing the motor from Delta to Star connection, the total power delivery by the motor will be less than the original design power. The total torque delivery by the motor will be only 1/3 of the total full torque in Delta Connection.

Therefore, the motor will need higher ampere to delivery the full torque required by the connected load and sequentially, the motor winding will be over heated and burnt.

On other hand, if the motor is design to be run in Delta Connection but when it is change to Star Connection, it still can be used to drive the connected load but the total connected load MUST be 50% less than the total original connected load. In this case, the total power delivery by the motor still higher than 50% of the connected load.

Everyone agree, Please sign in.

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#30
In reply to #29

Re: Induction Motor Running in Star Connection

10/12/2009 8:56 PM

Think nobody disagrees with your statements Ms Simon, Just trying to apply them on a certain situation and give some verdict without knowing all the facts. So you understand answers can be nothing but general

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#31
In reply to #29

Re: Induction Motor Running in Star Connection

10/12/2009 9:38 PM

Agree with the first part of the statement by simon but you loss me on the 50% bit in last paragraph. Bottom line in my opinion is motor will burn out. I rest my case

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Anonymous Poster
#33
In reply to #29

Re: Induction Motor Running in Star Connection

10/15/2010 2:26 PM

i think soe times motors r provided with both rating

for example

y connection 230/delta 400V

both then

how to operate the motor

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