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### Operation of Motor in Star Connection

07/26/2012 1:53 AM

At one of the water schemes we are working on, we have a vertical turbine water pump powered by an induction motor of 250 kW at 3.3 kV.

The motor draws high current (58A) which is the full load current and gets hot. I noticed that the motor connection is in star. As far as I know, for currents above 30% FL, the connection should be delta since equivalent star connection has less voltage per phase and draws more current.

I look forward to the advise of our learned members of the forum. Thank you

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#1

### Re: Operation of Motor in Star Connection

07/26/2012 6:43 AM

That´s wrong! In star connection the current will be less, for the simple fact that the voltage per phase is minor.

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#6

### Re: Operation of Motor in Star Connection

07/26/2012 11:33 AM

That might confuse him! For a given motor, if the correct supply voltages are available to connect it in star or delta, star connection draws lower line current (as higher voltage). But the motor will get just as hot if it's highly loaded.

And it doesn't mean you can connect it in star when it should be delta.

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#2

### Re: Operation of Motor in Star Connection

07/26/2012 7:39 AM

Following #1↑ check where the pump is operating on its pump curve. Multiply the volumetric flowrate achieved by the pressure rise achieved to obtain required shaft power. Compare that figure to the motor's rated power and the observed operating power. If the thing is running at or above its power rating, then something must be done before it fails, for fail it will.

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#3

### Re: Operation of Motor in Star Connection

07/26/2012 7:58 AM

Thanks for your time. My main question is: should such a motor be wired in star or delta?

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#4

### Re: Operation of Motor in Star Connection

07/26/2012 9:00 AM

Many MV motors are wired in star.

It makes for less insulation in the windings. IE cheaper.

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#5

### Re: Operation of Motor in Star Connection

07/26/2012 11:22 AM

It depends on what it says on the nameplate.

If it says 3.3 kV delta, that's how to connect it. If you happen to have a supply at 3.3*√3 = 5.7 kV (that's phase - phase of course) you can use that and connect it in star.

If it says 3.3 kV star, that's how to connect it. If you happen to have a supply at 3.3/√3 = 1.9 kV you can use that and connect it in delta.

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#7

### Re: Operation of Motor in Star Connection

07/26/2012 12:01 PM

Codemaster is correct, it doesn't matter what anyone THINKS, it only matters that the motor nameplate SAYS on it. If it has dual coltages on the nameplate, the higher voltage supply would require connection of the motor windings in Star, the lower voltage supply would be connected to Delta.

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#8

### Re: Operation of Motor in Star Connection

07/26/2012 11:54 PM

the motor should stay connected in "STAR" when powering a pump. If connected in "DELTA" it may stall.

AS for as the motor becoming hot, yes it will if running at FL, if you are producing the correct flow rate from your pump then you have undersized your motor by at least 10%.

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#9

### Re: Operation of Motor in Star Connection

07/27/2012 12:12 AM

"the motor should stay connected in "STAR" when powering a pump. If connected in "DELTA" it may stall"

Nonsense,The connection should match the voltage not because it is a pump.Most likely it is a star connection being HV, but cant say for sure unless we can see the motor nameplate

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#11

### Re: Operation of Motor in Star Connection

07/27/2012 12:29 AM

Nonsense you say.. mind your words my friend, and think before you type. I suggest that you go back to school and learn the difference between "STAR" and "DELTA", it is not a good idea to connect any motor powering a pump in "DELTA".

If the OP has the motor running at FL and the pump is performing either at the BEP or above, then he has seriously undersized the motor and connecting it in "DELTA" will not get him out of the situation he dug himself into. Stalling is the worse case scenario, and I have seen it. Hope we are clear on that!

Remember this forum is here so we can SHARE our experiences.

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#16

### Re: Operation of Motor in Star Connection

07/27/2012 7:38 AM

Most of us here are happy to share and learn, but I've never heard of it is not a good idea to connect any motor powering a pump in "DELTA".

What difference does the type of load make? Any type of pump? The only thing I can think of is if it's a PD pump can't normally use star-delta starting as there's not enough torque, so there's no point insisting on a motor that runs in delta (at the voltage available).

Also I don't understand in your #8 the motor should stay connected in "STAR" when powering a pump. If connected in "DELTA" it may stall. Assuming there is just mains supply available (no soft-start etc) if the motor is wound star it's just DOL and it will start OK. If it's wound delta there's the option of star-delta starting, but in that case it's more likely to stall in star (before changeover to delta) as motor torque is lower.

I seem to remember recently on this forum somebody saying HV motors are always wired star, and only 3 terminals provided (and #4 refers). But OP implies (not totally clear) that he has the options. Is that right Mohamed?

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#19

### Re: Operation of Motor in Star Connection

07/27/2012 10:28 AM

Shame that you marked yourself as OFF TOPIC, you were not only ON TOPIC, but you also know what you are talking about....the other Guy simply does not understand induction motors, you do!!!!!!

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#21

### Re: Operation of Motor in Star Connection

07/27/2012 10:57 AM

Thanks for that. I don't know how I managed to mark myself off topic, didn't mean to.

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#22

### Re: Operation of Motor in Star Connection

07/27/2012 11:40 AM

You just did it again, you must remember to uncheck that OFF TOPIC box......or at least check it to make sure its not checked.

If you reply to an off topic post, its always set to OFF TOPIC automatically....

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#23

### Re: Operation of Motor in Star Connection

07/27/2012 12:08 PM

Ah yes, I see it now! I didn't realise that. Of course this one really is off topic, but I've cleared it anyway.

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#25

### Re: Operation of Motor in Star Connection

07/27/2012 2:35 PM

Its a bad day when it passes and I have not learnt something new!

Have a good one Codey....

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#10

### Re: Operation of Motor in Star Connection

07/27/2012 12:26 AM

If the motor is designed to be connected in star for that voltage, there is no harm in running it continuously in star. If you compare the current actually drawn by the motor it should be within the rated current value unless the motor is over loaded when current will exceed the rated value.

The change over you are referring to is for star- delta starter where the motor is connected in star during starting to reduce the starting current and then switch over to delta because the motor is designed for continuous operation in delta.

You may please find out the reason for overloading. Apparently there is no electrical issue, the problem may be on the load side.

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#12

### Re: Operation of Motor in Star Connection

07/27/2012 12:46 AM

It may not want to start in Delta.

Is there a Star-Delta switch on that motor of yours?

Is it rotating at the right speed? What is the cooling regime?

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#13

### Re: Operation of Motor in Star Connection

07/27/2012 3:55 AM

You might have the option for either a star of delta connection. You need to check the motor name plate and work out where the links have been fitted.

A motor wired in star will take less current than wired in delta for the same voltage.

You say the motor is hot but how hot. Is it hot that you can't touch it or is hot that it is bringing thermal protection out.

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#14

### Re: Operation of Motor in Star Connection

07/27/2012 5:04 AM

Without full motor details, we are all guessing......the OP must inform us fully if he wants some reasonable and usable information....

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#15

### Re: Operation of Motor in Star Connection

07/27/2012 6:04 AM

Hey Mo!,

Are you gonna start contributing to your own discussion?

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#17

### Re: Operation of Motor in Star Connection

07/27/2012 8:41 AM

Dear Mohamed Akram Khan,

The rated voltage and current of the motor is 3.3kV and 58Amp and star connected. If the motor is drawing full load current at rated voltage nothing will happen to the motor while running. But you must check the pump load, pump curve which must be compatible with the motor data and spped-torque curve. While running of the motor, there will be temperature rise and it must be within specified limit. Also you must check the input power quality, i.e.,line to line balance voltages, frequency, harmonics,THD/THVD etc.and please also check percentage impedances and resistances of motor windings to identify any defects in windings including crack in rotor bars.

Thanks,

Manindra

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#18

### Re: Operation of Motor in Star Connection

07/27/2012 8:53 AM

I think you're getting ahead of the game here! OP didn't say he thought he had a problem of overheating. He just said it gets hot. Any motor running at rated output gets hot, but if it's within limits it's OK.

Maybe he just started the discussion out of curiosity. Time he came back and clarified!

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#20

### Re: Operation of Motor in Star Connection

07/27/2012 10:52 AM

I understand that as of this moment we don't know if the motor was just hot, but within allowed tolerances, or it was actually overheating....

But I remembered a problem we were having with a computer card reader (1976 or thereabouts) motor that was always overheating and failing. I have to admit that it was a single phase induction motor and not a 3 phase, but I am pretty sure that the effects are similar.

I found that the reason was that it was a 240VAC motor, but due to incorrect transformer settings on the mainframe, the motor was only receiving 200 VAC. Once the transformer was correctly set up, the motor ran far far cooler and never did fail again.

I actually do not know if the motor was running at the rated speed or not (I suspect that it was running slower than it should and the reduced back emf was allowing a high current to flow), but it was not producing any read errors on the Hollerith cards passing through the unit....probably because only one motor was involved and every was still "Synced up" as all the various machine parts were all driven off the same belt....

Its a long shot.....but my thoughts when I read this blog was that if the 3 phase motor is a Star/Delta motor, in star configuration, but on Delta voltage.......it could be getting too hot because of low voltage/heavy load....

As we still do not appear to have full plate details of the motor, this is only a thought.....

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#24

### Re: Operation of Motor in Star Connection

07/27/2012 2:08 PM

I tend to agree with you Mohammed. We have pumps that run in star when the torque requirement is low and in delta when the torque requirement goes up.

We had issues like overload protection trip when the increase in load was not noticed by the operators for changing the winding connection to delta.

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#26

### Re: Operation of Motor in Star Connection

07/29/2012 8:28 AM

First of all, let us understand the pros & cons of Star & Delta connection of motor windings.

In a delta connected winding, for the same line current (and, thus for the same HP output), the windings would carry only 57% of the line current and thus the heating in the windings would only be 1/3rd of that had the windings been connected in star.

Then one might ask, why wouldn't all the motors be wound in Delta only? But, in HV motors, if the windings are connected in Delta, the advantages of lesser heat produced in the windings is greatly offset by the increased insulation cost. Remember! In delta, the phase to earth insulation has to be for the line-to-line voltage. But, if the high voltage motor is connected in star, the phase-to-earth insulation needs to be done only for the phase-to-neutral voltage.

In a LV Motor, say, 415V, there is no big difference between line voltage insultaion (415V) and phase voltage insultaion (240V). So, there is no big savings on the insulation cost by connecting the motor windings in Star. Hence, one would connect the LV motor windings in Delta, at least to reap the benefit of lesser heat generation in the windings.

In HV motors, say, 3.3kV, theere might be appreciable difference between line voltage insulation (3.3kV) and phase voltage (1.9kV) insultaion. Thus, you save more by reducing the insulation cost in HV motors than by the marginal savings in reduced heating, had the windings been connected in Delta here.

Hope it is clear!

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#27

### Re: Operation of Motor in Star Connection

07/30/2012 4:26 PM

Electricalexpert65 - you've repeated in several posts over a few years this idea that a star connected motor produces more heat in the windings than delta connected, and now you're doing it again. I've explained a number of times why it is not the case. Let me try again, using (round) numbers for the sake of clarity.

Consider a motor wound for 400v delta, 690v star, and drawing line current 100 amps in star. In star the current in each phase is also 100amps (obviously, as in series with the line) and the voltage across each phase is 690/√3 = 400v. In delta the line current is √3*100amps, the phase current is 100amps and the phase voltage is 400v. So each phase sees voltage 400v and current 100amps WHETHER THE MOTOR IS CONNECTED IN STAR OR DELTA, and the heating effect is the same for both.

I explained the flaw in your reasoning in #15 and #35 of attached thread.

Just to be clear, the above is for a given motor fed at the correct voltage for the connection type. It doesn't refer to connecting it in star but giving it delta voltage, or vice versa.

http://cr4.globalspec.com/browse/showtag?tn=Motor%20operation%20in%20STAR

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#28

### Re: Operation of Motor in Star Connection

07/30/2012 4:48 PM

Codemaster,You are are talking about different line voltages for you star and delta connections.I believe EE65 is talking about same Line voltages but different motor connections, thus different Phase Voltage and different motor behaviour.

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#29

### Re: Operation of Motor in Star Connection

07/30/2012 5:47 PM

Thanks, but no, I don't think he is talking about same Line voltages but different motor connections, see #13 and #15 of attached thread. In #26 above he refers to the pros & cons of Star & Delta connection. That seems to mean the practical alternatives. We'd all agree that by connecting it in star but giving it delta voltage, or vice versa it won't work properly, or for long.

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