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

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.

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.

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

Many MV motors are wired in star.

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

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.

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.

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%.

"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

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?

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.

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

Have a good one Codey....

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.

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?

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.

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

Hey Mo!,

Are you gonna start contributing to your own discussion?

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

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!

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.....

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.

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!

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.

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.