Star delta motor for the same power 4

[HanyHamed]

my qustion is:

if i am designing a motor for a load of 50 KW 380 volt with 1000 rpm direct coupling, and i have to design two motors one is star connection and the other is delta connection,
1-what ts the advantage and disadvantage of each connection
2-what is the phase current
as the output torque should be the same for both at this speed, is the phase current is the same?

 

[Skogsgurra]

Not much difference. But, of course, you must see to it that both motors have the correct rated voltage with their respective connections.

The only advantage with a delta connected motor would be that it is possible to wye/delta start is. But that is a very questionable advantage - and I would not consider it at all.

Yes, phase currents are the same. A star can be transformed into an equivalent triangle - not possible to measure the difference from outside. So everything should be the same.

Gunnar Englund

http://www.gke.org

 

[HanyHamed]

if we have the same power for both sqr(3) (V)line * (I)line cos(q), V(line) is the same for both, but I(phase) for the Delta motor will be less by sqrt(3), ok , then my question about the torque for both?

 

[Skogsgurra]

The current in each mains phase will be the same.

Gunnar Englund

http://www.gke.org

 

[HanyHamed]

i review the said text-box
for the same input power, sure phase current is not the same
the delta current is less by sqrt(3), thus current per phase for star connection is more than current per phase for delta connection.... is that true?
so phase current is not the same
relly i feel confused because torque is relay dependent on phase current.

thanl you for your answer

 

[rbulsara]

The line currents in both cases will remain the same, but the current in each winding (phase) will be more in the star windngs, 1.73 times that in a delta winding, so probably will use more copper.

While in theory, star connection will have less voltage per winding (L-L divided by 1.732) compared to a delta windings, but cost of making a winding for 400V or 230V is the same.

For same magnetics, the torque in star connection will be 1/3 of a delta, but in theory you can desing star connceted motor to give you any torque.

I am not a motor designer, but I would bet it is more cost effective and electrically (torque wise) superior to build a delta connected motor for the same HP.

 

[HanyHamed]

thank you so much for your answer. that is what i need for now.

 

[Skogsgurra]

READ AGAIN THEN!


Let's apply power principles:

You have same torque and shaft speed. Right?

Torque times speed is shaft power. Right?

Electrical input power is mains voltage times line current divided by power factor and efficiency. Right?

If both motors have the same efficiency and power factor (no reason to believe otherwise) then both motors will need same electrical input for same shaft output. Right?

With identical mains voltages, power factors and efficiencies, it follows that line currents must be equal to satisfy the facts given. QED

If you, the DriveMaster, has any other thinking about this, you should try to grasp the basics of three phase systems once and for all. There is nothing mythical about it - the sqrt(3 is just the ratio between phase-phase voltage and phase-neutral voltage. It is true that the windings in wye and delta have different currents, yes. But the line currents will be the same.



Gunnar Englund

http://www.gke.org

 

[HanyHamed]

yes yes yes line current is the same i know from the begining i was talking about phase current and its effect in the produced torque.
thank you so much for your attention.

 

[Skogsgurra]

Is there a difference between phase current and line current? In my world they are interchangeable. That may be where the misunderstanding is.

Gunnar Englund

http://www.gke.org

 

[rbulsara]

skogpurra:

Yes. There is the difference.

 

[itsmoked]

rbulsara; How can there be if in both cases the load is the same KW?!?

Phase current is the line current.

 

[davidbeach]

Phase current is line current in the supply line. In the motor, in the delta winding, phase current can be defined as being winding current (Ia-Ib, Ib-Ic, Ic-Ia) while line current is still line current. It all just depends on where one looks.

From outside the motor, looking in at the terminals, there is no electrical test that can distinguish a balanced wye from a balanced delta. Throw in a neutral connection or unbalances and it begins to be possible to tell the difference between winding configurations.

DriveMaster, within the windings, there is a difference in current between the wye and delta connections, but there is also a difference in voltage across the individual windings between the wye and delta connections. These two differences counteract each other when looking into the motor from out side. Within the motor, the wye connected windings will carry more current (by sqrt(3)), but will have a lower voltage across the windings (also by sqrt(3)) when compared to the delta windings.

Torque is proportional to the current times the number of turns in the winding. To produce two motors with the same torque, one delta and one wye, the motor with the delta windings will require more turns on the windings to make up for the lower current in the winding. But one would expect a higher voltage winding to have more turns, so it all balances out.

 

[itsmoked]

davidbeach; Well put. It's the 'ol inside the motor, outside the motor confusion...

 

[rbulsara]

the original question was all about inside the motor!! star or delta!

 

[itsmoked]

Good point.

 

[Skogsgurra]

Good that you sorted that out! Sorry that I lost my temper. We are getting used to see some very basic questions here sometimes. I thought that this was one of them.

Gunnar Englund

http://www.gke.org

 

[aolalde]

Hello DriveMaster, analyzing your statement:

“so phase current is not the same
really i feel confused because torque is relay dependent on phase current.”


The torque produced by a motor designed to operate with winding delta connected as compared to the torque produced by a motor designed with a winding to be wye connected is the very same.

For a given Power, Voltage, speed and frequency, depending on the connection (wye or delta) the number of turns per coil is different and the copper cross sections are inversely proportional to the turns per coil, so both windings can produce the very same flux per pole. That magnetic flux induces in the rotor bars current proportional to the rotor slip and the resultant magnetic force in the rotor bars is:
F= Bg*L*I.

The flux per pole ( phi) is related to the phase voltage (Vph) , effective turns in series per phase (N), frequency (f) and the winding factors for span , distribution and skew (Kw)


Certainly the phase voltage (Vph) for delta connection is the same as the line to line voltage (VL) and the phase voltage is the line to neutral voltage for wye connection. However N is different and proportional to 1/1.732 (the relation between line to neutral and line to line voltages). Then the resultant flux per pole (phi) will be the same wye or delta connected.

phi =10^8* E / (4.44*Kw*N*f)
Were E = Vph – I*Z1

Neglecting the voltage drop in the stator impedance Z1;

E = Vph.

 

[HanyHamed]

i do thank you all for your very valuable interaction, relly every thing now is cleard and more than cleared.

thank you all again.

 

[cswilson]

I see that this question has already been answered to the satisfaction of the original questioner, but for others reading this, I'd like to add a clarification.

Many people are taught early in their motor career about the difference between star and delta-connected windings, with star providing higher torque capabilities and delta providing higher velocity capabilities. But this comparison assumes identical internal winding patterns. I think that lesson is the source of a lot of confusion if you are not careful about what you are comparing.

There is an interesting class of 3-phase motors with all 6 leads brought out, permitting external connection. I'm mainly familiar with these for high-speed spindles in machine tools. One pattern of relay closures provides a star connection for high torque at low speed; another provides a delta connection for higher speed capability. Of course, the internal winding pattern does not change.