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Star delta motor for the same power
4

Star delta motor for the same power

Star delta motor for the same power

(OP)
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?
 

RE: Star delta motor for the same power

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
www.gke.org

RE: Star delta motor for the same power

(OP)
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?

RE: Star delta motor for the same power

The current in each mains phase will be the same.

Gunnar Englund
www.gke.org

RE: Star delta motor for the same power

(OP)
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


RE: Star delta motor for the same power

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.

RE: Star delta motor for the same power

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

RE: Star delta motor for the same power

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
www.gke.org

RE: Star delta motor for the same power

(OP)
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.


RE: Star delta motor for the same power

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
www.gke.org

RE: Star delta motor for the same power

skogpurra:

Yes. There is the difference.

RE: Star delta motor for the same power

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

Phase current is the line current.

RE: Star delta motor for the same power

3
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.

RE: Star delta motor for the same power

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

RE: Star delta motor for the same power

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

RE: Star delta motor for the same power

Good point.

RE: Star delta motor for the same power

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
www.gke.org

RE: Star delta motor for the same power

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.

RE: Star delta motor for the same power

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

thank you all again.

RE: Star delta motor for the same power

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.

RE: Star delta motor for the same power

Thanks cswilson for bringing that subject. I will distinguish three types of different performance of induction motors when wye or delta connection is involved:

A- Wye-start , Delta-Run . The winding is designed to operate on single speed and full voltage while delta connected. Under Wye connection the magnetic flux is reduced by 1/SQRT(3) and the torque and inrush current are reduced to 1/3 of those at full voltage delta connected.

B- Two speed single winding motors:

 Constant Torque; 2 Wye for high speed, 1 Delta for low speed. The motor halves the speed in delta connection and torque developed is constant.

 Variable Torque; 2 Wye for high speed, 1 wye for low speed. The motor halves the speed and torque for 1 wye connection.

 Constant Horsepower; 1 Delta for high speed, 2 Wye for low speed.  The motor halves the speed and doubles the torque for 2 wye connection.

C- Single speed motor.
  The winding connection is selected by the motor manufacturer ( Delta or Wye). Performance as discussed in this thread.

RE: Star delta motor for the same power

Hello DriveMaster.
I have read all the above answers to your simple question.
If you have a 50kw motor 380v 1000rpm.
One motor is star rated at 50kw 380v 1000rpm.The other motor is delta rated at 50kw 380v 1000rpm. the manufacturer has designed thse motors to run on these connections,it is still a 50kw motor.
The only diferance is if you were to run the motor in star when it was designed to run in dalta then you would be running on reduced speed.

Barry.

RE: Star delta motor for the same power

Quote:

The only diferance is if you were to run the motor in star when it was designed to run in dalta then you would be running on reduced speed.

And if you drew rated power under these conditions you would damage the rotor through overheating.

----------------------------------

If we learn from our mistakes,
I'm getting a great education!

RE: Star delta motor for the same power

Hi laundry:
Do you mean?:
" The only diferance is if you were to run the motor in star when it was designed to run in dalta then you would be running on reduced VOLTAGE(instead of speed)."

RE: Star delta motor for the same power

Hi aolalde
Yes I stand corrected my mistake.

Barry.

RE: Star delta motor for the same power

Hello

Watch with this in Star Delta Starting,  
In general we have five basic methods of starting induction motor.These include:
Direct on Line (DOL)-Star Delta-Autotransformer-Reactor and Soft Starting. Except for DOL the prime objective is reduce the supply voltage to the motor with the propouse of reducing the Inrush currents during the starting operation,since  the starting currents are directly proportional to the supply voltage.But the Starting torque is reduced by the square of supply voltage. In a DOL Starting the starting inrush current were in order of 6 to 8 times the full load nameplate current rating.The star Delta asumed that these starting currents would  be reduced roughly to 3 or 4 Times the motor full load current.This assumption ignores the transient inrush current that is now known to result from temporary desconnection of the motor from the supply line during star-delta swiching process, resulting from this momentary power interruption,the stator current drops to cero whilst the rotor current continues to flow. Since This surge currents can typically attain peak values of up to 20 times the motor full load current rating and last 10 to 40 miliseconds.One consecuence of such high current surges is that the starting torque could be reduced even further, and this is highly undesirable.This can be avoided by the use of additional circuitry connecting temporarily components like resistance and reactors to ensure that the motor remains connected to the power source while transferring from star to the run condition this is called closed circuit transition.

I think is better you use a DOL Starting.

Regards
PETRONILA   
 

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