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?

 

[aolalde]

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.

 

[laundry]

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.

 

[ScottyUK]

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.



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

 

[aolalde]

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)."

 

[laundry]

Hi aolalde
Yes I stand corrected my mistake.

Barry.

 

[petronila]

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