Star or Delta
Star or Delta
(OP)
Suspect this is a dumb question from an electrically inept mechanical guy!
We have a 415 volt 3 phase supply (415 volt between phases?). Can I connect a 3 phase motor in either star or delta configuration?
I think in star, the torque / current is greatly reduced. Correct?
If so, why would anyone want to use star, other than soft start. Does star allow you to connect at to a higher voltage supply?
Please be gentle with replies as very high chance of exceeding my electrical understanding!
We have a 415 volt 3 phase supply (415 volt between phases?). Can I connect a 3 phase motor in either star or delta configuration?
I think in star, the torque / current is greatly reduced. Correct?
If so, why would anyone want to use star, other than soft start. Does star allow you to connect at to a higher voltage supply?
Please be gentle with replies as very high chance of exceeding my electrical understanding!





RE: Star or Delta
Don(resqcapt19)
RE: Star or Delta
If a motor is connected in star it operates with the same power and torque if the voltage is sqrt(3) times higher than the voltage for delta connection.
I guess the rating usually used is
415 V star connected or
240 V delta connected
but
415 V delta connected or
720 V star connected
would be possible to.
RE: Star or Delta
Star Delta
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Hope this turns out O.K!!
RE: Star or Delta
Follow up - Have now connected three phase inverter (output 220V) to my motor which was previously connected DOL to 415V 3 phase supply and good things happen (motor turns, no smoke etc).
I did not change the coil connection links inside the terminal box so suspect that the motor was (and still is) connected in Delta configuration.
If I now change to star configuration, I think that the motor should still run, but at higher torque. Is this correct.
Thanks for previous posts.
RE: Star or Delta
RE: Star or Delta
The fact that it was working satisfactorily is 240 delta appears to be luck... as you say the torque for any speed would be 3 times too low. Rewiring to 240 wye will restore your torque/speed characteristic to normal.
If you are presently
RE: Star or Delta
I connected it 240V wye (and it rotates).
Is this matrix of connection against voltage correct?
connection voltage result
wye 414 ok
delta 240 ok
wye 240 reduced torque
delta 414 smoke, fire etc
Suspect I should now get into terminal box and change inter connections to delta.
Currently 3 phase supply is connected to terminals a1, b1 and c1 on the motor. Terminals a2, b2 and c2 are commoned.
For correct delta connection, do I join a2 to b1, b2 to c1, and c2 to a1?
Once again, many thanks for continuing patience and help.
RE: Star or Delta
RE: Star or Delta
It is very important to connect the motor correctly. If you connect a motor in star on a voltage that it was designed to run in delta, you have effectively reduced the rating of the motor considerably and operation at it's rated load will cause failure. Likewise, operating a motor in delta when it shoud run in star will over flux the iron and it will fail also. If you are unsure, and the nameplate gives no clue, try running it openshaft in star connection on the desgn voltage and measure the current. This open shaft current is essentially the magnetising current and for a small motor could be up around 40 - 60% of the rated current of the motor and a large motor could be 20% - 40% of the rated current of the motor. If the magnetising current is much less than this, then it is designed to run in delta at that voltage. Reconnect in dleta and verify the amgnetising current.
Regards,
Mark.
Mark Empson
http://www.lmphotonics.com
RE: Star or Delta
For a given line voltage a motor designed for this voltage in delta connection will give the same voltage dip as a motor designed for this voltage in star connection. The advantage of the motor designed for delta connection is that it can be switched to star connection during starting for reducing the starting current. Since the starting torque is also reduced this type of starting only can be used for certain applications.
RE: Star or Delta
RE: Star or Delta
RE: Star or Delta
The motor is intially started in star connection resulting in phase to neutral voltage across each winding. The current is reduced to one third of the delta starting current and the torque is reduced to one third of the delta torque. After a period of time, the motor is disconnected from the supply and then reconnected in delta connection. This is an open transition switching and results in a very high current and torque transient. If there is insufficient torque in star connection to get the load up to full speed, the transition happens at partial speed and the steady state current is equal to Full voltage Delta connection during start. - no advantage at all!!
see http://www.lmphotonics.com/motor control.htm for more information.
Note, motor must be designed to operate at the nominal line voltage in delta connection. Star connected motors can not be star/delta connected.
Mark Empson
http://www.lmphotonics.com
RE: Star or Delta
5 HP 3-phase motor has the Wye start - Delta Run
configuration in the motor starting circuit.
This Press has a 250 lb. flywheel (maybe more).
There was a timer which switched the winding configuration
after the press was running about 12 seconds.
I was thinking it started delta for torque, then switched
to Wye for less current draw, but now I am not so sure.
RE: Star or Delta
We have a 415 volt 3 phase supply (415 volt between phases?).
///Yes. Normally, one interprets 415V between phases (or lines).\\\
Can I connect a 3 phase motor in either star or delta configuration?
///Yes.\\\
I think in star, the torque / current is greatly reduced. Correct?
///Yes. Torque is reduced by (1/3) multiplier and line current is reduced by (1/sqrt3) multiplier.
If so, why would anyone want to use star, other than soft start.
///There are some inherent advantages of the star connection. The star connection alleviates voltage stresses on the motor winding insulation at higher voltages; it has simpler winding end connections, etc.\\\
Does star allow you to connect at to a higher voltage supply?
///Yes, it does, if the insulation level of y-windings permits.\\\
Please be gentle with replies as very high chance of exceeding my electrical understanding!
///Well, if you need the better explanation, you are welcome to post another posting(s).\\\