Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Star or Delta

Star or Delta

Star or Delta

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!

RE: Star or Delta

The windings in the motor are designed for a specific voltage or set of voltages.   When they are designed for 2 voltages the higher voltage is normally twice that of the lower.  When connected for use with a low voltage supply the windings are connected in parallel.  They are connected in series for the higher voltage.  Either type of motor will provide the nameplate horsepower and torque when connected to the design voltage.  If you reconnect the windings from delta to wye you have changed the voltage across the windings.  A motor designed for delta connection will have the windings designed for the full phase to phase system voltage.  In a delta motor connection each of the three sets of windings will see the full supply voltage.  In a motor designed to have its windings connected in the wye configuration, the windings will be designed for the phase to phase voltage divided by 1.73.  No single set of windings in a wye connection will see the full phase to phase voltage.  If you reconnect a wye motor's windings so that they are connected delta and connect the motor to the supply voltage, you will let the smoke out of the windings, not a good thing :)   Sometimes large motors are connected in wye for start and delta for run.  This will reduce the starting current, but this is done with a 12 lead motor that has the sets of windings designed for the full phase to phase voltage.

RE: Star or Delta

Have look on the nameplate.
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

415 V delta connected or
720 V star connected

would be possible to.

RE: Star or Delta

If the motor is rated at 415v 3 phase Star connection, you can connect the motor in Delta with 240V (1/sqrt3) 3 phase.  This way the voltage across the windings will remain the same to maintain the horsepower rating of the motor.
            Star              Delta

              |                /\
              |               /  \
              |              /    \
             / \            /      \
            /   \          /        \
           /     \        /______\

Hope this turns out O.K!!


RE: Star or Delta

Tiny Glimmer of light dawning.

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

no! you cannot choose which connection to use, you must use the connection which correspods to the voltage you are supplying to the motor. the motor nameplate will normaly have two voltage/current values, the lower voltage/higher current rating is the delta connection.

RE: Star or Delta

I believe you're saying it's a 415 delta motor (you should confirm with nameplate as cbarn says).  If so I agree with your logic should be able to connect it to 415/sqrt(3)=240 in wye.

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

As if to further proove my electrical incompetence, previous post was wrong. Motor was originally wired 415V wye.
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

Your post dated 25 Jan, 2002 describes the things correctly for the motor you have.

RE: Star or Delta

Typically, motors used on 415v 50Hz in europe are designed for star/delta starting. In this case they are rated for 415 volt delta connection and about 720 volt star connection. There are however, lower rated motors that are designed for dual voltage usage i.e. 220 / 415 volt and this is specified on the name plate. In this case they are connected in delta for 220 volt and star for 415 volt.
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.

Mark Empson

RE: Star or Delta

I'd like to add some further information to damanjeets post. Otherwise it might be a bit misleading.

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

Why does a motor start in star and then switch to delta? whats the benifits of star delta starting?

RE: Star or Delta

As pointed out before, A star connection is in series and would take a higher voltage to run the motor normally compared to delta. So if turn that around so your motor is rated at its voltage in Delta ... connecting it star, the motor would only see about half (57%) the voltage to the motor. What you basically did is make a "reduced voltage starter" without the cost of resisters or transformers. You just did it with the motor leads instead. As for the benefits, reducing the voltage limits the current draw so the power company and others don't complain about how the lights go dim when you start it.

RE: Star or Delta

The Star/Delta starter is a way of getting around the regulations that are in some areas where all motors greater than 4 HP (or similar) must use a reduced voltage starter. The star/delta starter is a form of reduced voltage starter that meetsa the requirements but in reality does mor damage to the equipment and supply that it is supposed to be protecting, than a Direct On Line or Full voltage starter.

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

RE: Star or Delta

The 1964 Heidelberg printing press "KORD" with
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

Suggestion to rads (Mechanical) Jan 24, 2002 marked ///\\\
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?
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).\\\

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


White Paper - PLM and ERP: Their Respective Roles in Modern Manufacturing
Leading manufacturers are aligning their people, processes, and tools from initial product ideation through to field service. They do so by providing access to product and enterprise data in the context of each person’s domain expertise. However, it can be complicated and costly to unite engineering with the factory and supply chain. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close