Lesson Five: Star And Delta Connection

Star and Delta connection

When connecting a motor to the electrical network, it can be done in two ways. Either use star connection or Delta connection. The U1-U2, V1-V2 and W1-W2 that marks the coil ends of the three different phases that is present on the terminal block of the motor. The L1, L2 and L3 represent the three phases coming from the electrical network.


Figure 13. Delta and Star connection


A Delta connection is when the phase coils are connected across with connection bars and then later connected to L1, L2 and L3 according to Figure 13.
A Wye connection is when one side of all three phase coils are connected together with connection bars and the other side is connected to L1, L2 and L3 according to the picture above
The difference between star and delta connection is the difference in voltage over the phase windings. For example when a 400V network is connected to L1, L2 and L3 on the delta connected motor there is 400V over the different phases, meaning 400V between U1 and U2, 400V between V1 and V2 and also W1 and W2. But when the motor connected in star, it will still have 400V between L1 and L2 but only have 230V volts over each phase winding U1 to U2 and so on.
So in the star connected motor the network voltage divided by the square root of 3 over each phase winding while the delta connection have the same voltage as the network over the phase windings.
A lower voltage over the phase windings will also give a lower current in the windings.
As the ratio between star and delta connection is always the square root of 3, this means that a motor with a winding optimised for 400V delta could be connected in star to a 690 volt network and give the same performance, or a motor made for 400 volt star could also be used for 230 volt delta.
This is many times used in the industry where part of the factory might be running on 690 volt and other parts on 400 volt.
If one were to connect the motor so it won’t get enough voltage, it will simply not start or it will not be able to handle the load. If one connect so the motor gets more voltage than it can handle, the circuit breakers will trip. (Electrical4u)

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Post time: Jun-25-2019
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