Electrical – Generation and transmission of electricity

powerthree phase

According to my textbook the three phase power is generated so that the power that is needed to generate electricity by the torque remain constant. Another reason is that the summation of the three signals are zero so only one return wire is needed when the loads are equal. Therefore, the number of wires get decreased from 6 to 4.

I have some questions about the basics of generating and transmitting electricity.

1- I don't see how can we assume that the loads are equal when that depends on the users and the number of appliances that they use.

2- So the electricity is generated and transmitted as three phase, could someone explain to me in one form it enters our home.

Best Answer

  1. The number of users connected to one generator is high and divided among the three phases about equally. The result is approximately equal usage from each phase. Large buildings and factories have three-phase service. A high percentage of their connected equipment is three-phase motors. The motors are balanced three-phase loads. The load at the supply generator is therefore fairly well balanced. The generator will tolerate some imbalance.

  2. Electricity enters our homes from a single-phase transformer that is connected between one phase and neutral. It reduces the voltage to the home voltage level of about 240 volts. In the USA, that is divided into two 120 volt parts for wall plugs. Both parts are used for larger loads like electric stoves, water heaters, and central air conditioners. More than one house is connected to each transformer. There will be two other transformers in the same neighborhood connected to the other two phases.

The following picture shows three phase lines on three insulators at the top of the pole at the top right of the picture. The neutral wire is on a small insulator a short distance down the pole. At the next pole, three wires going down the pole are connected to the three phase wires. I believe that those three wires go under the ground to connect high voltage from a small wind generation system on the other side of the road. The poles that go on down the road have only one phase wire and a neutral wire. Near the bottom of the picture towards the left, you can see a transformer mounted on a pole. That supplies single phase power to one or more local users.

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