The following is a review of Three-Phase Electric. I wish I knew who to give credits to for this info but I can't post credits as I looked at too many books, websites, and timeless discussions with my electrician brother to try to understand this subject. I welcome any corrections or additions!
The Generation of Electricity:
Electricity is generated at power stations by high-pressure steam turbines drive a generator. Inside the generator are 3 windings and each is at 120 angular degrees from the other. The use of three windings at 120-degrees means that the power generated is actally 3-phase power.
In the middle of the generator there is a rotor with north and south poles. This rotor spins at a speed that produces 50 cycles a second. As one end of the rotor passes one winding - say the north pole passes the 'red' winding, it induces a voltage by the interaction of magnetic flux between the rotor and the stationary winding. While it is doing this, the other end of the rotor (the south end) is passing one of the other windings and so on and so forth.
Basically, the voltage is rising in one winding as it is falling in another. This is how we get 3 phases; each winding is a 'Phase' – red, yellow and blue. The provides exceptional efficiency as the generator is continuously transferring it’s energy to the power grid without lapses in power.
Power is then supplied to power sub stations as high voltage 3-phase power and transformed down for the low voltage distribution. Inside each transformer are 3 separate windings with each supplying approximately 240-volts between itself and neutral. The electricity supply usually leaves the transformer as 3 phases with customers being supplied with a single line with one of the three phases each. To balance the loading, customers are supplied from alternate phases from the transformer.
Single Phase vs. Three Phase Electric Motors
With a Single Phase electrical motor, the AC Power current supplied to the motor alternates in a sine-curve manor (Imagine a single curve on figure below). When the sin curve is in the positive or negative energy is utilized to drive the motor. When the current crosses zero on the curve there is no voltage and no energy is provided to allow the motor to work. During this very brief period, the motor uses momentum to advance it’s timing so that energy is again available. What this means is that in a single stage motor you have two power strokes per revolution of the motor.
With a three phase motor, power is fed to the motor much like the way that a generator creates electricity at the power plant except with the motor three separate power lines bring the power to the motor. Each of the three power lines carry the same voltage but their phases are 120-degrees off set so that each of the three phases are reaching their peak at a different time as shown in the image below. This means that three-phase motor provides six power stokes to each rotation of the motor and the timing of these power strokes such that the motor is continuously experiencing a power stroke. This in turn allows the three-phase motor to do significantly more work then a comparably sized single-phase motor.
Image borrowed from:
Three Phase AC Motor - By Dr. Xing M. Wang
Threaded Mode
