As a new electrical student or a common man this questions comes in mind that why we prefer 3 phase power system. In recent years 3 phase electric wiring has become common and mostly used worldwide for power generation, power transmission, distribution and three phase engines or motors.
A single-phase electric power supply is the distribution of alternating current electric power where all the voltages of the supply vary in unison. Single phase distribution is used when loads are mostly lighting and heating, with few large electric motors.
A single-phase power supply has two hot legs or conductors, which have a sine wave that are 180 degrees apart. Single-phase comes to the home with two wires: active and neutral. The neutral wire is connected to earth (water pipe, earth stake, etc.) at the switchboard.
3 phase power supply system consists of four wires i.e. three “active” phases, A, B, C and one “ground”, or “neutral”.
The color code for all wires is Phase A Red, Phase B Black, Phase C Blue and Neutral White or Gray.
In 3 phase power supply system there are three sinusoidal voltages of equal amplitude and frequency out of phase with each other by 120° Known as “balanced”. Phases are labeled A, B, and C and are sequenced as A, B, C (positive) or A, C, B (negative).
The three-phase generator has three induction coils placed 120° apart on the stator. The three coils have an equal number of turns, the voltage induced across each coil will have the same peak value, shape and frequency.
Why 3 phase power is preferred?
Wherever possible, it is always preferable to use three phase over single phase for load. This gives following benefits.
- Load is always balanced between phases.
- Motor / transformer wires and also other equipment are more efficient.
- Load on transmission lines and generator systems is balanced. This ensures full and proper utilization of lines and equipment.
- Current in cables is lesser.
- Cable losses go down.
- Switchgear current capacity is lower.
In technical languages as an engineer 3 phase power system has the following advantages as compare to single phase system:
Power to weight ratio of 3 phase alternator is high as compared to single phase alternator. That means for generation for same amount of Electric Power, the size of 3-phase alternator is small as compared to 1-ø alternator. Hence, the overall cost of alternator is reduced for generation of same amount of power.
Moreover, due to reduction in weight, transportation and installation of alternator becomes convenient. In addition it is easy to accommodate the alternator in small size power houses.
The amount of conducting material required to transfer a given amount of power is minimum in a three-phase system.
The instantaneous power in a three-phase system never falls to zero resulting in smoother and better operating characteristics of the load.
Thinner conductors can be used to transmit the same kVA at the same voltage, which reduces the amount of copper required (typically about 25% less). Also lighter lines are easier to install, and the supporting structures can be less massive and farther apart.
Use of 3 phase power for Motors
Three-phase equipment and motors have preferred running and starting characteristics compared to single-phase systems because of a more even flow of power to the transducer than can be delivered with a single-phase supply. In general, most larger motors are three phase because they are essentially self- starting and do not require a special design or additional starting circuitry.
3 phase power supply is required by three-phase induction motors which are widely used in industry because of their ruggedness, longer life, higher torque, low initial and maintenance costs.