Electrical Engineering ⇒ Topic : Reasons for the Use of 3-phase System

Reasons for the Use of 3-phase System

The following are the advantages of 3-phase system over the single phase system

(1) Constant power. In a single-phase circuit, the instantaneous power varies sinusoidally from zero to a peak value at twice the supply frequency. This pulsating nature of power is objectionable for many applications.

However, in a balanced 3-phase system, the power supplied at all instants of time is constant.Because of this, the operating characteristics of 3-phase apparatus, in general, are *superior to those of similar single-phase apparatus.

(2) Greater output. The output of a 3-phase machine is greater than that of a single-phase machine for a given volume and weight of the machine. In other words, a 3-phase machine is smaller than a single-phase machine of the same rating. This is a distinct advantage of 3-phase system over single-phase system.

(3) Cheaper. The three-phase motors are much smaller and less expensive than single-phase motors because less material (copper, iron, insulation) is required. Moreover, 3-phase motors are self-starting i. e. they do not require any special provision to get them started. However, single phase motors require internal starting device.

(4) Power transmission economics. Transmission of electric power by 3-phase system is cheaper than that of single-phase system, even though three conductors are required instead of two. For example, to transmit the same amount of power over a fixed distance at a given voltage, the 3-phase system requires only 3/4th the weight of copper than that required by the single-phase system. This means a saving in the number and strength of transmission towers.

(5) Three-phase rectifier service. Rectified 3-phase voltage is smoother than rectified single phase voltage. As a result, it is easier to filter out the ripple component of 3-phase voltage than that of a single-phase voltage. This is especially useful where large a.c. power is to be converted into steady d.c. power e.g. radio and television transmitters.

(6) Miscellaneous advantages. Other advantages of three-phase system over the single-phase system are

• A 3-phase system can set-up a rotating uniform magnetic field in stationary windings. This cannot be done with a single-phase current.
• The 3-phase motors are more efficient and have a higher power factor than single phase motors of the same capacity.

A knowledge of 3-phase power and 3-phase circuits is, therefore, essential to an understanding of power technology. Fortunately, the basic circuit techniques used to solve single-phase circuits can be directly applied to 3-phase circuits. It is because the three phases are identical and one phase (i.e., single phase) represents the behaviour of all the three.