At the end of this week, the students are expected to:
- Define polyphase system
- Explain the basic differences between single phase and three phase systems
- Explain phase sequence of a three phase system
- State the advantages of three phase circuits.
Circuits or system in which the ac sources operate at the same frequency but different phases are known as polyphase systems.
2.2 BASIC DIFFERENCES BETWEEN SINGLE PHASE AND THREE PHASE SYSTEMS
2.2.1 Single-Phase Systems A single phase a.c power system consists of a generator connected through a pair of wires (a transmission line) to a load. Figure 2.1 depicts a single phase two wire system, where VP is the magnitude of the source voltage and – is the phase. The power in a single phase system is pulsating (not constant). For large motors, pulsating power supply causes excessive vibration. Besides, it has neither delta nor star type of connection.
Fig 2.1: single-phase system two-wire type.
2.2.2 Three Phase Systems
Figure 2.2 shows a three phase four wire systems. As distinct from a single phase system, a three phase system is produced by a generator consisting of three sources having the same amplitude and frequency but out of phase with each other by 1200.
When one phase or two phase inputs are required, they are taken from the three phase system rather than generated independently. The instantaneous power in a three phase system can be constant. This result in uniform power transmission and less vibration of three phase machines. Generally, three-phase systems have two type of connection, which are star and delta connections
Fig 2.2: Three phase four wire-systems
2.3 PHASE SEQUENCE OF A THREE PHASE SYSTEM
Consider the waveform of three phase e.m.f shown in fig 2.3. From the waveform it is seen that the e.m.f of phase A lead that of B by 1200, and in similar way, the e.m.f of phase B lead that of C by 1200. Hence, the order in which the e.m.fs of phases A, B and C attain their maximum values is ABC. It is called phase sequence ABC. Therefore, Phase sequence is the order in which the three phases (A,B and C) reach their maximum value.
Fig 2.3: three phase waveforms
2.4 ADVANTAGES OF 3-PHASE CIRCUITS
- The instantaneous power in a three phase system can be held constant. This results in uniform transmission.
- Three phase systems are more economical than single phase systems.
- Three phase systems are more efficient