Polyphase circuits involve the use of multiple alternating current (AC) voltage sources that are out of phase with each other, typically by one-third or 120 degrees. These circuits are commonly used in electrical power systems to transmit and distribute electricity more efficiently.
In a three-phase AC system, there are three separate voltage waveforms that are generated by three separate AC voltage sources. These sources are typically represented by three sinusoidal waveforms that are 120 degrees out of phase with each other. The three phases are labeled as Phase A, Phase B, and Phase C.
When it comes to the interconnection of three phases, there are two primary configurations: the star (Y) connection and the delta (Δ) connection.
Star (Y) Connection:
In a star connection, one end of each phase is connected to a common point known as the neutral or star point (N). The other ends of the phases are connected to the load or other components. This configuration is also known as a "wye" connection. The voltage between any two line conductors is the phase-to-phase voltage (usually denoted as V_line-line), while the voltage between any phase conductor and the neutral is the phase-to-neutral voltage (usually denoted as V_phase-neutral).
Advantages of the star connection:
Provides a neutral point for grounding and easy connection of single-phase loads.
Phase-to-neutral voltages are lower than phase-to-phase voltages, which can be advantageous for some types of loads.
Delta (Δ) Connection:
In a delta connection, the phases are connected in a closed loop or triangle. There is no neutral connection in this configuration. The voltage between any two phases is the phase-to-phase voltage (V_phase-phase), which is typically higher than the phase-to-neutral voltage in a star connection.
Advantages of the delta connection:
Higher phase-to-phase voltage can be advantageous for transmitting power over longer distances without significant voltage drop.
In some cases, the absence of a neutral connection can simplify grounding and protective measures.
When designing power systems or connecting loads to three-phase circuits, the choice between a star and delta connection depends on various factors such as the type of load, the distance of transmission, voltage requirements, and safety considerations.
It's important to note that in practical applications, transformers can be used to convert between delta and star configurations, allowing for flexibility in adapting the system to different load requirements. Additionally, phase rotation is critical in polyphase systems to ensure proper operation and prevent undesirable effects like reverse rotation of motors.