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These are two core voltage concepts in three-phase AC circuits. Their key difference lies in different measurement objects, and the numerical relationship is determined by the wiring mode of the three-phase circuit.
- Basic Definitions
1.1 Phase Voltage
It refers to the voltage between any phase line (live wire) and the neutral line (fir neutru) in a three-phase power supply, denoted by $U_{ph}$.
Scenarii de aplicare: Three-phase four-wire systems with star (Y) connection, used to supply power to single-phase loads (de ex., lighting, aparate electrocasnice).
Common Value: In China’s low-voltage power distribution system, the effective value of phase voltage is 220V.
1.2 Line Voltage
It refers to the voltage between any two phase lines (live wires) in a three-phase power supply, denoted by $U_{l}$.
Scenarii de aplicare: Three-phase systems with star (Y) or delta (△) connection, used to supply power to three-phase loads (de ex., three-phase motors, industrial frequency converters).
Common Value: In China’s low-voltage power distribution system, the effective value of line voltage is 380V.
- Comparison Table of Core Differences
| Comparison Dimension | Phase Voltage (Uph) | Line Voltage (Ul) |
| Measurement Object | Phase line ↔ Neutral line | Phase line ↔ Phase line |
| Symbol | Uph | Ul |
| Symmetrical Star (Y) Connection | Ul=3⋅Uph≈1.732⋅Uph | The value is 3 times the phase voltage |
| Delta (△) Connection | Ul=Uph | Equal to the phase voltage in value |
| Load Adaptation | Single-phase loads (220V equipment) | Three-phase loads (380V equipment) |
| System Dependence | Depends on the neutral line, existing only in three-phase four-wire systems | Does not depend on the neutral line, existing in both three-phase three-wire and four-wire systems |
- Key Application Notes
3.1 Voltage Relationship in Star (Y) Connection
This is the most commonly used connection method in industrial and civil power distribution, where the ends of the three-phase windings are connected to form a neutral line (N).
Example: Phase voltage $U_{ph}=220V$ → Line voltage $U_{l}=1.732 \times 220V \approx 380V$.
Caracteristici: It can provide both single-phase 220V and three-phase 380V power supplies to meet the needs of different loads.
3.2 Voltage Relationship in Delta (△) Connection
The three-phase windings are connected end-to-end in sequence without a neutral line, and the line voltage is equal to the phase voltage.
Example: If the phase voltage of the winding is 380V, the line voltage is also 380V.
Caracteristici: It is mostly used for direct starting of three-phase motors, without the need for a neutral line, and has high requirements for the symmetry of loop current.
3.3 Precauții
The above numerical relationship is only applicable to symmetrical three-phase circuits (three-phase voltages with equal amplitude and 120° phase difference between each other). This relationship is not valid in asymmetrical circuits.
The function of the neutral line is to balance the voltage of three-phase loads. If the neutral line is disconnected, the phase voltage of the star connection will be unbalanced, leading to load damage.
- Relevance to Low-Voltage Electrical Appliances
Take the Siemens 6SL3210-1SE23-2UA0 power module you are concerned about as an example:
The module’s input voltage is marked as 3AC 380-480V, which refers to line voltage. It needs to be connected to three-phase lines without a neutral line.
If the module needs to control single-phase 220V loads, additional step-down or conversion circuits must be configured, and it cannot be directly connected to the phase voltage.
