Three-Phase Connections: Star and Delta

The three-phase connections are ways to join the three coils of a machine to obtain different voltage and current values.

Suppose you have the three coils of the generator or motor separated. They have three starts (U₁, V₁, W₁) and three ends (U₂, V₂, W₂). Depending on how we connect these terminals, the machine will behave radically differently.

Star connection (Y)

This connection is named so because, if drawn on paper, it looks like a “Y” or a three-pointed star (like the Mercedes-Benz logo).

How is it done? We join all the ends of the coils (U₂, V₂, W₂) at a common central point. The starts (U₁, V₁, W₁) are connected to the phases (L1, L2, L3).

Characteristics

1. The Neutral (N): The central point is the center of the star. It can be connected to the source neutral; in a balanced system, no current flows through this conductor. A floating star point must not be confused with ground or assumed to be 0 V.

2. Lower phase voltage: In a balanced system, each coil receives the line voltage divided by .

3. Use: It is used in distribution lines (to extract 230V for houses and 400V for industry) and for smooth motor starting.

Delta connection (Δ)

This connection takes its name from the Greek letter Delta.

How is it done? We connect the end of one coil to the start of the next, forming a closed loop.

• End of U to start of V.
• End of V to start of W.
• End of W to start of U.

There is no central point. There is no Neutral.

Characteristics

1. Raw Power: Each coil is connected directly between two Phases (L1-L2, etc.).
2. Maximum Voltage: The coil receives the full line voltage (e.g., 400V).
3. Use: It is used to obtain the maximum power from a motor during normal operation.

The √3 factor

In three-phase systems, the number 3 does not appear alone. It appears under a square root. In a balanced three-phase system, the relationship between line magnitudes and branch magnitudes includes the factor.

• In Star: We divide the voltage (230V at coil vs 400V at network).
• In Delta: We multiply the current.

The motor nameplate: how do I connect it?

This is rookie electrician mistake number 1. Burning a motor by connecting it in Delta when it should be in Star.

Always check the motor’s nameplate. It may indicate two voltages, for example: 230/400 V Δ/Y.

• The LOWER voltage (230V) is the maximum a single coil can withstand.
• The HIGHER voltage (400V) is for line calculations.

Practical Example: You are in Spain. The three-phase network is 400V between phases. You have a motor rated 230/400V.

1. If you connect it in Delta: Each coil receives 400V. But the nameplate says the coil can handle 230V. RESULT: The motor burns out in seconds → 🔥

2. If you connect it in Star: The network supplies 400V, but the Star connection divides by 1.73. The coil receives . RESULT: The motor runs perfectly. → :ok_hand:

Star-delta starting

A motor can draw several times its rated current during startup. Star-delta starting reduces the voltage applied to each coil during this phase:

1. We start the motor in star. Each coil receives less voltage, so the starting current and torque are reduced to approximately one-third compared to direct delta starting.
2. Once the motor has accelerated, contactors switch to delta for rated operation.