What is a Star-Delta circuit and How does it work?

Star-delta circuit working principle – One type of starter control commonly used to start a 3-phase electric motor other than the DOL circuit is Star-Delta. Star-delta starter is an electric motor starting method used to reduce starting current and torque in three-phase induction motors. We can usually use this model of starter in industrial applications, where large motors (More than 10 kW) need to be started smoothly to prevent damage to the motor windings and to reduce strain on the power supply system.

In this article, we will be discussing the Star-Delta circuit especially about:

• How Star-Delta circuit can reduce the starting Current?
• Components and description of the Star-Delta power circuit
• Components and description in the Star-Delta control circuit
• The figure of the Star-Delta (power and control circuit) and Working principle

How Star-Delta circuit can reduce the starting Current?

Initially, the motor is connected in a star configuration, which reduces the voltage applied to the motor windings, thereby reducing the initial inrush current drawn by the motor. After a predetermined time, the contactors are switched to the delta configuration, which increases the voltage applied to the motor windings, allowing the motor to run at full speed with reduced current draw.

To understand in more detail about this, pay attention to the explanation below carefully.

Phasor diagram theory in star connections

First of all, let’s look at the 2 figures below. We will start with the STAR connection.

Let’s see the Figure number 2. Phasor diagram where the phase voltages EAN, EBN and ECN have the same magnitude but are separated from each other by 120º.

The line voltage EAB is the sum of the vectors EAN and – EBN as well as the line voltages EBC and ECA and are also 120º apart from each other. EAB = EBC = ECA = 2 EAN cos 30º

Meanwhile, we can calculate the flowing current using the equation:

IA = IB = IC = Iph(Ia, Ib, Ic) (magnitude)

In the equation above, it can be seen that the current flowing in the motor coil (Ia) is the same as the incoming current (IA). and the phasor diagram can be seen in the following figure:

The conclusion from the explanation above is:

• The current flowing on the line side (source) is the same as the current flowing on the motor coil (I line = I phase).
• The voltage on the line side is equal to (Root 3) x Voltage on the motor coil (V line = 1.73 x V phase).

Phasor diagram theory in star connections

Phase a voltage (coil a) = line voltage = VCA, while the current in the circuit can be calculated using the equation:

We can obtain line currents (IA, IB, IC) by applying Kirchhoff’s law:

IA = Iab – Ica
IB = Ibc – Iab
IC = Ica – Ibc

From the equation above, we can see that the magnitude of the current flowing in line (IA) is √3 times the magnitude of the phase current Iph.

The following is a comparison of the line current flowing in the two connection methods above:

From the equation above, we can see that the magnitude of the current flowing in line (IA) is √3 times the magnitude of the phase current Iph.

The following is a comparison of the line current flowing in the two connection methods above:

The conclusion from the explanation above is:

• The current flowing on the line side (source) is equal to (Root 3) x current in the motor coil (I line = 1.73 x I phase)
• The voltage on the line side is the same as the voltage on the motor coil (V line = V phase).

Returning to the question, why can the star delta circuit reduce the starting current? I can explain it as follows.

We can see in the equation above that the current entering the motor coil when it is star-connected is smaller than when it is delta-connected. Likewise with the voltage.

Current:

In a star connection, the current entering the motor coil is the same as the current at the source. Meanwhile, in a delta connection, the current in the coil is 1.73 times the current in the source.

Voltage

In a delta connection, the voltage on the source side is the same as the voltage on the motor coil. Meanwhile, in a star connection, the voltage on the source side is 1.73 times the voltage on the motor coil. This means that the voltage on the motor coil is smaller than the voltage on the source side.

For Example:

When connected to Star, the starting current on motor A is, for example, 35 Amperes. What is the current on the source side? The answer is the same 35 Amperes. If this motor is delta-connected, then the starting current will be 1.73 x 35 = 65 Amperes. It can be seen that the starting current in the delta connection is almost twice the star connection.

Likewise with the amount of voltage. When connected to a delta, the voltage on the motor coil is the same as the source voltage, for example 400 volts, whereas when connected to a star, the voltage is 400/1.73 = 230 Volts.

This is the reason why motors with delta connection have greater speed and torque than those with star connection.

Power circuit in Star-delta connection

Above, we already understand the reasons why induction motors with delta connection have greater speed and torque than those with star connection. Next we will discuss the power circuit and control circuit in the Star-Delta Circuit.

The power circuit refers to the electrical connections and components responsible for supplying power to the three-phase induction motor. It also controls the switching between the star and delta configurations during motor starting.

Power Supply (A)

The power circuit starts with the three-phase AC power supply (RST), usually at a standard voltage level (e.g., 400V or 380V in many industrial settings). This power supply will later be connected to the Contactor.

Main & Star Contactor (B1, B2)

The main contactor is a heavy-duty switch that controls the power supply to the motor windings. In the star configuration, the main contactor connects the motor windings in a star (Y) arrangement to reduce the voltage applied to the motor during startup.

Delta Contactor (C)

The delta contactor is another heavy-duty switch that connects the motor windings in a delta (Δ) configuration once the motor has reached a predetermined speed. This switch changes the motor’s connection to receive full line voltage for normal operation.

Overload Relay (D)

When functioning as a power circuit, An overload relay is a protective device that monitors the motor’s current draw. If the motor draws excessive current, indicating a fault or overload condition, the overload relay will trip and disconnect power to the motor, protecting it from damage.

Control circuit in star-delta connection

The control circuit in a star-delta connection will be responsible for managing the switching of the motor windings between the star (Y) and delta (Δ) configurations, as well as providing safety and control features to ensure the proper operation of the motor. The control circuit works in conjunction with the power circuit to start and stop the motor while ensuring smooth transitions and protection against faults.

Here are the key components and functions of the control circuit in a star-delta starter:

Emergency stop button

The function of the Emergency Stop Button in the Star-Delta circuit is to quickly and immediately halt the operation of the electric motor in case of an emergency or hazardous situation.

MCB (Miniature Circuit Breaker)

The function of an MCB is to protect the Star-Delta circuits from overcurrents. Overcurrents can occur due to various reasons, such as short circuits (a sudden, unintended connection between two conductors), overloads (excessive current drawn by devices), or faults in the wiring.

Start/Stop Push Buttons

These are manual push-button switches that allow an operator to start and stop the motor. The start button initiates the motor starting sequence, while the stop button halts the motor operation.

Timer or Controller

The timer or controller is a critical component of the control circuit. It determines the timing of the transition from star to delta configuration. The timer ensures that the changeover occurs after the motor has reached a certain speed, typically around 70-80% of its rated speed. This prevents a sudden voltage jump during startup, ensuring a smooth transition.

Interlocking Mechanism

An interlock system prevents both the star and delta contactors from closing simultaneously, which could result in a short circuit. The interlock ensures that only one contactor is closed at any given time.

Overload Relay

As part of the control circuit, an overload relay monitors the current drawn by the motor. If the current exceeds a preset threshold (indicating an overload or fault), the overload relay will trip, opening the control circuit and stopping the motor.

Control Wiring

Control wiring consists of electrical wires that connect all the control components (push buttons, timer/controller, relays, overload relay, etc.) in the control circuit. It provides the pathways for signals to travel between these components to control the operation of the starter.

The figure of the Star-Delta (power and control circuit) and Working principle

Finally, we come to the last section, The Example of wiring diagram for a Star-Delta Starter. Below I have prepared the Star-Delta circuit, both the power circuit and the control circuit.

Note!

I use the Omron timer, H3CR-A8 in this case. Setting mode E, and 10 seconds).

Starting in Star Configuration (B2)

• Initially, the motor windings are connected in a star (B1-B2) configuration. In this configuration, each phase winding of the motor is connected in series with one another at a common point called the neutral or star point (usually denoted as N or Y).
• When the motor is started in the star configuration, each phase winding receives the full line voltage, but the current is reduced to approximately 1/√3 (approximately 58%) of the full-load current, as the winding phases are connected in series.
• This reduced current allows the motor to start smoothly without drawing excessive current from the power supply, which could cause voltage sags and damage to the motor or other connected equipment.

Transition to Delta Configuration (B1-C)

• After a predetermined time period (typically a few seconds to a couple of minutes, depending on the motor and application), the motor is transitioned from the star configuration to the delta configuration.
• In the delta configuration, the motor windings are reconnected in a delta (B1-C) pattern. This means that each phase winding is connected directly to the next phase winding without any connection to the common neutral point.
• When the transition to the delta configuration occurs, the motor receives the full line voltage, and the current drawn by the motor increases to its full-load value.

Control circuit component and working principle!

First of all, we start with a single-phase power source with a sinusoidal symbol. First of all, we start with a single-phase power source with a sinusoidal symbol. Then there is an emergency stop button (EMG) which functions to stop the circuit in an emergency.

An MCB is installed after the EMG for the purpose of ON-OFF the circuit in normal conditions and at the same time as protection when a short circuit occurs.

After the MCB, the next circuit goes to the NO and NC switches on the Thermal Overload relay (OL). The NC contact functions as protection when an overload occurs. The NO contact will function as a TRIP indicator light.

The Stop button (OFF) is installed after Overload followed by the start button (ON).

Working Principle of star-delta circuit

Switch ON the MCB, so that current flows to the OL contacts (NO and NC). Press the ON button, then current will flow and turn on Contactor M (Main Contactor) and timer TM (Timer). This two components are hold by NO Contact M. At the same time, the Star contactor (K1) is ON for a few seconds (according to user settings, usually 10 seconds).

After the time is reached, the contact on the timer (TM) changes position, opening the current flow to the coil K1 (STAR Contactor) and closing K2 (DELTA Contactor). At the same time the DELTA Connection indicator light will also turn on.

If an overload occurs, the NO contact on the OL will close and the Trip indicator light will light up.