Star-Delta Starter PLC Program and Wiring: Motor Basics Explained - Part 2

Wiring a 3-Phase Motor Terminal Box for Star-Delta Starter Control

The terminal arrangement mirrors what was previously described for motor windings, with a slight variation. For instance, in the motor shown below, three terminals with incoming power are labeled U1, V1, and W1. The lower terminals, however, differ slightly from the standard winding schematic.

In the schematic, the lower ends are labeled U2, V2, and W2,

but on the motor’s terminal box, they appear as W2, U2, and V2.

This configuration serves as a typical schematic for a three-phase motor’s terminal box. With these winding terminals, if you want to activate the motor in a star connection, you can connect these three terminals together, as previously explained.

If starting the motor in a delta-connection, you can link U1 to W2, V1 to U2, and W1 to V2, following the steps covered earlier.

Automating the Switch from Star to Delta Connections with Contactors

To limit the high initial current of a three-phase motor, it’s best to start the motor in a star connection. This star configuration reduces the inrush current, protecting the motor windings and electrical system. Once the motor reaches the desired speed, switching to a delta-connection ensures optimal power and efficiency. Manually changing these junction while the motor is energized isn’t practical, so an automated star-delta starter setup is essential.

Using a PLC control circuit allows the transition from star to delta to happen seamlessly, without interrupting the motor’s operation. This star-delta wiring approach not only reduces starting current but also ensures a smooth switch to delta configuration for full-power performance, safeguarding the motor’s longevity and reliability..

Guide to Using PLC Controls for Efficient Star-Delta Switching in Motor Starters

This is where contactors come in.

Standard Wiring Methods for Connecting Three-Phase Motors

Under standard conditions, running a three-phase motor involves connecting three-phase power to the motor’s incoming connection located in the terminal box..

Using Contactors for Enhanced Control in Motor Starter Circuits

To gain more precise control over the motor’s operation, a contactor can be installed between the incoming power source and the motor.

Setting Up Star Connections in 3-Phase Motor with Contactors

For a star junction setup, a contactor can be added with its power contacts wired to the terminals W2, U2, and V2.

By short-circuiting the opposite ends of these contactor contacts, when they close, all three motor connection will connect, resulting in a star configuration.

How Star and Main Contactors Coordinate for Efficient Motor Start-Up

In this arrangement, both the main and star contactors must be energized simultaneously to initiate the motor in a star connection.

Here, the main contactor supplies power to the motor, while the star contactor links the right-side motor terminals together, achieving the star configuration.

Switching to a Delta Connection with an Additional Contactor

To connect the motor windings in a delta configuration, an additional contactor, called the delta contactor, is necessary. For this delta-connection, similar to the star configuration, the main contacts of the star-delta controller should be connected to the appropriate motor connection. This setup involves wiring the three main contacts to the right-side connection of the three phase motor, allowing for a seamless switch from star to delta as needed. Proper wiring and control of the delta contactor ensures that the motor operates efficiently in 3-phase delta mode, supporting optimal performance and reduced starting current.

Connecting Delta Contactor Ends to Power Phases

The other ends of the delta contactor’s contacts are then connected to L1, L2, and L3.

How to Activate Star and Delta Connections for Efficient Motor Control

With both the main and delta contactors activated and their contacts closed, U1 connects to W2, V1 to U2, and W1 to V2, forming the necessary delta winding configuration.

Therefore, when the main and star contactors are both engaged, the motor operates in star mode. Conversely, with the main and delta contactors energized, the motor operates in delta.

Controlling Contactors Using a PLC

These contactors lack physical buttons for manually opening or closing the contacts, making them reliant on external control circuits for proper operation. To activate these contactors and manage their switching functions, they need to be integrated with an industrial controller, such as a PLC. By connecting the contactor control circuit to the PLC, operators can achieve precise automation in switching between the star and delta configurations for a 3-phase motor.

Star-delta starter

This star-delta starter setup within the PLC allows for seamless management of contactor activation, enabling efficient control over the motor’s starting current. The PLC is programmed to send signals to each contactor in sequence, ensuring that the motor windings transition smoothly between the star and delta configurations without manual intervention. Using a PLC control circuit also provides flexibility in adjusting the timing for the switch from star to delta based on the motor’s load and speed requirements.

Integrating a PLC with contactors not only automates the motor’s operation but also increases the system’s reliability and safety by reducing the need for physical adjustments. In the upcoming lesson, we’ll dive into the steps required to program the PLC for controlling star-delta wiring and managing each contactor effectively, ensuring the 3-phase motor operates smoothly under varied load conditions. This automated control is essential for three phase motor systems that require regular switching between star and delta modes, particularly in industrial environments focused on motor protection and optimized performance.

Controlling Star to Delta Transition in Three-Phase Motor Starters with PLC

To efficiently control the transition from a star connection to a delta-connection in a three phase motor, a PLC control circuit plays a critical role. In a star-delta starter setup, the motor initially starts in a star configuration to reduce the starting current. This configuration lowers the initial load on the motor windings, ensuring the induction motor experiences less stress at start-up. Once the motor reaches a stable speed, the PLC automatically switches the wiring from star to delta, creating a smooth transition to a delta configuration. This diagram setup allows the 3-phase motor to achieve full operating power and efficiency while minimizing initial current draw. The control circuit thus ensures optimal performance, connecting each phase at the correct time for both star or delta modes and ensuring longevity for the three phase motor.

Star-Delta Control in Three-Phase Motors: Why Use It?

A 3 phase induction motor started direct on line (DOL) draws a starting current of several times its rated current. In large motors this causes voltage dips in the supply network and mechanical shocks in the drive train. The star delta starter is the simplest reduced voltage starter for such motors and allows the motor to reduce the start current without any power electronics.

When the motor is connected in a star, each motor winding receives the line voltage divided by the square root of 3, for example 230 V instead of 400 V. Because of the lower voltage applied to the motor, the starting current drawn from the line and the starting torque both drop to about one third of the DOL values. In practice, the motor is started in star connection, which reduces the current that the motor draws and the mechanical stress on the motor and the driven machine. Once the motor reaches a certain speed, close to its rated speed, the starter makes the connection from star to delta. From then on the motor is connected in delta, gets full voltage and can deliver its full power.

Compared with a direct on line starter, the star-delta starter reduces the starting current, but it also has a drawback: during the change-over the windings are briefly disconnected, so a short current and torque peak appears when the delta connection closes.

Two conditions must be met to use star-delta starting:

• The voltage rating of the motor must allow delta operation at the supply voltage, for example a 400/690 V motor on a 400 V network. A 230/400 V motor, which runs in star on a 400 V network, cannot be started star-delta on that network.
• The load torque during start-up must be low, because the available torque is only about one third of the DOL value. Fans, pumps and machines started without load are typical applications. Star-delta starting is used only with three-phase motors; a single phase motor cannot be started this way.

Star-Delta Starter Wiring Diagram: Power and Control Circuit

A complete star-delta starter consists of a power circuit and a control circuit. In the power circuit you need three contactors and an overload relay:

• Main contactor (K1) - supplies L1, L2 and L3 to the U1, V1 and W1 terminals.
• Star contactor (K3) - shorts the W2, U2 and V2 terminals together, so the motor is connected in star.
• Delta contactor (K2) - connects W2, U2 and V2 to the phases, so the windings are connected in delta.
• Overload relay - usually placed after the main contactor in the winding circuit; in this position it is set to about 0.58 times the motor rated current (rated current divided by the square root of 3).

Because both ends of each winding are used, six cables run to the terminal box, and the links in the box must be removed. In the control circuit diagram, the star and delta contactors must be interlocked so they can never be energized at the same time, otherwise a phase-to-phase short circuit occurs. Use both an electrical interlock (NC auxiliary contacts) and a mechanical interlock between the two contactors. Classic starters use a star-delta time relay; in a PLC-based solution the timer is part of the program, which is the topic of the next section.

Automatic star-delta starter using timer

Before PLCs became common, every automatic star-delta starter used a timer relay with a dedicated star-delta function. The power and control circuits are the same as above: the timer energizes the star contactor first, and after the set time it switches to the delta contactor with a short break. The PLC solution simply moves this timer into software, which makes it easy to change the star time, add diagnostics and display the motor starting state on an HMI.

Starting methods compared

Star-Delta Starter PLC Program

The sequence in the PLC is simple: after the start command, K1 and K3 switch on together. After the set time, K3 switches off, and after a short dead time K2 switches on. The dead time ensures that the arc in K3 is extinguished before K2 closes. The example below is written in Structured Text (IEC 61131-3) and works in TIA Portal (SCL) or CODESYS with minor changes:

// Start/stop latch: xStop and xOverloadOk are NC contacts (TRUE = OK)
xRun := (xStart OR xRun) AND xStop AND xOverloadOk;

tStar(IN := xRun, PT := T#6S);        // star time, adjust to the load
tPause(IN := tStar.Q, PT := T#60MS);  // dead time between star and delta

xMainContactor  := xRun;
xStarContactor  := xRun AND NOT tStar.Q;
xDeltaContactor := xRun AND tPause.Q AND NOT xStarFeedback;

Practical tips for the PLC version:

• Keep the hardware interlock between K2 and K3 in the control circuit, even if the program already prevents simultaneous switching.
• Use the feedback (auxiliary contact) of K3 as a condition for switching on K2.
• Set the timer to the real start-up time with the actual load. Switching too early causes a high current peak, similar to a DOL start.
• Wire the overload relay contact to a PLC input and use it to stop the sequence and show an alarm on the HMI.

If you want to see how TON timers work in a Siemens controller, read our guide to timers in TIA Portal. The basics of motor connections are explained in part 1 of this series, and the Industrial Automation Basics course covers contactors, motor circuits and PLC wiring in practice.

Star-Delta Starter - FAQ

Why use a star-delta connection for a three-phase motor?

To reduce the starting current. In star, each winding gets a voltage lower by the square root of 3, so the current drawn from the network during motor starting is about one third of the direct on line value.

How many wires are connected to the motor in a star-delta circuit?

Six. Both ends of each of the three windings (U1, V1, W1 and U2, V2, W2) are connected to the starter, plus the protective earth conductor. Check the wiring diagram on the inside of the terminal box cover before the motor is connected.

What happens if you connect a delta motor in star?

Each winding works at a reduced voltage, so torque drops to about one third. Under load the machine slows down, draws excessive current and can overheat. That is why this connection is used only for start-up.

Should a 3 phase motor be connected in star or delta?

It depends on the nameplate and the supply voltage. A 230/400 V motor on a 400 V network is connected in star; a 400/690 V motor on a 400 V network is connected in delta and can be started star-delta.

Thank you for reading! If you're interested in learning more, explore our courses for in-depth training: Control Byte Courses.