3-Phase Automatic Star-Delta Starter using Timer – Power, Control & Wiring Diagrams

 

Starting & Controlling of 3-Phase Motors Using Automatic Star-Delta Starter (Y-Δ)

A Star Delta starter is an electromechanical device used to start and control the speed of a three-phase induction motor. This starter employs the star-delta (Y-Δ) method for starting the motor, which involves changing the motor’s winding connection from a Star configuration to a Delta configuration once the motor reaches a certain speed.

Ladder Diagram of Y-Δ Starter using PLC

The Star-Delta starter is a commonly used method for controlling the starting of three-phase AC induction motors, particularly in high-power applications. It consists of a control circuit that includes key components like a timer, contactors, and overload relays. When the motor starts, it is initially connected in a Star (Y) configuration, which reduces the starting current. Typically, this current can be as high as six times the motor’s full-load current. After the motor reaches a certain speed, the timer switches the connection to a Delta (Δ) configuration, allowing the motor to run efficiently at full power.

This type of starter is frequently used in industries such as oil and gas, mining, and manufacturing, where large motors are required. The Star-Delta starter offers several benefits, including reduced inrush current at startup, better management of starting current, and less strain on the motor windings. However, there are also some drawbacks, such as increased complexity and cost, longer startup times, and lower starting torque.

In this tutorial, we will walk through the automatic Star-Delta (Y-Δ) starting method for three-phase AC induction motors. We will provide detailed information, including schematics, power and control circuits, PLC ladder logic, and wiring diagrams. Additionally, we will explain how the Star-Delta starter works, outline its applications, and discuss its pros and cons in more detail.

Operation & Working of Automatic Y-Δ Starter

1. The phase current flows from L1 to the thermal overload contact through an MCB/MCCB or general fuse, then to the OFF push button, On push button interlocking contact 2, and then to MC3. The circuit is thus completed, and both contactor coil C3 and timer coil (T) are energized simultaneously. As a result, the motor winding is connected in Star, and when MC3 is energized, its auxiliary open links will close, and the close links will open.

1. Consequently, Contactor MC1 is also energized, and the Three Phase Supply reaches the motor. Since the winding is connected in Star, each phase will receive √3 times less than the line voltage, which ensures safe motor starting. The close contact of MC3 in the Delta line opens, preventing the activation of contactor 2 (MC2).

1. After the push button is released, Timer coil and coil 3 will receive a supply through Timer contact (2 and 7), Holding contact 3, and the close contact 2 of MC2. When Contactor 1 (MC1) is energized, the two open contacts in the line of MC1 and MC2 will close.

1. For a specific time (generally 5-10 seconds), the motor will be connected in Star. After that, the Timer contact (T) will open (which can be adjusted by rotating the timer knob to set the time again), and as a result, Contactor 3 (MC3) will turn off, and the open link of MC3 (in the line of MC2) will close, causing MC2 to energize. When MC3 is off, the star connection of the winding will also open, and MC2 will close, connecting the motor winding in Delta. Contact 2 (which is in the line MC3) will also open, preventing the activation of coil 3 (MC3).

1. Now that the motor is connected in Delta, each phase will receive full line voltage (415V), and the motor will start to run at full speed.

Legends and its Abbreviations:

Advantages & Disadvantages

Advantages:

• Simple design and operation.
• Comparatively cheaper than other voltage controlling methods.
• The torque and current performance of the Y-Δ starter is good.
• It draws two times the starting current of the FLA (Full Load Ampere) of the connected motor.
• It reduces the starting current to approximately one-third compared to a DOL (Direct ON Line Starter)

Disadvantages

• Starting Torque is also reduced to one-third because the starter reduces the starting current to one-third of the rated current [as Line voltage is also reduced to 57% (1/√3)].
• It requires six leads or terminals for a Delta-connected motor.
• For Delta connection, the supply voltage must be the same as the rated motor voltage.
• At switching time (from Star to Delta), if the motor does not reach at least 90% of its rated speed, then the current peak may be equally high as in a Direct ON Line starter( D.O.L), thus causing harmful effects on the contactor’s contacts, making it unreliable.
• We should not use a star-delta starter if the required (application or load) torque is more than 50% of the three-phase induction motor’s rated torque.