Understanding the Star/Delta Starter in Induction Motor Control
A star delta starter is used to start three phase induction motors so that the starting current will reduce. The motor first starts in the star configuration to make the current low and safe on the supply line.
As the motor attains some speed, it switches to the delta configuration to run with full voltage at high speed. This helps protect the motor and prevents high inrush current from flowing.
Because of this, star delta starter is one of the common methods used for starting bigger size induction motors.
Main Components of a Star/Delta Starter
- Main Contactor (KM)
- On-Delay Timer Relay (T1)
- Star Contactor (K1)
- Delta Contactor (K2)
- Overload relay
- Single start push button
- Single stop push button
- Protective devices, such as a three-pole breaker and a control breaker, to safeguard the circuit
- Indicator lights (optional) for monitoring:
- Red: Stop / Not running
- Green: Start / Running
- Amber: Tripped overload or fault
Operation of a Star/Delta Starter
Star/Delta starter is made up of two main circuits. The control circuit and the power circuit. The control circuit consists of the stop push button, start push button, and the coils and auxiliary contacts of main contactor (KM), star contactor (K1), delta contactor (K2), on-delay timer relay (T1), and the overload relay, and sometimes indicator lamps for monitoring. The power circuit on the other hand, consists of the star configuration made up of the main contactor and the star contactor, whiles that of the delta configuration is made up of the main contactor and the delta contactor.
Principles of Operation
When the start push button is pressed, current flows through the NC contact (95-96) of the overload relay, through the stop push button which is normally closed, and then through the start push button to the A1 terminal of the main contactor (KM) and the on-delay timer relay (T1). At the same time, current also flows through the NC contact of the on-delay timer relay and through the NC interlock contact of the delta contactor (K2) to the A1 terminal of the star contactor (K1).
Because the A2 terminals of the main contactor, the timer, and the star contactor are all connected to the neutral line, the main contactor (KM), the on-delay timer relay (T1), and the star contactor (K1) all energize at the same time, and the timer starts counting. The motor now starts running in the star configuration at a reduced speed. The hold-on or auxiliary contact (13-14) of the main contactor also closes as the main contactor energizes, so that when the start push button is released, the circuit will still remain energized.
As the timer is counting, current cannot flow to energize the delta contactor because the NO contact of the timer is still open, and the NC interlock contact of the star contactor is open since K1 is energized. When the preset time of the timer is reached, the timer contacts change their states. The NC contact on the star line opens, cutting current to the star contactor coil, so the star contactor de-energizes and its NC interlock contact closes back.
At the same time, the timer NO contact on the delta line closes, and now that the NC interlock contact of the star contactor has closed, current flows to energize the delta contactor (K2). The motor now runs in the delta configuration at a full speed. As the delta contactor energizes, its NC interlock contact on the star line opens to prevent the star contactor from energizing. This is what we call the electrical interlock.
When the stop push button is pressed, current flow to the entire control circuit is cut, and all the components de-energizes, and the motor stops running. Same way, when an overload occurs, the overload relay heats up and opens its NC contact (95–96). This also cuts the current to the control circuit, causing the main contactor, the timer, the star, and the delta contactors to all de-energize, thereby protecting the motor from damage.
Why the Overload Relay?
The overload relay protects the motor from excessive current caused by:
- Motor overload
- Seized bearings
- Misalignment on couplings
When excess heat is detected due to overcurrent, the relay deenergizes the contactor, stopping the motor to prevent damage.
Why the Circuit Breakers?
Circuit breakers are installed to cut the supply voltage to the motor in case of a short circuit, protecting both the motor and the circuit.
Advantages of a Star/Delta Starter
- It reduces the starting current of the motor.
- It helps prevent voltage drop on the supply line.
- It gives the motor a smooth start.
Disadvantages of a Star/Delta Starter
- The starting torque is low in star configuration.
- It cannot start a motor that is carrying a heavy load.
- It is very complex to install.
Applications
- Water pumps
- Fans and blowers
- Conveyors
- Other small to medium industrial machines such as crushers, mixers and compressors.
Learn How to Wire a Star/Delta Starter
Do you want to know how to wire this Star/Delta starter? Click the link below to watch the full tutorial on my YouTube channel:
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