A star triangle switch is a switch that has two circuits connected together. One circuit is the star circuit and the other is the delta circuit. The motor is powered in the star circuit when the switch is closed and powered in the delta circuit when the switch is opened.
The first step is to wire the switch and the power source together. Then the circuit must be made to connect the motor armatures in a triangle. This is done using a fourth contactor and a set of resistors.
These resistors are sized such that considerable current is able to flow round the motor windings while they are in circuit. Once the switch opens, this current can be shunted from the armatures to the supply by the delta contactor.
This system reduces the magnitude of switching transients by a factor of three, or 173%. In normal operation, these switching transients are a result of the auxiliary contactor’s closing in response to the timer’s de-energizing of its coils Kl and Ks.
To ensure that the motor is never switched between star and delta at the same time, an electrical lock is provided to prevent this. This is achieved by connecting a normally open contact on the star coil opposite of the interlock contacts on the delta contactor. The latching contact on the delta coil is then connected in parallel to this contact.
The interlock contacts are then connected to a switch that can be activated with the help of an electronic device. This switch can be an LED or a relay.
Alternatively, the switch can be a single cylinder type with a solenoid that closes when a button is pressed. The switch can also be connected to a switch that actuates the rotor of the motor.
In addition, the armatures of the motor can be switched in a delta triangle by the use of a single mechanical connection (a toggle switch). This is usually used with high horsepower electric motors.
When the ON push button is pressed, a timer’s auxiliary contact (KT)(3) in the star coil circuit changes its position from NC to NO. This is done to provide a latch which will hold the main contactor coil activated until the time delay reaches its preset value.
Once the time delay is reached, a separate timer coil (C3) and the timer’s de-energizing Ks and energizing Kt contacts are activated. These are then shunted across the contactor that is connected in parallel to the ON switch.