What is an auto-transform starter?
The Autotransformer starter is best to use for both star and delta connection circuit motors. Here we made a circuit to limit the starting current with the use of three three-phase autotransformers to decrease the voltage applied at the stator.
There are many toppings on the circuit. The starter is connected with one certain tapping to get the best value of the starting voltage for the motor. The double-through switch makes the connection of the autotransformer to perform starting processes.
If the switch is in starting condition, the primary side of the transformer makes a connection with the power supply, and the motor is connected to the autotransformer. When the motor gets to a speed of about 80% of its rated speed, the switch handle moves to run the state. So the autotransformer is disconnected from a circuit, and the motor makes a connection with the line and gets its rated voltage. The handle of the switch stays connected to the run state by a voltage relay.
If the supply voltage is less than a certain point, the handle is released and moved to the off state. Thermal overload relay as protection if needed.
Autotransformer starter diagram
Autotransformer
Main Switch
This switch is used to turn on and off the power supply connected to the starter. It can be MCCB or any other type of breaker, and in some cases, fuses are used. The main power supply is connected to the main switch as an input. The output is connected to the input terminal of the starter. If this main switch is off, no power will start, and the connected circuit
Shorting Contactor
Transformer Contactor
Running Contactor
It is configured between the incoming power supply of the main switch and the motor points. It makes the connection between the motor and the input supply. If the motor is running, it is in closed condition, and the motor will operate at full input voltage, known as a running contactor.
Autotransformer Starter Working
Here is the step explained.
- First of all, the shorting contactor is in a closed state, and the transformer contactor gets on. When this contactor is on, the motor will get a power supply and start operating. This supply voltage based on tapping is configured. Normally, the initial voltage used is 50%, 60%, or 70% of the full voltage value. This shorting contactor makes connection transformer windings in a star configuration.
- After a set time, the shorting contactor becomes open, and a coil of the transformer works as a series inductor with a motor.
- When the motor has a stable speed, the contactor gets closed. It is set by the timer. In this condition, the transformer works as a shunt for the motor.
- The transformer contact is open, and the autotransformer is insulated from the circuit and motor operating at full load voltage. In a running condition, the running contactor will be in the closed state, and other contactors will be in the open state.
Advantages of an autotransformer starter
- It is a low-cost circuit.
- Its high power ratio is between 65 and 80 percent.
- It helps set the starting voltage with the use of a tap on the transformer.
- It has maximum torque per ampere of supply current.
- The motor has a higher current than the supply current.
- It operates for a long start-up duration.
- The inrush current is decreased with this starter.
- For motors of high rating, it is used that cannot be started directly.
- It increases regulation.
Disadvantages
- It is complicated and uses a costly autotransformer.
- It needed a larger control panel for the transformer, making it costly.
- Its physical structure is larger, so it is not easy to connect it in a limited space.
Applications
- It is best to use it for reducing voltage at the start of an induction motor.
- It is used as a booster for the long transmission line.
- It is used in fluorescent lamp fixtures.
- It boosts the circuit input voltage.
- It is used for slip rings and squirrel cage induction motors.
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