Discover the working principles of auto transformer starters and how they effectively reduce the starting current in induction motors, enhancing efficiency and protecting electrical systems.
One of the main challenges with large motors is the high inrush current that happens during startup. This initial starting amps can cause damage to the equipment, lead to voltage dips and affect the integrity of the power supply network.
To overcome this issue, an auto transformer starter is the solution. It lowers the inrush current appreciably and produces sufficient torque to start the motor without stalling. In this article, we’ll discuss the working of auto transformer starter, and how it causes the starting current to drop.
What is an auto transformer starter?
Auto transformer starter is another type of reduced voltage starter used to start squirrel cage induction motors. During the startup of the electric motor, it applies a reduced voltage using an auto transformer. After a certain speed is reached (typically 70-80% of rated speed) the auto transformer is switched from the circuit and full voltage is applied.
We must reduce starting current
The start-up current of an induction motor can be 6 to 8 times its full-load current. This high current can:
- Draining the electrical grid
- Cause voltage dips
- Heat up the windings
- Shorten the motor’s life span
- Damaging other equipment on the same supply
It is essential to identify the start current drop, maintaining the torque required to activate the motor to avoid this situation.
How the Auto Transformer Starter Reduces the Starting Current
Auto transformer starter working principle: The basic concept of auto transformer starter is to reduce voltage at the time of starting. Here’s how it works:
1. Soft Starters (Initial Start with Reduced Voltage)
The motor is first connected through the taps of an auto transformer. These taps supply the motor with a lower voltage (typically 50%, 65% or 80% of the line voltage).
2. Reduced Current Draw
As a result, the current drawn by the motor is high. Even lower than that, is the current drawn from the main supply due to transformer action. This is what makes the auto transformer different from other reduced-voltage starters.
For example:
- The motor current at 65% voltage tap is approximately 65% of the direct-on-line (DOL) starting current.
- The line current (current pulled from the supply) is then reduced further down to 42% of DOL starting current.
3. Switching to Full Voltage
As the motor approaches approximately 80% of its full speed, the auto transformer disconnects and the motor is connected to the supply with full voltage for normal operation.
Benefits of Auto Transformer Starter
- The starting current is greatly reduced
- Improved voltage regulation at the transmission level
- Voltage taps at adjustable (typically 50%, 65%, 80%)
- Motor acceleration of the engine in a smooth manner
- Ideal for large motors requiring high start torque
Applications
Auto transformer starters are being used for:
- Pumps
- Compressors
- Conveyors
- Large fans and blowers
- Heavy duty induction type motors with high starting torque
Conclusion
This article gives a detailed overview of Auto Transformer Starter which is an energy efficient and reliable solution for limiting the starting current of large motors.
It does not only protect the motor and power system during startup by applying a reduced voltage and transfer smoothly to the full voltage, but it also improves operational efficiency. This kind of starter is a practical alternative for to make motor start at safer and more controlled manner for robust industries.
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