Starting asynchronous motors is the process of connecting the motor to the mains and putting it into operation. However, this process is not as simple as closing a circuit. The high current drawn by the motor during its initial start-up can damage both the mains and the motor’s mechanical system. Therefore, the starting methods used during startup are critically important for the system’s efficiency and lifespan. Various starting methods are available that can be preferred depending on different power levels, application types, and operating conditions.
Direct Connection (Direct Connection to the Grid)
The electric motor is connected directly to the three-phase mains via a contactor. This is the simplest and oldest method. Electric motors draw a current approximately 6–8 times their rated current at startup. This can cause significant voltage drops, especially in high-power motors. It is generally preferred for motors under 4 kW and in systems where the mechanical load is very low.
Star-Triangle Giving Way
When the motor starts, the voltage applied to the windings is reduced by 1/√3 (approximately 0.577 times) using a star connection. This reduces the starting current by about one-third. After a certain period, the motor is switched to delta connection to operate at full voltage. The star-delta control circuit automatically performs this transition via a time relay or PLC. Star-delta contactors must be carefully selected considering the motor’s power and current draw. The installation of three contactors—main, star, and delta—and a time relay is absolutely necessary. Starting current and torque are significantly reduced. Sudden voltage changes may occur during the transition. It cannot start under heavy load. It is generally used in asynchronous motors above 5.5 kW operating under light loads.
Giving Way with Auto-Transformer
At startup, the voltage is reduced via an autotransformer and applied to the motor. After a certain period, the motor is directly connected to the grid. The starting current and torque can be reduced to adjustable levels. The equipment cost is high. It is used in medium and high-power motors when starting under load is required.
Starting the Engine with a Soft Starter
A soft starter gradually increases the voltage applied during motor start-up. This allows the motor to engage slowly, preventing sudden mechanical stresses and voltage fluctuations. It is generally preferred in applications requiring heavy starting, such as pumps, compressors, and presses. It only engages during start-up and does not continuously adjust the speed.
Driver and Yielding Method
Using a frequency converter (motor driver), both the frequency and voltage are increased in a controlled manner during startup. The starting current can be reduced to the nominal level. Simultaneously, the motor’s speed, direction, and torque can be adjusted via the driver. The driver-based starting method provides efficient and precise control not only during startup but throughout the motor’s entire operating life. It is the most advanced but costly solution. It is used in critical processes and applications where energy savings are desired.
The Difference Between Star-Delta Yielding and Motor Driver Yielding
The star-delta method is only active during startup; it does not control speed and torque.
The drive system, however, provides complete control both during startup and throughout operation.
The star-delta system is an economical solution, but the drive technology is much more flexible.
Which type of yielding maneuver should I choose?
Asynchronous motor starting methods should be selected according to the application to be performed in the field.
If it is a low-power motor and the starting load is light, direct starting is sufficient.
For motors in the medium power range, star-delta starting is generally preferred as a safe and economical starting solution.
If the electric motor requires continuous speed control or has frequent start-stop cycles, it is beneficial to opt for starting with a drive.
When the starting type is chosen correctly, the motor’s lifespan is extended, energy efficiency is increased, and the system becomes safer.