Why is the star-delta starting method not possible for a delta connected induction motor?

The reason why an induction motor with a delta-connected configuration cannot use the star-delta starting method mainly involves the electrical characteristics of the motor and the relationship between current and voltage during the starting process. Here is a detailed explanation:

Differences in Electrical Characteristics

Voltage Relationship

In a triangle (Δ) connection, the voltage on each winding is equal to the line voltage of the source.

In a star (Y) connection, the voltage on each winding is equal to the phase voltage of the source, which is approximately 1/√3 times the line voltage.

Current Relationships

When connected in a triangle, the line current is equal to the phase current.

When connected in a star configuration, the line current is equal to the phase current times the square root of 3.

Impacts during startup

Starting Current

If the original triangle-connected motor is changed to a star connection for starting, the phase voltage at startup will be reduced to about 1/√3 times, resulting in insufficient starting current to drive the motor to normal speed.

In this case, the motor may not start properly, and even if it does start, it may be damaged due to overload.

Insufficient Power

Due to the lower phase voltage in a star connection, the starting torque of the motor will also be significantly reduced, unable to provide sufficient torque to overcome the starting load.

Practical Application

In practical applications, star-delta starting is mainly used for motors that have lighter loads during startup and heavier loads during operation.

For a motor that has been designed for delta connection, changing the connection method will not only fail to bring about any starting advantage, but may also lead to starting failure or damage to the motor.

In summary, the induction motor with a delta-connected winding cannot use the star-delta starting method, mainly because this operation leads to insufficient starting current, inadequate starting torque, and potential motor overload and damage. The correct approach is to select an appropriate starting method based on the specific motor design and load characteristics.