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

An induction motor (Induction Motor) draws a high current during startup due to several factors working together. Here is a detailed explanation:

1. High Starting Torque Requirement

Starting Torque:

• An induction motor needs to generate sufficient torque to overcome the static inertia and start the rotor rotating. This requires a large amount of current to produce a strong magnetic field and torque.

2. Low Power Factor

Power Factor:

• The power factor of an induction motor is very low during startup. The power factor is the ratio of real power to apparent power, indicating the efficiency of the load. During startup, since the rotor is not yet rotating, the phase difference between the magnetic field and the current is large, resulting in a low power factor. A low power factor means that most of the current is used to generate the magnetic field rather than doing actual work, thus leading to a high starting current.

3. Low Back EMF

Back EMF (Counter EMF):

• In normal operation, the rotating rotor generates a back EMF (counter EMF) that opposes the source voltage, reducing the current. However, during startup, the rotor is not yet rotating, so the back EMF is almost zero. As a result, the full source voltage is applied to the stator windings, causing a significant increase in current.

4. Motor Impedance Characteristics

Motor Impedance:

• The impedance of an induction motor is low during startup. At the beginning of the startup, the rotor speed is zero, and the induced EMF in the rotor windings is also very low, making the impedance of the rotor windings low. Low impedance means that more current can flow through the windings, leading to a higher starting current.

5. Electromagnetic Induction Principle

Electromagnetic Induction:

• According to Faraday's law of electromagnetic induction, when the current in the stator windings changes, it induces a current in the rotor. During startup, since the rotor is not yet rotating, the rate of change of the magnetic field produced by the stator is the highest, leading to the highest induced current in the rotor. These induced currents further increase the starting current.

6. Grid Characteristics

Grid Characteristics:

• The power grid has limited capability to handle high currents over a short period. When an induction motor starts, the high current can cause a significant voltage drop, affecting the operation of other devices on the same grid.

Summary

An induction motor draws a high current during startup for the following reasons:

1. High Starting Torque Requirement: A large amount of current is needed to generate sufficient torque.

2. Low Power Factor: During startup, the power factor is low, and most of the current is used to generate the magnetic field.

3. Low Back EMF: During startup, the back EMF is almost zero, and the full source voltage is applied to the stator windings.

4. Motor Impedance Characteristics: The motor impedance is low during startup, leading to a higher current.

5. Electromagnetic Induction Principle: The rate of change of the magnetic field is the highest during startup, leading to the highest induced currents in the rotor.

To reduce the starting current, various starting methods can be used, such as star-delta starting, autotransformer starting, soft starters, and variable frequency drives (VFDs).