Why does an induction motor start without using a starter?

Induction motors (Induction Motors) typically require a starter to control the starting process, ensuring that the motor starts safely and smoothly. However, some small induction motors can be started directly without a dedicated starter. Here are the main reasons and explanations:

1. Direct-On-Line Starting (DOL)

Definition: Direct-on-line starting is the simplest starting method, where the motor is directly connected to the power supply and starts immediately at full voltage.

Applicability: This method is suitable for small induction motors, especially those with low starting current and starting torque requirements.

Advantages:

Simplicity: The circuit is simple and cost-effective.

Reliability: There are no complex control circuits, ensuring high reliability.

Disadvantages:

High Starting Current: The starting current can reach 5-7 times the rated current, potentially causing a voltage drop in the power grid, affecting the normal operation of other devices.

Mechanical Shock: The high starting current can cause significant mechanical shock, potentially shortening the life of the motor and mechanical equipment.

2. Characteristics of Small Motors

Low Inertia: Small motors have low inertia, so the mechanical shock during starting is relatively small, and the motor and load can more easily withstand it.

Low Starting Torque: Small motors typically require low starting torque, resulting in less mechanical stress during the starting process.

Low Starting Current: Although the starting current is still high, the impact on the power grid is relatively small due to the low power of the motor.

3. Grid Capacity

Grid Capacity: In situations where the power grid has a large capacity, even though small motors generate significant starting currents, the grid can handle it without causing significant voltage drops.

Other Equipment: If other devices on the same power grid are not sensitive to voltage fluctuations or are few in number, the direct starting of small motors will not cause noticeable impacts.

4. Load Characteristics

Light Load Starting: If the motor starts under a light load, the mechanical and current shocks during the starting process are reduced, allowing the motor to start directly without a starter.

Soft Starting Requirement: For loads that require soft starting, even small motors may need a starter to smooth the starting process and reduce mechanical and current shocks.

5. Safety and Protection

Overload Protection: Even with direct starting, small motors are typically equipped with overload protection devices (such as thermal relays) to prevent overloading and overheating.

Short-Circuit Protection: Circuit breakers or fuses can provide short-circuit protection, ensuring the motor operates safely during starting and running.

Summary

Small induction motors can be started directly without a dedicated starter primarily because their starting current and starting torque are relatively low, their impact on the power grid is limited, and the mechanical shock is minimal. However, for larger motors or applications with special starting requirements, using a starter is still necessary to ensure the motor starts safely and smoothly.