How To Choose The Motor Starting Method

Choosing the right starting method for a motor is a crucial engineering issue, primarily depending on grid capacity, motor type, load characteristics, and cost. Different starting methods exhibit significant differences in starting current, torque characteristics, and smoothness.

Step 1: Determine the Motor Type

Squirrel-cage asynchronous motor: The most common type, offering the widest range of starting methods (full voltage, star-delta, soft start, frequency converter).

Wound-rotor asynchronous motor: Typically starts using a rotor series resistor or a frequency-sensitive rheostat to achieve high starting torque.

Synchronous motor: Usually uses frequency converter starting or asynchronous starting (damped winding).

Step Two: Key Decision-Making Basis

1. Power Grid Capacity
This is the core factor determining whether direct start is possible.

If the transformer is large enough, direct start has minimal impact on the power grid, so direct start is sufficient.

If the transformer is small and the motor is large, reduced-voltage start or soft start must be used; otherwise, the voltage drop at startup will cause the entire workshop's equipment to shut down.

2. Load Type

Light Load: Fans, Pumps: These have high power but no special requirements for the start-stop process. -Star-delta or soft start are options.

Heavy Load: Conveyor Belts, Mechanical Handling Equipment: To avoid severe impact on gears and chains during startup. -High starting torque is required; direct start, rotor resistance, or frequency converter start are options.

Constant Torque Load: Air Compressors, Extruders. -Frequency converter start is the best choice.

3. Motor Power

Low power (typically < 5.5kW or 7.5kW): Direct start. Low cost, simple protection.

Medium power (11kW ~ 90kW): Star-delta (light load) or soft starter (light/medium load).

High power (> 90kW or 110kW): Soft starter or frequency converter. High-voltage motors are usually directly equipped with a high-voltage starter cabinet or frequency converter.

4. Process Requirements
Is speed regulation required? If speed regulation is required during production, such as for conveyor belts, elevators, and CNC machine tools, only frequency converters can be selected.

Is smooth starting required? If there are strict requirements for mechanical shock, such as for cableways transporting tourists or precision equipment, avoid using star-delta (due to secondary inrush current); choose soft starters or frequency converters.

Is frequent starting required? Frequent starting will cause the motor to overheat. For frequent starting applications, frequency converters are recommended to reduce thermal shock.