Analysis of Starting Methods for Squirrel-Cage Three-Phase Asynchronous Motors

Squirrel-cage three-phase asynchronous motors are widely used in equipment such as pumps, blowers, conveying machinery, agricultural machinery, and mining machinery, where there are no special requirements for slip or other performance characteristics. During startup, motors generate a significant inrush current. Typically, direct starting current is 4 to 8 times the rated current, and can reach up to 10 times the rated current at maximum. If the starting method is chosen improperly, it can not only prevent the driven motor from starting and operating but also cause impacts on the control circuit and other electrical equipment, potentially damaging the power grid and other components in the control circuit. Therefore, constrained by grid capacity and the need to protect other electrical devices, one of three different starting methods—direct starting, reduced-voltage starting, or variable frequency starting—must be selected based on motor capacity, supply line requirements, and the impact tolerance of the equipment.

Additionally, to meet operational requirements, speed regulation of the motor is necessary. There are two main speed control methods for squirrel-cage three-phase asynchronous motors: variable frequency speed control and pole-changing speed control. Variable frequency speed control uses a frequency converter to change the motor's speed by altering the power supply frequency, offering stepless speed regulation. Pole-changing speed control adjusts speed by altering the number of pole pairs in the motor's stator winding, providing stepped speed regulation. 

The following sections elaborate on and analyze issues that arise with the three commonly used starting methods and two speed control methods for squirrel-cage three-phase asynchronous motors.

1. Direct Starting of Single-Speed Squirrel-Cage Three-Phase Asynchronous Motors

Direct starting involves applying the rated voltage directly to the motor's stator winding via a knife switch or circuit breaker, allowing the motor to start under full voltage. This method is suitable for small-capacity motors or motors whose capacity is significantly less than that of the supply transformer.

During direct starting, the voltage drop caused in the grid should be within 10% to 15% of the grid's rated voltage. Whether a low-voltage squirrel-cage three-phase asynchronous motor can be started direct-on-line depends on grid capacity, starting frequency, permissible line disturbance levels, and the motor's installation type. When powered by a dedicated independent transformer, the rated power of infrequently started motors should not exceed 30% of the transformer capacity, while for frequently started motors, it should not exceed 20%. When powered by a public low-voltage grid, motors up to 11 kW can generally be direct-started; for those supplied from a residential substation's low-voltage distribution panel, the limit is typically 5 kW.

The main issue with direct starting is the excessively high inrush current, which can trip the control circuit and prevent the motor from starting. Sometimes, due to high line voltage and inherently high motor starting current, the inrush can exceed 10 times the rated current. To address this, protection device parameters in the motor control system must be set appropriately to avoid activation during start-up. Often, the setting of the overcurrent relay is too low, causing unnecessary trips. Typically, the relay should be set to about 1.5 times the motor's starting current.

The starting current is essentially a fixed manufacturing parameter of the motor. Higher voltage during start leads to higher starting current, and vice versa. A heavier load during start results in a longer starting time, prolonged high current, excessive motor heating (potentially causing burnout), and a longer period of low system voltage. The large starting current increases voltage drop across system impedance, lowering the voltage at the system end. Therefore, excessive system voltage drop must be prevented.

As long as the motor's starting torque exceeds the load torque, the motor will eventually reach rated speed. However, the starting process can introduce adverse effects: 1) The high starting current causes significant system voltage drop, affecting the normal operation of the motor and other connected equipment. 2) Prolonged starting time leads to overheating of the motor and related electrical components, potentially causing insulation breakdown and performance degradation.

2. Common Reduced-Voltage Starting Methods for Squirrel-Cage Three-Phase Asynchronous Motors

If the acceleration torque and impact current from direct starting significantly affect the driven machinery or power supply system, reduced-voltage or reduced-frequency starting should be considered. Besides limiting starting current, these methods also aim to reduce starting torque to minimize mechanical shock and ensure smooth acceleration.

Common reduced-voltage starting methods include:

   ● Star-Delta (Y-Δ) Starting: For motors designed to run with a delta-connected stator winding, the winding is first connected in star during start-up and then switched to delta after starting. This reduces starting current and grid impact. Common issues include: excessively high starting current causing protection trips (potentially due to high starting torque in star mode or wiring errors preventing proper starting, leading to a direct delta start), and successful star start but trip upon switching to delta (often due to incorrect delta connection).

   ● Autotransformer Starting: Suitable for starting under heavier loads. It reduces both starting current and starting torque by the same ratio. For a given starting torque, the starting current is lower compared to other methods. Conversely, for a given starting current, a higher starting torque can be achieved, making it versatile for different load types.

   ● Soft Starter Starting: Utilizes the phase-angle control principle of thyristors to provide voltage-controlled starting. Parameters are adjusted based on the motor's load characteristics, enabling smooth, impact-free acceleration through voltage reduction and current limiting. Widely used in large units and critical applications. Soft starters protect the driven system, ensure reliable starting, reduce starting冲击, and often include computer communication interfaces for intelligent control. They can operate continuously under rated load or be bypassed by a contactor after starting (see Figure 1). When starting multiple motors with one soft starter, its capacity should be slightly greater than or equal to the total rated capacity of the motors.

3. Variable Frequency Starting and Speed Control for Squirrel-Cage Three-Phase Asynchronous Motors

Using a frequency converter for starting and speed control is feasible for single-speed motors, but frequency operation is limited—it cannot be too high or too low. For prolonged operation at low or high frequencies, a dedicated inverter-duty motor is required. The combination forms a variable frequency drive (VFD) system.

A VFD uses power semiconductors to convert mains frequency power to a variable frequency/variable voltage supply. It represents advanced technology in motor control, offering comprehensive functionality and superior performance. It provides not only starting capability but also stepless speed regulation during operation, enabling energy savings.

During variable frequency starting, the motor can start with high torque while the starting current is significantly lower than with direct starting. This reduces electromagnetic stress on the stator winding ends and extends motor life. VFD-based control systems represent the development trend for starting, speed control, and system energy efficiency in low-voltage squirrel-cage motors.

A VFD-controlled motor typically starts at low frequency, accelerating smoothly as frequency and voltage increase (resulting in a longer, controlled start time). Figure 2 shows the torque-speed characteristics during starting, constant torque operation, and constant power operation for a VFD-started motor. Preset starting modes can utilize the motor's maximum torque to meet acceleration/deceleration demands. VFD capacity is generally selected at 1.1 times the motor's rated capacity.

4. Tables 1 and 2 below compare the performance and selection characteristics of common starting methods for squirrel-cage three-phase asynchronous motors.

Table 1: Comparison of Common Starting Methods for Squirrel-Cage Three-Phase Asynchronous Motors

Table 2: Comparison of Common Reduced-Voltage Starting Methods

5. Summary

For single-speed three-phase asynchronous motors, safe starting is a prerequisite for reliable operation. Considering maintenance cost and operational simplicity, direct full-voltage starting is the first choice. If direct starting is not permissible, reduced-voltage starting should be selected.

For applications requiring speed control, pole-changing offers lower cost and simpler control but cannot provide stepless regulation and may have current surges during pole switching. For applications requiring high precision speed control, variable frequency speed control is preferable. With advancements in VFD technology, it has become the mainstream method for AC motor speed control. Inverter-duty motors controlled by VFDs offer smooth starting, combined speed regulation and energy-saving advantages, leading to increasingly widespread application.