Methods to Control Surge Current in AC Motors

1. Utilize Component Characteristics

• Inductive Components: Inductors impede the rate of change in current flow, reducing current peaks. For AC motors, inductors connected in series in the circuit can suppress surge currents. When the current suddenly increases, the self-induced electromotive force generated by the inductor resists the rapid rise in current, thereby reducing the magnitude and duration of the surge current. For example, this method is often used in the starting circuits of large AC motors to protect circuit components from the impact of surge currents.

• Capacitive Components: Capacitors can store energy. By selecting the appropriate capacitance value, electrical energy can be stored in the capacitor and discharged slowly. When a capacitor is connected in parallel with the motor circuit in an AC motor circuit, it can act as a buffer, absorbing some of the electrical energy at the moment the circuit is turned on to prevent excessive current from flowing directly through the motor, thereby reducing peak voltage and peak current and achieving the purpose of controlling surge current.

• Negative Temperature Coefficient (NTC) Thermistor: When no current is flowing, the NTC resistor has a high value. When powered, the high resistance allows a small amount of current to pass, which initiates self-heating, causing its own resistance to decrease and gradually allowing more current to flow through the load. By placing an NTC thermistor in series with the starting circuit of an AC motor, its characteristics can be utilized to limit the surge current at startup. However, NTC performance is dependent on ambient temperature, making it less suitable for applications with a wide range of operating temperatures.

2. Adopt circuit control technology

• Switching Rate Control: Directly control the rate of voltage rise at the output by controlling the rate at which switches are turned on. For AC motors, reducing the switching speed (dVout/dt), with the motor load capacitance Cload fixed, will result in a decrease in inrush current Iinrush. This method effectively controls the inrush current when starting the motor.

• Linear Soft Start or dV/dt Control: Many integrated power switches feature a linear control output voltage rise time. For AC motors, linearly controlling the output voltage rise time (i.e., controlling a constant dVout/dt rate) ensures that Iinrush is also constant when Cload is constant. This allows for precise surge current calculation and may meet requirements in cases where a maximum surge current limit and maximum turn-on time are specified, especially if methods like RC time constant control do not suffice.

• Constant Current / Current Limit Regulation: When powering purely capacitive loads (which can be approximated as capacitive during motor startup), the constant current method of controlling inrush current will produce results similar to a linear soft starter. By using a constant Iinrush to charge the motor, for a given Cload, it will charge at a constant dv/dt, thereby controlling the inrush current. However, when introducing other loads in addition to the capacitor, it will differ from the linear soft starter method.

III. Using Special Circuit Components and Circuits

• TVS Diodes: TVS diodes are fast-responding suppressors. When the input voltage in an AC motor circuit exceeds a certain voltage, they provide a low-impedance path, momentarily absorbing a large amount of current to prevent overvoltage and thus avoid surge currents caused by overvoltage from damaging the motor and its circuit.

• Metal Oxide Varistor (MOV): Responsive to permanent fault voltage or momentary overvoltage. In AC motor circuits, it can suppress overvoltage by continuously existing with low resistance rate, thereby preventing surge current caused by overvoltage from harming the motor.

• Internal Power Suppression Circuit: This circuit suppresses surge currents by capturing them in the downstream lines. For example, on the circuit board where an AC motor is located, an internal power suppression circuit can be formed by setting up inductive components to suppress surge currents.

IV. Optimize Wiring Design

• When designing circuit boards or wiring related to AC motors, use a wiring method that counters surge currents. For example, arrange the board lines as parallel as possible and keep the distance between adjacent lines as consistent as possible. A reasonable wiring method helps reduce surge currents caused by factors such as electromagnetic interference, thereby controlling surge currents in AC motors to some extent.