We all know that the current flowing through the three-phase stator windings of an electric motor generates a rotating magnetic field. According to the principle of magneto-electric induction, an induced electromotive force (EMF) is generated on the motor casing. The magnitude of this EMF depends on the switching frequency of the IGBT in the frequency converter. High-performance control requires a high switching frequency, resulting in a fast switching speed and a larger DV/DT ratio. Consequently, the induced EMF is also larger, causing a sensation of electric shock when touched. Theoretically, the faster the IGBT switching speed, the higher the induced EMF on the motor casing, and the higher the control precision and response of the frequency converter. This results in a stronger electric shock sensation when touched. Conversely, a slower IGBT switching frequency results in a smaller induced EMF and a weaker sensation when touched. Therefore, low-end frequency converters in China are designed with lower switching frequencies, resulting in a smaller induced EMF after controlling the motor, making it difficult to feel anything when touched. However, they have poorer controllability and a slower dynamic response.
The inverter output is controlled by PWM (Pulse Width Modulation, similar to high-speed switching), which results in high-frequency leakage current. The following factors determine the magnitude of the system leakage current and determine the appropriate leakage circuit breaker and necessary measures to mitigate the tripping of the leakage circuit breaker after power is supplied. First, the PE points of the inverter, the upstream power supply transformer, and the motor must be connected together before being grounded uniformly.
Inverters output high-frequency square wave voltage. Due to the existence of an equivalent capacitance between the internal coils and the motor casing, leakage current is generated. If the motor is not grounded or the grounding is poor, leakage will occur.
Factors affecting the magnitude of leakage current include:
(1) Leakage current of the cable (in two parts)
Leakage current of cables for residual current circuit breakers and filters. Leakage current of cables for frequency converters and motors.
(2) Leakage current of the filter (including the frequency converter).
(3) Leakage current of the motor.
In some field applications where frequency converters are used to control motors, leakage current issues may occur, with leakage voltages ranging from tens to two hundred volts. The causes of this phenomenon are analyzed below:
According to the functional block diagram of the inverter controlling the motor operation (above), the three-phase power supply is rectified by the inverter rectifier bridge, filtered by the capacitor, and then sent to the inverter bridge (IGBT). The inverter bridge outputs three-phase AC power with adjustable frequency and voltage to control the operation of the motor.
Alternating current with a phase difference of 120 degrees flows through the three-phase stator windings of the motor, generating a rotating magnetic field, which causes the motor rotor to rotate automatically under the influence of the rotating magnetic field of the stator windings.
When current flows through the three-phase stator windings of an electric motor, a rotating magnetic field is generated. According to the principle of electromagnetic induction, an induced electromotive force (EMF) is generated in the motor's casing. The magnitude of this induced EMF depends on the switching frequency of the inverter's IGBTs and C×DV/DT (related to the IGBT's switching speed). If this induced EMF is large, touching it will feel like an electric shock. Theoretically, the higher the IGBT switching frequency, the higher the effective value (induced voltage) of the induced EMF in the motor casing, and the higher the inverter's control precision and dynamic response to the motor, resulting in a stronger electric shock sensation upon contact. Conversely, the lower the IGBT switching frequency, the lower the effective value (induced voltage) of the induced EMF in the motor casing, and the weaker the electric shock sensation upon contact.
Because asynchronous motors generate induced voltage (i.e., leakage current) on their casings, motor manufacturers install grounding terminals in the junction boxes of motors at the factory. This allows users to connect the motors to the ground during operation to eliminate the induced electromotive force (i.e., eliminate induced leakage voltage) and prevent the feeling of electric shock when a person comes into contact with the motor.
When a motor operates at mains frequency, the switching frequency is approximately 50Hz, which is very low. Therefore, under normal circumstances, there is almost no feeling of leakage current (unless the motor insulation is very poor). However, when controlled by a frequency converter, because its switching frequency is much higher than the mains frequency, the motor casing will feel like it is leaking current when the frequency converter controls the motor to rotate.
