I. Phenomenon
  When using a speed-regulation system with a frequency converter, especially in some special application environments, it will be found that the motor has a "leakage" phenomenon. Some may even generate a static voltage of more than 100V. Such a high static voltage can cause a stinging sensation, so that people may mistakenly think that the motor is "leaking electricity". These special application environments include: relatively high air humidity, a relatively low insulation grade of the motor, and a relatively large grounding resistance of the motor.
II. Causes
  The control principle of most frequency converters mainly adopts the Pulse-Width Modulation (PWM) control method. To ensure the load responsiveness and smooth operation of the variable-frequency speed-regulation system and, at the same time, to reduce the high-frequency noise generated when the motor is running, the frequency of the modulation wave generally reaches 4KHz or more. Especially for small- and medium- power frequency converters, because the output current is relatively small, the carrier frequency can reach more than 10KHz. Therefore, the voltage output by the frequency converter is actually a series of pulses with different widths. In addition, in some high-performance current-vector frequency converters, such as the Yaskawa G-series frequency converters, Yolico YD-series frequency converters, Danfoss VLT5000-series frequency converters, etc., these frequency converters are connected with large DC-filtering capacitors, which makes the rising and falling edges of the output pulses relatively steep.Since there is an equivalent capacitance between the motor coil and the motor case, a certain voltage is induced on the motor case. However, the induced voltage is different from the leakage voltage. Although the induced voltage may be relatively high, the induced current is usually relatively small and will not cause particularly serious consequences. Leakage, on the other hand, occurs when the insulation of the motor is damaged, the insulation resistance between the coil and the case becomes very small, and the output voltage directly passes through this reduced insulation resistance, making the motor case electrified and endangering personal safety.
III. Measures
  According to the national electrical installation safety regulations, the motor must be grounded safely and reliably. In a three-phase four-wire power supply system, the neutral wire (N) must be grounded repeatedly at the connection point in the workshop, and the grounding resistance should be less than 4 ohms.
  Similarly, in a speed-regulation system using a frequency converter, the grounding of the motor is necessary. This can not only provide a release circuit for the induced voltage but also protect against leakage accidents caused by insulation faults of the motor. It can also reduce the interference of the frequency converter on other electrical equipment, enabling all electrical equipment to operate safely and reliably.