Some customers reported frequent tripping of the leakage protector when starting or running the Variable Frequency Drive (VFD). After multiple inspections, the customer identified the cause of the failure. Many people believed the VFD was the issue, but, in fact, it was not the key problem.
Working principle of leakage protection switch
The leakage protection switch detects the input common mode current, which is the leakage current to the ground. The current transformer detected by the leakage current passes through the R/S/T triple live and neutral wires simultaneously. This occurs regardless of multiple three-phase loads or single-phase loads. The sum of the currents flowing in the 4 rows of R/S/T and N wires is always zero. When there is a short circuit on the load side or a large capacitance on the ground, the current on the output side will return to the grid through the earth. At this time, the sum of the current transformers is not zero, which is called leakage current. When the detected current is too large to a certain extent, the protective switch is triggered to trip.
Analysis of causes of ground leakage current using VFD
Why does a large leakage current occur in applications?
There is a large distributed capacitance between the windings and the casing of ordinary motors. When powered by the power grid, there is only a 50Hz power frequency voltage on the power line. Because the frequency is very low, the leakage current through the distributed capacitance is very small. However, when using a VFD to drive a motor, since the VFD outputs a PWM (high-frequency pulse width modulation) voltage waveform of several kHz, the output voltage is a pulsating voltage that rapidly jumps between 0V and 530V. This pulsating voltage generates harmonics. , these harmonics will increase the leakage current by more than a hundred times for the same motor and the same distributed capacitance, so it is easy to cause the leakage switch to trip as soon as the frequency conversion is running.
The role of the safety capacitor at the VFD input end
The main function of the input-end safety capacitor is to reduce the interference effect of the VFD on the external power grid. The VFD is equipped with several sets of capacitor protection. This allows the VFD to withstand large voltage surges from the power grid, such as lightning strikes, without being damaged.
Given the small value of the safety capacitor in the VFD (4700PF), its impedance to power frequency is notably high (1.4M). Additionally, its contribution to leakage current is minimal—approximately 0.15mA per phase. The sum of the fundamental leakage currents, when the three phases are balanced, is zero.
However, if the voltage harmonics in the power grid are very high, the leakage current poured into the VFD from the power grid will increase significantly, and the three phases will not cancel out. The value of the leakage current is proportional to the frequency of the voltage harmonics, and is proportional to the frequency of the harmonic voltage. The amplitude is directly proportional. Therefore, it is easy for the leakage switch to trip as soon as it is closed.
The above leakage current may be much greater than 50mA, and the actual specific data will be related to the following factors:
1.The length of the motor cable;
2. Whether the motor cable is shielded;
3. VFD modulation frequency;
4. Whether to use radio frequency interference (RFI) filters;
5. Whether the motor is grounded.
Methods to reduce leakage current
Carrier frequency: The higher the carrier frequency, the greater the leakage current. The effective value of the leakage current has an approximately square root relationship with the frequency.
Output frequency: At very high output frequencies, the leakage current is not large. However, when the frequency approaches zero, the three-phase output leakage current of the VFD is superimposed, causing an increase in the effective value of the leakage current.
The influence of “zero-ground-in-one” wiring method on leakage current
The term “zero” refers to the PE terminal of the grid as the VFD. Since the housing’s leakage current returns to the grid through the neutral line, the leakage protection switch theoretically does not detect this partial leakage current. Theoretically, if the VFD and motor load are not connected to the shell (mainly the motor shell), the leakage current can be zero, but this is only theoretical. From a practical perspective, the leakage current cannot reach zero.
The function of the output reactor: adding a reactor at the output end of the VFDU/V/W increases the high-frequency impedance of the load and reduces the leakage current.
More: Do You Know What Protection Functions The Frequency Inverter Has?
Some customers reported frequent tripping of the leakage protector when starting or running the Variable Frequency Drive (VFD). After multiple inspections, the customer identified the cause of the failure. Many people believed the VFD was the issue, but, in fact, it was not the key problem.
Working principle of leakage protection switch
The leakage protection switch detects the input common mode current, which is the leakage current to the ground. The current transformer detected by the leakage current passes through the R/S/T triple live and neutral wires simultaneously. This occurs regardless of multiple three-phase loads or single-phase loads. The sum of the currents flowing in the 4 rows of R/S/T and N wires is always zero. When there is a short circuit on the load side or a large capacitance on the ground, the current on the output side will return to the grid through the earth. At this time, the sum of the current transformers is not zero, which is called leakage current. When the detected current is too large to a certain extent, the protective switch is triggered to trip.
Analysis of causes of ground leakage current using VFD
Why does a large leakage current occur in applications?
There is a large distributed capacitance between the windings and the casing of ordinary motors. When powered by the power grid, there is only a 50Hz power frequency voltage on the power line. Because the frequency is very low, the leakage current through the distributed capacitance is very small. However, when using a VFD to drive a motor, since the VFD outputs a PWM (high-frequency pulse width modulation) voltage waveform of several kHz, the output voltage is a pulsating voltage that rapidly jumps between 0V and 530V. This pulsating voltage generates harmonics. , these harmonics will increase the leakage current by more than a hundred times for the same motor and the same distributed capacitance, so it is easy to cause the leakage switch to trip as soon as the frequency conversion is running.
The role of the safety capacitor at the VFD input end
The main function of the input-end safety capacitor is to reduce the interference effect of the VFD on the external power grid. The VFD is equipped with several sets of capacitor protection. This allows the VFD to withstand large voltage surges from the power grid, such as lightning strikes, without being damaged.
Given the small value of the safety capacitor in the VFD (4700PF), its impedance to power frequency is notably high (1.4M). Additionally, its contribution to leakage current is minimal—approximately 0.15mA per phase. The sum of the fundamental leakage currents, when the three phases are balanced, is zero.
However, if the voltage harmonics in the power grid are very high, the leakage current poured into the VFD from the power grid will increase significantly, and the three phases will not cancel out. The value of the leakage current is proportional to the frequency of the voltage harmonics, and is proportional to the frequency of the harmonic voltage. The amplitude is directly proportional. Therefore, it is easy for the leakage switch to trip as soon as it is closed.
The above leakage current may be much greater than 50mA, and the actual specific data will be related to the following factors:
1.The length of the motor cable;
2. Whether the motor cable is shielded;
3. VFD modulation frequency;
4. Whether to use radio frequency interference (RFI) filters;
5. Whether the motor is grounded.
Methods to reduce leakage current
Carrier frequency: The higher the carrier frequency, the greater the leakage current. The effective value of the leakage current has an approximately square root relationship with the frequency.
Output frequency: At very high output frequencies, the leakage current is not large. However, when the frequency approaches zero, the three-phase output leakage current of the VFD is superimposed, causing an increase in the effective value of the leakage current.
The influence of “zero-ground-in-one” wiring method on leakage current
The term “zero” refers to the PE terminal of the grid as the VFD. Since the housing’s leakage current returns to the grid through the neutral line, the leakage protection switch theoretically does not detect this partial leakage current. Theoretically, if the VFD and motor load are not connected to the shell (mainly the motor shell), the leakage current can be zero, but this is only theoretical. From a practical perspective, the leakage current cannot reach zero.
The function of the output reactor: adding a reactor at the output end of the VFDU/V/W increases the high-frequency impedance of the load and reduces the leakage current.
More: Do You Know What Protection Functions The Frequency Inverter Has?
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