Frequency Inverters are using high- speed switching devices for PWM control.
When a relatively long cable is used for power supply to an inverter , current may leak from the cable or the motor to the ground , because of its capacitance,adversely affecting peripheral equipment.The intensity of such a leakage current depends on the PWM carrier frequency, the lengths of the input and output cables,etc. , of the inverter, and it is related as by product to the nature of DC chopped voltage fed to the AC motor.
To prevent current leakage , it is recommended to use pure sine wave voltage inverters as NFO Sinus from NFO Drives AB
or to take the following measures , as a part solution.
Types of leakage current
1- Leakage due to the capacitance between the ground and the noise filter (EMC filter)
2- Leakage due to the capacitance between the ground and the inverter itself
3- Leakage due to the capacitance between the ground and the cable connecting the inverter and the motor.
4- Leakage due to the capacitance of the cable connecting the motor and any inverter in another power distribution line .
5- Leakage due to the grounding line common to the motors.
6- Leakage to another line because of the capacitance of the ground.
Those mentioned above leakage currents may cause the following troubles
Malfunction of a leakage circuit breaker in the same or another power distribution line
Malfunction of a ground-relay , installed in the same or another power distribution line.
Noise produced at the output of an electronicdevice in another power distribution line
Activation of an external thermal relay installed between the inverter and the motor , at a current below the rate current value.
Measures to eliminate the leakage currents or to reduce their effectsThe best solution to eliminate the leakage currents is to feed the AC motor with pure sine wave voltage , but additional measures can be used against the effects of leakage currents as follows:
1- Measures to prevent the malfunction of leakage cicuit breakers
(1)-Decrease the PWM carrier frequency of the inverter, but that results in increasing the harmonic content in the output voltage signal fed to the motor and will last in additional motor heating.
(2)-Use radio- frequency interference- proof earth leakage circuit breaker( ELCBs) as ground-fault interrupters , not only in the system into which the inverter is incorporated but also in other systems ( that is expensive) . When ELCBs are used, the PWM carrier frequency needs to be increased to operate the PWM frequency inverter – this measure is in opposite to (1) mentioned above , so it is necessary to choose optimal carrier switching frequency !!
(3)- When connecting multiple inverters to a single ELCB, use an ELCB with a high current sensitivity ( expensive) or reduce the number of inverters connected to the ELCB.
2- Measures against malfunction of ground – fault relay :
(1) decrease the PWM carrier frequency of the inverter (results in additional heating of the driven motor)
(2) Install ground-fault relays with a high – frequency protective function ( means – grounding wire of each system separately to the grounding point).
(3) Ground ( shield) the main cicuit wires with metallic conduits – additional cost
(4) Use the shortest possible cables to connect the inverter to the motor , other wise it needs dV/dt , or sine wave filter
(5) If the inverter has a high-attenuation EMI filter ,turn off the grounding capacitor detachment switch to reduce the leakage current – Doing that leads to a reduction in the noise attenuating effect .
Ground fault
Before beginning operation , thoroughly check the wiring between the motor and the inverter for incorrect wiring or short circuit. Do not ground the neutral point of any star- delta connected motor.
Radio Interference ( noise produced by inverters)
All frequency converters PWM types produce noise ( NFO Sinus from NFO Drives AB exception – it is pure sine wave voltage output) and sometimes affects near by instrumental devices ,electrical and electronic systems, etc. The effects of noise greatly vary with the noise resistance of each individual device ( p.s EN 61000-6-3 directive for using electrical equipment in residential , commercial and light industry environment, EN61000-6-2 in industrial environment , EN 60601-1-2 in hospitals), its wiring condition, the distance between it and the inverter , etc.
Measures against noises
According to the route through which noise is transmitted, the noises produced by PWM frequency converter are classified into transmission noise , induction noise and radiation noise.
Examples of protective measures
separate the power line from other lines, such as weak- current lines signal lines , and install them apart from each other.
Install a noise ( EMC) filter in each inverter. It is effective for noise preventation to install noise filters in other devices and systems , as well. But the best is ti include the noise (EMC) filter inside the inverter itself.
Shield cables and wires with grounded metallic conduits ( expensive , not needed with NFO sinus inverter), and cover electronic systems with grounded metallic cases.
Separate the power distribution line of the inverter from that of other devices and systems.
Install the input and output cables of the inverter apart from each other.
Use shielded twisted pair wires for wiring of the weak- current and signal circuits,and always ground one of each pair wires.
Ground the inverter with grounding wires as large and short as possible, separately from other devices and systems
It is recommended to have built in noise ( EMC) filters ,which significantly reduce noise.
Power Factor improvement capacitors
It is not recommended to install power factor improvement capacitors on the input or output of the inverter .
Installing a power factor improvement capacitor on the input or output side causes current containing harmonic components to flow into the capacitor, adversely affecting the capacitor itself or causing the inverter to trip.To improve the power factor,install an input AC reactor or a DC reactor on the primary of the PWM frequency converter.
Installation of input reactors
These devices are used to improve the input power factor and suppress high harmonic currents and surges. Install an input AC reator when using frequency converter under the following conditions:
(1) When the power source capacity is at least 10 times or more greater than the frequency converter capacity .
(2) When the PWM (FC)frequency converter is connected to the same power distribution system as a thyristor – committed control equipment.
(3) When the FC is connected to the same power distribution system as that of distorted wave-producing system, such as arc furnaces and large-capacity frequency converters .
It is not recommended to install power factor improvement capacitors on the input or output of the inverter .
Installing a power factor improvement capacitor on the input or output side causes current containing harmonic components to flow into the capacitor, adversely affecting the capacitor itself or causing the inverter to trip.To improve the power factor,install an input AC reactor or a DC reactor on the primary of the PWM frequency converter.
Installation of input reactors
These devices are used to improve the input power factor and suppress high harmonic currents and surges. Install an input AC reator when using frequency converter under the following conditions:
(1) When the power source capacity is at least 10 times or more greater than the frequency converter capacity .
(2) When the PWM (FC)frequency converter is connected to the same power distribution system as a thyristor – committed control equipment.
(3) When the FC is connected to the same power distribution system as that of distorted wave-producing system, such as arc furnaces and large-capacity frequency converters .
