Anti-jamming measures of inverter control loop

The inverter is composed of two parts: the main loop and the control loop. Because the main loop is nonlinear (switching action), the inverter itself is a harmonic interference source, so it will affect the equipment on the power supply side and the output side. Compared with the main loop, the control loop of the inverter is a small energy, weak signal loop, which is easily subject to interference from other devices, causing the inverter to fail to work. Therefore, when the inverter is installed and used, it must take anti-jamming measures on the control circuit.

1. The basic loop of the inverter has the analog and digital two basic circuits that communicate with the outside:

(1) 4 to 20mA current signal loop (analog); 1 to 5V/0 to 5V voltage signal loop (analog).

(2) Switch signal loop, inverter start/stop command, forward/reverse command, etc. (digital).

The external control command signal is introduced into the inverter through the above basic circuit, and the interference source also generates an interference potential on its loop to control the cable as the media invades the inverter.

2. Basic type of interference and anti-interference measures (1) Electrostatic coupling interference: refers to the electrostatic capacitance coupling between the control cable and the surrounding electrical circuit and the potential generated in the cable.

Measures:

- Increase the distance from the interference source cable. When the diameter of the conductor is more than 40 times, the degree of interference is not obvious.

- Set the shield conductor between the two cables and ground the shield conductor.

(2) Electrostatic Induction Disturbance: The potential induced in the cable by changes in the magnetic flux generated by the surrounding electrical circuit. The size of the interference depends on the amount of magnetic flux generated by the interference source cable, the closed loop area formed by the control cable, and the relative angle between the interference source cable and the control cable.

Measures:

*Generally separate the control cable from the main circuit cable or other power cable. The separation distance is usually more than 30cm (the minimum is 10cm). When the separation is difficult, the control cable is laid through the iron pipe.

* Twist the control conductors, the smaller the twist spacing, the shorter the laying route, the better the anti-jamming effect.

(3) Radio interference: It means that the control cable becomes an antenna, and the external incoming wave generates a potential in the cable.

Measures: Same as described in (1) and (2). If necessary, place the inverter in the iron box to shield the electric wave. The shielded iron box should be grounded.

(4) Bad contact interference: It refers to the electrical contacts of the control cable of the inverter and the faults caused by the poor contact of the relay and the resistance changes to occur in the cable.

Measures:

- Poor contact of relay contacts, using parallel contacts or gold-plated contact relays or sealed relays.

- Tighten and tighten the cable connection points periodically.

SSX Safety and EMC Networks (5) Power Line Conducted Interference: Refers to the fact that when various electrical equipment is supplied with power from the same power supply system, the other equipment generates the potential directly in the power supply system.

Measure: The inverter's control power is supplied by another system; a line filter is installed on the input side of the control power supply; an insulation transformer is installed and the shield is grounded.

(6) Grounding interference: refers to the body grounding and signal grounding. For the weak voltage current loop and any unreasonable grounding can induce all kinds of unexpected interference, such as setting more than two grounding points, the ground potential will produce a potential difference, resulting in interference.

Measures:

- The speed given control cable should be grounded at 1 point. The grounding wire should not be used as a signal path.

- The grounding of the cable is performed on the inverter side. Use a dedicated grounding terminal and do not share the grounding terminal. And to minimize the resistance of the ground terminal contact point, generally not more than 100d.

3. Other precautions (1) The control cabinet with the inverter should be far away from the large-capacity transformer and motor. The control cable lines should also avoid these large flux leakage devices.

(2) Weak voltage and current control cables should not approach circuit breakers and contactors that are prone to arcing.

(3) The control cable is recommended to use 1.25mm2 or 2mm2 shielded stranded insulated cable.

(4) The shielding of the shielded cable must be the same length as the cable conductor. When the cables are connected in the terminal box, the shielding terminals are connected to each other.