7.7.1 Interference on meter switches and sensors

Interference phenomenon:

Pressure, temperature, displacement, and other signals of a sensor are collected and displayed by a human-machine interaction device. The values are incorrectly displayed as follows after the VFD is started:

1.    The upper or lower limit is wrongly displayed, for example, 999 or -999.

2.    The display of values jumps (usually occurring on pressure transmitters).

3.    The display of values is stable, but there is a large deviation, for example, the temperature is dozens of degrees higher than the common temperature (usually occurring on thermocouples).

4.    A signal collected by a sensor is not displayed but functions as a drive system running feedback signal. For example, a VFD is expected to decelerate when the upper pressure limit of the compressor is reached, but in actual running, it starts to decelerate before the upper pressure limit is reached.

5.    After a VFD is started, the display of all kinds of meters (such as frequency meter and current meter) that are connected to the analog output (AO) terminal of the VFD is severely affected, displaying the values incorrectly.

6.    Proximity switches are used in the system. After a VFD is started, the indicator of a proximity switch flickers, and the output level flips.

Solution:

1.    Check and ensure that the feedback cable of the sensor is 20 cm or farther away from the motor cable.

2.    Check and ensure that the ground wire of the motor is connected to the PE terminal of the VFD (if the ground wire of the motor has been connected to the ground block, you need to use a multimeter to measure and ensure that the resistance between the ground block and PE terminal is lower than 1.5 Ω).

3.    Try to add a safety capacitor of 0.1 μF to the signal end of the feedback signal terminal of the sensor.

4.    Try to add a safety capacitor of 0.1 μF to the power end of the sensor meter (pay attention to the voltage of the power supply and the voltage endurance of the capacitor).

5.    For interference on meters connected to the AO terminal of a VFD, if AO uses current signals of 0 to 20 mA, add a capacitor of 0.47 μF between the AO and GND terminals; and if AO uses voltage signals of 0 to 10 V, add a capacitor of 0.1 μF between the AO and GND terminals.

Note:

1.    When a decoupling capacitor is required, add it to the terminal of the device connected to the sensor. For example, if a thermocouple is to transmit signals of 0 to 20 mA to a temperature meter, the capacitor needs to be added on the terminal of the temperature meter.; if an electronic ruler is to transmit signals of 0 to 30 V to a PLC signal terminal, the capacitor needs to be added on the terminal of the PLC.

2.    If a large number of meters or sensors are disturbed. It is recommended that you configure an external C2 filter on the input power end of the VFD. For models of filters, see section D.7 "Filters".

7.7.2 Interference on communication

Interference phenomenon

The interference described in this section on 485 communication mainly includes communication delay, out of sync, occasional power-off, or complete power-off that occurs after a VFD is started.

If the communication cannot be implemented properly, regardless of whether the VFD is running, the exception is not necessarily caused by interference. You can find out the causes as follows:

1.  Check whether the 485 communication bus is disconnected or in poor contact.

2.  Check whether the two ends of line A or B are connected reversely.

3.  Check whether the communication protocol (such as the baud rate, data bits, and check bit) of the VFD is consistent with that of the upper computer.

If you are sure that communication exceptions are caused by interference, you can resolve the problem through the following measures:

1.  Simple inspection.

2.  Arrange the communication cables and motor cables in different cable trays.

3.  In multi-VFD application scenarios, adopt the chrysanthemum connection mode to connect the communication cables between VFDs, which can improve the anti-interference capability.

4.  In multi-VFD application scenarios, check and ensure that the driving capacity of the master is sufficient.

5.  In the connection of multiple VFDs, you need to configure one 120 Ω terminal resistor on each end.

Solution:

1.    Check and ensure that the ground wire of the motor is connected to the PE terminal of the VFD (if the ground wire of the motor has been connected to the ground block, you need to use a multimeter to measure and ensure that the resistance between the ground block and PE terminal is lower than 1.5 Ω).

2.    Do not connect the VFD and motor to the same ground terminal as the upper computer. It is recommended that you connect the VFD and motor to the power ground, and connect the upper computer separately to a ground stud.

3.    Try to short the signal reference ground terminal (GND) of the VFD with that of the upper computer controller to ensure that ground potential of the communication chip on the control board of the VFD is consistent with that of the communication chip of the upper computer.

4.    Try to short GND of the VFD to its ground terminal (PE).

5.    Try to add a safety capacitor of 0.1 μF on the power terminal of the upper computer (PLC, HMI, and touch screen). During this process, pay attention to the voltage of the power supply and the voltage endurance capability of the capacitor. Alternatively, you can use a magnet ring (Fe-based nanocrystalline magnet rings are recommended). Put the power L/N line or +/- line of the upper computer through the magnet ring in the same direction and wind 8 coils around the magnet ring.

7.7.3 Failure to stop and indicator shimmering due to motor cable coupling

Interference phenomenon:

1.    Failure to stop

In a VFD system where an S terminal is used to control the start and stop, the motor cable and control cable are arranged in the same cable tray. After the system is started properly, the S terminal cannot be used to stop the VFD.

2.    Indicator shimmering

After a VFD is started, the relay indicator, power distribution box indicator, PLC indicator, and indication buzzer shimmers, blinks, or emits unusual sounds unexpectedly.

Solution:

(1)   Check and ensure that the exception signal cable is arranged 20 cm or farther away from the motor cable.

(2)   Add a safety capacitor of 0.1 μF between the digital input terminal (S) and the COM terminal.

(3)   Connect the digital input terminal (S) that controls the start and stop to other idle digital input terminals in parallel. For example, if S1 is used to control the start and stop and S4 is idle, you can try to connect connect S1 to S4 in parallel.

Note: If the controller (such as PLC) in the system controls more than 5 VFDs at the same time through digital input terminals (S), this scheme is not available.

7.7.4 Leakage current and interference on RCD

VFDs output high-frequency PWM voltage to drive motors. In this process, the distributed capacitance between the internal IGBT of a VFD and the heat sink and that between the stator and rotor of a motor may inevitably cause the VFD to generate high-frequency leakage current to the ground. A residual current operated protective device (RCD) is used to detect the power-frequency leakage current when a grounding fault occurs on a circuit. The application of a VFD may cause misoperation of a RCD.

1.    Rules for selecting RCDs

(1)   VFD systems are special. In these systems, it is required that the rated residual current of common RCDs at all levels is larger than 200 mA, and the VFDs are grounded reliably.

(2)   For RCDs, the time limit of an action needs to be longer than that of a next action, and the time difference between two actions need to be longer than 20 ms. For example, 1s, 0.5s, and 0.2s.

(3)   For circuits in VFD systems, electromagnetic RCDs are recommended. Electromagnetic RCDs have strong anti-interference capability, and thus can prevent the impact of high-frequency leakage current.

2.    Solution to RCD misoperation (handling the VFD)

(1)  Try to remove the jumper cap at "EMC/J10" on the middle casing of the VFD.

(2)  Try to reduce the carrier frequency to 1.5 kHz (P00.14=1.5).

(3)  Try to modify the modulation mode to "3PH modulation and 2PH modulation" (P8.40=0).

3.    Solution to RCD misoperation (handling the system power distribution)

(1)   Check and ensure that the power cable is not soaking in water.

(2)   Check and ensure that the cables are not damaged or spliced.

(3)   Check and ensure that no secondary grounding is performed on the neutral wire.

(4)   Check and ensure that the main power cable terminal is in good contact with the air switch or contactor (all screws are tightened).

(5)   Check 1PH powered devices, and ensure that no earth lines are used as neutral wires by these devices.

(6)   Do not use shielded cables as VFD power cables and motor cables.

7.7.5 Live device chassis

Phenomenon

After a VFD is started, there is sensible voltage on the chassis, and you may feel an electric shock when touching the chassis. The chassis, however, is not live (or the voltage is far lower than the human safety voltage) when the VFD is powered on but not running.

Solution

1.    If there is power distribution grounding or ground stud on the site, ground the cabinet chassis of the drive system through the power ground or stud.

2.    If there is no grounding on the site, you need to connect the motor chassis to the ground terminal PE of the VFD, and ensure that the jumper at "EMC/J10" on the middle casing of the VFD is shorted.