Frequency Inverter ST600 Series High-Performance Multifunction VFD Troubleshooting
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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.
Requiring highly sensitive, accurate, and stable zero-
phase sequence current transformer, using
permalloy high-permeability materials, complex
process, high cost, not susceptible to voltage
fluctuation of the power supply and ambient
temperature, strong anti- interference capability
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