In the electrical system of the frequency converter, the choice of shielded and unshielded cables directly affects the operational stability of the equipment, today we will take Siemens’ senior engineers to provide you with a detailed analysis from the three dimensions of structure, characteristics and actual impact.
1. Structural difference:
the “presence or absence” of the shielding layer determines the difference
between the core wires The structure of unshielded cables is relatively simple, only containing conductive cores, and the cores are separated by an insulating layer; On
top of the unshielded cable, a metal shield (usually copper mesh or aluminum foil) is added to the outermost layer of the shielded cable, and
this layer of metal conductor becomes the key to distinguishing the two.
2. Characteristics difference:
capacitance distribution is significantly different 1
. The formation principle of distributed capacitor capacitors
is the combined structure of “conductor + insulator + conductor”. For unshielded cables, there is only a distributed capacitance between the cores; Because the outer layer of shielded cable has an additional metal shield (conductor) that creates an additional distributed capacitance section between the core and shield, the total distributed capacitance of shielded cables of the same length is greater than that of unshielded cables.
2. Schematic Intuitive Embodiment Unshielded
Cable:
The system diagram only shows the distributed capacitance (CP) of the core-to-ground (PE).
Shielded cable:
After the shield is grounded, in addition to the capacitance between the core, the capacitance between the shield and the core is integrated into the system to form a larger total distributed capacitance (CP), which is equivalent to adding an additional capacitance component on the output side.
3. The actual impact on frequency converters and motors
The difference in distributed capacitance will directly affect the system voltage, which is manifested as:
1. The voltage rise is more obvious
Due to the large distribution capacitance of the shielded cable, under the same cable length, its grounding distribution capacitance has a stronger impact on the voltage increase, which may lead to two types of problems:
Inverter bus overvoltage:
For example, In the case of centrifugal pumps, F30002 may be triggered, causing the drive to shut down immediately after booting and not working properly.
Increased reflected voltage at the motor end:
Long-term high reflected voltage can reduce the insulation performance of the motor and even lead to insulation breakdown and motor damage.
2. It may cause the inverter to report an overcurrent fault
according to the formula
Due to the increase in distributed capacitance, the current on the motor cable will also be higher, which may cause a current spike to trigger the output current limit of the inverter, which in turn will cause the inverter to report an F7801 or F30001 fault.
【Summary】
The shielded cable achieves electrostatic shielding through a metal shielding layer, reducing interference with weak current equipment, but also increasing the distributed capacitance, which may cause voltage abnormalities. Unshielded cables have small distribution capacitance and good voltage stability, but lack anti-interference ability. In practical applications, it is necessary to weigh anti-interference requirements and voltage stability according to the scenario. Have you ever encountered similar voltage issues when
choosing a cable? Or there are other questions about the impact on distributed capacitance, which can be discussed further~
