How to Solve the Effect of Electromagnetic Interference on Multi-Core LV Shielded Control Cable?

Strong electromagnetic interference on electronic systems, electronic equipment interference, especially sensitive electronic equipment due to overload and damage. Among the many models of cables, even shielded control cables with metal braided shielding, it is difficult to escape electromagnetic and radiation interference on the cable.

To avoid the influence of external electric fields on the instrumentation, a cavity conductor is used to shield the external electric fields from the inside.

Multi-core low-voltage shielded control cables in the electrical control cable shielding layer are not properly handled, the control cable and power cable are confused together, and no separation is done.

If the control cable is connected to the inverter, the inverter grounding effect is not good control power grounding effect is not good.

Control cable where the place where the motor is more, and the motor shell is not grounded, can not export the excitation residual current generates a magnetic field, but also travel the phenomenon of electrical interference.

So how to solve this problem, today ZMS cable editorial give you a detailed talk.

General electromagnetic interference

Shielded Control Cable Prevent Electromagnetic Interference

Can be used when using a control cable grounding.

Weak and strong circuits do not use the same control cable at the same time, split-phase control circuits in the control loop should not use the same control cable.

Recommendations for practical application:

Carefully check the grounding point of the equipment;

The distribution cabinet shell and distribution panel are required to have grounding treatment;

The equipment frame needs to have overall grounding treatment, to be connected to the plant grounding network;

The control part of the grounding of a separate grounding bus, is not shared with the power part, if necessary, the isolation transformer can be used as a control circuit power supply;

Control cables must be shielded cables and the shielding layer must have good grounding treatment.

The shield layer is connected by the specifications and can play a role in shielding electromagnetic fields, its suppression effect and circuit balance on the space electromagnetic field of common mode interference suppression effect is additive.

ZMS Cable specializes in wire and cable production and sales, the company has imported production equipment and testing equipment, a strong technical force, with strong new product development capabilities.

Mainly produces high and low-voltage cross-linked power cables, mineral fire cables, aluminum alloy cables, plastic power cables, control cables, overhead conductors, flame-retardant, fire-resistant cables, low-smoke halogen-free cables, prefabricated branch cables, shielded cables, high-temperature-resistant cables, environmentally friendly cables and a variety of special cable products.

About Multi-Core Shielded Control Cable

Shielded control cables are suitable for the connection line of AC-rated voltage 450/750V and below electrical instruments and the transmission line of the automatic control system.

This kind of cable has excellent performance of oil resistance, waterproof, wear resistance, acid and alkali resistance and all kinds of corrosive gases, aging resistance, non-combustible, etc. It is suitable for metallurgy, electric power, petrochemical, and other industries.

Radiation Interference with Shielded Control Cables

In addition to electromagnetic interference, the most common is the cable is subject to radiation interference.

Cable radiation is one of the most common problems in the project, more than 90% of the equipment can not pass the radiation emission test due to cable radiation.

The cable radiation mechanism has two kinds: the signal current in the cable (differential mode current) loop generated by the differential mode radiation, and the cable in the conductor (including shielding) on the common mode current generated.

Radiation from cables comes mainly from common mode radiation.

Common mode radiation is generated by the common mode current, the loop area of the common mode current is formed by the cable and the earth (or adjacent to other large conductors).

Thus having a larger loop area produces stronger radiation.

How Common Mode Currents are Generated

To understand this, it is first clear that common mode voltage is the root cause of common mode current.

The common mode voltage is the voltage between the cable and the earth or other large conductors in the vicinity.

From the common-mode voltage, it is easy to find the cause of the common-mode current.

Common mode currents on cables can arise for several reasons: common mode currents due to differential mode current leakage.

Even if the cable contains a signal return line, there is no guarantee that 100% of the signal current will return to the signal source from the return line, especially at higher frequencies, where the various stray parameters in space provide a third, or even more, return path for the signal current.

This common-mode current, although a small percentage, but due to the large area of the radiation loop, the radiation can not be ignored.

Generally the closer the signal line is to the return line, the lower the impedance of the differential mode current loop.

A typical example is coaxial cable.

Since the return current of the coaxial cable is uniformly distributed on the outer jacket, its equivalent current coincides with the axis, so the loop area is zero and the differential mode impedance is close to zero.

Almost 100% of the signal current returns to the signal source from the outer jacket of the coaxial cable, and the common mode current is almost zero, so there is very little common mode radiation.

On the other hand, since the area of the differential mode current loop is almost zero, the differential mode radiation is also very small, so the radiation of the coaxial cable is very small.

For high-frequency signals, radiation can be avoided by transmitting with coaxial cables.

This is of the same essence as the purpose of traditionally transmitting high-frequency signals with coaxial cables to minimize the loss of the signal.

Because the loss of the signal is small, naturally indicates that the leakage of the component is less, and this part of the leakage is the radiation of the cable.

Shielded Control Cable types

Improvement Methods for Cable Interference

Under the premise of meeting the requirements of use, try to use short cables.

However, the cable length is often limited by the connection distance between the equipment, and can not be arbitrarily shortened.

Moreover, when the length of the cable cannot be reduced to less than half of the wavelength, reducing the length of the cable has no significant effect.

Increasing the Impedance of the Common Mode Current Loop

After the equipment is assembled, the common mode voltage generated on the equipment cable is also certain.

The way to reduce the common mode current in the cable is to increase the impedance of the common mode current loop.

However, how to increase the impedance of the common-mode loop is a confusing problem for many engineers.

They often try to increase the common mode loop impedance by disconnecting the connection between the circuit board and the chassis, or the connection between the chassis and the safety ground to increase the common mode loop impedance, but the results are often disappointing.

This is because these methods are only effective at low frequencies, where common mode currents are not the main cause of radiation.

A practical and effective method is to connect a common-mode choke in series with the cable. The common-mode choke is simple to use as it creates a large impedance to the common-mode currents and has no effect on the differential-mode signals.

Common mode chokes do not need to be grounded and can be added directly to the cable.

The whole bundle of cables through a ferrite ring constitutes a common-mode choke, according to the needs of the cable can also be wound on the ring several turns.

For engineering convenience, many manufacturers offer split magnetic rings, which snap easily onto the cable. After the cable set of the ferrite magnetic ring, the amount of improvement in radiation intensity depends on the impedance of the original common mode current loop.

The above is about the shielded control cable by electromagnetic and radiation interference when the specific situation and improvement methods, I hope to help you in the daily use of cables. If you have any concerns about the purchase and use of cables, Please consult us.