In a system design, we often consider cabling as the last step. However, cables play a major role in the performance of a system and an entire system can fail due to improper cabling. For example, a data communication system depends on cabling and in case of inappropriate cables the control could be lost. Also, cabling has different effects depending on a particular application. Therefore, we will discuss about the different effects of cabling on applications and how to select the right type of cable.
What is a Cable?
Cable, or cabling, consists of two or more insulated conductors wrapped up together. There is a wide variety of multi-conductor cables with different configurations including multiple pairs, twisted pairs, etc. There are also several parallel constructions including woven flat cables and ribbon cables.
Why Choose Right Type of Cable?
Cable reliability is very important for a long lasting system that is based on signal integrity and durability. The characteristics of cables are imposed on the carrying signals. Ideally, cables should behave as ideal transmission lines with no effect on the signal, but practically it is not possible.
Effects of Cabling
The alternating current flowing through the cable has magnetic field associated with it that becomes a source of self inductance of the cable. The self inductance is inversely proportional to the conductor radius and results in lower impedance in the outer rings of the conductor;
Where, f is AC frequency in hertz.
For a cable composed of one or more concentric circular elements in a conductor, a greater magnetic flux is enveloped at the conductor’s center portion compared to the outside part. Consequently, the center of the conductor has greater self-induced back emf, causing the current density to be greater on the conductor surface. The greater concentration at the surface is commonly known as “Skin Effect”. This effect results in incremental increase in the effective resistance of the conductor. Skin effect becomes more apparent with increasing frequency.
Skin effect in a current carrying conductor
The following formula gives the effective resistance;
Where, Ys shows the incremental increase in resistance due to skin effect.
Two cables placed close to each other and carrying current will have magnetic fields that will interact with each other. Magnetic field of each cable links with the adjacent cable causing a force to exist between the two cables. If the currents are varying with time and flowing in the same direction, the adjacent cable will have an induced voltage.
The current distribution of the conductors placed closed to each other will be altered by the mutual reactance. This will increase the resistance and the phenomenon is known as “Proximity Effect”.
Proximity Effect-Conductors carrying current in the same direction
Proximity Effect-Conductors carrying current in the opposite direction
In the formula given for resistance,
the factor Yp shows the increase in resistance due to proximity effect.
Magnetic Conduit Effect
The cables installed in conduit or pipe made of magnetic materials will have a further effect causing an increase in AC resistance. This effect is known as magnetic conduit effect and is calculated by multiplying the factors of skin effect and proximity effect by 1.7.
Cable Length Effect
With the increase in cable length, certain losses are introduced that tend to have a negative impact on the transmission and produce the attenuation of the circuit. The reason is that operation over long cables introduces distortion and noise and may affect frequency response when the available current is insufficient to drive cable capacitance. With frequencies less than 10 kHz, this signal distortion is not a problem. However, there exists a possibility of signal distortion for higher frequency over cables longer than 30 meters or 100 ft. For a given cable length, the transmitted maximum frequency is a function of cable capacitance.
How to Select the Right Type of Cable?
So far we read about the effects of cabling and how they can affect the performance of a system. Therefore, it is necessary to choose the right cable for any particular application. In order to select a cable, we need to consider a few things that are given as follows:
1 – Type of Load
Firstly, it is necessary to consider the type of load to use with the cable for making the right cable selection. Once you have determined the type of load, evaluate the desired safety factor that will vary according to the application. It is also important to consider the environmental conditions and mechanical affects on the cable, such as shock, vibration, etc. It gets easier to select the diameter cable by determining the type of load and conditions with which a cable will be used.
2 – Cable Construction
The second step in selection of cable is to consider the construction. Certain applications may require flexibility in cabling that may range from moderate to high flexible cables. The breakaway strengths of cables will help in determining the cable construction for a particular application.
3 – Cable Composition
Cable composition is another important factor considered for the right selection of cable. Primarily, service life, cost and environment are considered for cable composition. The use of galvanized and non-galvanized cables varies depending on the application and environmental conditions.
4 – Cable Shielding
Lastly, determine the need of coated cables. There are a few reasons to select coated cables that are; cable protection from undesired materials, to increase the size appearance of the cable, to reduce friction, to use colored cables for aesthetic reasons, and to protect cable from abrasion.
Application Example of Cabling Effects
Let us now consider a practical application example of cabling in variable frequency drive (VFD). We will consider the effects caused by the cabling in this particular example.
The frequencies of electrical energy flowing through the cable are about more than 30 MHz. Energy with such high frequency should be contained within the cable, otherwise it will radiate out interfering with the operation of nearby equipment. Sometimes, it is difficult to find and eliminate the problem of noise emission associated with VFD systems. Without proper cable shielding design, the noise emission can disrupt the system operations. In addition, length of the cable is also a limitation because longer cable radiates more noise.
The selection of cable in VFD systems will impact the system’s reliability. VFD is connected with the motor through a cable, as shown in Figure below, which plays a significant role in optimizing the performance and longevity of VFD system.
Typical VFD System (belden.com)
Commonly, there are four issues related to cable that affect variable frequency drive systems. The cable issues affecting the performance of VFD systems are as follows:
Reflected Wave Voltage: The longer motor leads create a mismatch between the impedance of the cable and motor causing voltage reflection. This reflection potentially doubles the terminal voltages of motor and creates a standing wave effect. The higher voltage increases the probability of drive or motor failure. These stresses can be potentially reduced by using the lowest capacitance cables that will increase the critical distances.
Capacitive Coupling and Cable Charging: These effects are caused due to capacitive interaction of adjacent cables or other motor leads consuming some of the output current and power of the drive that results in reduced induced voltages and motor torque. The length of the cable is proportional to the losses caused by capacitive charging. In order to reduce the losses due to cable charging and capacitive coupling, it is important to use a cable with practically lowest capacitance and prevent capacitive interaction by effectively shielding the motor lead sets from each other.
Common Mode Current: Common mode current is any current that creates noise by returning through any ground path. A cable used in a VFD system is responsible for providing a safe path to these potentially harmful currents and prevent them from disturbing the surround equipment. Common mode currents are reduced effectively by the cable providing the lowest impedance path.
Installation Safety and Reliability: Cabling also ensure system’s safety and reliability. Inappropriate cable selection can risk operational reliability and personal safety. Therefore, it is critical to select cable system that can provide highest degree of safety and can withstand electrical and environmental challenges.