Electrical Noise and Interference

ELECTRICAL NOISE AND INTERFERENCE :

Definition of electrical noise
 
Noise, or interference, can be defined as undesirable electrical signals, which distort or interfere with an original (or desired) signal. Noise could be transient (temporary) or constant. Unpredictable transient noise is caused, for example, by lightning. Constant noise can be due to the predictable 50 or 60 Hz AC 'hum' from power circuits or harmonic multiples of power frequency close to the data communications cable. This unpredictability makes the design of a data communications system quite challenging.

Noise can be generated from within the system itself (internal noise) or from an outside source (external noise).
Examples of these types of noise are:

Internal noise

• Thermal noise (due to electron movement within the electrical circuits)
• Imperfections (in the electrical design). 
• Shot noise is generated by individual electrons "jumping" across some sort of barrier potential as they travel through a conducting substance. Shot noise is proportional to the amount of electric current going through a conductor.

• Thermal noise, also known as Johnson noise, is caused by the random motion of electrons due to thermal energy. As one might guess, this type of noise is proportional to conductor temperature.

• Flicker noise, or 1/f noise, is characterized by a magnitude that is inversely proportional to frequency. Little is known about the origins of this type of noise, but it is proportional to the amount of DC current, just like shot noise, and so may be mitigated using the same controls.

External noise

• Natural origins (electrostatic interference and electrical storms)
• Electromagnetic interference (EMI) - from currents in cables
• Radio frequency interference (RFI) - from radio systems radiating signals
• Cross talk (from other cables separated by a small distance).

From a general point of view, there must be three contributing factors before an electrical noise problem can exist. These are:

1. A source of electrical noise
2. A mechanism coupling the source to the affected circuit
3. A circuit conveying the sensitive communication signals.

Typical sources of noise are devices, which produce quick changes (spikes) in voltage or current or harmonics, such as:

• Large electrical motors being switched on
• Fluorescent lighting tubes
• Solid-state converters or drive systems
• Lightning strikes
• High-voltage surges due to electrical faults
• Welding equipment. 

 Electrical systems are prone to such noise due to various reasons. As discussed in the previous chapter, lightning and switching surges are two of these. These surges produce high but very short duration of distortions of the voltage wave. Another common example is 'notching', which appears in circuits using silicon-controlled rectifiers (power thyristors). The switching of these devices causes sharp inverted spikes during commutation (transfer of conduction from one phase arm to the next).Figure 1 shows the typical waveform with this type of disturbance.

 

Harmonics in supply system is yet another form of disturbance.

The following general principles are applicable for reducing the effects of electrical noise:

• Physical segregation of noise sources from noise-sensitive equipment
• Electrical segregation
• Harmonic current control
• Avoiding ground loops which are a major cause of noise propagation (including measures such as zero signal reference grid, explained later in this chapter)
• Shielding/screening of noise sources and noise-susceptible equipment including use of shielded/twisted pair conductors.

Alternatives to Help Sort Out the Best Course of Action for Resolving Electrical Noise Interference from Motors and Drives That Affect Sensor Signals and HMI

• Electrical noise interference from motors and drives is affecting our sensor signals and HMI too often.One solution suggested is to start using fiberoptic cable for signal transmission, but that's not cheap. Alternatively, there also are dozens of power filters, uninterruptible power supplies and other power conditioning devices. 

• Separation between the low voltage control and high voltage power lines: If those lines need to cross, they should do so at 90° angles.Use shielded cables and/or shielded cable trays. It is important to note that simply using what is typically sold as shielded sensor cable could actually increase the problem, especially when using cables with M12 connectors. Most such cables do not connect the shield to the coupling nut and, as a result, do not provide a clean path to ground.

• Provide for solid machine ground interconnections. This is a must when using shielded cables or the shield will serve to equalize the potential between the different machine sections.

• Use ferrite chokes.

• Reduce the length of the sensor cables. In this case the sensors are connected to field-mounted I/O modules that communicate over a network with the PLC.

Good Earth Ground

• Make sure that all the equipment is grounded to a single point, also known as “star” point. This star point should go back to the power supply ground. This will help to reduce ground loop currents. In conjunction with this, use in-line toroid filters with the power supply lines to each piece of equipment. For sensors, use L-C feedthrough filters between the sensor and controllers or PLC. The filter frequency range should be DC to 50 MHz with attenuation of 30 dB or more. The filter ground must be connected to Earth ground.

Electrical Isolation Is the Answer

•  To reduce the effect of electrical noise on signals controlling machinery and other equipment, a combination of two technologies prove to be the most effective. These technologies are optical coupling and inductive coupling.

• An additional way isolation can be improved is effective wiring practices. Depending on length of sensor cabling run, voltage signals are easy to use but provide voltage drops when cable lengths exceed even a few feet and can be expensive if larger conductors are used.

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