PLUG-MAGNETIC 8T6332 - Caterpillar

Introduction
[Appendix C reproduces Woodward application note 50532.]Electronic governing systems are required to operate in proximity to other electrical and electronic devices. Many of these devices emit electromagnetic energy. The emissions may be incidental to the operation of the device, such as from rectifiers (SCRs), motors, generators, relays, and switching power supplies. The emissions may be deliberate, such as from communication transmitters, radar, and navigation aids. When these emissions adversely affect another device, it is called electromagnetic interference (EMI).This application note covers some of the ways EMI occurs and some ways in which interference can be prevented. Only the practical aspects of EMI in the typical prime mover wiring installation are covered.Interference Paths
These are two basic ways an interfering signal (noise) can be introduced into an electronic device:* When two or more devices share a common ground, current drawn by one device creates a ground voltage seen by the other devices.* Wires running through a noisy environment can pick up noise and conduct it into the electronic circuits.Methods of Noise Reduction
There are two primary ways of reducing interference:* Grounding* ShieldingThe application of these techniques will prevent EMI in almost all systems employing electronic governors.Grounding
There are two important reasons for grounding:* To prevent a high voltage shock hazard from lightning or component failure.* To provide a common reference to the system. The ground system can also provide effective protection against EMI if properly designed.Ground
Ground is defined as an equipotential point or plane used for the system reference. Ground can be at earth potential, as required for safety grounds, or floating with respect to earth. The important point of this definition is equipotential. In the real world, all conductors have some impedance. When carrying current, no two points on the conductor are at the same potential. A good ground, then, is one in which the ground potentials and their effects are minimized.Ground Systems
The objectives of a good ground system are:* To prevent ground potential noise coupling caused by currents from two or more circuits flowing through a common ground impedance.* To avoid ground loops which are affected by magnetic fields and differences in ground potentials.The first step in proper grounding is to establish a separate signal ground for the system electronics. A typical system will have a minimum of three separate grounds:* Hardware Ground-This is for connecting enclosures, chassis, equipment racks, etc. In ac power systems, this is where the green safety wires are connected.* Power Ground-This is the return for noisy equipment such as motors, relays, and SCRs.* Signal Ground-This serves as the reference for the electronic devices in the system. Signal ground may be connected to the other grounds at a single point near the primary ground point.Figure C-1 illustrates why a separate signal ground is important.
Figure C-1. Series Connected Single Point Ground SystemThe resistors shown represent the impedances of the ground conductors. Currents from circuits 2 and 3 combine with the current from circuit 1 to develop the ground potential.V1=R1 (I1+I2+I3)The potential at 2 isV2=R1 (I1+I2+I3) + R2