Electromagnetic Compatibility Description

Electromagnetic Compatibility Description

Overview

Vehicles are typically subject to certain legal requirements that limit the amount of electromagnetic interference (EMI) that can be generated by the vehicles electronic devices. Additionally, the electronic devices within the vehicle must be able to withstand a certain amount of EMI without effecting their operation. EMI is generated whenever electrical current flows through a circuit. The amount of EMI generated is usually dependent upon the amount of current flow and the switching frequency of current flow through the circuit. The EMI requirements are generally referred to as electromagnetic compatibility.

There are many ways of ensuring the vehicle meets electromagnetic compatibility requirements. These include:

* Adding capacitors and resistors to certain electrical circuits

* Regulating the frequency at which a component operates

* Shielding wires, cables and components

Circuit Design

The generator control module, often referred to as the drive motor control module, and the 14V power module each contain filter capacitors connected to the high voltage circuits. These capacitors are necessary to reduce the voltage spikes that occur as a result of the switching of current. Reducing voltage spikes reduces EMI. The switching frequency of the current is also closely regulated. Too high of a frequency can cause an increase in EMI generation.

Wiring/Cable Design

Different types of wire/cable shielding methods are utilized in the vehicle. Common types of shielding include twisted-pair and internal braid or foil. Twisted pair is typically used in circuits such as serial data circuits. The wire pair is twisted together at a particular turns-per-length ratio. Shielded cable is utilized for all other circuits requiring either protection from external EMI or to reduce EMI radiation of the cable itself into other nearby components or circuits.

High Voltage Cable

Drive Motor Generator Battery 3 Phase Cable Assembly

The high voltage cables utilize internal braid shielding. Typically, both ends of the internal braid shield are attached to chassis ground. All of the shielded high voltage cables are grounded at their cable end attachment points.

Low and Intermediate Voltage Wiring

The drive motor position sensor and temperature sensor circuits utilize internal foil shielding.

Component Shielding

Certain components utilize their structure to effectively shield EMI. Metal covers, chassis grounded metal cases and electro-magnetically conductive gaskets may all be part of a component's electromagnetic compatibility design.

Shielding Loss

A loss of proper shielding may result in poor AM band radio reception and/or incorrect sensor circuit readings depending upon the location of the shield loss. Damage that has penetrated to the insulated conductor of high voltage cables is not repairable. Certain Low and Intermediate voltage shielded wiring harnesses may be repairable. Refer to Wiring Repairs Wiring Repairs and Splicing Twisted or Shielded Cable Splicing Twisted or Shielded Cable.