Electromagnetic Compatibility (EMC)
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, or amplitude, is usually dependant upon the amount of current flow, amperage, and the on-off pattern of current flow through the circuit, frequency. The EMI requirements are generally referred to as electromagnetic compatibility (EMC).
There are many ways of ensuring the vehicle meets EMC requirements. These include:
| • | Adding capacitors and resistors to certain electrical circuits |
| • | Regulating the frequency at which a component may operate |
| • | Shielding the wires, cables and components |
Circuit Design
The drive motor generator power inverter module (PIM) and the accessory DC power control module assembly (APM) 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 On and Off. Reducing voltage spikes reduces EMI. The frequency of current switching is also closely regulated. Too high 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 circuit shielding include twisted-pair and internal braid or foil. Twisted pair is typically used in circuits such as high speed GMLAN communication 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
| • | Battery Positive and Negative 300V Cable Assembly |
| • | Drive Motor Generator Power Inverter Module 3 Phase Cable Assembly |
| • | Air Conditioning Compressor 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 high voltage, internally shielded cables are grounded at their cable end attachment points. Mounting blocks, where used, perform the shield to chassis ground connection. Connection points not serviced with a mounting block utilize a separate ring terminal.
Low and Intermediate Voltage Wiring
The signal circuits for the transmission sensors utilize shielding protection. The drive motor generator position sensor and temperature sensor circuits utilize internal foil shielding. The wiring harness external of the transmission assembly is connected to chassis ground with ring terminals at the drive motor control module assembly. The internal transmission wiring harness is attached to chassis ground with a ring terminal at the valve body assembly.
The auxiliary transmission fluid pump (ATFP) 3 phase cables utilize internal foil shielding. The wiring harness shield is connected to chassis ground within the ATFP control module.
The power steering intermediate voltage wiring utilizes internal foil shielding. The wiring harness shield is connected to chassis ground at the APM connector and within the power steering motor assembly.
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 components EMC 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. High voltage cables are not repairable. No attempt should ever be made to repair any portion of high voltage cables. Certain Low and Intermediate voltage shielded wiring harnesses may be repairable. Refer to Wiring Repairs.
