Impact of Motor Capacitance on Vehicle Electrical System Transients

The electrical architecture of today's automobiles employs a significant number of fractional horsepower motors to control wipers, windows, seats, etc. The typical motors are permanent magnet DC brush-commutated motors, often referred to as BM motors. These BM motors, while simple in design, have the inherent issue of creating short-duration, high-frequency electrical noise (caused by the constant interruption, or commutation, of the motor current). This electrical noise can readily lead to radio reception interference. In order to protect against this risk, a typical solution is to install a radio frequency (RF) filter internal to the motor. This filter generally includes a high-frequency ceramic or metal film capacitor across the motor terminals that connect to the vehicle electrical system. This capacitor, which was placed to solve one electromagnetic compatibility (EMC) issue related to radiated and/or conducted RF emissions, has the potential to create a secondary EMC issue related to conducted transients. If the motor is energized via a conventional electromechanical relay and its inherent contact bounce characteristics, a significant transient current pulse can be generated on the power bus of the vehicle which can adversely impact other electronic modules. This paper will explore the performance of BM motors with capacitor-based filters, both from a radio interference and transient risk/mitigation perspective.