Retard system solution on electric mining trucks

Loss of dynamic brake is one of the important issues of AC electric mining trucks. The lack of dynamic brake reduces the safety and worries the truck driver. The driver may be forced to use the service brake, which increases the cost of repair and maintenance. Also, the service brake is designed for truck speeds less than 3-6 mph and is not proper for high speeds. The AC electric truck has retard contactors in series with grid resistors. When the driver presses the retard pedal to reduce the speed, the contactors are closed and power of traction motors are transferred to the inverter cabinet and then dissipated on grid resistances. Each contactor includes main contacts for its energy dissipative network. A set of auxiliary contacts that are used to control the main contacts provides feedback to a vehicle system control circuit. Auxiliary contacts can be subjected to dust, carbon monoxide and other contaminants which prevent the auxiliary contacts from operating properly. Improperly operating auxiliary contacts can result in a fault or a loss of retard braking for a short period of time. This paper describes the innovative design, implementation, and introduction of Unit Rigs´ successful AC haul truck drive system without using auxiliary contacts on the main retard contactors.