There is a posting about a runaway engine which is controlled by a DCS system. MTH uses a tachometer to monitor the rpm of the motor. If this tachometer fails the engine will attempt to accelerate to the DCS controller "speed" setting. As a result the engine rpm will ramp up to the maximum speed as determined by the track voltage. This could be a problem for any control system that uses a tach tape and tach reader (sensor) to set the motor rpm.
One way to ensure that the engine can never run at an excessive speed is to install diodes in series with each motor to limit the upper voltage that can be applied to the motor. I routinely add up to 8 diodes in series with the motor wiring for each "can" motor. I actually add two sets of diodes (one for each polarity) for each motor.
The result is that the maximum voltage which can be applied to each motor is reduced by about 5 volts. However the voltage applied to the control boards in the engine is not affected. With 18 volts applied to the "track" the maximum voltage which can be applied to each motor is limited to about 13 volts.
Note: This maximum voltage setting is the same whether the engine is operated under DCS command control or under conventional control.
I routinely add these diodes to my Williams engines since I want to have the headlights and sound system operating at all times. I tap off the "diode string" to provide constant intensity lighting with directional control of the headlights as well.
