Whenever I have seen a diesel in a movieclip online or in person and the motors are putting out a lot of torque, the generators audibly rev. It seems the loco should use pulse width modulation to drive the motors for the most torque at low speeds.
So, why does a locomotive rev, and how do they drive the traction motors (PWM, rheostat etc.)
The revving you hear is the diesel prime mover, speeding up so the alternator will provide more power.
The alternator output is rectified to DC. In an AC loco, it is then re-converted to modulated pulse AC to turn the commutator-free AC traction motors. DC traction motors use it direct.
If you can hear the traction motors through the internal combustion racket, you have better ears than I have!
Chuck
thanks chuck
I had always wondered about that and had never been able to put 2 and 2 together until now. I now assume the rectifiers can produce the PWM frequencies to vary the speed of the motors.
yo-ya37 wrote: thanks chuckI had always wondered about that and had never been able to put 2 and 2 together until now. I now assume the rectifiers can produce the PWM frequencies to vary the speed of the motors.
The rectifiers (actually huge silicon diodes!) just produce DC. To get the "not out of the wall socket" AC for the traction motors, there are a bunch of special computer-controlled inverter units (connected to wheel slip detectors, the engineer's throttle and ??? else) that deliver the exact wave form, voltage and frequency needed by each motor, updated several times a microsecond. Rather more sophisticated (and humongously more powerful) versions of model railroading DCC decoders with BEMF.
Chuck (modeling Central Japan in September, 1964 - with diesel-hydraulic locomotives)