| 摘要 |
A propulsion system for an electrically driven traction vehicle includes a chopper, a d-c traction motor, and means effective when the system is operating in a motoring mode for connecting the chopper in series with the armature and the field windings of the motor to a d-c electric power source that includes a filter capacitor. Cyclically operative means provides periodic gating signals for alternately turning on and turning off the chopper, and it can be smoothly changed from a constant frequency, variable pulse width mode to a variable frequency, constant (minimum) off time mode so as to vary the "duty factor" of the chopper over a wide range extending up to 100 percent on time. Brake set up means is operative in response to a motoring-to-braking command for reconnecting the chopper in parallel with the motor and the filter capacitor and for reversing the polarity of the connection of the series field winding relative to the armature. In response to this operation of the brake set up means, current in the field of the motor is momentarily boosted, and an extended chopper turn on signal having a duration substantially longer than that of the aforesaid periodic gating signals is supplied to the chopper to ensure that it turns on and conducts armature current to begin the braking mode of operation even if the command happens to take place at low speeds when the electromotive force of the motor is correspondingly low. |
