Traction Motor Field Control

Traction motors, such as those used in railway locomotives, are invariably of series-wound construction. This gives high starting torque, since the armature current passes through the field windings resulting in maximum flux under heavy load conditions.

It is possible to duplicate this characteristic in a separately- excited motor by controlling its field current by a thyristor drive configured as a current regulator, the reference being derived from the motor's armature current through either a shunt or a D.C. current transformer (DCCT).

The advantages of this technique include increased motor output (since the resistance of the field windings is not connected in series with the armature) and the facility to set maximum and minimum limits of field current, thus preventing saturation of the magnetic circuit and improving performance under light and overhauling load conditions, e.g. downhill running.

The speed amplifier needs to be used as a buffer amplifier by reducing its gain to unity. This allows the ramps, speed limits and lower current limits to be used to control rate of change, minimum and maximum current, respectively. A flywheel diode needs to be connected across the output terminals to provide a path for circulation...