DC SERVOMOTORS AND TRACTION MOTORS BASIC TUTORIALS

DC servomotors are high-performance motors normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops must be made quickly and accurately.

Servomotors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these motors results in a low electrical time constant (typically 0.05 to 1.5 ms) that further sharpens motor response to command signals.

Servomotors include permanent-magnet, printed-circuit, and moving-coil (or shell) motors. The rotor of a shell motor consists of a cylindrical shell of copper or aluminum wire coils. The wire rotates in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core.

The field is provided by cast Alnico magnets whose magnetic axis is radial. The motor may have 2, 4, or 6 poles.

Each of these basic types has its own characteristics, such as inertia, physical shape, cost, shaft resonance, shaft configuration, speed, and weight. Although these motors have similar torque ratings, their physical and electrical constants vary considerably.

The choice of a motor may be as simple as fitting one into the space available. However, this is generally not the case since most servosystems are very complex.

DC Traction Motors. These are dc series motors typically rated 140 hp, 310 V, 2500 r/min. Four motors are used in each transit car, two on each axle.

The power supply is 600 to 1000 V dc from the third rail, which is powered by 2500- to 5000-kW rectifier sets in rectifier substations located along the track. Starting and speed control are by either a cam controller or a chopper controller on board the transit car.