Adjustable-Voltage System

(also called the adjustable-potential system, the generator-motor system, or the Ward Leonard system), an electrical drive system in which the DC driving motor with separate excitation is energized by a current generator that also has separate excitation. The adjustable-voltage system is mainly used for electrical drives operating under stress with frequent actuation, speed regulation over a wide range, or special demands on the regulation of speed, torque, or other characteristics. These systems are most widely used in the metallurgical industry.

In an adjustable-voltage system (see Figure 1) the generator (G) is rotated by an asynchronous or synchronous electric motor (AM). The machines in the system are usually excited by the exciter (E); in high-powered equipment ion excitation, as well as thyristor devices, is also used. The motor (M) is started by the gradual increase in the voltage of G by a rheostat (RhG) in the field circuit or by switching on the field winding (fwG) of the generator straight to full voltage or even to increased voltage. The reversing of M accomplished by changing the polarity of G, reversing the current in fwG by switching the directional contacts ƒ and r. By reducing the excitation of G or by disconnecting fwG, M passes into the regenerative-braking mode, and G passes into the motor mode, in which it reduces the load of AM or converts it to the generator mode, delivering energy to the circuit. In an adjustable-voltage system, the speed is regulated by varying the voltage in the armature of M (downward from the primary voltage) or by attenuating the magnetic flux in M (upward from the primary voltage). The full range of speed regulation reaches 1:30. This range can be broadened by automatic control methods—for example, electrical-machine, semiconductor, and magnetic devices.

Figure 1. Diagram of an adjustable-voltage system: G is the generator; M is the electric motor; E is the exciter; RhE, RhG, and RhM are rheostats; AM is an asynchronous motor; fwE, fwG, and twM are field windings; rrG and rrM are regulating resistances; r and f are groups of rotation-direction contacts (reverse and forward).

The advantages of an adjustable-voltage system include good dynamic properties (which permit the production of various characteristics in transitional regimes), simplicity and economy of control, and wide range and great smoothness of speed control. Among its shortcomings are comparatively low efficiency (0.6-0.8), high rated power of mechanisms and high equipment cost, and increased service and repair expenses.