This article focuses on a motor intended for motion control—the servomotor, or servo. Whereas stepper motors rotate through an angle and halt, many servo motors rotate continuously. Properly controlled, a servo can do everything a stepper can and more. In addition to setting the rotation angle, the controller can configure the servo’s rotational speed and […]
The twentieth century brought about a revolution in electronics, as researchers used solid state physics to miniaturize transistors and other circuit elements. With these new integrated circuits, engineers were able to design systems more complex than anything dreamed of before. As circuit designs grew in complexity, so did the designs of electric motors. In 1962,
Operation of DC Servo Drives The precise meaning of the term ‘servo’ in the context of motors and drives is difficult to pin down. Broadly speaking, if a drive incorporates ‘servo’ in its description, the implication is that it is intended specifically for closed-loop or feedback control, usually of shaft torque, speed, or position. Early
Chopper Fed DC Motor Drives If the source of supply is d.c. (for example in a battery vehicle or a rapid transit system) a chopper-type converter is usually employed. We know that with the help of chopper circuit, the average output voltage can be varied by periodically switching the battery voltage on and off for
Control Arrangements for DC Drives The most common arrangement, which is used with only minor variations from small drives of say 0.5 kW up to the largest industrial drives of several megawatts, is the so-called two-loop control. This has an inner feedback loop to control the current (and hence torque) and an outer loop to
Operation of Thyristor DC Drives The thyristor d.c. drive remains an important speed-controlled industrial drive, especially where the higher maintenance cost associated with the d.c. motor brushes is tolerable. The controlled (thyristor) rectifier provides a low-impedance adjustable ‘d.c.’ voltage for the motor armature, thereby providing speed control. Until the 1960s, the only really satisfactory way
Four Quadrant Operation and Regenerative Braking of DC Motor The beauty of the separately excited d.c. motor is the ease with which it can be controlled. Firstly, the steady-state speed is determined by the applied voltage, so we can make the motor run at any desired speed in either direction simply by applying the appropriate
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Characteristics of DC Motor Before variable-voltage supplies became readily available, most d.c. motors were obliged to operate from a single d.c. supply, usually of constant voltage. The armature and Weld circuits were therefore designed either for connection in parallel (shunt), or in series. As we will see shortly, the operating characteristics of shunt and series
Current Surges in DC Motor The steady-state armature current depends on the small difference between the back e.m.f. E and the applied voltage V. In a converter-fed drive it is vital that the current is kept within safe bounds, otherwise the thyristors or transistors (which have very limited overcurrent capacity) will be destroyed. In order
Base Speed and Field Weakening of DC Motor When the field flux is at its full value the speed corresponding to full armature voltage and full current (i.e. the rated full-load condition) is known as base speed (see Figure 1). The motor can operate at any speed up to base speed, and at any torque