Motors And Drives

TOY MOTORS (Motors And Drives)

The motors used in model cars, trains etc. are rather different in construction from those discussed so far, primarily because they are designed to be cheap to make. They also run at high speeds, so it is not important for the torque to be smooth. A typical arrangement used for rotor diameters from 1 cm […]

D.C. MOTOR DRIVES

INTRODUCTION 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 (c.f. induction motor) 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 of […]

THYRISTOR D.C. DRIVES – GENERAL (Motors And Drives)

For motors up to a few kilowatts the armature converter can be supplied from either single-phase or three-phase mains, but for larger motors three-phase is always used. A separate thyristor or diode rectifier is used to supply the field of the motor: the power is much less than the armature power, so the supply is […]

CONTROL ARRANGEMENTS FOR D.C. DRIVES (Motors And 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 control speed. When position control […]

CHOPPER-FED D.C. 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. The basic operation of a single-switch chopper was discussed in topic 2, where it was shown that the average output voltage could be varied by periodically switching the battery voltage on […]

DIGITALLY CONTROLLED DRIVES (Motors And Drives)

As in all forms of industrial and precision control, digital implementations have replaced analogue circuitry in many electric drive systems, but there are few instances where this has resulted in any real change to the structure of existing drives, and in most cases understanding how the drive functions is still best approached in the first […]

D.C. SERVO DRIVES (Motors And 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 servomechanisms were developed primarily for […]

INDUCTION MOTORS – ROTATING FIELD, SLIP AND TORQUE (Motors And Drives)

INTRODUCTION Judged in terms of fitness for purpose coupled with simplicity, the induction motor must rank alongside the screwthread as one of mankind’s best inventions. It is not only supremely elegant as an electromechanical energy converter, but is also by far the most important, with something like one-third of all the electricity generated being converted […]

THE ROTATING MAGNETIC FIELD (Motors And Drives)

Before we look at how the rotating field is produced, we should be clear what it actually is. Because both the rotor and stator iron surfaces are smooth (apart from the regular slotting), and are separated by a small air gap, the flux produced by the stator windings crosses the air gap radially. The behaviour […]

TORQUE PRODUCTION (Motors And Drives)

In this section we begin with a brief description of rotor types, and introduce the notion of ‘slip’, before moving onto explore how the torque is produced, and investigate the variation of torque with speed. We will find that the behaviour of the rotor varies widely according to the slip, and we therefore look separately […]

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