Motors And Drives

Nearly all motors exploit the force which is exerted on a current-carrying conductor placed in a magnetic field. The force can be demonstrated by placing a bar magnet near a wire carrying current (Figure 1.1), but anyone trying the experiment will probably be disappointed to discover how feeble the force is, and will doubtless be […]

MAGNETIC CIRCUITS (Motors And Drives)

So far we have assumed that the source of the magnetic field is a permanent magnet. This is a convenient starting point as all of us are familiar with magnets, even if only of the fridge-door variety. But in the majority of motors, the working magnetic field is produced by coils of wire carrying current, […]

TORQUE PRODUCTION (Motors And Drives)

Having designed the magnetic circuit to give a high flux density under the poles, we must obtain maximum benefit from it. We therefore need to arrange a set of conductors, fixed to the rotor, as shown in Figure 1.11, and to ensure that conductors under a N-pole (at the top of Figure 1.11) carry positive […]

SPECIFIC LOADINGS AND SPECIFIC OUTPUT (Motors And Drives)

Specific loadings A design compromise is inevitable in the crucial air-gap region, and designers constantly have to exercise their skills to achieve the best balance between the conflicting demands on space made by the flux (radial) and the current (axial). As in most engineering design, guidelines emerge as to what can be achieved in relation […]

ENERGY CONVERSION – MOTIONAL EMF (Motors And Drives)

We now turn away from considerations of what determines the overall capability of a motor to what is almost the other extreme, by examining the behaviour of a primitive linear machine which, despite its obvious simplicity, encapsulates all the key electromagnetic energy conversion processes that take place in electric motors. We will see how the […]

GENERAL PROPERTIES OF ELECTRIC MOTORS (Motors And Drives)

All electric motors are governed by the laws of electromagnetism, and are subject to essentially the same constraints imposed by the materials (copper and iron) from which they are made. We should therefore not be surprised to find that at the fundamental level all motors – regardless of type – have a great deal in […]

EQUIVALENT CIRCUIT (Motors And Drives)

We can represent the electrical relationships in the primitive machine in an equivalent circuit as shown in Figure 1.15. The resistance of the conductor and the motional e.m.f. together represent in circuit terms what is happening in the conductor (though in reality the e.m.f. and the resistance are distributed, not lumped as separate items). The […]

POWER ELECTRONIC CONVERTERS FOR MOTOR DRIVES

INTRODUCTION In this topic we look at examples of the power converter circuits which are used with motor drives, providing either d.c. or a.c. outputs, and working from either a d.c. (battery) supply, or from the conventional a.c. mains. The treatment is not intended to be exhaustive, but should serve to highlight the most important […]

VOLTAGE CONTROL – D.C. OUTPUT FROM D.C. SUPPLY (Motors And Drives)

For the sake of simplicity we will begin by exploring the problem of controlling the voltage across a 2 V resistive load, fed from a 12 V constant-voltage source such as a battery. Three different methods are shown in Figure 2.2, in which the circle on the left represents an ideal 12 V d.c. source, […]

D.C. FROM A.C. – CONTROLLED RECTIFICATION (Motors And Drives)

The vast majority of drives of all types draw their power from constant voltage 50 or 60 Hz mains, and in nearly all mains converters the first stage consists of a rectifier which converts the a.c. to a crude form of d.c. Where a constant-voltage (i.e. unvarying average) ‘d.c.’ output is required, a simple (uncontrolled) […]