Motors And Drives

INVERTER SWITCHING DEVICES (Motors And Drives)

As far as the user is concerned, it does not really matter what type of switching device is used inside the inverter, but it is probably helpful to mention the four most important families of devices in current use so that the terminology is familiar and the symbols used for each device can be recognised. […]

A.C. FROM D.C. SP – SP INVERSION (Motors And Drives)

The business of getting a.c. from d.c. is known as inversion, and nine times out of ten we would ideally like to be able to produce sinusoidal output voltages of whatever frequency and amplitude we choose. Unfortunately the constraints imposed by the necessity to use a switching strategy means that we always have to settle […]

CONVERTER WAVEFORMS AND ACOUSTIC NOISE (Motors And Drives)

In common with most text topics, the waveforms shown in this topic (and later in the topic) are what we would hope to see under ideal conditions. It makes sense to concentrate on these ideal waveforms from the point of view of gaining a basic understanding, but we ought to be warned that what we […]

COOLING OF POWER SWITCHING DEVICES (Motors And Drives)

Thermal resistance We have seen that by adopting a switching strategy the power loss in the switching devices is small in comparison with the power throughput, so the converter has a high efficiency. Nevertheless almost all the heat which is produced in the switching devices is released in the active region of the semiconductor, which […]

CONVENTIONAL D.C. MOTORS (Motors And Drives)

INTRODUCTION Until the 1980s the conventional (brushed) d.c. machine was the automatic choice where speed or torque control is called for, and large numbers remain in service despite a declining market share that reflects the move to inverter-fed induction motors. Applications range from steel rolling mills, railway traction, to a very wide range of industrial […]

TORQUE PRODUCTION (Motors And Drives)

Torque is produced by the interaction between the axial current-carrying conductors on the rotor and the radial magnetic flux produced by the stator. The flux or ‘excitation’ can be furnished by permanent magnets (Figure 3.2(a)) or by means of field windings (Figures 3.1 and 3.2(b)). Permanent magnet versions are available in motors with outputs from […]

MOTIONAL E.M.F. (Motors And Drives)

Readers who have skipped topic 1 are advised to check that they are familiar with the material covered in Section 1.7 before reading the rest of this topic, as not all of the lessons discussed in topic 1 are repeated explicitly here. When the armature is stationary, no motional e.m.f. is induced in it. But […]

TRANSIENT BEHAVIOUR – CURRENT SURGES (Motors And Drives)

It has already been pointed out that 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, […]

SHUNT, SERIES AND COMPOUND MOTORS (Motors And Drives)

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 field 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 machines differ widely, and […]

FOUR-QUADRANT OPERATION AND REGENERATIVE BRAKING (Motors And Drives)

As we saw in Section 3.4, 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 magnitude and polarity […]