TABLE OF CONTENTS We saw in Chapter 6 that the induction motor can only run efficiently at ow slips, i.e. close to the synchronous speed of the rotating field. The best method of speed control must therefore provide for continuous smooth variation of the synchronous speed, which in turn calls for variation of the supply frequency. This is achieved using an inverter (as discussed in Chapter 2) to supply the motor. A complete speed control scheme which includes tacho (speed) feedback is shown in block diagram form n Figure 8.1.
We should recall that the function of the converter (i.e. rectifier and variable-frequency inverter) is to draw power from the fixed-frequency constant-voltage mains, and convert it to variable frequency, variable voltage for driving the induction motor. Both the rectifier and the inverter employ switching strategies (see Chapter 2), so the power conversions are accomplished efficiently and the converter can be compact.
Variable frequency inverter-fed induction motor drives are used in ratings up to hundreds of kilowatts. Standard 50 Hz or 60 Hz motors are often used (though as we will see later this limits performance), and the inverter output frequency typically covers the range from around 5-10 Hz up to perhaps 120 Hz. This is sufficient to give at least a 10:1 speed range with a top speed of twice the normal (mains frequency) operating speed. The majority of inverters are 3-phase input and 3-phase output, but single-phase input versions are...
