Appendix15.1.1 Principle of an Inverter Drive15 -215.1 Inverter Drive BasicsThis section explains the basics of an inverter drive.15.1.1 Principle of an Inverter DriveAn inverter is a frequency conversion device that converts a commercial frequency power supply to a vari-able frequency power supply. Fig. 15.1 show an operation diagram for a 3-phase voltage Inverter consist-ing of switches, such as relay contacts. S1 and S4, S3 and S6, and S5 and S2 work as pairs to repeatedly turnON and OFF each half-cycle. Their ON/OFF timing is staggered by one-third of a cycle, so a square waveAC voltage can be obtained as the output, as shown in Fig. 15.2. The frequency of the AC output voltageis proportional to the speed at which the switches are turned ON and OFF; in other words, inversely propor-tional to the cycle. This is the basic operating principle of an inverter.S 1EdS 3 S 5S 4 S 6 S 2UVWFig 15.1 Inverter Operating PrincipleS 1, S 4EdS 3, S 6S 2, S 5SwitchesS 1 ON ON ONON ON ONON ON ONS 4 S 1S 6 S 6S 3S 5 S 5S 2Vu-v0OutputvoltageVv-w0Vw−u00 60° 120°180° 360°Fig 15.2 Switch Operation and Output VoltageIn reality, a motor drive inverter requires a variable voltage and variable frequency (VVVF), so sine wavepulse width modulation (PWM) control is used, as shown in Fig. 15.3. When the carrier frequency is in-creased, sine wave current flows through the motor.15
