55 Frequency Converters A freewheeling diode is connected in parallel with each transistor, because high induced voltages can occur across the inductive output load. The diodes force the motor currents to continue fl owing in their direction and protect the switching components against imposed voltages. The reactive power required by the motor is also handled by the freewheeling diodes. 2.6 Modulation Principles The semiconductors in the inverter either conduct or block according to the signals generated by the control circuit. The variable voltages and frequencies are generated using two basic principles (types of modulation):
• Pulse Amplitude Modulation (PAM) and • Pulse Width Modulation (PWM) Fig. 2.13 Modulation of amplitude and pulse width 2.6.1. Pulse Amplitude Modulation (PAM) PAM is used in FC’s with variable intermediate circuit voltage or current. In FC’s with uncontrolled or half-controlled rectifi ers, the amplitude of the output voltage is generated by the intermediate circuit chopper, shown in Fig. 2.9 Variable DC voltage intermediate circuit. In a case where the rectifi er is fully controlled, the amplitude is generated directly. This means that the output voltage for the motor is made available in the intermediate circuit.
The intervals during which the individual semiconductors should be on or off are stored in a pattern, and this pattern is read out at a rate dependent on the desired output frequency. This semiconductor switching pattern is controlled by the magnitude of the intermediate circuit variable voltage or current. If a voltage-controlled oscillator is used, the frequency always follows the amplitude of the voltage.