Introduction

International English

The supply, which can be single phase (usually at low power) or three-phase is fed to a full wave rectifier which supplies the DC link capacitors. The capacitors reduce the voltage ripple (especially on single-phase supplies) and supply energy for short breaks in the input supply. The voltage on the capacitors is uncontrolled and depends on the peak AC supply voltage.

The DC voltage is converted back to AC using Pulse Width Modulation (PWM). The desired waveform is built up by switching the output transistors (Insulated Gate Bipolar Transistors (IGBTs) on and off at a fixed frequency (the switching frequency). By varying the on and off time of the IGBTs the desired current can be generated, but the output voltage is still a series of square wave pulses. Pulse Width Modulation is shown in Figure 1-5.

Voltage

Current

Time

0 V

Figure 1-5 Pulse Width Modulation

There are many complex aspects of inverters which need to be considered during the design:

·The control system to calculate the PWM requirements is very complex and specially designed integrated circuits (ASICs) are needed.

·The control electronics are often connected to the DC link, which is connected to the supply, so the customer connections, display etc. must be safely isolated from this.

·The output current must be carefully monitored to protect the inverter and the motor during overload and short circuit.

·At first switch on the DC link capacitors are discharged, and the inrush current must be limited, usually using a resistor which is bypassed by a relay after a few seconds.

·All connections to the inverter, especially the supply and control connections, may carry a lot of interference and must be fitted with suitable protection components.

·An internal power supply with several different output voltages is needed to supply the control electronics.

·The inverter, especially the IGBTs and rectifier diodes, produce heat which must be dissipated using a fan and heatsink.

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MICROMASTER Applications Handbook