60 Frequency Converters With switch combinations 000 and 111, the same potential occurs at all three output terminals of the inverter. This will be either the positive or negative potential from the intermediate circuit, as shown in Fig. 2.15 Inverter switch states. As far as the motor is concerned, this is the equivalent to a terminal short circuit and so a voltage of 0 V is applied to the motor windings. Generation of motor voltage Steady-state operation involves controlling the machine voltage vector Uωt on a circular path.
The length of the voltage vector is a measure of the value of the motor voltage and the speed of rotation, and corresponds to the operating frequency at a specifi c time. The motor voltage is generated by briefl y pulsing adjacent vectors to produce an average value. Some of the features of the Danfoss SFAVM method are as follows: • The amplitude and angle of the voltage vector can be controlled in relation to the preset reference without deviations occurring • The starting point for a switching sequence is always 000 or 111.
This enables each voltage vector generated to have three switch states • The voltage vector is averaged by means of short pulses on adjacent vectors as well as the zero vectors 000 and 111 1,00 U-V V-W W-U 0,50 0,00 0 -0,50 -1,00 Switching pattern of phase U Phase voltage (0-point & half the intermediate circuit voltage) Combined voltage to motor Fig. 2.16 With the synchronous 60° PWM principle the full output voltage is obtained directly 18 0 30 6 0 1 20 24 0 30 0 3 60