Homage inverter output voltage must be
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What does this have to do with the PWM voltage waveform in Figure 2b? Instead of letting the current pulse rise on the same order as the applied voltage pulse, the current will start to rise slowly. Whether a current is increasing or decreasing, an inductor will oppose the change. A characteristic of induction is its resistance to current changes. What makes this work is that, as the name implies, an induction motor is a big inductor of sorts. Note that sometimes the voltage by frequency (V/f) ratio can be quite nonlinear, which is typical for fans, pumps, or centrifugal loads that do not require constant torque but instead favor energy savings. The resulting voltage waveform applied to the motor winding is not sinusoidal (see Figure 2b).
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Normally, the relationship between voltage and frequency is kept linear so that a constant torque can be produced. When the output frequency increases, the output voltage should also increase at the same rate to keep the ratio constant and thus the motor flux constant. In general, there is a fixed relationship between the two. The ratio of the output RMS voltage to output frequency determines the flux developed in the ac motor. Because the dc voltage is inverted into ac, the output stage is also called "the inverter." The duration for which each IGBT switch in a given pair is turned ON or held OFF can be controlled, which determines the RMS value of the output voltage. The inversion process is carried out using three insulated gate bipolar transistor (IGBT) pairs-one pair per output phase (see Figure 2a). The dc bus voltage is then inverted to yield a variable voltage, variable frequency output. A VFD typically rectifies the 3-phase input to a fixed dc voltage, which is filtered and stored using large dc bus capacitors. What happens at the output of a VFD is an entirely different story. This applied voltage creates a sine wave current waveform with the same frequency if checked at the motor leads. Each phase is a sine wave with a frequency of 60 Hz and usually has a voltage amplitude of 230 V, 460 V, or 575 V in the U.S. With a starter, starting the motor involves connecting the 3-phase leads of the motor to 3-phase power.
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People tend to be more familiar with running their ac induction motors using motor starters.