The Inverter and PWM Drive
Vary the on-time fraction sinusoidally — the average voltage is…
An inverter connects its output terminal to +Vdc or -Vdc, alternating very fast. Left as it is, that is a rough pulse train, but the switching is so fast that the motor inductance filters out the high-frequency components and only a current close to the average flows. The modulation-index slider sets how widely the ON-time ratio swings sinusoidally; pick first what shape the average (gold) running through the rough pulses will take, then move the slider.
The inverter switches DC into AC
A three-phase inverter is made of three legs, each a pair of semiconductor switches, one above the other, across the DC bus. Treating the DC bus as two rails, +Vdc and -Vdc, about its midpoint, each leg connects its output to +Vdc when the upper switch is on and to -Vdc when the lower switch is on. Running the three legs 120° apart gives three-phase AC; switching each switch only once per cycle would give a rough square wave, but in practice the switches turn on and off very fast, at several kilohertz. The inductance of the motor winding filters out this high switching-frequency content, so the current carries mainly the fundamental, which corresponds to the average, with only a small ripple.
PWM averages into a sine
The fraction of one switching period during which the output stays at +Vdc is called the duty. With a duty of one half the average is 0; a larger duty moves the average toward +, a smaller one toward -. Varying this duty sinusoidally over one output cycle makes the average voltage trace a sine wave, and the method of setting the duty this way is pulse-width modulation (PWM). It is usually done by comparing a reference sine with a fast triangle wave (the carrier) and turning the upper switch on while the reference is above the carrier. The amplitude of the reference sine, the modulation index m, sets the size of the output voltage and the frequency of the reference sine sets the output frequency; the two are independent, so voltage and frequency can be held separately. Once the reference amplitude exceeds the carrier, in the overmodulation region, the average starts to depart from a sine wave.
One inverter runs every machine
V/f control of an induction machine means making the modulation index m proportional to the output frequency to keep the flux constant, and the variable speed of synchronous machines and servos and the electronic commutation of the BLDC are likewise all this inverter shaping voltage and frequency to a command or to the rotor position. Fast switching brings switching loss and harmonics, so the carrier frequency is chosen as a compromise between loss and harmonics, and modulation methods such as space-vector modulation use the DC bus voltage more efficiently. In effect, the inverter generalizes with semiconductor switches the commutation that the DC machine's commutator did with mechanical contacts. Every machine in this course, starting from the magnetic circuit, is in the end controlled through voltage and frequency on top of this one device.
Back to the first screen
Varying the ON-time ratio sinusoidally made the average trace a sine wave. The inverter only switches its output quickly between +Vdc and -Vdc, but the motor inductance filters out the high-frequency components and lets only a current close to the average flow, so modulating the duty sinusoidally turns that average into a smooth sine wave. The amplitude of the reference sine (the modulation index) sets the size of the voltage and the reference frequency sets the output frequency, holding voltage and frequency separately from a single DC source, and the V/f of the induction machine, the variable speed of the synchronous machine and the electronic commutation of the BLDC all run on this one device. The story of electric machines, which set out from a single strand of flux in a magnetic circuit and passed through transformers and rotating machines, comes to a close here, where semiconductor switches shape voltage and frequency.
Starting from the flux and reluctance of the magnetic circuit (A), we have come through the transformer that passes that flux along (A), the DC machine whose commutator turns the armature's AC into DC at the terminals (B), the induction machine that turns in a rotating field carrying its slip (C), the synchronous machine that binds speed and frequency as one (D), and the special machines and inverter shaped by permanent magnets and semiconductor switches (E). Every machine stood on one stem: it turns by the force that flux and current exchange, and commutation and control govern that force. With this map drawn over twenty units in hand, whatever rotating machine you meet, you start by asking where the flux comes from, where the current flows, and what controls them.