The first stage of a VFD is the converter, which comprises six
diodes, which are similar to check valves used in plumbing systems. These allow
current to flow in only one direction; the direction shown by the arrow in the
diode symbol. For example, whenever A-phase voltage (voltage is similar to
pressure in plumbing systems) is more positive than B- or C-phase voltages,
that diode opens and allows current to flow. When B phase becomes more positive
than A phase, B-phase diode opens and A-phase diode closes. The same is true
for the three diodes on the negative side of the bus. Thus, we get six current
pulses as each diode opens and closes. This is called a 6-pulse VFD, which is
the standard configuration for current VFDs. We can get rid of AC ripple on DC
bus by adding a capacitor. A capacitor operates in a similar fashion to a
reservoir or accumulator in a plumbing system. It absorbs AC ripple and
delivers smooth DC voltage. The diode bridge converter that converts AC to DC
is sometimes just referred to as a converter. The converter that converts DC
back to AC is also a converter, but to distinguish it from the diode converter,
it is usually referred to as an inverter. It has become common in the industry
to refer to any DC-to-AC converter as an inverter. When we close one of the top
switches in the inverter, that phase of the motor is connected to the positive
DC bus and voltage on that phase becomes positive. When we close one of the
bottom switches in the converter, that phase is connected to the negative DC
bus and becomes negative. Thus, we can make any phase on the motor positive or
negative at will and can thus generate any frequency that we want. So we can
make any phase positive, negative or zero. Notice that, output from the VFD is
a rectangular waveform. VFDs do not produce a sinusoidal output. This
rectangular waveform would not be a good choice for a general-purpose
distribution system, but is perfectly adequate for a motor. If we want to
reduce motor frequency, we simply switch the inverter output transistors more
slowly. But if we reduce frequency, we must also reduce voltage in order to
maintain V/Hz ratio. Pulse width modulation (PWM) does this. Imagine, we could
control the pressure in a water line by turning the valve on and off at high
speed. While this would not be practical for plumbing systems, it works very
well for VFDs. Notice that, during the first half-cycle, voltage is on half the
time and off the rest. Thus, the average voltage is half of 480V, that is,
240V. By pulsing the output, we can achieve any average voltage on the output
of the VFD.
