Answer Posted / edgar segovia

In DC current we have no frequency so instead of VVVF
drives we use CONVERTERS (rectifiers) and control
armature, field currents and voltages to get motor speed
control. In AC we use VVVF drives which means VARIABLE
VOLTAGE AND VARIABLE FREQUENCY, i.e. we have to control
both frequency and voltage to avoid SLIP thus not losing
torque. With a CONVERTER we go from AC to DC and then to
power electronics to get motor control through the above
mentioned parameters. With a VVVF drive we go a little bit
further since we first go from AC to DC, then we pass
through a FILTERING device and then go to the real
inverter which goes from filtered DC back to AC (nowadays
by meas of PWM). Forgot to mention in AC drives we also
have external resistors to absorve regenerative power (IGBT
controlled as well), in this way we avoid lines pollution
as well (harmonics).

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