All of the ac drives compared here allow the use of economical,
robust ac induction motors. Scalar frequency control also
offers the advantage of operation without an encoder. On the
negative side, torque and flux are neither directly nor indirectly
controlled. Control is instead provided by a frequency and
voltage reference generator with constant volts per hertz
output, which then drives a pulsewidth-modulated (PWM)
modulator. Although simple, this arrangement provides limited
speed accuracy and poor torque response. Flux and torque
levels are dictated by the response of the motor to the applied
frequency and voltage and are not under the control of the
drive.