Scalar Control (V/f Control) of Variable Frequency Drive (VFD, variable speed drive, VSD) (part 2)
Continued from the previous post: Scalar Control (V/f Control) of Variable Frequency Drive (VFD, variable speed drive, VSD) (part 1) , we still introduce scalar control (v/f control). There are a number of ways to implement scalar control (v/f control). Sinusoidal Pulse Width Modulation (PWM) & Six-Step Pulse Width Modulation (PWM) are described in the following sections.
Sinusoidal Pulse Width Modulation (PWM)
In this method of Sinusoidal Pulse Width Modulation (PWM), the sinusoidal weighted values are stored in the PICmicro microcontroller and are made available at the output port at user defined intervals.
The advantage of Sinusoidal Pulse Width Modulation (PWM) is that very little calculation is required. Only one look-up table of the sine wave is required, as all the motor phases are 120 electrical degrees displaced.
The disadvantage of Sinusoidal Pulse Width Modulation (PWM) is that the magnitude of the fundamental voltage is less than 90%. And the harmonics at Pulse Width Modulation (PWM) switching frequency have significant magnitude.
Six-Step Pulse Width Modulation (PWM)
The inverter of the frequency converter (ac drive) has six distinct switching states. When it is switched in a specific order, the three phase AC induction motor can be rotated.
The advantage of this method is that there is no intermediate calculation required and thus, is easiest to implement. Also, the magnitude of the fundamental voltage is more than than the DC bus. The disadvantage of Six-Step Pulse Width Modulation (PWM) is higher low order harmonics which cannot be filtered by the motor inductance. This means higher losses in the motor, higher torque ripple and jerky operation at low speed. (to be continued)
