This mode of control is suitable when the speed is to be controlled at higher values; for example above base speed.
At higher speed the motor will not response to torque harmonics (because of filtering due to electromagnetic time constant) and hence we can apply simple square wave voltage input. The motor then responses to the fundamental component of voltage and (virtually) ignores harmonic voltages.
Switching pattern for squarewave voltage output
Ts - Switching cycle time.
Switching signals for squarewave voltage output
Output voltage V is found as,
Vd when Q1 and Q2 ON
-Vd when Q3 and Q4 ON
Current waveform I
All 8 devices (Q1-Q4, D1-D4) participate to deliver current waveform I. The figure of current waveform I shows different time intervals in the cycle when different devices carry current.
Mathematically (e.g. Fourier expansion wise) voltage V is given by
Current Id drawn from the DC input contains ripple.
I when Q1 and Q2 ON
-I when Q3 and Q4 ON
Current Id drawn from the DC input contains ripple
Total power absorbed from the DC input = Vd(Id)mean
Current Id has instantaneous negative values, which indicates reversals of Id. The DC link capacitor helps to absorb this return current. (If the capacitor is not used the return current has no where to go and the converter operation will not be what expected.)
Voltage and frequency control