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

V =

-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

Hence, The r.m.s. value of fundamental voltage

Current Id drawn from the DC input contains ripple.

I when Q1 and Q2 ON

Id =

-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**

rms values of fundamentals (e.g. 20V - 300V)

f = 1/Ts frequency of fundamentals (e.g. 10-100 Hz)

To control the speed of the motor, we need to adjust V and f. Adjustment of f is straightforward, (e.g. adjust Ts). To adjust voltage V, we should alter input voltage Vd. This demands a DC voltage controller at the input.

DC voltage controller

The control unit thus, gives switching signals S1-S4 for the DC-AC invertor with cycle time Ts and a separate switching signal S0 for the DC-DC regulator to fix Vd.DC - DC regulator circuits

We can show that,Vd = Vdc * (ON-state duty factor of switching signal S0)

ON-state duty factor

D = Ton / T The range of D is, 0 <= D <= 1

Switching frequency of DC-DC unit (e.g. of Q0) is usually large of the order of 20-40 KHz. Such higher switching frequency is used for good regulator of Vd (and for small filter component).

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