Test: Inverters- 2 - Electrical Engineering (EE)

• By using adjustable voltage inverter, speed range of motor is limited because of motor cogging at 6 Hz and below.
• In pulse width modulated inverter, input p.f. is good at all frequencies.
• A CSI has the capability of regeneration back to the a.c. line because d.c. link polarity can be reversed.
• In VSI, the misfiring of switching devices may cause short circuit across the source and create serious problems.

In a CSI, load current rather than load voltage is controlled, and the inverter output voltage is dependent upon the load impedance and the output voltage waveform. The load current and hence its waveform is independent of load impedance due to which a CSI has inherent protection against short-circuit across its terminals.

In case of singte-putse width modulation (PWM), the width of the pulse is adjusted to reduce the harmonic. However, a single phase IGBT bridge inverter produces a square wave. This square wave contains  harmonic, harmonic andharmonic.

The rms value of amplitude of harmonic voltage of a single, pulse modulated wave is given by

(where, p = width of pulse an V_dc = supply dc voltage)
If the 3^rd harmonic Is to be eliminated, then
E_L3 = 0
i.e. 
or, 
or, 
= Required pulse width

In a 1-φ full bridge inverter if RLC load is underdamped, then the two thyristors (namely T₁ and T₂) shown in figure will get commutated naturally and therefore no commutation circuitry will be needed. Thus, load commutation will be possible.

(Here, 2 pulse width = 120°)
∴ 

A voltage source inverter is one in which the d.c. source has small or negligible impedance. Due to low internal impedance, the terminal voltage of a VSI remains substantially constant with variations in load. Reason is also a correct -statement because due to low time constant of internal impedance any short-circuit across the terminals of a VSI causes current to rise very fast. Hence, both assertion and reason are true but reason is not the correct explanation of assertion.