Previous Year Questions- Inverters - 2 | Power Electronics - Electrical Engineering (EE) PDF Download

Q21: In the 3-phase inverter circuit shown, the load is balanced and the gating scheme is 180°-conduction mode. All the switching devices are ideal.
The rms value of load phase voltage is       (2012)
(a) 106.1V
(b) 141.4V
(c) 212.2V
(d) 282.8V
Ans: (b)
Sol: RMS value of line voltage
Therefore, RMS value of load phase voltage

Q22: A three phase current source inverter used for the speed control of an induction motor is to be realized using MOSFET switches as shown below. Switches S₁ to S₆ are identical switches.
The proper configuration for realizing switches S₁ to S₆ is      (2011)
(a) (b) (c) (d) Ans: (c)
Sol: Device used in current source inverter (CSI) must have reverse voltage bloking capacity. Therefore, device such as GTOs, power transistors and power MOSFETs canot be used in a CSI. SO, a diode is added in series with the devices for reverse blocking.

Q23: The Current Source Inverter shown in figure is operated by alternately turning on thyristor pairs (T₁, T₂) and (T₃, T₄). If the load is purely resistive, the theoretical maximum output frequency obtainable will be     (2009)
(a) 125kHz
(b) 250kHz
(c) 500kHz
(d) 50kHz
Ans: (b)
Sol: The circuit shown in the figure is a single phase bridge auto sequential commutated inverter (1-phase ASCI).
Thyristor pairs T₁, T₂ and T₃, T₄ are alternatively switches to obtain a nearly square wave load current. Two commutating capacitors, one C₁ in the upper half and the other C₂ in the lower half are connected as shown.
Diodes D₁ to D₄ are connected in series with each SCR to prevent the commutation capacitors from discharging into the load.
THe inverter output frequency is controlled by adjusting the period T through the triggering circuits of thyristors.
The theoretical maximum output frequency obtainable

Q24: A single phase source inverter is feeding a purely inductive load as shown in the figure The inverter is operated at 50 Hz in 180° square wave mode. Assume that the load current does not have any dc component. The peak value of the inductor current i₀ will be      (2008)
(a) 6.37A
(b) 10A
(c) 20A
(d) 40A
Ans: (b)
Sol: Load current increases linearly from −I_P to I_P during 0 < ωt ≤ π, where I_P = peak value of i₀

Q25: A 3-phase voltage source inverter is operated in 180° conduction mode. Which one of the following statements is true ?       (2008)
(a) Both pole-voltage and line-voltage will have 3^rd harmonic components
(b) Pole-voltage will have 3^rd harmonic component but line-voltage will be free from 3^rd harmonic
(c) Line-voltage will have 3^rd harmonic component but pole-voltage will be free from 3^rd harmonic
(d) Both pole-voltage and line-voltage will be free from 3^rd harmonic components
Ans: (b)
Sol: In 3-phase VSI working in 180° mode
Pole voltage is a squre wave.
Therefore 3^rd harmonic is present in pole voltage.
In line voltage waveform even and tripplen harmonics are not present.

Q26: A voltage source inverter is used to control the speed of three-phase, 50 Hz, squirrel cage induction motor. Its slip for rated torque is 4%. The flux is maintained at rated value. If the stator resistance and rotational losses are neglected, then the frequency of the impressed voltage to obtain twice the rated torque at starting should be      (2007)
(a) 10 Hz
(b) 5 Hz
(c) 4 Hz
(d) 2 Hz
Ans: (c)

Q27: A single-phase voltages source inverter is controlled in a single pulse-width modulated mode with a pulse width of 150° in each half cycle. Total harmonic distortion is defined as   where V₁ is the rms value of the fundamental component of the output voltage. The THD of output ac voltage waveform is      (2007)
(a) 65.65%
(b) 48.42%
(c) 31.83%
(d) 30.49%
Ans: (c)
Sol: Pulse width = 2d = 150°
Output voltage in fourier series,
RMS value of fundamental component of output voltage,
Total harmonic Distortion,

Q28: "Six MOSFETs connected in a bridge configuration (having no other power device) must be operated as a Voltage Source Inverter (VSI)". This statement is     (2007)
(a) True, because being majority carrier devices MOSFETs are voltage driven.
(b) True, because MOSFETs hav inherently anti-parallel diodes
(c) False, because it can be operated both as Current Source Inverter (CSI) or a VSI
(d) False, because MOSFETs can be operated as excellent constant current sources in the saturation region.
Ans: (b)
Sol: As MOSFET have internally antiparallel diodes, MOSFET can not withstand reverse voltage , due to which MOSFET can not be as current source inverter (CSI).

Q29: A single-phase inverter is operated in PWM mode generating a single-pulse of width 2d in the centre of each half cycle as shown in figure. It is found that the output voltage is free from 5th harmonic for pulse width 144°. What will be percentage of 3rd harmonic present in the output voltage (V_o3/V_o1max) ?        (2006)
(a) 0.00%
(b) 19.60%
(c) 31.70%
(d) 53.90%
Ans: (b)
Sol: 

Q30: The output voltage waveform of a three-phase square-wave inverter contains     (2005)
(a) only even harmonics
(b) both odd and even harmonic
(c) only odd harmonics
(d) only triple harmonics
Ans: (c)
Sol: The output voltage in both 180° and 120° conduction mode can be described by the fourier series as follow:
In 180° conduction mode, line output voltage
line to neutral output voltage
where k = 0, 1, 2...
It is clear from equation (i) to (iv), output voltage wave form contains only odd-harmonic.

Q31: With reference to the output wave form given in the figure, the output of the converter will be free from 5th harmonic when      (2003)
(a) α = 72°
(b) α = 36°
(c) α = 150°
(d) α = 120°
Ans: (a)
Sol: To eliminate fifth harmonic,

Q32: An inverter has a periodic output voltage with the output wave form as shown in figure
When the conduction angle α = 120°, the rms fundamental component of the output voltage is      (2003)
(a) 0.78 V
(b) 1.10 V
(c) 0.90 V
(d) 1.27 V
Ans: (a)
Sol: Output voltage,
R.M.S. value of fundamental component, n = 1

Q33: Figure shows an inverter circuit with a dc source voltage V_s. The semiconductor switches of the inverter are operated in such a manner that the pole voltages V₁₀ and V₂₀ are as shown in figure. What is the rms value of the pole-to-pole voltage V₁₂?    (2002)
(a)
(b)
(c)
(d)
Ans: (b)
Sol: 
Q34: A single-phase full-bridge voltage source inverter feeds a purely inductive load, a shown in figure, where T₁, T₂, T₃, T₄ are power transistors and D₁,  D₂,  D₃,  D₄ are feedback diodes. The inverter is operated in square-wave mode with a frequency of 50 Hz. If the average load current is zero, what is the time duration of conduction of each feedback diode in a cycle?
(a) 5 msec
(b) 10 msec
(c) 20 msec
(d) 2.5 msec
Ans: (a)
Sol: Conduction time = T/4 = 20/4 = 5 ms.