Power electronics and drives miscellaneous
- A voltage source inverter is used to control the speed of a 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
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T = sR2 × 60 M K2 V12 [R2 + (sn2)2] 2 π Ns
Trated ∝ sf2 = sf1 ⇒ s = f2 f1
In first case, f2 = 2Hz
For twice rated torque f2 will be twice, so f2 = 4 Hz.Correct Option: C
T = sR2 × 60 M K2 V12 [R2 + (sn2)2] 2 π Ns
Trated ∝ sf2 = sf1 ⇒ s = f2 f1
In first case, f2 = 2Hz
For twice rated torque f2 will be twice, so f2 = 4 Hz.
- A single-phase voltage 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
THD = √Vrms² - V1² × 100 V1
where V1 is the rms value of the fundamental component of the output voltage. The THD of output ac voltage wave form is
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Pulse width, 2d = 150°d = 75° = 75° radian = 1.315 radian 57°
RMS value of output voltage,
Output voltage in terms of fourier series,
RMS value of fundamental component of output voltage,V1 = 1 
4Vs sin 75° 
= 0.78 Vs √2 π
then, total harmonic distortion,
= 31.83%Correct Option: A
Pulse width, 2d = 150°d = 75° = 75° radian = 1.315 radian 57°
RMS value of output voltage,
Output voltage in terms of fourier series,
RMS value of fundamental component of output voltage,V1 = 1 4Vs sin 75° = 0.78 Vs √2 π
then, total harmonic distortion,
= 31.83%
- “Six MOSFETs connected in a bridge configuration (having no other power device) MUST be operated as a Voltage Source Inverter (VSI)”. This statement is
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NA
Correct Option: D
NA
- A single phase full-wave half-controlled bridge converter feeds an inductive load. The two SCRs in the converter are connected to a common DC bus. The converter has to have a freewheeling diode
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Fig. Single-phase bridge type semi-converter: (a) circuit diagram, and (b) waveforms.
During the free-wheel period, π to (π + α), and 2π to (2π + α), energy stored in Inductance during (π + α) to π and α to π, gets dissipated partially in R and partially to the energy stored in load emf E. No energy is fed back to the source during free-wheeling period.
Therefore, in the absence of free-wheeling diode, T1 and T2 may conduct simultaneously which will came short-circuit of supply voltage VS.Correct Option: B
Fig. Single-phase bridge type semi-converter: (a) circuit diagram, and (b) waveforms.
During the free-wheel period, π to (π + α), and 2π to (2π + α), energy stored in Inductance during (π + α) to π and α to π, gets dissipated partially in R and partially to the energy stored in load emf E. No energy is fed back to the source during free-wheeling period.
Therefore, in the absence of free-wheeling diode, T1 and T2 may conduct simultaneously which will came short-circuit of supply voltage VS.
- A three - phase, fully controlled thyristor bridge converter is used as line commutated inverter to feed 50 kW power at 420 V dc to a three - phase, 415 V (line), 50 Hz ac mains. Consider dc link current to be constant. The rms current of the thyristor is
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P = Vd × Id
∴ 50 × 1000 = 420 × Id , Id = 119.05rms current of the thyristor = 119.05 = 39.68 3 Correct Option: D
P = Vd × Id
∴ 50 × 1000 = 420 × Id , Id = 119.05rms current of the thyristor = 119.05 = 39.68 3
