# Recent questions and answers in Power Electronic Controlled Drives

In a three phase controlled bridge rectifier with an increase of overlap angles the output dc voltage

In a thyistorised converter a free wheeling diode is used to  A) Add to the conduction current ofthe thyristor B) Oppose the thyristor conduction C) Conduct current during the off period of the thyristor D) Protect the thyristor by providing a shunt path

In d.c. choppers, for periodic time T, the output voltage can be controlled by frequency modulation by varying

For speed control of ac drive, the preferred method using thyristors is  A) Phase control B) Integral cycle control C) Single pulse PWM control D) Sinusoidal PWM control

A step-down chopper, fed from a 120 volt DC source, operates a DC motor whose armature e.m.f. and armature resistance are 100 volt and 0.5Ω respectively. With the magnitude control ratio 0.6, the quadrant of operation DC motor is  (a) First (b) Second (c) Third (d) Fourth

The ripple factor of a bridge rectifier is  1.0.48 2.0.812 3. 1.11 4. 1.21

A DC chopper is used for regenerative braking of a separately excited DC motor. The DC supply voltage is 400V. The armature current during regenerative braking is kept constant at 300 A with negligible ripple. For a duty cycle of 60% the power return to the DC supply is  (a) 78 kW (b) 58 kW (c) 38 kW (d) 48 kW

In static Kramer drive, motor turns ratio is 0.6, the firing angle is 120o. The motor slip is   (a) 0.633 (b) 0.533 (c) 0.833 (d) 0.733

In v/f control method,   (a) The maximum torque decreases (b) The motor speed increases (c) The maximum torque is constant (d) The maximum torque increases

Specific energy consumption of an electric train is the ratio between (A) Specific energy output at driving wheels and efficiency of the traction motor (B) Specific energy output at driving wheels and efficiency of the transmission gear (C) Specific energy output of driving motor and efficiency of the driving wheels (D) Specific energy output at driving wheels and efficiency of the (motor + transmission gear)

In electric traction for a trapezoidal speed-time curve, the time period is (A) free running speed period = constant speed period (B) free running period = coasting period (C) free running period + coasting period = constant speed period (D) average speed period = scheduled speed period

The no-load speed of a single-phase SCR bridge converter fed separately excited dc motor operating at a firing delay angle, α and flux, Φ (A) directly proportional to α and Φ (B) inversely proportional to α and Φ (C) directly proportional to α and inversely proportional to Φ (D) directly proportional to Φ and inversely proportional to α

The direction of rotation of an inverter fed 3-phase ac motor is reversed by (A) a mechanical reversing switch (B) reversing the input dc link voltage (C) operating the inverter as a rectifier (D) changing the sequence of switching

A moving coil meter has a resistance of 3 Ω and gives full scale deflection with 30 mA. To measure voltage up to 300 V, the external resistance to be connected in series with the instrument is (A) 9997 Ω (B) 10000 Ω (C) 997 Ω (D) 10003 Ω

RC triggering is preferred over resistance triggering because (A) it provides triggering at a precise instant (B) it gives a longer duration gate pulse (C) it enables triggering at a larger value of firing angle (D) it protects the SCR against high dv/dt

In an adjustable speed ac drive which quantity gets reduced above rated speed ? (A) developed torque (B) stator frequency (C) reduced starting torque (D) stator current

Scherbius Drive System