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Correct CHOICE is (d) 2 auxiliary thyristors and 2 diodes

Explanation: Two auxiliary thyristors are connected in the chopper CIRCUIT ALONG with two diodes, one INDUCTOR, one capacitor in series with the voltage supply which is then connected in PARALLEL with diode and DC motor armature.

Correct option is (a) 2 auxiliary thyristors

Best explanation: Two auxiliary thyristors are connected in the CHOPPER CIRCUIT along with one DIODE, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC MOTOR armature.

Correct choice is (b) Resistance commutation

Best explanation: Thyristor in a chopper has to be turned off by auxiliary commutation. This can be DONE by TWO methods-FORCED commutation and Load commutation. Forced commutation can ALSO be done by two ways, current and voltage.

Correct OPTION is (b) V/(1-α)

Explanation: With the help of CONVENTIONAL chopper one can reduce the average voltage, so by doing slight MODIFICATION in the circuit one can use chopper METHOD to increase the average voltage APPLIED, by the formula V/(1-α).

Right choice is (c) By using CONSTANT and variable frequency systems

To ELABORATE: The average VOLTAGE of the chopper OUTPUT depends on α, thus, α can be varied by either changing the COMPLETE time period or by changing an on time or off time. These two methods are named as constant and variable frequency system.

Correct option is (a) αV

Explanation: Here the average value of the CHOPPER output VOLTAGE waveform is given by Va=V tON/(tON+tOFF) = V (tON/T) =αV. So, by varying DUTY cycle α ONE can VARY the average voltage at output.

The correct option is (c) Thyristor switch in series with load

The EXPLANATION: A CHOPPER is essentially a thyristor switch in series with the load. A shunting diode is PROVIDED ACROSS the load because free-wheeling the load current when thyristor is off. The thyristor can be shown enclosed by a dotted square can be turned-off over a time period and the cycle is REPEATED.

Right option is (d) AC to DC control

Best explanation: The chopper PROVIDES stepless and efficient control of DC voltage and it is less expensive. It is having another advantage of faster response and regeneration. The chopper is RELATIVELY NEW technology and equivalent to DC-DC TRANSFORMER.

Correct choice is (a) DC CHOPPER

For explanation: A chopper is essentially an ELECTRONIC switch that turns on the fixed-voltage DC source for SHORT time intervals and applies source potential to motor terminals in series of pulses. This process controls the average voltage APPLIED to the motor.

The correct CHOICE is (d) DC chopper

The explanation is: All methods listed above are the methods which can be used to control DC motor, but resistive method makes lots of WASTAGE of ENERGY, while motor-generator set and inverter RECTIFIER set are bulky and costly.

Correct option is (d) Semi converters and full converter

The BEST EXPLANATION: Semi-converters and full-converters are also employed for control of a DC series motor. The total circuit inductance is HIGH because of the series field, as a consequence of which the ARMATURE current is mostly continuous.

Right option is (c) INVERSION operation

For explanation: When α is greater than 900, the average motor voltage becomes negative. If the motor terminals are reversed, then the CIRCUIT will act as a GENERATOR in order to feed power back to the AC supply. This is the inversion operation of the converter and used in regenerative BRAKING.

Right answer is (d) 4

The explanation: Full converter requires 4 thyristors. The only different thing about the full converter is, it does not REQUIRE free-wheeling DIODE. A full converter can be used to feed a separately excited DC motor. Each THYRISTOR is fired accordingly.

The correct option is (b) Ratio of peak to AVERAGE and RMS to average armature current INCREASES

To explain I WOULD say: The armature current sometimes DUE to some reasons may become non-uniform. In this conditions motor performance deteriorates with discontinuous armature current. So, the ratio of peak current to average current increases. Also, rms to average armature current increases.

Correct answer is (b) Large firing angles

Explanation: There are VARIOUS reasons when armature current becomes discontinuous armature current, prominent reasons are large VALUES of firing angle, high speed and SOMETIMES low torque can ALSO AFFECT.

Correct choice is (b) 2

Easiest explanation: Semi-converter REQUIRES two thyristors. Thyristor TH1 is fired at angle of α and Th2 at angle π+α with respect to the supply voltage V and the process is repeated continuously. Circuit also requires two diodes and one free-wheeling DIODE.

The correct option is (a) True

For explanation: According to the one-quadrant converter, it GIVES voltage and current of one polarity at DC terminals. Therefore, it does not provide support for regenerative braking, i.e. POWER flow from DC motor to the AC SUPPLY.

Right choice is (a) 1

To elaborate: Only ONE thyristor is sufficient for the half WAVE converter purpose, this is employed for small MOTORS only. This thyristor is connected to the single-phase AC supply and with PARALLEL COMBINATION of free-wheeling diode with motor armature.

Correct choice is (a) Below 400 W

To elaborate: Half WAVE converter requires a single thyristor and a free-wheeling diode. In half wave converter CIRCUIT the motor current will always be discontinuous. Ultimately it results in poor motor performance. This TYPE of converter is employed only for motors below 400W.