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The correct ANSWER is (b) Terminal voltage and LOAD current

The explanation: The load CHARACTERISTIC of a dc generator at a PARTICULAR speed is the relationship between its terminal voltage and load current (line current) and is also TERMED as the external characteristic. The internal characteristic is the plot between the generated emf and load current.

The CORRECT answer is (a) More than the normal SPEED

The explanation: When two DC series motor are connected in parallel then the resultant speed is INCREASE the normal speed. If the voltage across each motor is assumed to be V, then CURRENT through each motor in parallel connection will be I/2. So, speed α (voltage/current) α (V/Ia/2). Hence the resultant speed is more than normal speed.

Correct answer is (d) Generator 2 will START RUNNING as motor

The explanation is: If LOAD current in G1 INCREASES, load current in G2 decreases. Series excitation and INTERNAL voltage in G1 increases and for G2, these quantities decrease. Finally, all load shifts, which may turn to run G2 as a motor. All this leads to heavy overloading of G1.

Right answer is (b) External characteristics

The explanation is: TWO GENERATORS are connected in PARALLEL such that, summation of CURRENT carried by both generators is equal to load current. In such cases we need to see external characteristics on LOADED condition.

Right CHOICE is (a) For large DC load

To ELABORATE: We connect two generators in PARALLEL for supplying large DC load. It is DESIRABLE to use more than one generator in parallel. This arrangement provides the security that if one generator gives way, the other(s) can feed part load.

Correct choice is (d) Same current rating

Explanation: Same voltage rating, same percentage voltage REGULATION, same percentage SPEED regulation are the DESIRABLE conditions for CONNECTING two generators in parallel, THOUGH these conditions are not a must.

Correct answer is (a) 7

Easiest explanation: From the EXCITATION balance EQUATION, If + [Nse/Nf] Ia – Ifd =If (NET).

5 + 505(Nse /1000) – 0.95 = If (net).

Calculating for Nse by substituting If (net), we GET Nse = 6.8 that is 7 turns.

Right OPTION is (C) Same voltage rating

To explain: For connecting two DC GENERATORS in parallel we EQUAL voltage ratings. As the generator voltage is easily adjustable in a range, so the condition stated above is not a must. But, it is desirable condition.

The correct option is (c) 214 V

The best I can explain: Speed is directly proportional to the BACK emf of a machine. So, Ea = 225 V at 1000 rpm

Ea (950 rpm) = 225 x 950 / 1000, this will give speed of given DC machine at 950 rpm. UPON calculations we GET, Ea = 213.7 V= 214 V approx.

The correct answer is (c) 1^st and 2^nd quadrant

To ELABORATE: When external CHARACTERISTICS are plotted on graphs, and if effect of armature reaction is not considered GRAPH LIES in 1st quadrant only. When effect of armature reaction is considered we shift the ORIGIN thus, characteristics lies in 2^nd quadrant as well.

Correct option is (d) By shifting origin towards -ve x AXIS

The explanation is: The demagnetization CAUSED by armature REACTION can be quantified by equivalent FIELD current Ifd which can be taken as proportional to the armature current Ia. So, by shifting the origin towards -ve x axis by Ifd we can show armature reaction graphically.

The CORRECT choice is (c) Maximum Distance between OCC and v+ IaRa line is taken

The best I can EXPLAIN: OCC when starts from some RESIDUAL voltage on y axis, goes on increasing TILL some maximum value and then starts reducing slightly. The BULK is formed where maximum distance between OCC and v+ IaRa line is taken, to get effective frop in armature.

The correct CHOICE is (a) 1^st quadrant

Easiest explanation: Rf LINE is a straight line passing through the ORIGIN and having constant slope, so rising always in a positive direction. OCC STARTS from some positive value on Y axis and increases till maximum point, AFTERWARDS it starts becoming constant, where generally it intersects with Rf line.

The correct choice is (b) 0 V

To EXPLAIN: Here, DC SERIES motor fails to excite as addition of armature RESISTANCE, load resistance and field resistance is greater than the critical resistance of the machine by 20 Ω. Thus, machine fails to self-excite, as a result we’ll get zero terminal voltage.

Correct option is (a) Intersection of OCC and Rf LINE

The explanation is: Intersection of OCC with FIELD resistance line gives the no-load POINT. All the value so obtained from x and y axis respectively gives the TERMINAL voltage and field current of a DC generator at no-load.

Correct option is (c) (RSE+Rse+RL) > RC

For EXPLANATION I would say: Summation of armature resistance, series field resistance, load resistance must be LESS than the critical resistance. As seen in self-excited DC generator characteristics if this value is greater than critical resistance, VOLTAGE build-up will not be possible.

Right choice is (b) Zero voltage

For explanation: For all other DC generators other than SERIES DC generators, the external and internal CHARACTERISTIC as WELL, START from non-zero positive value of voltage. While in series DC generator both internal and external characteristic start from origin.

The correct choice is (a) Ia *(Ra + RSE)

Explanation: In series DC generator series resistor RSE is added with ARMATURE CIRCUIT, thus total drop in terminal VOLTAGE from generated armature voltage is equal to Ia *(Ra + RSE). While drawing internal characteristics presence of armature resistance and series resistance is not taken into ACCOUNT.

The correct OPTION is (d) Series DC generator

For explanation I would say: UNLIKE Self EXCITED DC shunt generator, separately excited generator, compound DC generator the VOLTAGE at zero load current for DC series motor doesn’t start from some positive non-zero value, instead it starts from ORIGIN.

Correct CHOICE is (c) High drop down in voltage

The best EXPLANATION: At a given value of load current, differential compound machine GIVES lowest voltage output. As the load current increases drop down in terminal voltage of DC differential compound GENERATOR is maximum.

Right answer is (d) ARMATURE reaction

Explanation: In SPITE of fixed excitation, Ea drops off with load OWING to the DEMAGNETIZING EFFECT of the armature reaction. As the voltage drop is caused by magnetic saturation effect, it increases with load non-linearity.

Right CHOICE is (b) Field CURRENT and SPEED both variable

The explanation is: The operation considered here assumes that the armature is driven at constant speed (by means of prime mover) and the field excitation (If) is adjusted to give RATED voltage at no-load and is then held constant at this value throughout the operation considered.

The correct choice is (B) NEGATIVE feedback

For explanation I would say: The field connection to the ARMATURE is such that the induced emf due to the residual MAGNETISM tends to destroy the residual magnetism, is called negative feedback. In the absence of residual magnetism machine will FAIL to self-excite.

Correct answer is (a) N1 < N2

Easiest explanation: For Field resistance kept constant, field resistance line will intersect OCC at different points as speed variation MAY shift OCC up or down. When speed is INCREASED OCC SHIFTS up indicating that no-load VOLTAGE will also rise up.

Right choice is (c) RF1 > Rf1

For explanation I WOULD say: When field RESISTANCE value increases the line of field resistance shifts towards more +ve slope, proving that field CURRENT decreases. This line will now intersect OCC at LOWER regions indicating lower values of terminal voltage.

The correct option is (b) Field RESISTANCE is more than CRITICAL resistance

For explanation: The no-load VOLTAGE is undefined for a field resistance WHOSE line COINCIDES with the linear portion of the magnetization curve. With field resistance even slightly more than this value, the machine does not excite to any appreciable value.

Right answer is (c) Field resistance line coincides the MAGNETIZATION linear line

For explanation I would say: We get no-load VOLTAGE when magnetization CURVE intersects with field resistance line. If that INTERSECTION is not OCCURRING, we’ll not get no load voltage. Thus, when both lines coincide we get undefined value of no-load voltage.

Right choice is (B) No load VOLTAGE will increase

The explanation is: When field resistance is decreased, it will increase field CURRENT by Ohm’s law. Thus, field resistance line will LIE below the older line of field resistance. Eventually, it’ll intersect OCC AFTERWARDS causing increase in no-load voltage.

The CORRECT answer is (b) IL + IL

To elaborate: For a self-excited DC shunt GENERATOR field winding is connected in parallel with ARMATURE winding so that, armature current splits into LOAD current and field current. Load current will be given out as the machine is working as a generator.

The correct choice is (C) Vd + IA *Ra

The best explanation: The difference in the OBSERVED VALUE and rated value of voltage can be obtained from the y-axis that is, from Voc. This difference is equal to Vd + Ia *Ra. Thus, by subtracting voltage drop in armature Vd can be found.

Correct CHOICE is (B) Field winding is connected in parallel of armature

The best explanation: Rather than arranging a separate dc source for excitation purposes, practical GENERATORS are always EXCITED from their own armature terminals, this method of excitation being KNOWN as self-excitation. A self-excited generator with such connection known as a shunt generator.

The CORRECT option is (b) Almost constant

The explanation: It is seen from the CHARACTERISTIC that at low values of Ia, the INCREASE in If is very SMALL to provide for increasing Ia *Ra drop. At large values of Ia there is a sharp increase in If to compensate for voltage drop caused by ARMATURE reaction.

The CORRECT choice is (a) Ia vs If

To explain: Armature characteristic is drawn ACROSS Ia vs If. This characteristic is on load characteristic, drawn when terminal VOLTAGE and speed of the GENERATOR is kept constant. Armature characteristic is regulation characteristic.

Right answer is (c) Both Field current and Speed

The explanation is: External characteristic is a load characteristic. In this, variation of terminal VOLTAGE ACROSS ARMATURE current is DRAWN keeping speed and field current constant. Load is generally KEPT variable.

The CORRECT answer is (b) Load characteristic

Explanation: It is the presentation of Ia vs If with Vt held constant (at rated VALUE) and GENERATOR run at constant n and load varied. It REVEALS the armature reaction effect on the flux/pole. It is ALSO called regulation characteristic.

Right ANSWER is (c) Below

Explanation: Load characteristics with at Ra =0 will lie below the load characteristics DRAWN at Ra not equal to 0. To the load CHARACTERISTIC we add IaRa drop to GET Ea induced emf with load. Thus, it will lie above.

Right option is (a) No-load characteristics

For explanation I would SAY: Under load conditions Ea cannot be determined from the OCC for If in the SATURATION REGION because of the demagnetizing effect of armature REACTION. We must therefore DETERMINE experimentally the equivalent demagnetizing ampere-turns ATd due to armature reaction under actual load conditions.

The correct choice is (b) Unsaturated

For explanation: The EXTENSION of the liner portion of the magnetization curve, is KNOWN as the air-gap LINE as it represents mainly the MAGNETIC behaviour of the machine’s air-gap, the iron being unsaturated in this region consumes negligible ampere-turns; in any CASE the effect of iron is also linear here.

Right answer is (b) +ve direction

For explanation I would say: In conducting the OCC test, If must be raised gradually only in the FORWARD direction OTHERWISE the curve would exhibit local HYSTERESIS loops. In OCC at If =0 there EXISTS small residual voltage shown by non-zero Voc.

The CORRECT option is (a) Load characteristics

To EXPLAIN: Since we have Ia value which is EQUAL to rated i.e. non-zero, it is indeed not a no-load characteristic. Axes given are Vt and If, hence it is not an armature characteristic. Thus, it’s called as load characteristic or magnetization characteristic on load.

The correct answer is (B) Ea VS If, Ia=0

Explanation: OPEN circuit characteristics is also CALLED as no-load characteristics or magnetization characteristics. No load clearly states that armature current will equal to 0. Thus, OCC is drawn at Ea vs If, Ia=0.

Correct choice is (a) Magnetization characteristics

The best explanation: With Ia = 0 (no load) at constant n, it is the PRESENTATION of Vt (=Ea) vs If. This is the most important CHARACTERISTIC as it reveals the nature of the magnetization of the MACHINE. It is easy to determine as the generator is on no load and so only low rated PRIME mover will serve the PURPOSE. It is commonly called the open–circuit/magnetization characteristic.

Right answer is (d) Changing the shaft direction

To elaborate: Negative feedback can be remedied simply by REVERSING the field CONNECTION to the ARMATURE or reversing the direction of rotation. For large dc generators with permanent connections and a fixed direction of rotation, the problem is overcome by TEMPORARILY EXCITING the field from a battery source.

Correct option is (c) Both a and b

The BEST explanation: When OCC is plotted against terminal voltage and field current where, field RESISTANCE line is also drawn, an INTERSECTION point will arise when OCC almost becomes constant DUE to saturation effect. So, corresponding value field current and no-load voltage can be observed from x and y AXIS resp.