Most Competitive DC Generator ( DC Machines ) MCQs – Latest DC Machines MCQs
Latest DC Machines MCQs
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DC Machines MCQs – DC Generator ( DC Machines ) MCQs
The most occurred mcqs of DC Generator ( ) in past papers. Past papers of DC Generator ( DC Machines ) Mcqs. Past papers of DC Generator ( DC Machines ) Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related DC Generator ( DC Machines ) Mcqs. The Important series of DC Generator ( DC Machines ) Mcqs are given below:
No Load with On Load Characteristics of DC Generator
1. Which of the following is not the operating characteristics of Dc generator?
a) No-load characteristics
b) Load characteristics
c) External characteristics
d) Internal characteristics
Explanation: The relationship between various parameters has to be presented graphically because of the magnetic saturation effect. Four characteristics of importance are the following: 1) No load characteristics 2) Load characteristics 3) External characteristics 4) Armature characteristics.
2. Characteristics drawn at Ia = 0 is also called as ____________
a) Magnetization characteristics
b) Non-magnetization characteristics
c) Anti-magnetization characteristics
d) Cannot be determined
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.
3. Open circuit characteristics (OCC) is generally drawn across __________
a) Ea vs If, Ia=constant (not equal to rated)
b) Ea vs If, Ia=0
c) Ea vs If, Ia=constant
d) Ea vs If, Ia=constant (rated)
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.
4. Characteristics of a DC generator drawn across Vt vs If at rated armature current and constant speed, is called as ____________
a) Load characteristics
b) No-load characteristics
c) External characteristics
d) Armature characteristics
Explanation: 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.
5. In an OCC at If =0, graph starts from origin.
Explanation: As the machine would have been previously subjected to magnetization, a small residual voltage would be present with field unexcited. As will be seen practically, this is necessary for generator to self-excite. So, graph will start from just above the origin on Voc axis.
6. While conducting OCC, in order to avoid hysteresis loop, in which direction If should be increased?
a) -ve direction
b) +ve direction
c) In any direction
d) In both direction there exists hysteresis loop
Explanation: 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.
7. Air gap line is drawn at iron _________
c) Moderately saturated
d) Variable saturation
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.
8. If suppose OCC is conducted at speed n1, where n1< nrated, OCC will lie ____________
a) Above OCC at nrated
b) On OCC at nrated
c) Below OCC at nrated
d) Can’t comment by only speed information
Explanation: For a less speed than the rated one, residual voltage appearing at terminal call Voc will also be less than Voc at rated value, it will vary in parallel manner but will never intersect OCC at rated speed.
9. Ea can be determined using __________
a) No-load characteristics
b) Load characteristics
c) Cannot be determined
d) Above OCC
Explanation: 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.
10. If load characteristics are drawn on OCC itself, we get curve ________
a) Above OCC
b) On OCC
c) Below OCC
d) Intersecting OCC
Explanation: Since on load operation of a DC machine, we’ll get terminal voltage less than the terminal voltage obtained in OCC, graph will start from below OCC. On load, the effect of armature reaction will draw load characteristics parallel to the OCC below it, causing no intersection.
11. Load characteristics drawn at Ra =0 and Ra not equal to 0, will lie _____
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.
12. OCC is drawn at two different speeds both less than rated speed. OCC drawn at speed N1 lies below OCC drawn at speed N2. Which of the following relation is correct?
a) N2 = N1
b) N2 < N1
c) N2 >> N1
d) Can’t comment
Explanation: As a speed on which OCC is taken decreases, the residual voltage appearing on Voc axis also decrease and OCC starts from below, compare to first one. Thus, N1 is less comparatively, as its OCC lies below than the OCC drawn at other speed.
13. Why No-load or load characteristics are also called as magnetization characteristics?
a) Ea α If
b) Ea α φ
c) If α φ
d) Cannot be determined
Explanation: As the generated voltage in the armature in the case of DC generator is proportional to terminal voltage, which also proportional to magnetic flux, as seen by residual voltage appearing at 0 field current. No-load and load characteristics are called as magnetization curves.
External and Armature Characteristics of DC Generator
1. Armature characteristic is _____________
a) No-load characteristic
b) Load characteristic
c) OCC Current
d) Cannot be determined
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.
2. External characteristics is drawn with constant term/s ____________
a) Field current
c) Both Field current and Speed
Explanation: 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.
3.Armature characteristic is drawn across ________
a) Ia vs If
b) Ia vs Vt
c) If vs Vt
d) Ea vs Ia
Explanation: 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.
4. what is the first step in drawing the armature characteristic?
a) If is made 0
b) Field current is adjusted to give Voc equal to rated value
c) Switch connecting armature circuit with external circuit is made open
d) Cannot be determined
Explanation: While drawing armature characteristic, switch of armature circuit is made open so that armature current is set to 0. Then, field current is adjusted to give Voc equal to rated value. Switch its then closed.
5. In armature characteristic, nature of If at lower values of armature current is ______
a) Very large
b) Almost constant
c) Decreasing rapidly
d) Increasing then decreasing
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.
6. At some point If On OCC, the value of voltage obtained which is more than rated voltage is _______
a) Ia *Ra + If *Rf
b) Ia *Ra
c) Vd + Ia *Ra
d) Can’t tell
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.
7. Which of the following is the correct formula for cumulative compound generator?
a) Nse= Nf (ΔIa/If)
b) Nse= Nf (ΔIf/Ia)
c) Nse= 2Nf (ΔIa/If)
d) Nse= Nf (ΔIa/If)/2
Explanation: According to proportionality constant obtained by analysis of OCC, various value of armature current can be obtained. From ΔIf values we can compute number of turns in series of armature in cumulative compound machine.
8. External Characteristic is a plot of _____________
a) Generated emf and load current
b) Terminal voltage and load current
c) Generated voltage and field current
d) Armature current and field current
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.
9. External characteristics for all separately DC generators (except series generator) can be summarized as ____________
a) Parabola opening in +x direction
b) Parabola opening in -x direction
c) Parabola opening in +y direction
d) Parabola opening in -y direction
Explanation: All parabolas for separately excited DC generator, shunt generator, compound generator are parabolas which are opening in -x directions, only centres of all parabolas lie at different places depending on the type of generator.
Characteristics of Self Excited Generator
1. What is Self-excitation in DC shunt generator?
a) Field winding is connected in series of armature
b) Field winding is connected in parallel of armature
c) Field winding is not connected to the armature
d) Field Winding is not excited
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.
2. For a self-excited DC shunt generator Ia = _____________
a) IL – If
b) IL + IL
c) – IL – IL
d) – IL + IL
Explanation: 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.
3. For self-excited DC generator, field circuit curve in the OCC will be __________
a) Straight line not passing through the origin with +ve slope
b) Straight line passing through the origin
c) Straight line not passing through the origin with -ve slope
d) Increasing curve
Explanation: Field is connected such that this current increases the field mmf and therefore the induced emf, the machine will continuously build up. For the field circuit V = If *Rf, which is a straight-line relationship, called the Rf -line, in V-If plot.
4. Intersection of Rf line and magnetization curve will give _____________
a) No load terminal voltage
b) Field current at no load
c) Both a and b
d) Not specific data
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.
5. What will happen if field resistance is decreased?
a) No load voltage will decrease
b) No load voltage will increase
c) No change in no load voltage
d) Field current will decrease
Explanation: 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.
6. No-load voltage can’t be defined where _____________
a) Field resistance line lies below the magnetization linear line
b) Field resistance line intersects the magnetization linear line
c) Field resistance line coincides the magnetization linear line
d) Defined everywhere
Explanation: 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.
7. Machine does not excite to an appreciable value when ___________
a) Field resistance is less than critical resistance
b) Field resistance is more than critical resistance
c) In both cases a and b
d) Depends on other parameters
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.
8. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in field resistance values?
a) Rf1 < Rf2
b) Rf1 = Rf1
c) Rf1 > Rf1
d) Can’t determine from no-load voltage
Explanation: 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.
9. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in speed if field resistance is kept same for both cases?
a) N1 < N2
b) N1 = N1
c) N1 > N1
d) Can’t determine from no-load voltage
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.
10. DC generator will not self-excite in __________
a) Positive feedback
b) Negative feedback
c) Zero Feedback
d) Doesn’t depend on feedback
Explanation: 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.
11. Which of the following is not a reason, behind failing of self-exciting machine?
a) Residual magnetism is absent
b) Negative feedback
c) Field resistance value is more than critical resistance value
d) Armature resistance is non-zero
Explanation: In case a and b, machine will tend to destroy the residual magnetism present in a core, which will not excite machine. When field resistance value is more than critical value as seen from OCC, machine will not excite to appropriate value.
12. By changing the polarity of the armature of failed self-exciting machine, machine can be started.
Explanation: By changing the polarity of field connections to the armature negative feedback can be turned into positive feedback, thus it will help to build up the voltage and machine will act good in self-excitation mode.
13. Which of the following will be useful to generate positive feedback?
a) Increasing the field resistance
b) Increasing the armature resistance
c) Decreasing the armature resistance
d) Changing the shaft direction
Explanation: 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.
Characteristics of Separately Excited Generators
1. Characteristics of separately excited DC generator are drawn by keeping _____
a) Field current and speed both constant
b) Field current and speed both variable
c) Field current constant and speed variable
d) Field current variable and speed constant
Explanation: 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.
2. What is the reason behind dropping down of Ea with load?
a) Field resistance
b) Load resistance
c) Internal factors
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.
3. External characteristic differ from an internal characteristic in separately excited DC generator by _________
a) Ia *Ra
b) If *Ra
c) IL *RL
d) If *Rf
Explanation: Internal characteristic is drawn across armature generated voltage, it doesn’t account the presence of armature resistance. External characteristic takes into account the presence of armature resistance as it is drawn across terminal voltage.
4. The variation in terminal voltage of DC shunt generator with respect to variation in separately excited DC generator is ___________
a) Much rapid
b) Much slower
c) Remains constant
d) Can’t say
Explanation: The terminal voltage drops off much more rapidly with load in a shunt generator than in a separately-excited generator because of fall in field current with terminal voltage. The external characteristic is a double-valued curve with a certain IL (max).
5. Which of the following characteristic lies above of all others?
a) Differential compound
b) Under compound
c) Level compound
d) Over compound
Explanation: All the graphs when drawn across voltage and load current, start from the same point but with increase in values of load current all machines show different elevation according to the values of series field resisitor.
6. Why differential compound generator is not used in practice?
a) High cost
b) High maintenance
c) High drop down in voltage
d) Difficult construction
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.
7.How compounding level in a compound machine is adjusted?
a) By adding variable resistance in series with series field resistance
b) By adding variable resistance in parallel with series field resistance
c) By adding fixed resistance in parallel with series field resistance
d) By adding fixed resistance in series with series field resistance
Explanation: When fixed resistance is added in parallel with series field resistance we get only one other compounding level. So, by adding variable resistance in parallel with series field we can get various other compounding levels.
8. Which of the following have different external characteristic than other?
a) Self excited DC shunt generator
b) Separately excited generator
c) Compound DC generator
d) Series DC generator
Explanation: 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.
9. External characteristic differ from internal characteristic in DC series motor by ______
a) Ia *(Ra + RSE)
b) Ia *Ra
c) Ia *RSE
d) If *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.
10. For series DC generator, internal/external characteristic start from ____________
a) Positive non-zero voltage
b) Zero voltage
c) Negative non-zero voltage
d) Can start from anywhere
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.
11. For a DC series generator what is the condition for self-excitation?
a) (Ra+Rse) > RC
b) (RSE+Rse+RL) < RC
c) (RSE+Rse+RL) > RC
d) (Rse+RL) > RC
Explanation: 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.
12. For a given DC series generator with critical resistance equal to 100 Ω, armature resistance is equal to 50 Ω, and series field resistance is equal to 20 Ω, is connected across load of 50 Ω. What will be the load voltage?
a) 20 kV
b) 0 V
c) 2 kV
d) Data insufficient
Explanation: 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.
Graphical Representation of External Characteristics
1. No load point of DC generator is __________
a) Intersection of OCC and Rf line
b) Point on the Y axis at rated field current
c) Point on the X axis at rated terminal voltage
d) Can’t find through graphical interpretation
Explanation: 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.
2. Rf line intersects with OCC in ________
a) 1st quadrant
b) 2nd quadrant
c) 3rd quadrant
d) 4th quadrant
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.
3. How armature resistance effect is shown graphically?
a) By adding IaRa product horizontally with Rf line
b) By subtracting IaRa product vertically with Rf line
c) By adding IaRa product vertically with Rf line
d) By subtracting IaRa product horizontally with Rf line
Explanation: For representing the voltage drop in armature resistance, we add product IaRa vertically with Rf line at minimum 2 points and draw line parallel to Rf to get line with V + IaRa = constant.
4. For determining IaRa maximum _____________
a) Distance between Rf line and v+ IaRa line is taken
b) Distance between OCC and v+ IaRa line is taken
c) Maximum Distance between OCC and v+ IaRa line is taken
d) Can’t calculated graphically
Explanation: 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.
5. How demagnetization effect of armature reaction is shown graphically?
a) By shifting origin towards +ve y axis
b) By shifting origin towards +ve x axis
c) By shifting origin towards -ve y axis
d) By shifting origin towards -ve x axis
Explanation: 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.
6. External characteristics of DC shunt motor lies in ______________
a) 1st quadrant
b) 2nd quadrant
c) 1st and 2nd quadrant
d) 4th quadrant
Explanation: 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 2nd quadrant as well.
7. From magnetization characteristic at If = 7.1 A, Ea = 225 V at 1000 rpm. What will be the terminal voltage at speed 950 rpm?
a) 225 V
b) 235 V
c) 214 V
d) 220 V
Explanation: 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.
8. For a given compound DC machine, Net field current obtained from characteristic is equal to 7.5 A, where shunt field current is equal to 5 A, armature current is 505 A, demagnetizing current equal to 0.95 A and shunt field winding of 1000 turns at rated speed of 1000 rpm. What will be the series field turns?
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.
Parallel Operation of DC Generator
1. For connecting two generators in parallel, they should have __________________
a) Same current rating
b) Same ohmic rating
c) Same voltage rating
d) All ratings must be same
Explanation: 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.
2. While connecting two DC generators in parallel, which of the following is not a desirable condition?
a) Same voltage rating
b) Same percentage voltage regulation
c) Same percentage speed regulation of the prime movers
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.
3. We connect two generators in parallel _____________
a) For large DC load
b) For small DC load
c) For any DC load
d) For any AC or DC load
Answer: 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.
4. Load sharing of two generators connected in parallel is determined by __________
a) Internal characteristics
b) External characteristics
c) Both internal and external characteristics
d) It doesn’t depend on load sharing
Explanation: 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.
5. For a parallel operation of 2 DC shunt generators, we get net external characteristics ___________
a) Starting from the same no-load point and between generator 1 and generator 2
b) Starting from the same no-load point and below generator 1 and generator 2
c) Starting from the same no-load point and above generator 1 and generator 2
d) Can’t be determined
Explanation: The load sharing by these generators is determined by addition of external characteristics of both of generators. Thus, at no load generators will have common point, on y- axis so parallel characteristic will start from same point and will lie above of both.
6. When two compound generators are connected in parallel, when load current in generator 1 is increased __________
a) Both generator will share same load
b) Generator 1 will start running as motor
c) Generator 2 will be overloaded
d) Generator 2 will start running as motor
Explanation: 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.
7. While running two compound motors in parallel, we connect equalizer ring between ___________
a) Two armatures
b) Two fields
c) Two load points
Explanation: A low-resistance equalizer connection is made directly between the two armatures before the series fields. Any emf variations of the armatures causes equalizing circulating current which do not affect the current through the series windings. Thereby the parallel operation is stabilized.
8. When two DC series motors are connected in parallel, the resultant speed is __________
a) More than the normal speed
b) Loss than the normal speed
c) Normal speed
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.