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Multiple choice question for engineering

Set 1

1. In PMDC motors we use magnetic material with ___________
a) Low residual flux
b) Low coercivity
c) High residual flux
d) All magnetic materials

View Answer

Answer: c [Reason:] In market magnetic materials with various properties are available, but for DC motor application we need magnetic materials with high residual flux in order to get high no load voltage. Along with residual flux, coercivity of material should be high.

2. In PMDC magnetic material is placed instead of _____________
a) Armature
b) Rotor
c) Stator
d) Can be placed anywhere

View Answer

Answer: c [Reason:] In PMDC motors we keep magnetic material or magnetic poles in the place of field winding of original DC machine. That is we are changing stator windings with two permanent magnetic poles bounded on casing.

3. PMDC motors are _________________
a) Smaller than corresponding rated field wound motor
b) Larger than corresponding rated field wound motor
c) Almost same size that of corresponding rated field wound motor
d) Can be smaller or greater than corresponding rated field wound motor

View Answer

Answer: a [Reason:] PMDC motors are always of smaller size compare to field wound motors of same rating. Here, field winding is replaced by small block of magnet so that power output remains same. PMDC motors are costly compare to DC motor of same rating.

4. PMDC motor offers ___________
a) Cumulative compound type characteristic
b) Differential compound type characteristic
c) Series characteristic
d) Shunt characteristic

View Answer

Answer: d [Reason:] The stator is made of permanent magnet in the PMDC motor hence, there is no need for separate field excitation for stator. So, PMDC acts as a shunt motor with fixed field. This can be seen from the construction also.

5. Which speed control method is employed in PMDC?
a) Field control method
b) Armature control method
c) Armature Field control method
d) Cannot be determined

View Answer

Answer: b [Reason:] Unlike in DC motors, field is provided by permanent magnets in PMDC. This implies that field provided in PMDC motor is permanent and can’t be altered unless whole construction is modified. Thus, armature voltage control method is employed here.

6. What is the air gap flux density for PMDC motor?
a) Bg= µ0(tm/lg) Hm
b) Bg= – µ0Hm
c) Bg= – µ0(tm/lg) Hm
d) Bg= – µ0(lg/tm) Hm

View Answer

Answer: c [Reason:] Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2*lg* Hg + 2*tm Hm = 0 and we know that air gap flux density is equal to µ times the magnetic field intensity.

7. Which of the following is most favoured choice for PMDC motor?
a) Neodymium-iron-boron
b) Iron-nickel
c) Nickel-neodymium
d) Cannot be given

View Answer

Answer: a [Reason:] Obvious choice of PMDC motor is neodymium-iron-boron which has high coercivity and high retentivity. Its characteristic is almost a straight line. Hence, neodymium-iron-boron combination is preferred over the other materials.

8. Speed-torque characteristic of PMDC motors is _______________
a) Starting from zero increasing continuously
b) Starting from some positive value and remaining constant
c) Starting from some positive value and decreasing exponentially
d) Starting from some positive value and decreasing on straight line

View Answer

Answer: d [Reason:] The material chosen for PMDC motor is such that speed torque characteristic of a given motor remains straight line for wider range of torque, for wider range of armature voltage. The straight line is such that it makes positive intercepts on both axes.

9. PMDC produces high torque on ______________
a) At low speeds
b) Only at maximum speed
c) Torque is constant
d) Cannot say

View Answer

Answer: a [Reason:] PMDC produces high torque even at very low speeds. In DC shunt motor torque produced at very low speeds is lower than the torque produced at the same speeds in PMDC motor.

10. PMDC gives better speed regulation than DC shunt motor.
a) True
b) False

View Answer

Answer: a [Reason:] PMDC motor exhibits better speed regulation and efficiency than dc shunt motor. The main problem of dc shunt motor is going to run-away when the field terminals are opened. But in PMDC there is no run-away problem, so it gives practical benefit to the industry applications.

11. If higher terminal voltage is applied to PMDC what will happen?
a) Speed at 0 torque will be reduced
b) Torque at 0 speed will be reduced
c) Torque at 0 speed will be increased
d) Torque will remain constant

View Answer

Answer: c [Reason:] The straight-line characteristic of PMDC is such that it produces positive intercepts on both axes. If terminal voltage is increased both of these intercepts increases, slope remaining constant. Thus, speed and torque at initial conditions both will increase.

Set 2

1. As the load is increased the speed of DC shunt motor will ___________
a) Reduce slightly
b) Increase slightly
c) Increase proportionately
d) Reduce rapidly

View Answer

Answer: a [Reason:] As the load is increased, speed of the DC motor will reduce slightly, the change is so negligible that in many cases it is assumed that speed of the DC motor remains constant. Hence, characteristic is also called as shunt characteristic.

2. The armature torque of the DC shunt motor is proportional to
a) Field flux only
b) Armature current only
c) Field flux and armature current
d) Field current

View Answer

Answer: b [Reason:] Torque of the DC shunt motor is directly proportional to the armature current. It’s almost a straight line if effect of armature reaction is neglected. If armature reaction is taken in consideration then torque will increase but non-linearly with armature current.

3. If a DC shunt motor is working at full load and if shunt field circuit suddenly opens _____________________
a) Will make armature to take heavy current, possibly burning it
b) Will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
c) Nothing will happen to motor
d) Motor will come to stop

View Answer

Answer: a [Reason:] At no load or lower loads, there is possibility of excess speed in such cases but here, when full load is given, armature circuit will draw very high current in order to maintain back emf. So, if fuses or circuit breakers are not used then, armature circuit may burn.

4. Speed of DC shunt motor is directly proportional to___________
a) Flux
b) Back emf
c) Terminal voltage
d) Armature resistance drop

View Answer

Answer: b [Reason:] The armature circuit equation for DC shunt motor is given by Ea = Vt -IaRa. But back emf is also equal to kφn. Thus, kφn= Vt -IaRa, which further gives n = (Vt -IaRa)/ kφ. Hence, speed is directly proportional to back emf.

5. Speed torque characteristic of DC shunt motor is _______________
a) Starting from origin
b) Starting from speed axis and increasing
c) Starting from speed axis and decreasing
d) Starting from speed axis and constant

View Answer

Answer: c [Reason:] Speed-torque characteristic falls as torque increases, because armature reaction causes decrease in flux whose square is in inverse proportion with speed, causing decrease in speed rapidly compare to increase in torque.

6. Correct equation of speed-torque characteristic of DC shunt motor is____________
dc-machines-questions-answers-operating-characteristics-dc-shunt-motor-2-q6

View Answer

Answer: a [Reason:] Speed-torque characteristic equation is obtained by n = (Vt -IaRa)/ kφ and Torque = k1φIa. Thus, by combining these equations and by assuming all constants equal to k and kl we get the final equation shown above.

7. For some percentage increase in the torque, which DC motor will have the least percentage increase of input current?
a) Series motor
b) Shunt motor
c) Cumulative compound motor
d) Separately exited motor

View Answer

Answer: a [Reason:] The mechanical torque T is directly proportional to the product of flux per pole φ and armature current. In case of DC series motor, up to saturation point flux is proportional to field current because Ia = If. Now if small percentage increase in armature current (before saturation) will occur the same percentage of torque will increase. Whereas in DC series motor the torque is proportional to square of the armature current (before saturation).

8. DC Shunt Motor has a disadvantage __________
a) Constant speed
b) Less expensive
c) Not suitable for rapidly changing loads
d) Cannot be determined

View Answer

Answer: c [Reason:] In DC shunt motor, due to shunt characteristics speed variation depends not only upon the controlling resistance but on the load current also. This double dependence makes it impossible to keep the speed sensibly constant on rapidly changing load, which is in contrast with characteristic.

Set 3

1. The speed of a DC shunt motor can be increased by ______
a) Increasing the resistance in armature circuit
b) Increasing the resistance in field circuit
c) Reducing the resistance in the field circuit
d) Reducing the resistance in the armature circuit

View Answer

Answer: b [Reason:] Speed of the DC motor is directly proportional to the back emf and inversely proportional to the flux produced by field. Where, flux produced is directly proportional to the current passing through the field winding (linear magnetization).

2. What will happen if excitation of DC shunt motor is changed?
a) Torque will remain constant
b) Torque and power both will change
c) Torque will change but power will remain constant
d) Torque, power and speed, all will change

View Answer

Answer: c [Reason:] The motor will accelerate the mechanical load connected during this period but no increase in the mechanical load as Pload = T1W1 = T2W2 where W2 >W1. So, at the higher speed there is less electrical torque for the same mechanical load / power.

3. If the speed of a DC shunt motor is increased, the back emf of the motor will ___________
a) Increase
b) Decrease
c) Remain same
d) Become zero

View Answer

Answer: a [Reason:] From, the speed-current characteristics of DC shunt motor we know that speed of the motor is directly proportional to the back emf and inversely proportional to the flux. So, for more speed there will be more back emf generated.

4. The speed of a DC shunt motor can be made more than full load speed by __________
a) Reducing the field current
b) Decreasing the armature current
c) Increasing the armature current
d) Increasing the excitation current

View Answer

Answer: a [Reason:] Speed of the DC motor obtained from speed equation is inversely proportional to flux produced by the field. So, reducing the field current flux produced by armature will decrease, and speed will increase.

5. Speed regulation of DC shunt motor is calculated by ratio of difference of full load speed and no-load speed with full load speed.
a) True
b) False

View Answer

Answer: b [Reason:] Speed regulation is defined as a ratio of difference of no-load speed with full load speed with no-load speed. Here, no-load speed is more than the full load speed. Thus, we divide difference by no-load value and not by full load value.

6. Which speeds can be obtained from field control of DC shunt motor?
a) Lower than rated speeds
b) Greater than rated speeds
c) Lower and greater than rated speeds
d) Neither lower nor greater than rated speeds

View Answer

Answer: b [Reason:] Speeds greater than rated speeds can be obtained by lowering the flux of shunt field motor. Field cannot be made any stronger, it can only be weakened by this method. Thus, speed lower than the rated speed can’t be obtained.

7. No load speed of the DC shunt motor is 1322 rpm while full load speed is 1182 rpm. What will be the speed regulation?
a) 12.82 %
b) 11.8 %
c) 16.6 %
d) 14.2 %

View Answer

Answer: b [Reason:] Speed regulation is equal to (No-load speed – Full load speed) / (Full load speed). By substituting all the values, speed regulation= (1322-1182)/ 1182. Speed regulation is given by 0.118. In percentage notation SR= 11.8 %.

8. Speed regulation of a DC shunt motor is equal to 10%, at no load speed of 1400 rpm. What is the full load speed?
a) 1233 rpm
b) 1273 rpm
c) 1173 rpm
d) 1123 rpm

View Answer

Answer: b [Reason:] Speed regulation is equal to 0.1 which is also equal to (no-load – full load speed) divided by full load speed. Thus, by substituting all known quantities we get full load speed = 1400/1.1 = 1272.7 rpm so, speed equal to 1273 rpm.

9. Where will speed-torque characteristics will lie when armature reaction is considered?
a) Below the speed-torque characteristics when armature reaction is not considered
b) Above the speed-torque characteristics when armature reaction is not considered
c) On the speed-torque characteristics when armature reaction is not considered
d) Can be anywhere with the speed-torque characteristics when armature reaction is not considered

View Answer

Answer: b [Reason:] The speed-torque characteristic which has a small linear drop due to the second term (Ra effect) and translates upwards as the field is weakened due to the armature reaction. The demagnetizing effect of the armature reaction causes the characteristics to somewhat bend upwards with increasing torque (increasing load current).

10. Working range of the speed-torque characteristic, with increasing speed will ___________
a) Reduce
b) Increase
c) Remain same
d) Cannot comment

View Answer

Answer: a [Reason:] The working range of the speed-torque characteristic reduces with increasing speed in order for the armature current not to exceed the full-load value with a weakening field. Thus, armature current gives the bound limit for curve.

11. For speed x rpm, we get field current If1 and for speed y rpm, we get the field current If2. If y is greater than x then, ________________
a) If1 f2
b) If1 >If2
c) If1 =If2
d) Cannot comment on If1, If2

View Answer

Answer: b [Reason:] When speed-torque characteristic for different speeds is plotted on the same graph, we get the curve limited by armature currents also. For any value of field current flux through the field is directly proportional current, while flux is inversely proportional to speed.

12. 400-V dc shunt motor takes a current of 5.6 A on no-load and 68.3 A on full-load. Armature reaction weakens the field by 3%. What is the ratio of full-load speed to no-load speed? Given: Ra = 0.18 Ω, brush voltage drop= 2 V, Rf = 200 Ω.
a) 1.2
b) 0.8
c) 1.4
d) 1

View Answer

Answer: d [Reason:] If = 400/200= 2 A No-load: Ia0 = 5.6 – 2 = 3.6 A Ea0 = 400 – 0.18 3.6 – 2 = 397.4 V Full-load: Ib>a (fl) = 68.3 – 2 = 66.3 A Ea (fl) = 400 – 0.18 / 66.3 – 2 = 386.1 V n (fl)/n (nl) = [386.1/397.4] [1/0.97] = 1.

13. In which of the following method, effect of armature reaction is more?
a) Field weakening method
b) Armature resistance control
c) Same in both methods
d) Cannot be determined

View Answer

Answer: a [Reason:] In field weakening method we are reducing the working flux to increase the speed, by reducing the field current. Therefore, effect of armature flux on main field flux will increase in case of field weakening method.

Set 4

1. Why starters are required in a DC motor?
a) Back emf of these motors is zero initially
b) These motors are not self-starting
c) These motors have high starting torque
d) To restrict armature current as there is no back emf at starting

View Answer

Answer: d [Reason:] At the time of starting (n=0), the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be Ia= V/Ra. Since armature resistance is very low, armature current drawn is very high and will damage the machine.

2. For which motors DOL starter can be used?
a) Up to 5 H.P
b) Up to 10 H.P
c) Up to 15 H.P
d) Up to 20 H.P

View Answer

Answer: a [Reason:] DOL starters are limited to the small rating motors where distribution system (mains supply) can withstand high starting currents without excessive voltage dips. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points, increases the life of starter.

3. A three-point starter is used for _________
a) Shunt motors
b) Shunt as well as compound motors
c) Shunt, compound and series motors
d) Not for DC motors

View Answer

Answer: b [Reason:] 3-point starters are used only for shunt and compound motors, they are not used for series motors. Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed.

4. The starting resistance of a DC shunt motor is generally ______
a) low
b) Around 0.5 kΩ
c) Around 5 kΩ
d) Infinitely large

View Answer

Answer: a [Reason:] Starting resistance of a DC shunt motor and DC compound motor is low. Well, that’s the reason why we use starters in a DC motors, in order to limit the armature current flowing through the armature and to protect machine circuitry.

5. In a manual shunt motor starter relay positions are ________________________
a) Over load relay is connected in series and no volt relay in parallel with the load
b) Over load relay is connected in parallel and no volt relay in series with the load
c) Over load relay and no volt relay are both connected in series with the load
d) Over load relay and no volt relay are both connected in parallel with the load

View Answer

Answer: a [Reason:] In simple manual shunt motor starter like 3-point starter over load relay coil is kept in series with DC mains while no volt coil is kept in parallel with DC mains. Both the coils are equally important in a motor starter circuit.

6. What will happen if DC motor is used without starter?
a) Heavy sparking at brushes
b) It’ll start smoothly
c) Will not start at all
d) Depends on load

View Answer

Answer: a [Reason:] It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life. Such heavy current cannot be generally permitted to be drawn from the source of supply.

7. Motor will start quickly when used without starter.
a) True
b) False

View Answer

Answer: a [Reason:] The only thing in favour of direct starting must be mentioned here. Since the torque of the motor with direct start is much higher, the motor starts much more quickly. As a consequence, the Joule input per start is much less than that with resistance start.

8. In shunt and compound motor starting the shunt field should be made on with full starting resistance in ____________
a) Series with field
b) Parallel with field
c) Series with armature
d) Parallel with armature

View Answer

Answer: c [Reason:] In shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. A short time delay in this position allows the field current to build up to the steady value of the inductive field transients.

9. A starter is required for a 220-V shunt motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required?
a) 5
b) 4
c) 6
d) 8

View Answer

Answer: c [Reason:] I1=55 A, I2 =35 A So, γ= Ratio of upper limit to the lower limit = 55/35 = 1.57, R1= 200/55= 4 Ω Now, γn-1 = 4/0.4 = 10. By substituting γ value, we get n = 6.

10. γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance?
a) 0.235
b) 0.370
c) 1.476
d) 2

View Answer

Answer: a [Reason:] γ is given as 1.585. So γ-1 is equal to 0.631. R1 is provided and it is equal to 4 Ω. r1= (1-0.631) * 4 = 1.476 Ω r2= 1.476*0.631= 0.931 Ω, similarly calculating till r5= 0.235 Ω.

11. Four-point starter is used when ______________
a) Motor field current is varied in narrow range
b) Motor speed is varied in small range
c) Motor field current is varied over wide range
d) Can be used anywhere

View Answer

Answer: c [Reason:] Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.

Set 5

1.Swinburne’s test can be carried out on all DC motors.
a) True
b) False

View Answer

Answer: b [Reason:] Swinburne’s test is a no-load test; thus, it is performed only on shunt and compound motors. DC series motor is not advisable to start at no-load so, this test can’t be performed on series motor.

2. Which of the following test will be suitable for testing two similar DC series motors of large capacity?
a) Swinburne’s test
b) Hopkinson’s test
c) Field test
d) Brake test

View Answer

Answer: c [Reason:] As Swinburne’s test and Hopkinson’s test are no-load test, one can’t perform these tests on DC series motor. For DC series motor with large capacities, we conduct field test to find various losses occurring in a machine.

3. Which losses can be identified from Swinburne’s test?
a) No-load core loss
b) Windage and friction loss
c) No-load and windage and friction loss
d) Stray load loss

View Answer

Answer: c [Reason:] We get total rotational losses occurring in a machine, which are equal to no-load core losses (iron + copper) and windage and friction losses. We also get shunt field losses and variable loss occurring in armature resistance.

4. While carrying out Swinburne’s test at rated armature voltage motor will run at ____________
a) Speed equal to rated speed
b) Speed greater than rated speed
c) Speed less than rated speed
d) Can run anywhere

View Answer

Answer: b [Reason:] The motor is set to run on a rated speed while conducting Swinburne’s test by adjusting field current to rated value. When rated armature voltage is applied, motor runs with slightly greater speed than the rated one as some o the losses are not taken into account.

5. In order to run motor on rated speed while carrying out Swinburne’s test we add ______________
a) Resistance in parallel with armature
b) Resistance in series with armature
c) Inductor in series with armature
d) Capacitor in parallel with armature

View Answer

Answer: b [Reason:] The machine would run at higher than rated speed with a rated armature voltage. Therefore, a series in the armature circuit is employed to reduce voltage applied to the motor armature such that it runs at rated speed.

6. What is the purpose of performing retardation test after Swinburne’s test?
a) To find stray load loss
b) To find variable losses
c) To separate out windage and friction losses
d) To find shunt field losses

View Answer

Answer: c [Reason:] While performing Swinburne’s test we get addition of all losses. So, if we want to treat each loss individually, one needs to separate them. Retardation test is thus used by disconnecting both armature and field of the running motor.

7. Efficiency calculated by Swinburne’s test is _____________
a) Exactly equal
b) Over-estimated
c) Under-estimated
d) Depends on the manual errors

View Answer

Answer: b [Reason:] The stray-load loss cannot be determined by this test and hence efficiency is over-estimated. Correction can be applied after assuming the stray-load loss to be half the no-load loss, which is done generally. Also, temperature may effect on resistance value which is not considered in this test.

8. Which of the following is not a disadvantage of a Swinburne’s test?
a) The stray-load losses can’t be determined by this test
b) Steady temperature rise can’t be determined
c) Does not give results about satisfactory commutation
d) Machine gets damaged

View Answer

Answer: d [Reason:] For increasing life and durability of a machine we carry out such tests. But stray load losses, steady temperature rise is not determined by this method. Also, results about satisfactory commutation are not given by this method.

9. While carrying out retardation test, if t is equal to time constant then _____________
a) Speed increases to 36.8% of its initial value
b) Speed reduces to 36.8% of its initial value
c) Speed reduces to 26.8% of its initial value
d) Speed reduces to 46.8% of its initial value

View Answer

Answer: b [Reason:] While carrying out retardation test, speed of the motor decreases. At time t equal to time constant then speed reduced is given by 36.8% of its initial value. Retardation test result is used to determine initial slope of ω(t).

10. In retardation test _____________
a) Motor switch is made ON and various speed readings are taken
b) At rated speed various speed readings are taken out at different times
c) Motor switch is made OFF at rated speed and various speed readings are taken
d) Some readings are taken while speed is building up and some readings while speed is lowering down

View Answer

Answer: c [Reason:] The motor is run to rated speed (or any high speed) and the supply is switched-off. As the motor decelerates (retards), several speed-time readings are taken, by a speedometer and watch with seconds hand. Initial readings are taken at small time intervals and the time interval is increased as the motor slows down.

11. Retardation curve is ______________
a) Starting from origin
b) Starts from some positive value and increasing
c) Starts from some positive curve and stays constant
d) Starts from some positive value and decreases

View Answer

Answer: d [Reason:] Retardation curve is a plot of speed vs. time at various retarding speeds. Thus, curve starts at some positive value and shows exponentially decaying nature with time. From various values of speed we calculate windage and friction loss at each point.