Answer: d [Reason:] DC motors are used for traction as, according to the characteristics of DC motors speed is inversely proportional to torque and square of armature current as well, if linear magnetization is concerned. Thus, DC motors are perfectly suitable for traction.

Answer: b [Reason:] Conveyors need high starting torque initially, and constant torque later. Thus, DC series motor is used in conveyors as it provides very high starting torque, which is practically 5 times the rated torque.

Answer: d [Reason:] Rotary compressor generally demand constant speed operation throughout the load. Sometimes, DC machines are not able to produce constant speed throughout the process hence, synchronous machine is used.

Answer: c [Reason:] Cumulative compound DC motor is used with flywheel carrying peaks and so to smooth out the load on the motor as well as to reduce peaks on power system. Without flywheel the motor construction will be much larger.

Answer: b [Reason:] Separately excited DC generators are still used in wide output voltage control like in Ward Leonard speed control. In all power plants today, generally AC generators are used due to low cost and less maintenance required.

Answer: a [Reason:] For a dc machine, of course, the main attraction lies in its flexibility, versatility and ease of control. This explains why in spite of its rather heavy initial investment it still retains its charm in strong competitive industrial applications.

Answer: a [Reason:] While for all other motors maximum torque is restricted to certain value as various losses in other motors lead to heating of the core materials. In DC machines for maximum torque commutation time will obviously decrease and beyond some point commutation process can’t be fastened.

Answer: c [Reason:] DE series motor’s closest rival is the wound-rotor induction motor with a rotor resistance control. But ultimately the availability and economics of a dc power is the deciding factor rather than the motor characteristic.

Answer: d [Reason:] Owing to the relative simplicity, cheapness and ruggedness of the squirrel cage induction motor, the shunt motor is less preferred for constant-speed drives except at low speeds. At high or medium speed applications we use induction motor, mostly squirrel caged.

Answer: b [Reason:] At low speeds, DC shunt motors are comparable with synchronous motors. The outstanding feature of a DC shunt motor however is its superb wide range flexible speed control above and below the base speed using solid-state controlled rectifiers.

Answer: d [Reason:] Lathes machines requires uniform torque which is provided with squirrel cage induction or DC shunt motors. Hence, they are preferred for lathes. DC shunt motor and induction motor of squirrel cage type follow same shunt characteristics.

Answer: b [Reason:] Normally in electric traction purposes DC series motors are employed. At above condition the back emf is greater than supply voltage hence, it will operate as series generator which will provide energy back to the supply.

Answer: c [Reason:] DC machine is used in tractions, because its characteristics suit perfectly with application. Also, its smooth braking capacity and ability to transfer from one mode to other is very good.

Answer: a [Reason:] A pneumatic motor (Air motor) is a motor which does mechanical work by expanding compressed air. Air motors generally convert the compressed air energy to mechanical work through either linear or rotary motion, and are preferred in highly explosive atmosphere.

Answer: c [Reason:] Due to the various speed control techniques available and with the help of electrical brakes various speed levels can be obtained in DC motor. While in AC motor for same purpose we require frequency change which requires another complex circuitry.

Answer: c [Reason:] In armature control method, we vary armature voltage by adjusting variable resistance value. So, when we define armature efficiency, it is equal to η = [ (V_t-I_aR_e) I_a] / V_tI_a= 1-I_aR_e / V_t. Thus, efficiency is reduced drastically for large speed reductions.

Answer: d [Reason:] In field control method we change the flux produced by machine with the help of field current. If applied voltage is changed, definitely speed will change. By adding resistance in series with armature voltage of the armature can be varied to change the speed.

Answer: b [Reason:] Armature control method provides a constant-torque drive. In the shunt motor case by keeping the field current at maximum value full motor torque can be obtained at full-load armature current at all speeds.

Answer: c [Reason:] According to terminology used in DC machines, emf induced in a machine when armature rotates is called as back emf. This back emf value is directly proportional to the speed of the motor.

Answer: a [Reason:] Drum controllers are used when an operator is controlling the motor directly/manually. The drum controller is used to start, stop, reverse, and vary the speed of a motor, by its mechanism. This type of controller is used on crane motors, elevators, machine tools, and other heavy applications.

Answer: b [Reason:] Unlike field control method, the main field ampere-turns are maintained at a large value, flux density distortion caused by armature reaction is limited. Unlike field control scheme, speed reversal can be easily implemented here.

Answer: b [Reason:] In rheostatic series control method, we add variable resistance in series with the armature so that armature voltage can be varied, by varying the voltage drop at series resistor. By increasing value of series resistor speed can be decreased.

Answer: c [Reason:] The drawback of armature speed control method is we get only speeds which are below the rated speed. As the value of series resistance increases, back emf decreases which result in lowering of the speed.

Answer: a [Reason:] Speed-torque characteristic will start from same point because at zero torque presence of series resistance is not taken into account. Afterwards speed torque characteristic with higher series resistance value will lie below the other.

Answer: c [Reason:] The speed regulation of the method is poor as for a fixed value of series armature resistance, the speed varies directly with load, being dependent upon the resistance voltage drop. In general, rheostatic control is economically feasible only for very small motors (fractional kW) or for short-time, intermittent show-downs for medium-sized motors.

Answer: c [Reason:] In shunted armature speed control method we add one variable resistance in series with armature and one variable resistance in parallel with armature, so by varying the ratio of this resistances we can get various different speeds.

Answer: b [Reason:] Speed regulation of shunted armature control method is better than that of rheostatic armature control. External power loss in shunted armature control method is very high compare to that of rheostatic armature control method.

Answer: c [Reason:] Here two identical motors are coupled together mechanically to a common load. Two speeds at constant torque are possible in this method—one by connecting the motors armatures in series and the other by connecting them in parallel.

Answer: c [Reason:] Series-parallel control method is superior to the rheostatic control insofar as efficiency is concerned. However, it is limited to two speed steps. The method is commonly employed for speed control of series traction motors.

Answer: a [Reason:] All the conductors on the armature periphery between adjacent brushes carry currents (of constant value, UNcIc) in one direction and the current distribution alternates along the periphery. Because of commutator action, armature current distribution is in the steps of UNcIc. Thus, mmf waveform can be generalized by joining peak points to get triangular wave.

Answer: a [Reason:] Direct axes is simply defined as the line passing through the axes of main poles. Maximum flux passes through this line. It’s also called as Direct Axis. Direct axes is always perpendicular to the geometrical neutral axis of machine.

Answer: d [Reason:] Flux density waveform is symmetrical and square wave with distortion at the zero points, causing wave to be in trapezoidal shape. The wave is flat topped, which get distributed due to armature mmf distribution, giving rise to the resultant flux distribution wave.

Answer: b [Reason:] The nature of armature reaction in a dc machine is cross-magnetizing with its axis (stationary) along the q-axis (at 90° elect. to the main pole axis). It causes no change in flux/pole if the iron is unsaturated but causes reduction in flux/pole (demagnetizing effect) in presence of iron saturation.

Answer: a [Reason:] Initially at unsaturated condition in a DC machine armature reaction lies along the q-axis. It will cause no change in flux/pole if iron is unsaturated. Now, when iron gets saturated axis gets shifted which will cause reduction in flux/pole.

Answer: a [Reason:] When the armature of a dc machine carries current, the distributed armature winding produces its own mmf (distributed) known as armature reaction. The demagnetizing component acts in the opposite direction, reducing flux/pole in a machine, which will ultimately reduce generator emf.

Answer: d [Reason:] Armature short circuit may occur due to contact of two commutator bars or due to contact in of two coil turns as commutators are connected to respective coil sides. If two or more turns of coil are grounded then they have common end which again leads to short circuit.

Answer: a [Reason:] Leading pole tip (LPT) and trailing pole tip (TPT) are the two edges of the pole, they depend upon the direction of motion of the armature (in case of DC). While performing a motion the armature first saws an edge of the pole, that edge is called leading pole tip. Thus, at leading pole tip there will be demagnetization.

Answer: d [Reason:] When the armature of a dc machine carries current i.e. load current in armature, the distributed armature winding produces its own mmf (distributed) known as armature reaction. The tothe field ampere-turns (AT_f) and armature ampere-turns (AT_a).

Answer: d [Reason:] When non-zero load current is passed through the armature winding, the distributed armature winding produces its own mmf known as armature reaction. According to its nature cross-magnetizing and demagnetizing, it will distort or reduce the main flux distribution.

Answer: a [Reason:] Brushes are generally located at 900 to direct axis. The axis 900 to the direct axis is called as quadrature axis (q-axis). Generally, q-axis is along the geometric neutral axis (GNA) of machine. The brushes in a DC machine are normally located along the q-axis.

Answer: c [Reason:] The armature reaction flux strengthens each main pole at one end and weakens it at the other end (cross magnetizing effect). Armature reaction with axis at 90° to the main field axis is known as cross-magnetizing mmf.

Answer: a [Reason:] If the main pole excitation is such that iron is in the saturated region of magnetization (this is the case in a practical machine), the increase in flux density at one end of the poles caused by armature reaction is less than the decrease at the other end, so that there is a net reduction in the flux/pole, a demagnetizing effect.

Answer: a [Reason:] Peak flux density is defined as ratio of total flux density given to the number of poles for a given DC machine. Peak flux density in terms of total flux density is given by AT_a (peak) = AT_a (total) /P.

Answer: b [Reason:] A DC machine is a heteropolar structure with stationary poles and the rotating armature. Field coil is mounted on stator. In an AC machine armature winding is located at stator while field coil is located at stator.

Answer: a [Reason:] Coil side current pattern is the same as the emf pattern. Only difference is that while the coil-side emf reduces towards the outer side of poles, the current remains the same in all the coil-sides except for alterations from pole to pole, while the coil side current reverses, the current exchanged with external circuit must be unidirectional.

Answer: a [Reason:] Since it is a DC machine the generated output must be DC. Any electrical machine works on induced emf concept which is AC in nature. Commutator and brush assembly of the DC machine performs the mechanical rectification process so; induced AC is converted into DC(Unidirectional).

Answer: b [Reason:] Brushes are located electrically in the magnetically neutral region. Due to their location adjoining brushes are at constant DC voltage and the coil in series between the constitute one parallel path.

Answer: c [Reason:] Brushes are at magnetically neutral region hence, induced emf due to armature coils at brushes will be equal to zero. As in the magnetically neutral region change in flux will be equal to the zero, emf will not be induced (Faraday’s law).

Answer: a [Reason:] Brushes are located electrically in magnetically neutral region. Adjoining brush pairs are at constant DC voltage and the number of coils tapped by brush pairs also remain constant. Their disposition relative to the poles is the same.

Answer: d [Reason:] Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.
Y_CS= 12/4= 3.

Answer: c [Reason:] Coil-side voltages around the coil are additive most of the time (except when coil-sides lie near the magnetic neutral region). Thus Y_CS= Nearest lower integer, which means that for non-integral S/P, the coils are short-pitched.

Answer: b [Reason:] Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Number of poles= 2, Slots in the armature winding= S= P* Y_CS. Here, Y_CS= 6, P=2. Thus S=12. C=S=Number of commutator segments.

Answer: c [Reason:] Pole pitch is called as center to center distance between two adjacent poles. When measured in electrical degrees one pole itch is equal to 1800. Coil span is simply a peripheral distance between two sides of a coil. If the coil span is equal to the pole pitch, then the armature winding is said to be full pitched coil.

Answer: b [Reason:] Unlike conventional DC motor, BLDC motor has its field winding rotating. Unlike PMDC motor field winding is present. Thus, it is placed on rotor. In BLDC motor we have fixed commutators, with brush gear rotating with speed of rotor field.

Answer: a [Reason:] In BLDC because of rotor position feedback triggering of thyristors/transistors, the stator and rotor field always remain in synchronism as the frequency of triggering automatically adjusts to motor speed. The length of on-time of the transistors determines motor torque magnitude.

Answer: d [Reason:] The brushless DC motor being more expensive compare to DC motor of same kW ratings, serves many other advantages like long life, less maintenance, rapid response, linear characteristic, no sparking and many more.

Answer: d [Reason:] The stator is connected to a variable voltage current source through an inductor and an inverter comprising six SCRs (S1 to S6). In place of SCRs, power transistor or FETs could be used according to power rating of the motor.

Answer: b [Reason:] BLDC motors are widely used in various applications of medical industry. Sensor less BLDC motor and with sensor BLDC motors are used because of easy operation and high reliability compare to conventional motors.

Answer: c [Reason:] Construction of BLDC motor is exactly similar to the PMSM i.e. permanent magnet synchronous motor. BLDC is considered as machine working like DC with construction similar to the AC synchronous machine.

Answer: a [Reason:] A typical brushless motor has permanent magnets which rotate around a fixed armature. An electronic controller replaces the brush-commutator assembly of the brushed conventional DC motor, which continually switches the phase to the windings to keep the motor turning.

Answer: b [Reason:] Brushless motors fulfil almost all functions originally performed by brushed DC motors, but cost and control complexity prevent brushless motors from replacing brushed motors completely in the lowest-cost areas. Brushless motors are now dominating many applications, particularly devices such as computer hard drives and CD/DVD players.