D_C_ Motors — Part 1 MCQs with Explanations

If a D.C. motor is connected across an A.C. supply, it will burn due to the heat produced in the field winding by eddy currents. The field windings in a D.C. motor are designed for direct current, and when subjected to alternating current, the changing magnetic fields induce eddy currents in the windings. These eddy currents generate excessive heat, leading to potential damage or burning of the motor.

In the case of D.C. shunt motors, the speed is dependent on back e.m.f. only because Flux is practically constant in D.C. shunt motors. In a shunt motor, the field winding is connected in parallel with the armature winding. This configuration ensures that the flux (magnetic field) remains relatively constant, regardless of changes in load or speed. As a result, the back electromotive force (e.m.f.) generated in the armature is primarily determined by the speed of the motor and not significantly affected by changes in the flux. Therefore, the correct option is Flux is practically constant in D.C. shunt motors .

A D.C. series motor is a type of direct current motor where the field winding is connected in series with the armature winding . Because the same current flows through both the field and the armature, the field winding must be designed to carry full armature current . To handle the high current, the field winding in a series motor is made of thick wire with fewer turns . This design allows the winding to carry large current without overheating. Other options are incorrect because: Option B: D.C. series motors have high starting torque, not poor torque. Option C: They should never be started without load, as they may accelerate to dangerously high speeds. Option D: They do not have constant speed; speed varies greatly with load. Correct Answer: Option A

When two DC series motor are connected in parallel then the resultant speed is more than the normal speed. If the voltage across each motor is V and current through each motor in parallel connection is $$\frac{I}{2}.$$ So $${\text{speed}} \propto \frac{{{\text{voltage}}}}{{{\text{current}}}} \propto \left( {\frac{{\text{V}}}{{\frac{{{I_{\text{a}}}}}{2}}}} \right) \Rightarrow 2\left( {\frac{{\text{V}}}{{{I_{_{\text{a}}}}}}} \right)$$ Hence the resultant speed is more than normal speed.

Self-loading property refers to a motor's ability to automatically increase its load when the speed decreases slightly. In simpler terms, it means how well a motor can handle sudden increases in the amount of work it needs to do. Option A: Series motor This statement is correct . Series motors have a high starting torque and naturally increase their torque output as the speed decreases. This makes them ideal for applications requiring heavy loads from the start, such as cranes or electric trains. The self-loading property is especially high in series motors because as the load increases, the current increases, which in turn increases the magnetic field and torque. This is why series motors are commonly used in high-torque applications where the motor needs to handle sudden increases in load. Option B: Shunt motor This statement is incorrect . Shunt motors maintain a relatively constant speed regardless of changes in load. While they are efficient and provide stable operation, they do not exhibit the self-loading property like series motors. The field winding in a shunt motor is connected in parallel to the armature, so the field current remains constant and does not increase as the load increases. This means that the torque does not increase significantly with load, leading to a poor self-loading property. Option C: Cumulatively compounded motor This statement is incorrect . Cumulatively compounded motors combine both series and shunt motor characteristics. They have a good balance between starting torque and speed regulation, but their self-loading property is not as high as a series motor. While they do have some self-loading features due to the series winding, they are not as efficient at increasing torque when speed decreases, making their self-loading property less pronounced compared to a series motor. Option D: Differentially compounded motor This statement is incorrect . Differentially compounded motors have poor starting torque and can even become unstable under heavy loads. They have the worst self-loading property of all the D.C. motor types. The field winding configuration in these motors causes the magnetic flux to oppose the armature flux, resulting in decreased torque and an inability to handle sudden increases in load effectively. Therefore, the correct answer is Option A: Series motor , as it has the maximum self-loading property due to its high starting torque and ability to automatically adjust to load changes by increasing its torque output as speed decreases.

At the instant of starting, when a D.C. motor is put on supply, it initially behaves like a highly resistive circuit . This behavior is primarily due to the low resistance of the armature windings and brushes, which offer minimal opposition to the flow of current. As a result, the motor draws a high starting current, often several times higher than its rated current, to overcome the initial inertia and start rotating. This high starting current, combined with the low resistance of the circuit, results in a situation akin to a highly resistive circuit. As the motor gains speed and reaches its operating condition, the resistance decreases, and the motor settles into its normal operating state. Options B and C are incorrect because: Option B: A low resistance circuit implies minimal opposition to the flow of current, which is not characteristic of a D.C. motor at the instant of starting. Option C: A capacitive circuit involves the storage and release of electrical energy in a capacitor, which is not relevant to the behavior of a D.C. motor at startup. Therefore, the correct answer is Option A: A highly resistive circuit, as this best describes the initial behavior of a D.C. motor when it is put on supply.

In a D.C. shunt motor , the field winding is connected in parallel (shunt) with the armature winding. Since the field winding is connected directly across the supply, the field current remains nearly constant regardless of the load. As a result, the magnetic field produced is stable, and any small change in armature current due to load variation has minimal effect on the speed . Therefore, even when the load is reduced, the motor speed does not change significantly. This is why D.C. shunt motors are widely used where constant speed is required . Correct Answer: Option C

Regenerative braking: This method is highly efficient and is widely used in elevators. In regenerative braking, the motor acts as a generator, converting the kinetic energy of the elevator into electrical energy, which is then fed back into the power supply. This results in a significant energy-saving benefit while ensuring a smooth and controlled stop. Why it's preferred for elevators: Efficiency: Regenerative braking does not waste energy as heat, unlike rheostat braking. It instead recovers the energy and returns it to the system, which is a major advantage in reducing operational costs and improving the system's overall efficiency. Safety and Reliability: Regenerative braking causes less wear and tear on the motor and other components compared to methods like plugging. It provides a reliable and smooth deceleration, which is critical for the safe operation of elevators. Comparison with other methods: Option A: Plugging: This involves reversing the polarity of the motor's voltage, generating a counter-torque that brings the elevator to a stop. While effective, it creates more wear on the motor and is not ideal for frequent use like in elevators. Option C: Rheostat braking: This method adds resistance to the motor circuit to dissipate energy as heat. While simple, it is less efficient because it wastes energy and is not as effective as regenerative braking in terms of energy recovery. Therefore, regenerative braking is the most efficient, safe, and reliable method for elevators, making it the best choice for frequent and smooth braking in such systems.

Variable losses = constant losses is the correct answer to “The condition for maximum efficiency for a D.C. generator is”. This question is standard in competitive examination syllabi for D_C_ Motors.

Increasing the resistance in field circuit is the correct answer to “The speed of a D.C. shunt motor can be increased by”. This question is standard in competitive examination syllabi for D_C_ Motors.

Constant torque drive is the correct answer to “The armature voltage control of D.C. motor provides”. This question is standard in competitive examination syllabi for D_C_ Motors.

To restrict armature current as there is no back e.m.f. while starting is the correct answer to “Starters are used with D.C. motors because”. This question is standard in competitive examination syllabi for D_C_ Motors.

Independent of Ia is the correct answer to “If I a be the armature current, then speed of a D.C. shunt motor is”. This question is standard in competitive examination syllabi for D_C_ Motors.

Up to 5 H.P. is the correct answer to “A direct on line starter is used: for starting motors”. This question is standard in competitive examination syllabi for D_C_ Motors.

Shunt motors is the correct answer to “The ratio of starting torque to full-load torque is least in case of”. This question is standard in competitive examination syllabi for D_C_ Motors.

It limits the starting current to a safe value is the correct answer to “For starting a D.C. motor a starter is required because”. This question is standard in competitive examination syllabi for D_C_ Motors.

Armature control method is the correct answer to “Which of the following method of speed control of D.C. machine will offer minimum efficiency”. This question is standard in competitive examination syllabi for D_C_ Motors.

All of the above is the correct answer to “Sparking at the commutator of a D.C. motor may result in”. This question is standard in competitive examination syllabi for D_C_ Motors.

Variable speed operation is the correct answer to “For which of the following applications a D.C. motor is preferred over an A.C. motor”. This question is standard in competitive examination syllabi for D_C_ Motors.

Series motor is the correct answer to “Which D.C. motor has got maximum self relieving property”. This question is standard in competitive examination syllabi for D_C_ Motors.

Over load relay is connected in series and no volt relay in parallel with the load is the correct answer to “In a manual shunt motor starter”. This question is standard in competitive examination syllabi for D_C_ Motors.

Residual magnetism of the generator is the correct answer to “In Ward-Leonard control the lower limit of speed is imposed by”. This question is standard in competitive examination syllabi for D_C_ Motors.

Series motor is the correct answer to “Which motor should not be started on no-load”. This question is standard in competitive examination syllabi for D_C_ Motors.

Reducing the field current is the correct answer to “The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by”. This question is standard in competitive examination syllabi for D_C_ Motors.

Compound motor is the correct answer to “D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this”. This question is standard in competitive examination syllabi for D_C_ Motors.

In series with armature winding is the correct answer to “In case of conductively compensated D.C. series motors, the compensating winding is provided”. This question is standard in competitive examination syllabi for D_C_ Motors.

Series motor is the correct answer to “If a D.C. motor is to be selected for conveyors, which rioter would be preferred”. This question is standard in competitive examination syllabi for D_C_ Motors.

All above factors is the correct answer to “Torque developed by a D.C. motor depends upon”. This question is standard in competitive examination syllabi for D_C_ Motors.

Low is the correct answer to “The starting resistance of a D.C. motor is generally”. This question is standard in competitive examination syllabi for D_C_ Motors.

Series motor is the correct answer to “D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the”. This question is standard in competitive examination syllabi for D_C_ Motors.

Reduce slightly is the correct answer to “As the load is increased the speed of D.C. shunt motor will”. This question is standard in competitive examination syllabi for D_C_ Motors.

500 W is the correct answer to “The total losses in a well designed D.C. generator of 10 kW will be nearly”. This question is standard in competitive examination syllabi for D_C_ Motors.

Commutator gets damaged is the correct answer to “Sparking, is discouraged in a D.C. motor because”. This question is standard in competitive examination syllabi for D_C_ Motors.

The speed is inversely proportional to the torque and the torque is proportional to square of armature current is the correct answer to “Why are the D.C. motors preferred for traction applications”. This question is standard in competitive examination syllabi for D_C_ Motors.

Line losses is the correct answer to “D.C. generators are installed near the load centers to reduce”. This question is standard in competitive examination syllabi for D_C_ Motors.

Armature copper loss is the correct answer to “The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator”. This question is standard in competitive examination syllabi for D_C_ Motors.

Conveyors is the correct answer to “Which of the following load normally needs starting torque more than the rated torque”. This question is standard in competitive examination syllabi for D_C_ Motors.

Reluctance motor is the correct answer to “Which of the following motors is most suitable for signaling devices and many kinds of timers”. This question is standard in competitive examination syllabi for D_C_ Motors.

Machine tools is the correct answer to “D.C. shunt motors are used for driving”. This question is standard in competitive examination syllabi for D_C_ Motors.

Back e.m.f. = $$\frac{1}{2}$$ × supply voltage is the correct answer to “The condition for maximum power in case of D.C. motor is”. This question is standard in competitive examination syllabi for D_C_ Motors.