WorksheetsDC Machines Worksheet — Questions Converted to Scorable Items
Total questions: 19
Worksheet time: 10mins
A DC motor used in a workshop is making excessive noise and vibration during operation. On inspection, the outer frame shows cracks and rust. Identify the affected part of the DC machine.
Yoke
Armature core
Commutator
Brushes
A DC motor used in a workshop is making excessive noise and vibration during operation. On inspection, the outer frame shows cracks and rust. Explain how damage to this part affects the magnetic circuit and mechanical strength of the machine.
It weakens the magnetic circuit, causes mechanical imbalance and vibration, and reduces efficiency.
It increases air-gap flux and improves cooling, raising efficiency.
It mainly affects electrical insulation only, with no influence on flux path or mechanics.
It reduces brush wear and improves commutation under load.
In a DC generator supplying power to a small industry, the output voltage fluctuates frequently. Engineers suspect improper magnetic flux distribution. Which constructional part is responsible for uniform flux distribution?
Pole shoes
Yoke
Commutator
Brush holders
In a DC generator supplying power to a small industry, the output voltage fluctuates frequently. Engineers suspect improper magnetic flux distribution. Explain the role of the responsible part in improving machine performance.
It spreads the magnetic flux evenly over the armature surface, reduces magnetic reluctance, and improves commutation and efficiency.
It merely supports bearings and has no effect on flux distribution.
It converts alternating emf to direct current, stabilizing voltage.
It increases brush pressure to reduce contact resistance.
A DC motor fails to develop sufficient torque even when the supply voltage is normal. Which winding should be inspected first?
Field winding
Armature winding
Compensating winding
Rotor bars
A DC motor fails to develop sufficient torque even when the supply voltage is normal. Explain how the condition of this winding influences torque production.
Weak or faulty winding reduces flux, leading to low torque.
Worn bearings increase torque by reducing friction.
Short-circuited commutator segments increase torque at low speed.
Higher brush resistance increases torque by boosting current.
During routine maintenance of a DC machine, overheating of the armature is observed. Which constructional feature of the armature core helps reduce heating?
Laminations in the armature core
A solid steel core
Additional bearing lubrication
Thicker commutator segments
During routine maintenance of a DC machine, overheating of the armature is observed. Explain why laminations are used instead of a solid core.
They reduce eddy current losses, minimizing heat generation and improving efficiency.
They increase hysteresis losses to raise torque.
They provide mechanical strength only, with no electrical effect.
They allow higher brush pressure without sparking.
A DC motor used in a conveyor belt shows reduced speed under load conditions. Identify the armature component responsible for current flow and emf generation.
Armature winding
Machine frame
Commutator hub
Shaft
A DC motor used in a conveyor belt shows reduced speed under load conditions. Explain how the identified component’s design affects motor performance.
It carries current, interacts with the magnetic field to produce torque, and is where emf is induced; a poor winding condition reduces torque and speed.
It only provides mechanical support and therefore does not influence torque or speed.
It converts AC to DC, so its geometry alone sets the speed.
It mainly controls cooling airflow and thus determines speed directly.
A DC motor shows heavy sparking at the brush–commutator interface during operation. Identify the function of the commutator in a DC machine.
It converts alternating emf in the armature into direct current at the output terminals.
It strengthens the yoke to carry mechanical loads.
It regulates field current automatically with speed.
It cools the brushes by increasing airflow.
A DC motor shows heavy sparking at the brush–commutator interface during operation. Explain how a faulty commutator causes sparking.
Worn segments, uneven surface, poor alignment, or improper commutation cause sparking.
Low magnetic permeability of the yoke increases brush pressure, creating sparks.
Excessive field flux eliminates commutation and prevents sparking.
High-quality brush material always prevents sparking regardless of commutator condition.
In a printing press DC motor, frequent replacement of brushes is required. What is the function of brushes in a DC machine?
They provide electrical contact between the rotating commutator and the external circuit.
They magnetize the yoke to increase flux.
They laminate the armature core to reduce losses.
They convert AC to DC at the output terminals.
In a printing press DC motor, frequent replacement of brushes is required. Explain how improper brush material or pressure affects machine operation.
It causes excessive wear, poor contact, and potential sparking, degrading operation.
It increases flux in the field winding, raising torque.
It improves commutation by lowering contact resistance regardless of surface condition.
It reduces carbon dust production while enhancing efficiency.
A DC motor in an industrial lifting mechanism is not starting smoothly and produces uneven torque. Which constructional parts could cause this problem? Select all that apply.
Field winding
Commutator
Brushes
Yoke
Pole shoes
A DC motor in an industrial lifting mechanism is not starting smoothly and produces uneven torque. Select the statements that correctly pair each identified part with its function and a plausible fault.
Field winding: produces the main magnetic field; a weak field reduces torque.
Commutator: converts armature AC to DC; an uneven surface causes poor commutation.
Brushes: provide electrical contact; improper contact leads to current fluctuation.
Yoke: carries coolant; dust accumulation increases torque.
During preventive maintenance, carbon dust is found accumulated near a DC machine. Which constructional parts are associated with this issue? Select all that apply.
Brushes
Commutator
Field winding
Armature core
During preventive maintenance, carbon dust is found accumulated near a DC machine. Select the statements that explain their function and why regular maintenance is required.
Carbon dust results from brush wear; maintenance prevents sparking, overheating, and insulation damage.
Brushes magnetize the yoke; maintenance increases flux.
Commutator segments create airflow; maintenance is unnecessary for dust.
Field winding produces mechanical strength; dust indicates normal operation.
Why are DC machine yokes usually made of cast steel or rolled steel instead of aluminium? Name the part and explain its magnetic and mechanical function in the machine.
Yoke; it must carry magnetic flux and provide mechanical strength.
Armature core; it must carry current and convert AC to DC.
Commutator; it laminates the core and reduces eddy currents.
Brushes; they provide mechanical rigidity to the frame.
