Capteurs et actionneurs électriques Practice Quiz (11 questions)

1. Which components are included in an industrial process involving electrical power conversion?

A. An electric actuator, a load, a transformer, and a protective relay

B. A power electronic converter, a mechanical gearbox, a generator, and a cooling system

C. A control system, a sensor network, a utility grid, and a storage battery

D. A power electronic converter, an electric actuator, a load, and control equipment

Answer: (D)

Explanation

The industrial process includes a power electronic converter, an electric actuator, a load, and control equipment. A transformer and protective relay may be present in an installation, but they do not replace the listed process components.

2. When defining an electro-physical power conversion for an industrial application, what factors should be considered?

A. The operating environment, the actuator’s materials, and the maintenance schedule

B. The motor’s rated speed, the load’s mass, and the converter’s purchase price

C. The application, the motor’s efficiency, and the available control interfaces

D. The application, the operating environment, and the conversion’s impact on that environment

Answer: (D)

Explanation

The definition should account for the industrial application, the operating environment, and the conversion’s impact on that environment. Rated speed and purchase price may matter in design, but they do not constitute the full set of factors specified here.

3. What is the air gap in an electrical machine?

A. The space between the rotor and its shaft, maintained by a coupling

B. The space between the stator and its frame, maintained by mounting brackets

C. The space between the stator and rotor, maintained by bearings

D. The space between the machine and its load, maintained by a flexible coupling

Answer: (C)

Explanation

The air gap is the space between the stator and the rotor, maintained by bearings or rolling bearings. The clearance between a rotor and its shaft is a different mechanical interface.

4. How is the operating point of a machine coupled to a load determined?

A. By the intersection of the motor’s mechanical characteristic and the load’s mechanical characteristic

B. By the difference between the motor’s maximum speed and the load’s rated speed

C. By the sum of the motor’s rated torque and the load’s rated torque

D. By the intersection of the motor’s electrical characteristic and the supply-voltage curve

Answer: (A)

Explanation

The operating point is where the motor and load mechanical characteristics intersect. Comparing rated torques does not identify that point because it does not account for how torque varies with speed.

5. What role do the graphite brushes play in a DC machine?

A. They carry current between the external circuit and the commutator, enabling current reversal in the armature conductors.

B. They connect the field winding to the stator poles and regulate the magnetic flux in the air gap.

C. They insulate the copper commutator segments from the armature winding during rotation.

D. They close the magnetic circuit by guiding flux through the rotor and machine frame.

Answer: (A)

Explanation

The graphite brushes provide the current path through the commutator and support reversal of current in the armature conductors. The commutator’s copper segments are connected to the armature windings; the brushes make sliding electrical contact with those segments.

6. How is a DC motor started when its current must be limited to a maximum value?

A. It starts at nominal voltage, which is gradually lowered to the current limit.

B. It starts with a negative torque, which is gradually changed to the rated torque.

C. It starts at reduced voltage, which is gradually raised to the nominal operating point.

D. It starts with reduced magnetic flux, which is gradually increased to the nominal value.

Answer: (C)

Explanation

Starting at reduced voltage limits the initial current, and the voltage is then increased progressively toward the nominal operating point. Reducing magnetic flux is used to extend speed above nominal, not as the described current-limited starting method.

7. What is the purpose of autopilotage in a synchronous machine?

A. To keep the stator field synchronized with the rotor while maintaining a constant angle between them

B. To switch the stator phases according to current measurements without tracking rotor position

C. To regulate rotor current so that the stator flux remains constant as load changes

D. To make the rotor run at a speed that differs from the stator field by a controlled slip

Answer: (A)

Explanation

Autopilotage synchronizes the speed of the stator field with the rotor while keeping their autopilotage angle constant. Controlling a speed difference between the rotor and field describes slip-based operation rather than synchronous autopilotage.

8. A four-pole asynchronous machine supplied at 50 Hz runs at about 1450 rpm. When does its rotor speed differ from the synchronous field speed?

A. When the rotor reaches the same speed as the field, producing maximum slip

B. Only after the supply frequency is changed to adjust the machine speed

C. Only during startup, before the rotor reaches its steady operating speed

D. Whenever the rotor turns below or above the field speed, producing nonzero slip

Answer: (D)

Explanation

Slip is the relative difference between the stator field speed and the rotor speed, so the speed difference exists during asynchronous operation. The startup-only interpretation is incorrect because the stated 1450 rpm operating speed remains below the 1500 rpm synchronous speed.

9. How does scalar control differ from vector control of an asynchronous machine?

A. Scalar control requires a speed sensor to regulate rotor speed; vector control avoids current regulation by directly setting stator frequency.

B. Scalar control separates flux and torque through current components; vector control sets stator-field speed while rotor speed varies with load.

C. Scalar control sets stator-field speed while rotor speed varies with load; vector control separates flux and torque through current components.

D. Scalar control adjusts rotor currents using a wound rotor; vector control derives commands from measured voltages and currents and a machine model.

Answer: (C)

Explanation

Scalar control imposes the stator-field speed, leaving rotor speed dependent on load, whereas vector control independently manages flux and torque using separate current components. Direct torque control is the approach that derives commands from measured currents and voltages compared with a machine model.

10. A motor driving a load has a large moment of inertia. What effect does this have when the motor accelerates or brakes?

A. It decreases the torque required during rapid changes in speed.

B. It increases the torque required to change the system’s speed.

C. It makes the motor’s steady-state speed depend on its mass.

D. It reduces the resisting torque once the system begins moving.

Answer: (B)

Explanation

A large inertia resists changes in rotational speed, so more torque is needed during acceleration and braking. Resisting torque describes the load’s opposition to motion, not the inertia’s effect on speed changes.

11. When selecting a drive for a centrifugal fan, which load-torque relationship should be used as the practical model?

A. The resisting torque increases in direct proportion to speed.

B. The resisting torque remains approximately constant as speed changes.

C. The resisting torque decreases in inverse proportion to speed.

D. The resisting torque increases in proportion to the square of speed.

Answer: (D)

Explanation

Centrifugal fans are classified as loads whose resisting torque is proportional to the square of their speed. A constant-torque model applies instead to examples such as conveyors and lifting equipment.