Electrical Circuits: Relays and Contactors
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Questions and Answers

What is the main reason shunts should be replaced as soon as possible?

  • They can improve circuit efficiency.
  • They are prone to mechanical wear.
  • They enhance the performance of other components.
  • Loose connections may cause overheating. (correct)
  • What happens to copper strands in a shunt when the terminals overheat?

  • They expand and improve connection.
  • They remain unchanged.
  • They become brittle and discolored. (correct)
  • They become more conductive.
  • What are contact tips primarily made of?

  • Silver
  • Aluminum
  • Copper (correct)
  • Steel
  • What condition do normally open interlocks indicate when the main contacts are closed?

    <p>Interlocks are opened.</p> Signup and view all the answers

    What type of contactor is preferred for heavy current traction circuits?

    <p>Air operated contactors.</p> Signup and view all the answers

    In what situation is an electromagnetic contactor NOT typically used?

    <p>When high current is present.</p> Signup and view all the answers

    What material is commonly used for the piston packing in air operated contactors?

    <p>Rubber or leather</p> Signup and view all the answers

    What happens in the unsaturated region of the magnetisation curve?

    <p>Flux density increases very rapidly with increasing magnetising force.</p> Signup and view all the answers

    Which of the following correctly describes the coercive force?

    <p>The amount of magnetising force needed to reduce residual magnetism to zero.</p> Signup and view all the answers

    What characterizes the saturation region of the magnetisation curve?

    <p>Flux density undergoes little or no change despite increased magnetising force.</p> Signup and view all the answers

    What does the area within the hysteresis loop represent?

    <p>The power losses per cycle of magnetism of the iron core.</p> Signup and view all the answers

    How does heat treatment affect the magnetisation curve?

    <p>It can affect the material's magnetic properties and curve shape.</p> Signup and view all the answers

    What is one of the primary functions of the lubricant used in air operated contactors?

    <p>It reduces wear and prolongs the life of the piston packing</p> Signup and view all the answers

    What happens to the operation of contactors if the cylinder walls become sticky?

    <p>Operation becomes sluggish</p> Signup and view all the answers

    Which pressure is typically required for the normal operation of air operated contactors?

    <p>70 psi</p> Signup and view all the answers

    How does the magnet valve affect the flow of air to the piston?

    <p>It controls the flow of air remotely by electricity</p> Signup and view all the answers

    What is a common issue that can occur with the ground valve seat of a magnet valve?

    <p>It can leak if not fitted properly</p> Signup and view all the answers

    What component is used to minimize the arc created when the contactor opens a circuit?

    <p>Blow out coil</p> Signup and view all the answers

    What role does the return spring play in the operation of a contactor?

    <p>It forces the piston back when air supply is cut off</p> Signup and view all the answers

    What effect does cutting off the current through the operating coil have on the magnet valve?

    <p>It causes the valve to shift positions</p> Signup and view all the answers

    What characteristic do contactor tips possess to aid in their operation?

    <p>They exhibit a rolling or knuckling action</p> Signup and view all the answers

    What primarily influences the capacitance of a capacitor?

    <p>Area of plates, dielectric constant, and thickness of dielectric</p> Signup and view all the answers

    Which of the following is a type of fixed capacitor?

    <p>Electrolytic capacitor</p> Signup and view all the answers

    Which type of capacitor is commonly used to prevent burning flash in relay contacts?

    <p>Electrolytic capacitor</p> Signup and view all the answers

    What does the breakdown voltage of a dielectric refer to?

    <p>Voltage at which the dielectric fails</p> Signup and view all the answers

    What happens to a capacitor when connected to a DC voltage?

    <p>It gets charged and can eventually discharge</p> Signup and view all the answers

    In the context of magnetism, what is true about electron spin within an atom?

    <p>Some electrons spin clockwise and some counterclockwise</p> Signup and view all the answers

    Which type of capacitor allows for variable capacitance based on construction material?

    <p>Variable capacitor</p> Signup and view all the answers

    What is the commonly used smaller unit of capacitance in practical applications?

    <p>Pico-farad</p> Signup and view all the answers

    Which of the following materials is not listed as a dielectric type for fixed capacitors?

    <p>Plastic</p> Signup and view all the answers

    What effect does increasing the thickness of the dielectric have on a capacitor rated for higher voltage?

    <p>It decreases the capacitance</p> Signup and view all the answers

    What is the product of the number of turns of a coil and the current flowing through the coil called?

    <p>Magnetomotive Force</p> Signup and view all the answers

    What does the symbol mmf stand for?

    <p>Magnetomotive force</p> Signup and view all the answers

    How is permeability (μ) defined?

    <p>The measure of magnetic conductivity of a substance</p> Signup and view all the answers

    If a coil produces 1000 lines of flux in air and 6000 lines with another material, what is the permeability of that material?

    <p>6</p> Signup and view all the answers

    What is reluctance in magnetic circuits analogous to in electric circuits?

    <p>Resistance</p> Signup and view all the answers

    What happens to the flux density at very low magnetizing forces according to the B-H curve?

    <p>It increases at a very low rate.</p> Signup and view all the answers

    What occurs beyond 5 ampere turns per inch on the B-H curve?

    <p>The rate of increase of flux drops off.</p> Signup and view all the answers

    What does the term 'saturated' refer to in the context of magnetic materials?

    <p>When no more flux can be accumulated</p> Signup and view all the answers

    What is the relationship between flux density (B) and magnetic force (H)?

    <p>B = μ x H</p> Signup and view all the answers

    What indicates that a material has high permeability?

    <p>It can easily conduct magnetic flux.</p> Signup and view all the answers

    What primarily defines the function of a relay in an electrical circuit?

    <p>To operate by electromagnetism and control the flow of current in response to another circuit</p> Signup and view all the answers

    Which component is directly responsible for closing the contact tips in an electromagnetic contactor?

    <p>The movement of the armature due to magnetic attraction</p> Signup and view all the answers

    What is a significant risk associated with the wear of the knife edge on the armature of a contactor?

    <p>It can result in faulty operation due to improper contact closure</p> Signup and view all the answers

    What is the consequence of an arc striking the shunt in a contactor?

    <p>Permanent damage leading to broken shunts</p> Signup and view all the answers

    In what manner do relays typically indicate their de-energized position in circuit diagrams?

    <p>By displaying contacts in their normally open or closed state</p> Signup and view all the answers

    What is a common characteristic of contactors as compared to relays?

    <p>They have a greater capacity for handling heavy electrical loads</p> Signup and view all the answers

    What factor influences the operational lifespan of an electromagnetic contactor's shunt?

    <p>The physical damage caused by arcs and environmental conditions</p> Signup and view all the answers

    How does the presence of DC current in the control winding affect the operation of the magnetic amplifier?

    <p>It allows for maximum AC current flow in the load winding.</p> Signup and view all the answers

    What is the mathematical expression for impedance in an AC circuit involving resistance and inductive reactance?

    <p>Z = √(R^2 + XL^2)</p> Signup and view all the answers

    What happens to inductive reactance as the core saturates due to the control current?

    <p>Inductive reactance decreases, allowing more AC current.</p> Signup and view all the answers

    According to Faraday's Law, what is induced in a circuit when there is a change in flux?

    <p>Voltage</p> Signup and view all the answers

    What is a direct consequence of the alternating current varying from maximum to minimum in a load winding?

    <p>High inductive reactance leading to reduced current flow.</p> Signup and view all the answers

    Which function of the lubricant in air operated contactors is essential for ensuring an airtight seal?

    <p>Helps the packing fit closely</p> Signup and view all the answers

    What happens to the contact tips when the current is cut off from the magnet valve?

    <p>The tips open due to the spring action</p> Signup and view all the answers

    What is a potential consequence of having a bent or sticky valve stem in a magnet valve?

    <p>Delayed operation of the contactor</p> Signup and view all the answers

    How does the blow out coil function to protect the contact tips?

    <p>By creating a magnetic field to move the arc away</p> Signup and view all the answers

    What is indicated by a hissing sound coming from a magnet valve?

    <p>Air escaping due to a leak</p> Signup and view all the answers

    Which factor can lead to sluggish operation of air operated contactors?

    <p>Incorrect air pressure levels</p> Signup and view all the answers

    What role does the current flowing through the operating coil of a magnet valve serve?

    <p>It energizes a coil to move the valve</p> Signup and view all the answers

    What is a critical factor that the blow out coil helps to manage when a contactor opens?

    <p>Dissipation of heat from the arc</p> Signup and view all the answers

    How does air pressure assist in the normal operating function of air operated contactors?

    <p>It builds pressure to push the piston away</p> Signup and view all the answers

    What determines whether a material is considered magnetic or non-magnetic?

    <p>The orientation of electron spins within atoms</p> Signup and view all the answers

    What occurs when an external force aligns the domains of magnetic materials?

    <p>The material is said to be magnetized</p> Signup and view all the answers

    Which of the following statements correctly describes a 'domain' in magnetic materials?

    <p>It is an aligned group of atoms having magnetic properties</p> Signup and view all the answers

    How is flux density (B) defined mathematically?

    <p>Flux density is total flux divided by area</p> Signup and view all the answers

    What does it mean for a magnetic material to be 'saturated'?

    <p>All domains are aligned in one direction</p> Signup and view all the answers

    Which term describes the force required to create a flow of magnetic flux in a material?

    <p>Magnetomotive Force</p> Signup and view all the answers

    What effect does the alignment of electron spins have on the magnetic properties of a material?

    <p>It can vary the degree of magnetic strength</p> Signup and view all the answers

    Which of the following best describes the internal arrangement of domains in a non-magnetic material?

    <p>Domains have no specific arrangement and are randomly oriented</p> Signup and view all the answers

    What is the significance of 'lines of flux' within a magnetic field?

    <p>They represent the flow of magnetic force in a given area</p> Signup and view all the answers

    What role does the magnetic flux unit 'Maxwell' play in magnetism?

    <p>It quantifies the total magnetic flux</p> Signup and view all the answers

    What is represented by the symbol mmf in the context of magnetism?

    <p>Magnetomotive force</p> Signup and view all the answers

    How is the permeability of a substance mathematically defined?

    <p>Permeability (μ) = Flux density (B) / Magnetic Force (H)</p> Signup and view all the answers

    In the magnetic circuit analogy, what does reluctance correspond to in electrical circuits?

    <p>Resistance</p> Signup and view all the answers

    What does the B-H curve indicate at very low values of magnetizing force?

    <p>Flux density increases at a very slow rate.</p> Signup and view all the answers

    What characterizes the linear region of the B-H curve between 2 and 4 ampere turns per inch?

    <p>The domains begin to align efficiently.</p> Signup and view all the answers

    If a material shows increased flux density when subjected to a magnetizing current, what may this suggest about its permeability?

    <p>It has high permeability.</p> Signup and view all the answers

    When a material reaches saturation, what does this signify regarding its magnetic domains?

    <p>They have all aligned directionally.</p> Signup and view all the answers

    What does the term 'ferromagnetic' materials refer to in terms of permeability?

    <p>They exhibit high permeability.</p> Signup and view all the answers

    What effect does an increase in magnetizing force have on an ideal magnetic material's saturation point?

    <p>The saturation point remains unaffected.</p> Signup and view all the answers

    In what scenario does the flux density stop increasing, despite increases in magnetizing force?

    <p>When the material reaches saturation.</p> Signup and view all the answers

    Study Notes

    Relays

    • A relay is a switch operated by an electromagnet, not manually.
    • It can open or close one or more circuits when energized.
    • Used when the operation of one circuit depends on another.
    • Can be calibrated to operate at specific current values.
    • A relay is represented in a circuit diagram as a coil and a set of contacts.
    • When the operating coil is energized, a normally open contact closes, and a normally closed contact opens.
    • Circuit diagrams show relays in their de-energized state.

    Contactors

    • Contactors are heavy-duty switches operated by relays or other switches.
    • They fall into two main categories: electromagnetic (EM) and electro-pneumatic (EP).
    • Electromagnetic contactors are operated by electricity.
    • When energized, the current flows through the operating coil; the coil becomes a magnet, and the armature is pulled toward the core, closing the contacts.
    • The armature spring returns the armature to its original position when the current is stopped, opening the contacts.
    • The knife-edge on the armature may wear out, causing faulty operation.
    • A shunt, made of very thin copper wire, is a flexible lead that improves operation but can be easily damaged through misdirected arcs.

    Contact Tips

    • Contact tips are the parts that make contact to complete the circuit.
    • They can wear out mechanically or due to electrical burning.
    • They are usually made of copper and bolted into place.

    Interlocks

    • Small contact fingers that operate simultaneously with the main contacts.
    • Typically connected to a low-voltage or battery circuit.
    • Used for signaling when the main contacts operate.
    • Normally open or normally closed types.
    • Usually have silver contacts to reduce electrical resistance, common in low voltage circuits.

    Air Operated Contactors

    • Commonly used in heavy current traction circuits.
    • Contain a piston, piston rod, spring, and cylinder.
    • Flexible piston packing (often rubber or leather).
    • Piston rod moves a contact tip, "movable tip."
    • The cylinder is smooth for easy piston movement.
    • Lubrication is crucial to reduce wear and create an airtight seal.
    • Operated by compressed air at 70 psi (5 kg/cm²).
    • Air enters the cylinder, the spring forces the piston back when the compressed air stops flowing, opening the contact tips.

    Magnetic Valves

    • Used to control air flow to the piston in air-operated contactors.
    • Position determined electrically, operated by a remotely controlled coil.
    • One position admits air to the cylinder, the other cuts off the air supply for exhaust.
    • Require minimal maintenance but should be checked for leaks or bent/sticky stems.

    Switches

    • Devices used to make or break electrical current.
    • Exist in varying sizes depending on the application.
    • A common example is turning on/off a light, switching low-current circuits.
    • Can also carry heavy currents.
    • Switch design includes a hinged blade attached to terminals of a switch, connected to one terminal portion, and a switch base to the other.

    Capacitors

    • Components that oppose changes in voltage.
    • Made of two conductors separated by an insulator, often thin plates separated by an insulator (dielectric).
    • Measured in farads (F), microfarads (µF), or picofarads (pF).
    • Capacitance depends on the plate area, dielectric constant, and dielectric thickness.

    Magnetic Amplifiers

    • Devices that use magnetism to regulate or control the flow of current to the load.
    • Their operation is based on the magnetic properties of materials (especially iron)
    • The domains in magnetic materials can be controlled/affected by external magnetic forces causing an increase or decrease of the flux density in the material.
    • This can alter the inductive reactance of the material, influencing current flow.

    Additional Notes

    • The text includes figures and diagrams in describing some component parts.
    • Additional details and specifications vary depending on the application, but the core components involved are similar.

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    Description

    This quiz covers the fundamentals of relays and contactors, including their operation and applications in electrical circuits. Learn about how relays are controlled by electromagnets and how contactors function as heavy-duty switches. Test your knowledge on their circuit representations and energizing mechanisms.

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