Mod 6 Electromagnetism Flashcards Preview

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Flashcards in Mod 6 Electromagnetism Deck (34)
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What is back emf in a DC motor

  • Induced emf in a DC motor 
  • Called back since it opposes the supply emf 


Why is there back emf in a DC motor

  1. A rotor coil turns in a uniform magnetic field
  2. The rotor coil experiences changing magentic flux
  3. An EMF is induced


What is relationship between EMF(back) and I(coil)

EMF(coil) = EMF(supply) - EMF(back) = I(coil)R


For an ideal motor at operating speed what is relationhsip between supply and back EMF

Supply EMF = Back EMF


What is expression for power loss

P(loss) = I^2 R


What is expression for resistance in relation to power loss

  • R = (Roh(L)/A)
  • Roh = resistivity of wire
  • L = length of wire
  • A = cross sectional area


What is V in regards to power loss durin transmission

V is voltage drop across the transmission lines


How is V(drop) calculated in a transmission line

  • V(drop) = IR
  • Or difference in voltage across the two wires


What is alternative word for EMF



How do most generators function

  1. External force and torque applied
  2. Armature and coil rotate in magnetic field
  3. Rotor coil experiences change in flux
  4. Alternating emf and current are induced in the coil


What are the features of an AC generator

  • Stator (pair of perm magnets or electromagnetic coils)
  • Rotor: consists of armature, rotor coils and axle
  • Contacts: slip rings and brushes


What is the armature and its role

  • Cylinder of laminated iron
  • Carries the rotor
  • Iron core concentrates the external magentic field and increases induced emf


What is result of AC generator spinning at a faster rate

  • Faster rotation flux through coil changes at a faster rate
  • as dt decreases the amplitude of EMF increases
  • Each cycle of flux completes faster
  • Hence higher frequency 
  • Therefore increased speed means increased ampitude and frequency


What is the role of the commutator

  • Reverses direction of current every half rotation
  • Ensures the direction of torque is maintained in the same direction to ensure motion is continuous


What is unit for torque

Newton metres Nm


What does graph of torque look like for standard DC motor starting with vector area perpendicular to magnetic field

  • Starts at tmax
  • Shape of cos wave that does not fall below at x-axis
  • Touches x-axis at T/4 and 3T/4


What is effect of adding coils in different planes to a DC motor

  • Coils in different planes will produce max torque at different times
  • Plane of one coil is parallel to magnetic field the plane of the other will produce torque of different magnitude
  • Each toque produced by the coils will result in a steadier torque as resultant torque will fluctuate between two non-zero values


3 magnets dropped through a:

  • A. copper tube,
  • B. copper tube with slits
  • C. plastic tube

which one hits the ground first and why

  • Magnet C will leave its tube first 
  • Faraday's law states that EMF will be induced in a circuit when it experiences a change in magnetic flux
  • Furthermore Lenz's law states that induced current is such that it will create a magnetic field to oppose the change in flux which created it
  • Both A and B are copper tubes and as such will experience an EMF as it falls through. 
  • Eddy currents are produced around the tube so that magnetic flux produced by them opposes the change in flux due to the falling magnet
  • As a result the bottom end of the falling magnet is repelled upwards and top end is attracted upward
  • This process is possible in magnet A since eddy currents can form around the whole circumference of the tube
  • Magnet B the slots break up the eddy currents path and thus it will fall faster
  • However, magnet C will fall the fastest since plastic is not conductive and therefore no EMF and current will be produced and will experience no braking force


How does an AC motor operate

  • Stator coils are connected to a 3-phase AC supply
  • Results in apparent rotating magentic field being produced 
  • Establishes Eddy currents along bars of the squirrel cage rotor
  • Opposing magnetic fields are set-up in the rotor
  • Interaction between magnetic fields from stator coils and eddy currents produce torque
  • The rotor is dragged along chasing the rotating magnetic field in the stator


What speed does an AC motor rotate at

  • Speed of rotor is always less than speed of rotating magnetic field RPM(R)
  • Rotor never reaches synchronous speed (speed when rotor and rotating magnetic field are equal)
  • Difference between speed of rotor and synchronous speed is called slip


What is purpose of laminated cylindrical iron core in an AC motor

  • Iron core intensifies the magnetic field of the stator
  • Hence induces larger current in the stator
  • Resulting in a larger torque
  • Laminations reduce formation of eddy currents in core


Why are AC motors the most common motor used toay

  • Require less maintenance (no contacts)
  • Self-starting
  • AC is most common type of power supply


What is operation of stator in an AC induction motor

  1. Opposing stator coils are connected to one phase of the three phase current
  2. AC continually changes the strength of each pair of electromagnets 120 degrees out of phase
  3. This creates apparent rotating magnetic field


What is operation of the rotor in an AC induction motor

  1. 4. Rotating magnetic field induces currents with opposing magnetic polarity's in bars of squirrel cage rotor
  2. Eddy currents produce magnetic fields which interact with rotating field of the stator
  3. A torque is exerted on rotor in direction of rotation of the stator field
  4. The rotor rotates in same direction of magnetic field


How is steadier EMF produced in DC generators

  • Radial magnets
  • Pair of perpendicular coils


What are different types of power loss in a transformer

  • Flux leakage
  • Resistance in coils (joule heating)
  • Eddy currents in the core
  • Hysteresis of the core


What is flux leakage in transformer and how is it mitigated

  • When not all magnetic flux links both the primary and secondary coil reducing efficiency of transformer
  • Use ferromagnetic core, reduce distance between coils


What is resistance in coils in transformers and how is it mitigated

  • The wires have finite resistance so they heat up
  • Further increasing resistance and power loss
  • USe low resistance wire and use cooling strategies such as vents and fans


How does Lenz's law relate to law of conservation of energy

  • Energy cannot be created nor destroyed only transferred or transformed
  • Lenz's law complies with conservation of energy as direction of induced current opposes the change in energy
  • Evident as work must be done for electromagnetic induction to occur
  • Furthermore, if current was induced in other direction magnet would be accerlated spontaneously towards the coil giving it infinite acceleration 
  • Maintains total energy level in a system at 0


How is steadier torque produced in a DC motor

  • Radial magnets
  • Set of perp coils