Mod 6 Electromagnetism Flashcards
(34 cards)
1
Q
What is back emf in a DC motor
A
- Induced emf in a DC motor
- Called back since it opposes the supply emf
2
Q
Why is there back emf in a DC motor
A
- A rotor coil turns in a uniform magnetic field
- The rotor coil experiences changing magentic flux
- An EMF is induced
3
Q
What is relationship between EMF(back) and I(coil)
A
EMF(coil) = EMF(supply) - EMF(back) = I(coil)R
4
Q
For an ideal motor at operating speed what is relationhsip between supply and back EMF
A
Supply EMF = Back EMF
5
Q
What is expression for power loss
A
P(loss) = I^2 R
6
Q
What is expression for resistance in relation to power loss
A
- R = (Roh(L)/A)
- Roh = resistivity of wire
- L = length of wire
- A = cross sectional area
7
Q
What is V in regards to power loss durin transmission
A
V is voltage drop across the transmission lines
8
Q
How is V(drop) calculated in a transmission line
A
- V(drop) = IR
- Or difference in voltage across the two wires
9
Q
What is alternative word for EMF
A
Voltage
10
Q
How do most generators function
A
- External force and torque applied
- Armature and coil rotate in magnetic field
- Rotor coil experiences change in flux
- Alternating emf and current are induced in the coil
11
Q
What are the features of an AC generator
A
- Stator (pair of perm magnets or electromagnetic coils)
- Rotor: consists of armature, rotor coils and axle
- Contacts: slip rings and brushes
12
Q
What is the armature and its role
A
- Cylinder of laminated iron
- Carries the rotor
- Iron core concentrates the external magentic field and increases induced emf
13
Q
What is result of AC generator spinning at a faster rate
A
- 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
14
Q
What is the role of the commutator
A
- Reverses direction of current every half rotation
- Ensures the direction of torque is maintained in the same direction to ensure motion is continuous
15
Q
What is unit for torque
A
Newton metres Nm
16
Q
What does graph of torque look like for standard DC motor starting with vector area perpendicular to magnetic field
A
- Starts at tmax
- Shape of cos wave that does not fall below at x-axis
- Touches x-axis at T/4 and 3T/4
17
Q
What is effect of adding coils in different planes to a DC motor
A
- 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
18
Q
3 magnets dropped through a:
- A. copper tube,
- B. copper tube with slits
- C. plastic tube
which one hits the ground first and why
A
- 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
19
Q
How does an AC motor operate
A
- 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
20
Q
What speed does an AC motor rotate at
A
- Speed of rotor is always less than speed of rotating magnetic field RPM(R)<rpm>
</rpm><li>Rotor never reaches synchronous speed (speed when rotor and rotating magnetic field are equal)</li><li>Difference between speed of rotor and synchronous speed is called slip</li>
</rpm>
21
Q
What is purpose of laminated cylindrical iron core in an AC motor
A
- 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
22
Q
Why are AC motors the most common motor used toay
A
- Require less maintenance (no contacts)
- Self-starting
- AC is most common type of power supply
23
Q
What is operation of stator in an AC induction motor
A
- Opposing stator coils are connected to one phase of the three phase current
- AC continually changes the strength of each pair of electromagnets 120 degrees out of phase
- This creates apparent rotating magnetic field
24
Q
What is operation of the rotor in an AC induction motor
A
- Rotating magnetic field induces currents with opposing magnetic polarity’s in bars of squirrel cage rotor
- Eddy currents produce magnetic fields which interact with rotating field of the stator
- A torque is exerted on rotor in direction of rotation of the stator field
- The rotor rotates in same direction of magnetic field
25
How is steadier EMF produced in DC generators
* Radial magnets
* Pair of perpendicular coils
26
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
27
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
28
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
29
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
30
How is steadier torque produced in a DC motor
* Radial magnets
* Set of perp coils
31
What is torque
* The tendency of a force to rotate the body to which it is applied
32
How does electromagnetic braking work
* Eddy currenst induced and create attractive and repulsive forces
* create drag force
* reduce speed
33
What is equation for emf in generators/solenoids
-N(BA)/dt = -Blv
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