Magnetism Magnetic Circuits and Transformers Flashcards
What is a solenoid
it is a current carrying wire formed into a long straight coil
Flux density symbol
B = flux/area =phi/A
flux symbol and units
phi in Wb (webers)
how to calculate flux
F/St where F is the mangetomotive force in amp turns and St is the reluctance
How do you calculate the reluctance
S=L/mu*A where mu is the permiability, L is the path length and A the the area of the path units of henrys to the minus 1, H^-1
How do you add reluctance in series
St = S1 + S2 + S3
How do you add reluctance in parallel
1/St = 1/S1 + 1/S2 + 1/S3
What is the standard equation for calculating flux
Use equation for mangetomotive force F=NI = flux * S (reluctance)
flux = NI/S
relative perimetivity of air
mur = 1 and mu = 4pi *10-7
If there is no change in the field because the flux linking the coil is constant then
there is no current induced in the coil
if there is no change in field because the current is constant
then there is no voltage induced in the second coil
if a conductor moves through a magnetic field then
an emf will be induced in the conductor
Emf generated =
BLv where v is velocity B is flux density and L is length in the field
which simplifies to rate of change of flux
How do you calculate the inductance of a coil
L = N^2/S
emf induced across a coil is and what is it called
E = L dI/dt back emf
what is the induced emf in a coil trying to do
resist the current; trying to slow down when the current is increasing and trying to prevent falling when it is decreasing
what will a real inductor have
a resistor across it as well
Draw the symbol of a transformer
two inductors facing each other with what looks like a capacitor in the middle
Transformer rations
N1/N2 = V1/V2 = I2/I1 Pin = Pout
A step up transformer
steps the voltage up
turns ratio of 10:1 means
10 turns on the primary to 1 turn on the secondary,
N1/N2 = 10
If the load on the secondary coil changes what happens
the voltage of the secondary coil is not dependent on the load therefore does not change but the current will, this will alter the current on the primary to match up
if the current on the secondary has a magnitude and a direction what will the current on the primary be
Dividing the current on secondary by the turns ratio will give the current on the primary; the magnitude will change the direction will remains the same
Prove the impedance ratio
see paper
What is a VA rating of a transformer
product of the rated voltage and rated full load current of a winding
to calculate multiple current and voltage together from one coil or the other
What is the equation relating max flux and Vrms and how might this be used to find the number of coils on primary and secondary
Vrms= 4.44Nf*phi(max)
given an input voltage maximum rated flux and req the coils on the primary can be found
if a known output voltage is required then V1/V2 = N1/N2 may be used
Force on a current carrying conductor =
F = BIL
When do you use left hand right hand for the flemings
Left hand is the motor rule -> for generating force
right hand is for generating current
Unit of flux density
T tesla = flux / area
Issues with dc motor
the current must be reversed every half turn in order to allow constant rotation
this is done using a rotary switch called a commutator
Draw a separately excited dc motor
see paper
current If flowing through field winding produces magnetic field which is cut by rotating dc armature
What is Ea
it is the back emf generated due to the magnetic field being cut
In a separately excited dc motor what is Ea approximately equal to and why
Va as losses in the field winding is relatively small
Draw a series DC motor
see paper
Advantages and disadvantages of PM motor
no losses in the field winding so more efficient
but once built cant alter the magnetic field -> can alter slope of torque speed graph (can alter its y intercept)
Advantages of a series dc motor
get high torques at low speeds and low torques at high speeds ideal for vehicle traction
speed control can be done by altering supply voltage as for a given torque speed is proportional to the voltage
but if no friction ie no torque have to be careful as speeds can be becomes very high so have to be careful to limit rotor speeds
issues when starting dc motors
back emf will initially be zero and initial currents will therefore be very high, okay for smaller motors -> armature resistance covers it, but for larger motors a starting resistance may be included
Advantages of Universal motor
Can work on dc or ac
get to use ac supply on dc motor characteristics -> high starting torques compact design and high running speeds
how does a universal motor work with ac
current in both the field and armature (hence the magnetic field) will alternate at the same time making mechanical torque generated always the same
disadv of universal motor
maintenance and reliability problems caused by commutator
Draw a permanent magnet dc motor
see paper
Power output from a motor
P = T * angular velocity(omega)
adv of brushless motor
no brushes to wear out or springs to pop out
no losses from brushes rubbing
no sparking brushes
better heat dissipation as stator can be placed externally
What are the copper losses of a transformer
losses due to resistance on the primary plus referred losses from the secondary (or vice versa should be equal)
Total resistance referred to the primary side
resistance of primary side + resistance referred to it by the secondary side (this total resistance can then be referred back to the secondary side to save work)
