Magnetism Magnetic Circuits and Transformers

Question 1

What is a solenoid

Answer 1

it is a current carrying wire formed into a long straight coil

Question 2

Flux density symbol

Answer 2

B = flux/area =phi/A

Question 3

flux symbol and units

Answer 3

phi in Wb (webers)

Question 4

how to calculate flux

Answer 4

F/St where F is the mangetomotive force in amp turns and St is the reluctance

Question 5

How do you calculate the reluctance

Answer 5

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

Question 6

How do you add reluctance in series

Answer 6

St = S1 + S2 + S3

Question 7

How do you add reluctance in parallel

Answer 7

1/St = 1/S1 + 1/S2 + 1/S3

Question 8

What is the standard equation for calculating flux

Answer 8

Use equation for mangetomotive force F=NI = flux * S (reluctance)
flux = NI/S

Question 9

relative perimetivity of air

Answer 9

mur = 1 and mu = 4pi *10-7

Question 10

If there is no change in the field because the flux linking the coil is constant then

Answer 10

there is no current induced in the coil

Question 11

if there is no change in field because the current is constant

Answer 11

then there is no voltage induced in the second coil

Question 12

if a conductor moves through a magnetic field then

Answer 12

an emf will be induced in the conductor

Question 13

Emf generated =

Answer 13

BLv where v is velocity B is flux density and L is length in the field
which simplifies to rate of change of flux

Question 14

How do you calculate the inductance of a coil

Answer 14

L = N^2/S

Question 15

emf induced across a coil is and what is it called

Answer 15

E = L dI/dt back emf

Question 16

what is the induced emf in a coil trying to do

Answer 16

resist the current; trying to slow down when the current is increasing and trying to prevent falling when it is decreasing

Question 17

what will a real inductor have

Answer 17

a resistor across it as well

Question 18

Draw the symbol of a transformer

Answer 18

two inductors facing each other with what looks like a capacitor in the middle

Question 19

Transformer rations

Answer 19

N1/N2 = V1/V2 = I2/I1 Pin = Pout

Question 20

A step up transformer

Answer 20

steps the voltage up

Question 21

turns ratio of 10:1 means

Answer 21

10 turns on the primary to 1 turn on the secondary,

N1/N2 = 10

Question 22

If the load on the secondary coil changes what happens

Answer 22

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

Question 23

if the current on the secondary has a magnitude and a direction what will the current on the primary be

Answer 23

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

Question 24

Prove the impedance ratio

Answer 24

see paper

Question 25

What is a VA rating of a transformer

Answer 25

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

Question 26

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

Answer 26

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

Question 27

Force on a current carrying conductor =

Answer 27

F = BIL

Question 28

When do you use left hand right hand for the flemings

Answer 28

Left hand is the motor rule -> for generating force

right hand is for generating current

Question 29

Unit of flux density

Answer 29

T tesla = flux / area

Question 30

Issues with dc motor

Answer 30

the current must be reversed every half turn in order to allow constant rotation
this is done using a rotary switch called a commutator

Question 31

Draw a separately excited dc motor

Answer 31

see paper

current If flowing through field winding produces magnetic field which is cut by rotating dc armature

Question 32

What is Ea

Answer 32

it is the back emf generated due to the magnetic field being cut

Question 33

In a separately excited dc motor what is Ea approximately equal to and why

Answer 33

Va as losses in the field winding is relatively small

Question 34

Draw a series DC motor

Answer 34

see paper

Question 35

Advantages and disadvantages of PM motor

Answer 35

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)

Question 36

Advantages of a series dc motor

Answer 36

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

Question 37

issues when starting dc motors

Answer 37

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

Question 38

Advantages of Universal motor

Answer 38

Can work on dc or ac

get to use ac supply on dc motor characteristics -> high starting torques compact design and high running speeds

Question 39

how does a universal motor work with ac

Answer 39

current in both the field and armature (hence the magnetic field) will alternate at the same time making mechanical torque generated always the same

Question 40

disadv of universal motor

Answer 40

maintenance and reliability problems caused by commutator

Question 41

Draw a permanent magnet dc motor

Answer 41

see paper

Question 42

Power output from a motor

Answer 42

P = T * angular velocity(omega)

Question 43

adv of brushless motor

Answer 43

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

Question 44

What are the copper losses of a transformer

Answer 44

losses due to resistance on the primary plus referred losses from the secondary (or vice versa should be equal)

Question 45

Total resistance referred to the primary side

Answer 45

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)