2nd Year Class Test Flashcards
What direction do magnetic fields take?
N to S
What is the right hand screw rule for?
shows direction of magnetic field around a coil. Thumb indicated current flow.
What is Flux Density?
flux/Cross section Area
B = Φ/A
Measured in Teslas
What is flux?
Φ
Measured in Webbers (Wb)
What is relative permeability?
ratio of permeability / permeability of free space.
Permeability easured in (Wb/Am)
What is permeability of free space?
4pi*10^-7 (Wb/Am)
What is reluctance?
(Fancy R)= length / perm*Area (At/Wb)
What is MMF?
Magnetomotive Force
Fancy F) = Φ(fancyR
MMF = flux * Reluctance
MMF =
N*I
This can be subbed back into the equation relating MMF Reluctance and Flux
What is the field intensity?
H = MMF / length
At/m
How is Field intensity (magnetising force) and flux density related?
B = perm * H (T)
What is hysteresis?
a non linear relationship which follows different paths depending on direction. This is for B and H relationship.
What is the left hand Rule?
Thumb = FORCE First = FIELD Second = CURRENT
What is the equation for the thrust force in a field?
F = BIL
What is the equation in a linear generator?
emf=BLv
(v=velocity)
also:
emf = dΦ/dt
What is the right hand rule?
Thumb = MOTION First = FIELD Second = EMF
What is Faraday’s Law?
emf = N* dΦ/dt
How does the power output look from a 3 phase system?
total power out is a steady value and does not oscillate.
How do you calculate the power from a 3 phase system?
P = 3V(rms)I(rms)
All in phase
What are the characteristics of a STAR connection?
I line = I phase
V line = Root3<30 * V phase
neutral point is zero voltage when loads balanced.
What are the characteristics of a DELTA connection?
V line = V phase
I line = Root3<-30 * I phase
No neutral point
How do you find the power from a connection? (star or delta)
S = root3 * V line(mag) * I line (mag)
How do you calculate the power in an AC circuit?
P = V(max)^2/R *cos^2(wt)
always above 0
How do you calculate the average power in an AC circuit?
Pav = v(max)^2/2R
How do you measure capacitance?
C = Q/V (Farads)
Where Q = charge and V = PD
Whats the reactance of a capacitor?
Xc = 1 / wC
V lags I by 90deg
(multiply I by -j for IMPEDANCE)
How do you calculate inductance?
L = N^2 / fancyR (Henry)
Fancy R = reluctance N=turns
Whats the reactance of an Inductor?
Xl = wL
V leads I by 90 deg
(multiply by j for IMPEDANCE)
Whats the impedance of a resistor?
Same as resistance mate.
Why are impedances important?
used instead of resistances in AC circuits. V = Z*I
How to remember leading and lagging?
CIVIL
Capacitor = I leads V
Inductor = V leads I
What are the units for S P and Q respectively?
S=VA
P=W
Q=VAr
