AC Systems Flashcards
constant speed drive purpose
alternator will produce constant output frequency and voltage
what is an IDG and what is its purpose
integrated drive generator
CSD and generator are mounted side by side to form 1 compact unit
prevents overheating & save space
difference between alternators and generators
alternators - field rotates
generators - armature rotates
advantages of brushless generators
low maintenance cost - no slip rings or brushes
minimize chance of sparking of high current
how to express emf VR, VY, VB in phasor form
VR = V < 0
VY = V < -120
VB = V < -240
impedance formula
Z = R + XL + XC
formula for capacitance and inductance
XC = 1/ (j2pi * f * C)
XL = j2pi * f * L
I phase formula using Z
I phase = V phase / Z
balance star load, neutral wire how many amps?
0 A
if is balance star load, what would be equal?
ZR = ZY = ZB
current will be similar
total apparent power formula
sqrt3 (V line * I line)
total reactive power formula
sqrt 3 (V line * I line * sin theta)
wattmeter formulas
W1 + W2 = sqrt 3 (VL * IL * cos theta)
W1 - W2 = VL * IL * sin theta
wattmeter formulas to find tan theta and cos theta (PF)
tan theta = sqrt 3 ( (W1 - W2)/(W1 + W2) )
power factor = cos theta = cos( arctan( sqrt3( (W1 - W2)/(W1 + W2) )))
what is a motor
motor takes in electrical energy and converts it to mechanical energy
why are electric motors not 100% efficient?
electric energy is lost in the form of heat, from I^2R losses
what equipment can be used to start synchronous motor?
pony motor starter
are synchronous motors self starting
no, needs power supplied to rotor to start rotating
how does induction motor works?
rotor flux interacts will rotating (main) flux, creating mechanical force that causes rotor to rotate in the same direction as magnetic field
why does induction motors need to have slip? and what is slip
slip is the difference between magnetic field rotation and rotor’s rotating speed
if induction motor have slip = 0, there will be no flux cut as both field and rotor will be stationary wrt e/o
advantages & disadvantages of single phase motor
adv:
1) small and cheap
2) no need 3 phase supply
disadv:
1) cannot self-start
2) creating a pulsating torque
3) less efficient
how does resistance-split motor works?
- starter winding’s current lags voltage by much less than 90deg
- phase difference causes starter winding to be energised before main winding
- centrifugal switch opens at 25% rated speed, disconnect starter winding
how does capacitor-split motor work?
- offsets inductance causes even greater phase shift between currents
- used where good starting torque is important
- centrifugal switch open at 25% rated speed
how does shaded pole motor work?
- increases inductance of copper ring’s shaded part
- energies unshaded parts of poles first, creating magnetic fields in these areas
- shaded magnetic field to lag behind unshaded magnetic field, creating rotational effect
what is VVVF and its purpose
variable-voltage variable-frequency drive
control the speed and torque of ac motor
parts of VVVF drive
input power bridge rectifier filter control circuit & inverter pulse-width modulated (PWM) power
what does a rectifier do
converts AC input to DC outputs
what does a capacitor do in a rectifier?
during +ve half cycles: stores charge
during - ve half cycles: discharges
maintain constant output voltage across R
to reduce ripple, discharge time constant is increased
silicon-controlled rectifier states of operation
1) forward blocking
2) reverse blocking
3) forward conducting
auto transformer used for
circuits requiring only a small step-up or step-down of voltage
an ac brushless generator is excited by the used of?
dc field exciter
parts of a AC brushless generator
DC exciter
AC exciter
Rotating Rectifier assembly (rotor)
Main generator (AC output stator)
why is alternator better than armature generator?
rotating assembly (field winding) is lighter so easier to rotate thicker conductors can be used to carry larger current and easier to cool down
AC generator +ve and-ve sequence
+ve: RYB
-ve: RBY
