Power Electronics Flashcards
What is Faraday’s Law of inductors?
vL = L(diL/dt)
What is the Ampere-Maxwell Law of capacitors?
iC = C(dvC/dt)
What is the average value of voltage across an inductor at steady-state?
Zero
What is the average value of the current of a capacitor at steady state?
Zero
Define power factor
A measure of how effectively the load draws the real power
What is the equation for power factor?
PF = P/S P = active power S = apparent power
Define total harmonic distortion
The quantified level of distortion of a current waveform
What is the formula for THD?
THD = sqrt[(Irms)^2-(I1rms)^2]/I1rms
How is the electric field in a diode created?
p type joined to n type electrons near the interface diffuse into the p regions and leave positive charged ions in n region. likewise for n to p but with negative charge.the regions now near the interface become charged. the electric field created bu the space charge region opposes the diffusion.
What are the external characteristics of a diode?
draw graph of I(d) by V(d) that shows breakdown voltage and threshold voltage
Draw the equivalent circuits for:
- froward bias power diode
- reverse bias power diode
- power diode for breakdown
1) diode with resistor Vf and Vd flowing through, iD opposing Vd> Vf
2) the same but -V(BR) < vD ( Vf
3) just a diode but facing other way with Vd following across and Vbr and Id opposing Vd < - V(BR)
What happens if a thyristor gate doesn’t receive pulses?
-thyristor has a forward and reverse blocking capabilities because there is always a p-n junction reverse bias
Why use a firing circuit for thyristors?
- pulse technique for precise control of the point on wave at which the thyristor is fired
- dissipates less energy in the gate than a continuous current
Why use a transformer with a thyristor?
- to turn on you need a pulse current which can create large losses
- transformer can change low v high i to low i high v
How do your turn off a thyristor?
- need an external circuit to turn off reverse polarity of supply voltage
- first decrease the conduction current to 0
- reverse bias for a time interval
Compare MosFET and IGBT
- MosFET have higher switching frequencies, limited y paretic capacitances and have inherent body diode
- IGBT higher blocking capabilities due to large depletion layer and the minor carrier injection. have a combination of transistor and MosFET characteristics
What is the communication overlap?
- time when two or more diodes are conducting at the same time because the current cannot switch instantaneously
- for 6 pulse 3 or more, every 60 degrees
How would you calculate the average voltage for a diode rectifier?
V = 1/T ∫ v(t) dt
shift limits by π/6
V = 3/π ∫ V(LL, max) cos (ωt) dωt
How do you calculate average current?
I = 1/T ∫ i dt
where i = V(t) / R
and V(t) = ∫ Ri + L di/dt dt
Define slip
slip is the difference between the rotational speed of the magnetic field and the rotational speed of the rotor
-for induction motors slip ≠ 0
What is the equation for slip?
s = (ω - pω(r)) /ω
What is the equation for electrical torque?
T(el) = P(mech) / ωr
What is the equation for P(mech)?
P(mech) = 3 * [(1-s)/s] * r’(r) * I²(r)
