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Inductance, capacitance and
resistance
- As previously discussed inductors and capacitors create loads on a circuit.
- This is called reactance.
- It varies depending on current and frequency.
- At no frequency, or DC there is no reactance.
- At low frequency capacitors create the most reactance
- At high frequency inductors create the most reactance
Inductance, capacitance and
resistance
- Since inductive reactance varies with frequency and inductance the formula for this is Xl=2π f L where f is frequency and L is Henrys and Xl is in Ohms.
- Ohms law for inductance is the same as that used to combine resistances in series and parallel circuits.
- An inductor will cause current to lag behind voltage because induced voltage resists current changes.
Inductance, capacitance and
resistance
- Therefore capacitive and inductive reactance
counter, or cancel each other.
- Their effect on phase counters the other’s
phase effect.
- ELI the ICEman
- E leads I with an L (inductor)
- I leads E with a C (capacitor)
Inductance, capacitance and
resistance
- Since resistance doesn’t effect phase the net
of the two reactances, with the lessor subtracted from the greater, will act upon total impedance at 90° to resistance.
- But since reactance is already expressed in
the form of Ohms in a purely reactive circuit Ohms laws applies normally for a purely inductive or capacitive circuit.
Inductance, capacitance and
resistance
- Ohms law works for AC circuits with
inductors, capacitors and resistances.
- Series circuits solve for impedance first, in
parallel solve for currents since the V-drop is the same across each leg.
Inductance, capacitance and
resistance
• Resonance is when the frequency is
such that a capacitor in series with an
inductor cancel each other’s reactance.
• Similar resonance in a parallel circuit
with an inductor and capacitor will have
infinite resistance at a resonant
frequency.
Inductance, capacitance and
resistance
- Apparent Power is that derived from
measuring voltage and current in an AC circuit and multiplying them.
- True power is the power actually used by the
resistive load and does not contain the power lost to reactance.
- Power factor = 100 X True Power / Apparent
Power
Inductance, capacitance and
resistance
110V 400hz
270 Ω
300 μf
31mH
Xl= 2π f L Xc= 1/(2π f C) Rt= R Z^2 = Rt^2 + (Xc-Xl)^2 It = E/Z
Transformers
- A transformer is a set of two or more inductors in close proximity whose purpose is to exchange voltage for current in an AC circuit.
- If the voltage or current is incorrect for a given application it can be transformed up or down.
- The catch is if one goes up, the other must go down.
- The other catch is this will lose some power within the circuit.
Transformers
Transformers
Transformers
Transformers
Transformers
- They can be cooled, often in an oil bath.
- They are limited by the apparent power being
driven through them.
- Excessive power input or output can overheat
them.
- They can have different cores from iron to air.
