Electrical- Transformers Flashcards
Transformer circuit symbol
Technically there are two separate circuits. They are next to each other by two long inductor symbols. Between them are two parallel lines representing the core. The left circuit is the primary and the second is the secondary. Often has turns ratio above this.
What can be said about the fluxes through each coil in an ideal transformer?
They are the same. This means that I1/I2=N2/N1
This is 1/turns ratio
What can be said for a transformer with no losses?
Power in equals power out.
Therefore V1/V2=N1/N2
This is the turns ratio
Why do only alternating currents work with transformers?
Relies in a change in flux linkage to generate voltages. Direct currents don’t change flux linkage so don’t induce a voltage on either coil and the transformer acts like a piece of wire.
Why is it important to keep the reluctance of the core in a transformer very low?
To reduce any leakage flux which would not couple with the other coil and would result in self-inductance.
How to do loop analysis for a circuit with a transformer
Do normal current loops. Label unknown voltages on coils. The current in each coil is only the current in its loop. Use turns ratio to relate the currents and voltages. These give you the two extra equations you need.
How to refer a resistor from the secondary to the primary
Times it’s resistance by the turns ratio squared (N1/N2 squared). Now just a series circuit and no transformer.
How to refer inductors or capacitors from secondary to primary
They are treated as impedances. Therefore can times by turns ratio squared like normal resistors. If it was an inductor, use formula for its impedance (jωL) to find new inductance by multiplying by turns ratio squared. If capacitor use impedance is 1/jωC to find new capacitance by dividing turns ratio squared.
What happens when the flux in a core material keeps increasing to high levels?
There is a maximum flux a core material can support (above this it’s relative permeability drops rapidly. Often shown on graph as relative permeability against flux density. It is a hill shape and decreases and stays at very levels for very high B.
Formula for Vrms in terms of flux and turns on coil
2πfNφmax/rt(2)
Where φ max is maximum flux because sinusoidal input voltage.
f is frequency
Also equal to 4.44Nfφmax
What is VA rating?
The maximum current x voltage (both RMS) that the transformer ca tolerate. It is the apparent power as it neglects phase difference
