Electric Machines (4) Flashcards
What is the basic transformer equation?
v1/v2 = N1/N2
where v1 and v2 are the induced voltages in the primary and secondary coils and N1 as well as N2 are the number of turns in each coil of the primary and secondary.
Note: this can be extended to mention current, (thats on another flash card but for ease i2/i1 = v1/v2 = N1/N2).
What is a transformer and its features?
What is a transformer?
* A transformer is a device that ‘steps-up’ (increases) or ‘steps-down’ (decreases) the voltage of an AC input to desired values.
Tranformer features:
* Made up of a primary coil, where the original AC voltage enters
* A core made up of (often) laminated sheets of steel to reduce eddy currents and reluctance, which carries the magnetic field generated by the primary coil to the seconary coil.
* The secondary coil where the magnetic flux in the core then generates a voltage corresponding to the ratio of turns of the primary and secondary coils, as the magnetic flux in the core fluxtuates similarly to the inital voltage.
What is the circuit symbol of a transformer?
What is the power-current relationship in an ideal tranformer? This creates a new transformer equation.
Ideally no power is lost so Pin = Pout.
Since Pin = i1V1
And Pout = i2V2
we can express the following relationship:
i2/i1 = v1/v2 = N1/N2
or for RMS values:
I2/I1 = V1/V2 = N1/ N2 (note they are now capitalised)
What is meant by an R.M.S value?
R.M.S value or Root-Mean-Square value is used in place of average values.
This is because the sin wave goes positive and negative but both are the same magnitude so the average would be 0. Therefore if we square the value and then find the mean, we get the R.M.S value
Why is R.M.S is useful?
R.M.S value is useful as it essentially works out the voltage/current/power/etc required from an equivant DC circuit to get the same state. i.e. if a bulb in an AC circuit with a fixed resistor gets to a bright state, then we can enter a DC source equal to the R.M.S value of the AC source to get the same brightness.
What is the circuit symbol for an AC source?
What are the main reasons for using AC instead of DC for power generation, transmission and distribution?
- AC is natural to electrical generators and motors
- Easier to switch-off as the current naturally falls to zero every half cycle
- AC voltage, v, can easily and efficiently be stepped up (increased) using a transformer.
–> Since power = vi, then by increasing v, the current i is going to decrease, hence transmission loss (I^2 R) is going to reduce.
–> This reduces the size of the transmission lines needed to carry the current
How can AC be converted to DC? Think in the example from a wall outlet.
A tranformer in the cable plug will lower the voltage to the desired value for the appliance, then this is converted to DC using a full wave rectifier.
What features are found in a coil rotating AC generator?
Feature:
* Magnetic poles that stay fixed
* The coil that rotates in the magnetic field
* Slip rings to tranfer the current from the rotating wires to the circuits
* Brushes to get the current from the slip-rings to the circuit.
