11.3 Transformers Flashcards
If an alternating voltage Vp is applied tot he primary coil what happens in the secondary coil?
Because the magnetic field i t produces is varying the flux linkage in the secondary coil is also changing and so an EMF Vx is induced such that Vp/Vs = Np/Ns
What is the power in a perfectly efficient transformer?
The power from the secondary coil equals the power into the primary coil so VpIp = VcIs
Therefore Ip/Is = Ns/Np
What do step up transformers do?
Increase the output voltage.
What do step down transformers do?
Decrease the output voltage.
What happens in real transformers?
They suffer from power losses, the output power is less than the input power.
What are the three reasons for power losses?
Heating of the coil wires
Eddy currents
Hysteresis
Flux loss
What are eddy currents?
Currents created when free electrons inside the iron core are forced to move due to the magnetic field they are exposed to. This creates heating in the core and power loss.
How are eddy currents reduced?
By laminating the core.
What is hysteresis?
As the current increase so does the magnetic field and the stored magnetic energy. As the current decreases the magnetic energy decreases as well but not all of it returned, some energy remains stored.
What is flux loss?
Some od the flux created in the primary coil does not link the secondary coil. The iron core reduces flux loss to a very large degree.
What is the power loss for transmission cables?
Plost = RI^2 as heat in the cables.
How can the heat lost in the cables be reduced?
By reducing the current, this means transmitting at very high voltage which requires transformers.
What are the different transformers used to reduce the current?
A step up transformer is used to raise the voltage, the current will be reduced down the cables so much less power is wasted. A step down transformers is then used to reduce the high voltage to a safe low value to be supplied to consumers.
