Summary
Motors and generators can have coils on shaft or casing or both. (both for universal).
Motors switch the field to be the wrong way with commutators.
Induction motors have designer eddy currents to create drag.
Energy is conserved, back torque and EMF with speed. High speed greater voltage.
Energy transforming devices can be compared:
They have different function:
Motors generate force from electricity
Generators make electricity from force
Transformers make electricity from electricity
They use different laws:
The motor effect describes current to produce force F = BilsinTheta
Faraday's Law v=-Dphi/DT describes changing magnetic flux to produce a voltage across a conductor. Either motion of the conductor or changing flux. Current flows if there is a circuit.
Conservation of energy applies to each:
When current flows in a generator, power is transferred, the motor effect resists motion W =Fxd, energy is conserved.
When a motor speeds up the Faradays law explains a back EMF
How do you describe a transformer?
A transformer is an electrically powered electricity generator.
A current in one circuit drives a magnet, that then drives the current in the next circuit .
The wires are drawn showing that electricity can not pass straight through the transformer. The primary is usually on the left or bottom of the diagram. The number of turns is N1. The more turns the stronger the magnetic field.
The core is drawn by a series of parallel lines to represent a laminated core. This core directs the magnetic field from the primary winding to the secondary winding. The iron core is permeable and intensifies the magnetic field inside the iron and reduces it outside therefore it acts as a channel for magnetic field. The energy transformation is electrical- magnetic-electrical.
N2 is the number of coils on the secondary. The voltage change across a transformer is given by
For AC currents. Faraday's law means that transformer time differentiates the input voltage. Having a rapid change in current, causes a voltage spike and spark. This is the principal of a cars spark plugs.
Step up transformers have many turns on the secondary coil. The wires on the secondary are small as the current for a particular power level is less. SUT are used for high voltages in long distance transmission, and in old TV sets, and street lighting.
Step down transformers have few turns on the secondary.
There is greater current in the secondary and the wires are thick to reduce resistive loss. SDT are used to get safe domestic voltage and again for Silicon Digital devices. They are also used for where high currents are needed in welding. Transformers are used to deliver high currents for speakers.
Managing energy losses in transformers.
There is ohmic or resistive energy loss as you can have kilometres of wire conductor in a transformer
The is mechanical vibration loss. The wires in a transformer are squeezed together at high current. This can be heard as transformer buzz. The mechanical vibration limit the matterials used for the core.
The are parasitic eddy currents. This is managed by using lamination to interrupt the eddy.
The is current leakage through the insulation of the wires. This is called dielectric break down. Special matterials are used PEEK and transformer oil.
The major problem of the losses is the increase in temperature leading to run away ohmic heating. Special transformer oil is cycled through the transformer similar to an oil heater. The oil moves through cooling fins, and if the air circulation is not enough fans are connected to force cooling. The oil is designed to expand when heated and so rise and push cool oil through the transformer. In large transformers only metal cores have the thermal conductivity. Transformer oil must withstand high voltages.
Major problems of failure in transformer is material degradation leading to break down. The oil channels can become clogged. Hot spots can cause run away failure. Dr Hill worked on optical fibre temperature sensors for inside transformers and optical fibre voltage sensors. You have sometimes as little as 1 second to shut a transformer down before it explodes.
Frustrated eddy current
