Radiologic Physics Ch. 9 - Electric Generators & Motors
Right Hand Motor Rule
shows the direction the armature (conductor) will be thrust based on the relationship between the magnetic field and direction of the current. Its force is proportional to the length of the conductor or number of turns, strength of the magnetic field and size of the current.
Impedance
the total resistance of an AC circuit. This depends on : inductive reactance, capacitive reactance, & true electrical resistance.
Motor Principle
when a conductor carrying an electric current is placed in a magnetic field there is a side thrust on the conductor This occurs due to the magnetic lines surrounding the conductor that interact with the magnetic flux between magnetic poles.
Advantages of Alternating Current
1. AC allows us to operate transformers 2. Less power loss with AC vs DC - this occurs due to the varying nature of the sine wave with AC
Name the essential parts of a generator (dynamo)
1. An electromagnet 2. Armature
A basic electric generator consists of:
1. Wire loop 2. Magnet 3. Slip ring 4. Brushes
What is a generator (dynamo)?
A device that converts mechanical energy into electrical energy using electromagnetic induction.
Synchronous Motor
A type of AC electric motor that rotates at a fixed speed. Operates at speeds identical to or multiples of the generator
AC vs DC
AC currents vary in strength (amplitude) & direction. Also the voltage/amperage vary over time. DC current voltage and amperage remain constant. The direction remains constant. Voltage and amperage never fall below zero.
Current measuring devices
All current measuring devices work on the motor principle
Sine Waves
Graphic representation of the angle between the armature and magnetic flux at different points in time. It demonstrates the number of cycles per second (a peak and valley of a current)
Types of Resistance Associated with AC
Inductive Reactance Capacitive Reactance True Electrical Resistance
Commutator
Is involved in pulsating DC and causes a current to flow in one direction in an external circuit. Consists of a split metal ring and insulator between metal.
Types of current measuring devices
Moving coil galvanometer - measures DC voltage/amperage Electrodynamometer - measures AC voltage/amperage and must have modifications to synchronize electromagnets with AC
Power loss equation
PL=I^2XR AC is transmitted over long distances using low amperage and high voltage because it reduces PL
True Electrical Resistance
Resistance caused by the conductor ( length, temp, cross-sectional area) Larger diameter, less resistance
Capacitive Reactance
Resistance to current flow when a capacitor is added to an AC circuit
Induction Motor
Supplied by AC. Are in x ray tubes and work off electromagnetic induction. Consists of rotators ( are inside the envelope attached to the anode and moves) and stators ( consists of copper wires that are stationary, supplied w AC out of step, sits outside the envelope)
AC current motors
Synchronous & Induction
Inductive Reactance
The bucking tendency of back EMF is caused by the back EMF created by the changing strength & direction of AC. Results in some resistance to current flow
Frequency
The number of cycles per second. It is measured in hertz (Hz) Each Hz consists of 2 pulses (pos & neg) The US electrical equipment operates on 60 Hz (120 pulses)
Mutual Induction
Two insulated coils setting beside each other, one supplied with AC will induce an EMF in the other coil due to the fluxuating magnetic field.
Electromagnetic Induction
When a moving conductor cuts across a magnetic flux or is cut by a magnetic flux, an electromotive force is induced in the conductor.
Self-Induction
a back EMF in a wire coil produced by the swelling magnetic flux that accompanies AC current in a wire coil that tends to oppose the applied emf.
Electric Motors
a device that converts electrical energy into mechanical energy
Thumb =
direction of conductor thrust (looking at thumb)
Middle Finger =
direction of electron current
Index Finger =
direction of magnetic field
