DC Generator and Motor AC G & M

Generators and Motors
Consider two poles of a magnet between which a rectangular coil of wire is rotated by using mechanical energy. This is an Electrical generator in which the varying magnetic field will create an electric field due to electromagnetic induction, which in turn will generate emf between generator terminals.


In a DC generator, the two ends of the coil are joined to two split-rings which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip rings. The special arrangement of brushes and slips allows to gather the emf on the terminals. Charge separation takes place due to electrons flowing away from one terminal and toward the other, until, in the open-circuit case, sufficient electric field builds up to make further movement unfavorable. The emf is countered by the electrical voltage due to charge separation. If a load is attached, this voltage can drive a current.

A motor

A motor is essentially same arrangement as a generartor.
If an Electric current flows through the coil, one side will experience an upward force while the other will experience a downward force, causing the loop to turn by 90 degrees and come to rest. The loop can be made to spin (or rotate ???) continuously by fixing a half circle of copper known as commutator, to each end of the loop. Current is passed into and out of the loop by brushes that press onto the strips. The brushes do not go round so the wire of the loop does not get twisted. This arrangement makes sure that the current always passes down on the right side and back on the left side so that the rotation continues. This is how a simple Electric motor is made.



When we use mechanical power to move the coil and generate emf on the terminals, it is a generator. But if we use a DC voltage supply as input, the contacts at the brushes will make the coil to rotate, which will create mechanical motion. As the motor is turning, it also acts as a generator and generates a "back emf". By Lenz's law, the emf generated by the motor coil will oppose the change that created it. If the motor is not driving a load, then the generated back emf will almost balance the input voltage and very little current will flow in the coil of the motor. But if the motor is driving a heavy load, the back emf will be less and more current will flow in the motor coil. Thus electric power being used is converted to the mechanical power to drive the load. The motors inside all domestic appliances which we see daily such as a fan, rotor, grinding machine etc are essentially such motors.

AC Generator and Motor

A hand-cranked generator can be used to generate voltage which in turn can run a motor. This is an example of energy conversion from mechanical to electrical energy and then back to mechanical energy.
The turning of a coil in a magnetic field produces motional emfs in both sides of the coil which add. Since the component of the velocity perpendicular to the magnetic field changes sinusoidally with the rotation, the generated voltage is sinusoidal or AC.



As the motor is turning, it also acts as a generator and generates a "back emf". By Lenz's law, the emf generated by the motor coil will oppose the change that created it. If the motor is not driving a load, then the generated back emf will almost balance the input voltage and very little current will flow in the coil of the motor. But if the motor is driving a heavy load, the back emf will be less and more current will flow in the motor coil and that electric power is converted to the mechanical power to drive the load.

Mutual Inductance
When two coils are placed nearby, a change in current through one will change the magnetic flux and such a change will induce a current in the second coil. But the current in the second coil also being changing will have a reciprocal effect on the current in first coil. This is called mutual inductance.



Imagine two coils wound on two arms of a core made of iron which allows smooth passage of magnetic flux. If an AC current is passed through one coil, called primary winding, then an AC current will be generated in the second coil. If the no of loops in the secondary winding is less then the voltage generated across the coil will also be less. But the current generated will increase such that the power in first coil is same as the power in second coil (neglecting small energy losses in the system). This is called transformer and depending on whether voltage in secondary circuit is less or more it is called step-down or step-up transformer.


Thus transformer is a unique way of transferring electric power from one system to another. This is used for power transmission. You step up the power, thus increasing voltage but reducing current. Then when you transmit the power, ohmic losses will be less as the current is low. At the other end you use a step down transformer to get a normal voltage of 220 volts. This is why transmission is done by high tension wires that is, wires carrying a very high voltage current.




Self induction –
Just as current is induced in the second coil kept nearby, similarly a voltage is also generated in the wire which carries the verying current because the changing magnetic flux has impact on the wire too. This is called Self inductance. The induced emf will tend to oppose the current which created it. Net effect is some kind of resistance to the current which is called inductive resistance as opposed to ohmic resistance. In circuit diagrams the inductive resistance is shown as a coil.
The voltage generated by self induction due to verying current can be written as

Where
VL = the induced voltage in volts
L = the value of inductance in henries
di/dt = the rate of change of current in amperes per second.

The term inductor is used to describe a circuit element possessing the property of inductance and a coil of wire is a very common inductor. In circuit diagrams, a coil or wire is usually used to indicate an inductive component.