In an AC system voltage level can be increased or decreased very easily with the help of a transformer, therefore, this system is exclusively used for generation, transmission and distribution of electric power.

Synchronous generator: The mechanical power or energy is converted into electrical power or energy with the help of an AC machine called alternator or synchronous generator.

Synchronous motor: when the same machine can be used to convert electrical power or energy into mechanical power or energy, then it is known as a synchronous motor.

Thus, the same machine can be operated as a generator or as a motor and in general called as a Synchronous machine, which rotates only at synchronous speed- \[N_s = \dfrac{120f}{P}\]

A SM is just an electro-mechanical transducer which converts mechanical energy into electrical energy or vice-versa.

The fundamental phenomenon which make these conversions possible are :

Law of electromagnetic induction:

production of emf induced in a conductor whenever it cuts across the magnetic field.

Law of interaction:

phenomenon of production of force or torque whenever a current carrying conductor is placed in the magnetic field

by the interaction of the magnetic fields produced by the current carrying conductor and the main field, force is exerted on the conductor and torque is developed.

- Synchronous Machines
- Alternators: primary source of electrical energy
- Synchronous Motors: Used as motors as well as power factor compensators (synchronous condensers).
- Alternators or A.C. Generators
- Principle:Electromagnetic Induction
- consists of armature winding and magnetic field
- Armature stationary and field rotating
- Opposite to dc machines
- Stator: Armature winding on stationary element
- Rotor Field winding on rotating element

ME \(\Rightarrow\) EE

rotation is due to mechanical torque, therefore, \(T_m\) and \(\omega\) are in the same direction.

The frictional torque \(T_f\) acts in opposite direction to rotation \(\omega\).

\(T_e\) acts in opposite direction to \(T_m\) so that \(\omega Tm = \omega Te + \omega T_f\)

\(E>V\)

the torque angle is leading

EE \(\Rightarrow\) ME

rotation is due to electromagnetic torque, therefore, \(T_e\) and \(\omega\) are in the same direction.

\(T_f\) in opposite direction to \(\omega\)

\(T_m\) in opposite direction to \(T_e\) so that \(\omega Te = \omega Tm + \omega T_f\)

\(V>E\)

the torque angle is lagging

Generator action

An emf is induced in the armature conductors when they cut across the magnetic field.

On closing the circuit, current flows through the armature conductors which produces another field.

By the interaction of this field and main field a force is exerted on the conductor which acts is opposite direction to that of rotation.

The mechanical power is converted into electrical power.

Motor Action

A current is supplied to the machine which flows through the armature conductors.

The armature conductors produce a field which interacts with the main field. Thus, a force is exerted on the conductors and rotation takes place.

Once rotation occurs, an emf is induced in the conductors due to relative motion. This emf acts in opposite direction to the flow of current.

The electrical power is converted into mechanical power.

Production of sinusoidal alternating EMF

When a conductor or coil cuts across the magnetic field an emf is induced in it by the phenomenon called electromagnetic induction.

This can be achieved by;

either rotating a coil in the stationary magnetic field

or keeping the coil stationary and rotating the magnetic field.

Conductor at \(X\) has \(B_{max}\), thus have maximum EMF induced in it

At interpolar gap, as at \(A\), has minimum induced EMF, because minimum flux density

One cycle of e.m.f. induced in a conductor when one pair of poles passes over it.

In other words, the e.m.f. in an armature conductor goes through one cycle in angular distance equal to twice the pole-pitch

Since, one cycle of emf = a pair of poles passes past a conductor

Number of cycles of emf produced in one revolution of the rotor = number of pair of poles

\[\begin{aligned} \mbox{No. of cycles/revolution} & =P/2\\ \mbox{No. of revolutions/second} & =N/60\\ \Rightarrow\mbox{frequency} & =\dfrac{P}{2}\times\dfrac{N}{60}=\dfrac{PN}{120}\\ & \boxed{f =\dfrac{PN}{120}}~Hz \end{aligned}\]

Magnetization from dc source at 125-600 volts

Exciting (or magnetizing) current from a small dc shunt generator mounted on the alternator shaft itself

Field rotating, current supplied thorough two slip-rings

Excitation voltage is relatively small, slip-rings and brush gear are of light construction

Brushless excitation system: 3-phase ac exciter and group of rectifiers supply dc

Hence, brushes, slip-rings and commutator are eliminated

Stationary stator conductors cuts the magnetic flux produced by rotating rotor, induced emf in stator

Because of alternate N and S pole, alternating emf (or current) is produced, whose

frequency depends on number of N and S pole moving past a conductor in one second

direction given by Flemingâ€™s RHR

Output current directly tapped from fixed stationary terminals without brush-contacts

Easier to insulate stationary armature winding for high ac voltage (30 kV or more)

Sliding contact (slip-rings) are transferred to the low-voltage, low-power dc field circuit, therefore, easily insulated

Armature winding can be more easily braced to prevent any deformation (due to mechanical stress produced by short-circuit current and high centrifugal force)