ALTERNATING CURRENT GENERATORS

(ALTERNATORS)

Engr. Sheena B. Vergara

ALTERNATING CURRENT GENERATORS (ALTERNATORS)

• An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.

• Most alternators use a rotating magnetic field with a stationary armature but occasionally, a rotating armature is used with a stationary magnetic field.

Parts of an Alternator

Two Possible Construction of an Alternator

1. Stationary armature and revolving field (Stator – armature / Rotor – field)

2. Stationary field and revolving armature (Stator – field/ Rotor – armature)

Types of Revolving FieldSalient Type – used for low and medium speed alternator

having many projecting (salient) poles. They are characterized by large diameter and short axial length. Poles and poles shoe are laminated to minimize heating due to the eddy currents.

Non-salient Type – used for high speed alternator. They are characterized by small diameter and very long axial or rotor length. The cylindrical construction of the rotor gives better balance and quieter operation and also less windage losses.

Prime Movers for Alternators

A. For large AC Generators

1. Steam Turbine2. Hydraulic Turbine3. Gas Turbine4. Internal Combustion Engine

B. For small AC Generators

1. Internal Combustion Engine

Speed and Frequency of Alternators

Formula:

where: f - frequency (Hz) P - number of poles N - speed of prime mover (rpm)

Sample Problems:

1. The number of cycles generated in a 10 pole alternator in one revolution is _____.

Solution : ( Note: To create a cycle, it requires a pair of poles. )

There fore,

2. What is the speed of a 100 kW, 230V, 3ø, 4 pole, 60 Hz alternator?

Solution : Speed (N) = ?

Substituting the values to the equation,

3. What is the speed of an alternator with 6 poles?

Solution : Speed (N) = ?

(But going back to the problem, there is no frequency given. Remember that when there is no given frequency, use the standard frequency in the Philippines which is 60 Hz. )

Substituting the values to the equation,

Generated EMF per Phase in the Alternator

Formula:

where: kp- pitch factor kd- distribution or belt factor f - frequency (Hz) ɸ - flux per pole ( Weber) Z - no. of conductors per phase

Formula: (In CGS )

where: kp- pitch factor kd- distribution or belt factor f - frequency (Hz) ɸ - flux per pole ( maxwells or lines ) Z - no. of conductors per phase

Pitch Factor - the ratio of the voltage generated in the fractional-pitch coil to the voltage generated in the full-pitch coil.

• Full-pitch coil - a coil having a distance between its two sides exactly equal to 180 electrical degrees.

• Fractional-pitch coil - a coil having a distance between its two sides less than 180 electrical degrees.

Formula:

where: kp- pitch factor Pᵒ - span of coil in electrical degrees

Sample Problem:1. A 6 pole, 3ø, alternator has 72 slots and coil span of 12.

What is the pitch factor?

Solution : Pitch factor (kp) = ?

Substituting the values to the equation,

Distribution or Belt Factor - the factor by which the generated voltage must be multiplied because the coils are distributed in several slots under the poles instead of being concentrated in single slots under the poles.

Formula:

where: kd- distribution or belt factor n - no. of slots per pole per phase β – no. of electrical degrees per slot

Sample Problem:1. A 3ø alternator has 6 slots per pole per phase.

Determine the distribution factor?

Solution : Distribution factor (kd) = ?

Getting β first,

Substituting the values to the equation of kd,

Sample Problems:

1. A 400kVA, 6.9 kV, 60 cycle Y-connected alternator has the following data : poles = 6, conductors per slot = 12, total slots = 54, pitch = 8/9, flux per pole = 4x10^7 maxwells. Determine a. Pitch factor b. Belt factor c. Conductors per phase d. Terminal voltage

Solution :

a. Pitch factor (kp) = ?

Substituting the values to the equation,

Solution : b. Distribution factor (kd) = ?

Getting β and n first,

Substituting the values to the equation of kd,

Solution : c. Conductors per phase = ?

Solution : d. Terminal voltage = ?

Substituting the values to the equation,

But,

2. A 4 pole, 3ø, 50 Hz, star-connected alternator has 24 stator slots. There are 6 conductors per slot and flux per pole of 63mWb. Find the terminal voltage if the coil span factor is unity.

Solution :

For Pitch factor (kp) ,

For Distribution factor (kd) ,

Getting β and n first,

Substituting the values to the equation of kd,

For the number of conductors per phase,

Substituting the values to the equation,

Getting the Terminal Voltage,

Thank You!