26SI Series Alternator – Normal Operation

The field winding uses DC voltage to create a magnetic field. The magnetic field is used to magnetize the rotor. The stator is a stationary component that surrounds the rotor. When the rotor rotates voltage is produced. The stator will then send the voltage to the rectifier. The rectifier changes the AC voltage into DC voltage. Part of the DC voltage returns to the field winding in order to maintain the magnetic field. The remainder of the DC voltage is supplied to the battery. Electrical components on the machine will then receive the voltage from the battery. The regulator limits the DC voltage at the battery terminals to 28 ± 1 volts by controlling the current in the field winding.

Alternator Operation Schematics



Illustration 1 g00866883

Electrical schematic of the N3 series alternator (35 amperes)

B- – Frame to battery ground

B+ – Alternator to battery positive terminal

D+ – Output for the excitation circuit and regulator input

D- – Ground for the alternator and regulator

DF – Alternator output and regulator input

G – Field winding

R – Auxiliary terminal

U – Stator winding

V – Stator winding

W – Stator winding




Illustration 2 g00866887

Electrical schematic of the N3 series alternator (50 amperes)

(1) Diodes for protection against load dump

B- – Frame to battery ground

B+ – Alternator to battery positive terminal

D+ – Output for the excitation circuit and regulator input

D- – Ground for the alternator and regulator

DF – Alternator output and regulator input

G – Field winding

R – Auxiliary terminal

U – Stator winding

V – Stator winding

W – Stator winding

Note: The “R” terminal may be used by one of the following components:

  • Charge indicator

  • Tachometer

  • Hour meter

  • Electronic control module

Illustrations 1 and 2 show the electrical schematics for the N3 series alternators.

The alternator has two circuits: the charging circuit and the excitation circuit. The charging circuit functions during normal operation. The excitation circuit functions during normal operation and during start-up. An independent circuit exists for each phase of the stator. Phases are noted as “U”, “V”, and “W”. The 50 amp alternator has two extra diodes. Diodes are connected to the neutral point of the stator. Diodes also add extra current to the output. However, the 35 ampere alternator lacks the extra diodes.

The descriptions that follow show the N3 series alternator (35 ampere).

Charging Circuit

The charging circuit supplies current to the battery and to the electrical systems during normal operation. Stator windings in the alternator generate three-phase AC voltage. Positive diodes and the negative diodes change the AC voltage into DC voltage. DC voltage allows the current to flow to the battery terminals.




Illustration 3 g00825118

Charging circuit with phase angle of 120°

The flow of current at a 120° phase angle is shown in Illustration 3. The polarity for the stator winding is described below:

  • Positive at the output of winding “U”

  • Zero at the output of winding “V”

  • Negative at the output of winding “W”



Illustration 4 g00825120

Charging circuit with phase angle of 150°

The flow of current at a 150° phase angle is shown in Illustration 4. The polarity for the stator winding is described below:

  • Positive at the output of winding “U”

  • Positive at the output of winding “V”

  • Negative at the output of winding “W”

Excitation Circuit (Normal)

The N3 series alternator is self-excited. The excitation circuit supplies current to the field winding (exciter) during normal operation. The rotor contains an iron core that acts as a rotating magnet. The rotating magnetic field induces voltages in the stator windings. Stator windings generate three-phase AC voltage. Exciter diodes and negative diodes change the AC voltage into DC voltage. DC voltage allows current to flow in the field winding. The current induces a stationary magnetic field in the field winding that will keep the rotor core magnetized. This process continues as the rotor operates normally.




Illustration 5 g00825122

Excitation circuit with phase angle of 120°

The flow of current at a 120° phase angle is shown in Illustration 5. The polarity for the stator winding is described below:

  • Positive at the output of winding “U”

  • Zero at the output of winding “V”

  • Negative at the output of winding “W”

Excitation Circuit (Start-up)

The exciter diodes and the negative diodes change the AC voltage into a uniform DC voltage. Stator windings generate three-phase AC voltage.

The excitation circuit also supplies current to the field winding (exciter) during start-up. N3 series alternators depend on the residual magnetism in the rotor core in order to achieve normal operation. Once the rotor begins to move, the residual magnetism induces a weak voltage in the stator windings. The voltages cause a small amount of current to flow through the excitation circuit. The current then produces a stationary magnetic field in the field winding. The stationary magnetic field strengthens the magnetism in the rotor core, and the voltage in the stator windings increases. The effect is cumulative. The regulator controls the output voltage by varying the current in the field winding (exciter).

The alternator functions normally when the excitation circuit produces the breakdown voltage of two diodes in series.

The rotor generates the breakdown voltage by 2000 rpm. When the rotor reaches 2000 rpm, the alternator generates an output.


Regulator Operation

N3 series alternators charge the battery. N3 series alternators will also supply the power for the electrical system. In order to prevent overcharging the battery or damaging the systems, the voltage regulator keeps the output voltage at a constant level. The regulator maintains the voltage regardless of variations in load or variations in rotor speed.

The alternator output voltage is directly related to the exciting current. For example, increasing the exciting current increases the output voltage. By controlling the exciting current, the regulator compensates for variations in load. As a result, the terminal voltage remains constant up to the maximum current output.

The alternator is regulated by periodically increasing and decreasing the exciting current. If the output of the alternator is below 29 volts, the exciting current rises and the voltage rises. If the voltage exceeds 29 volts, the regulator turns off the exciting current. The drop in current reduces the output voltage. When the voltage drops below the specified lower limit of 27 volts, the regulator turns on the exciting current. The rise in current increases output voltage to 29 volts. The cycle will then repeat. The cycles are repeated quickly in order for the voltage to remain constant at the desired level.

The alternator output voltage is directly related to rotor speed for a given electrical load. For example, increasing the rotor speed increases the output voltage. By controlling the exciting current, the regulator compensates for variations in rotor speed. At low speeds, a higher average exciting current results. At high speeds, a lower average exciting current results.

Illustration 6 shows the connection between the solid state regulator and the alternator. The regulator is totally enclosed and nonadjustable.




Illustration 6 g00866932

Connection between the alternator and the regulator



Illustration 7 g00866934

Solid state regulator

(D+) Input for the excitation circuit to the regulator

(DF) Output for the regulator to the field winding

(D-) Ground for the regulator

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