HDB Series and M1 Series Heavy Duty Brushless Alternators Caterpillar


Normal Operation

Usage:

769C 01X


The field winding uses DC voltage to create a magnetic field. The magnetic field magnetizes the rotor. The rotor rotates the magnetic field on the inside of the stator. The stator generates AC voltage. 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 and to the electrical systems through battery terminals.

Alternator Operation Schematics




Illustration 1g00825115

Electrical Schematic of the M1 Series Alternator (35 amp)




Illustration 2g00825116

Electrical Schematic of the M1 Series Alternator (50 amp)




Illustration 3g00829818

Electrical Schematic of the HDB Series Alternators

The illustrations above show the electrical schematics for the 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. The phases are noted as "u", "v", and "w". All alternator schematics are similar except for the 35 amp alternator. The 50 amp alternators, the 65 amp alternator, the 80 amp alternator, and the 95 amp alternator have two extra diodes. The diodes are connected to the neutral point of the stator. The diodes add extra current to the output. The 35 amp alternator lacks the extra diodes. The descriptions that follow show the 35 amp alternator.

Charging Circuit

The charging circuit supplies current to the battery and to the electrical systems. The stator windings generate three-phase AC voltage. The positive diodes and the negative diodes change the AC voltage into DC voltage. The DC voltage allows current to flow to the battery terminals.




Illustration 4g00825118

Charging circuit with phase angle of 120°

The flow of current at a 120° phase angle is shown in Illustration 4. The voltage is positive at the output of winding "u". The voltage is zero at winding "v". The voltage is negative at winding "w". The current path is described below:

Winding "u", positive diode "u", the "B+" terminal, the "B-" terminal, negative diode "w", winding "w" and the neutral point.




Illustration 5g00825120

Charging circuit with phase angle of 150°

The flow of current at a 150° phase angle is shown in Illustration 5. Voltage is positive at the output of winding "u", positive at winding "v" and negative at winding "w". The current path is described below:

Winding "u" and winding "v", positive diode "u" and positive diode "v", the "B+" terminal, the "B-" terminal, negative diode "w", winding "w" and the neutral point.

Excitation Circuit (Normal)

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




Illustration 6g00825122

Excitation circuit with phase angle of 120°

The flow of current at a 120° phase angle is shown in Illustration 6. The voltage is positive at the output of winding "u". The voltage is zero at winding "v". The voltage is negative at winding "w". The current path is described below:

Winding "u", exciter diode "u", the field winding (exciter), the regulator, negative diode "w", winding "w" and the neutral point.

Excitation Circuit (Start-up)

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

The excitation circuit also supplies current to the field winding (exciter) during start-up. The alternator depends on the residual magnetism in the rotor core in order to achieve normal operation. Once the rotor begins to move, the residual magnetism induces weak voltages in the stator windings. The voltages cause a weak current to flow through the excitation circuit. The current produces a stationary magnetic field in the field winding. The stationary magnetic field strengthens the magnetism in the rotor core. The voltage in the stator windings increases. The effect is cumulative. The voltage continues to rise until the regulator controls the output voltage. 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 diodes are one positive diode and one negative diode.

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

Regulator Operation

The alternator charges the battery. The alternator also supplies power to the electrical systems. 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 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 is repeated.

The cycles are repeated quickly. The output voltage remains 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 7g00825127

Electrical Schematic of the Regulator

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