PRINCIPLES OF OPERATION - TM-10-3930-644-14-P0252

TM 10-3930-644-14 & P

current is limited to a negligible value by the high

resistance of R2 and R3. As the alternator speed and

voltage Increase, the voltage between R2 and R3

increases to the point where zener diode 02 conducts.

Transistor TR2 then turns on and TRI turns off. With TRI

off, the field current and system voltage decrease, and

D2 then blocks current flow, causing TRI to turn back on.

The field current and system voltage increases, and this

cycle then repeats many times per second to limit the

alternator voltage to a preset value.

Capacitor C1 smoothes out the voltage across R3,

resistor R4 prevents excessive current through TRI at

high temperatures, and diode D3 prevents high-induced-

voltages in the field windings when TR1 turns off,

Resistor R2 is a thermistor which causes the regulated

voltage to vary with temperature, thus providing the

optimum voltage-for charging the battery.

Figure 2-2. Cross-Sectional View of Alternator

B. PRINCIPLES OF OPERATION

The principles of operation of the alternator are as

follows. See Figure 2-3.

When the Ignition switch is closed, current from the

battery flows through the 10-ampere fuse and resistor to

the alternator No. 1 terminal, through resistor R1, diode

DI, and the base-emitter to transistor TRI to ground. and

then back to the battery. This turns on transistor TR1,

and current flows through the alternator field coil and

TR1 back to the battery. The ammeter shows discharge.

The resistor in parallel with the ammeter reduces total

circuit resistance to provide higher field current for initial

voltage build-up when the engine starts.

With the alternator operating. a.c. voltages are

generated in the stator windings, and the stator supplies

d.c. field current through the diode trio, the field, TR1,

and then through the grounded diodes in the rectifier

bridge back to the stator. Also, the six diodes in the

rectifier bridge change the stator a.c. voltages to a d.c.

voltage which appears between ground and the

alternator "SAT" terminal.

As alternator speed

increases, current is provided for charging the battery

and operating electrical accessories. Also, with the

alternator operating, the same voltage appears at the

"BAT" and No. 1 terminals, and the ammeter shows

charge to Indicate the alternator is producing voltage.

Figure 2-3. Alternator Internal Circuits, Schematic

The No. 2 terminal on the alternator is always connected

Diagram

to the battery, but the discharge

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