Description

Front Panel Circuit Breaker
The circuit breaker over-current trip level is 30 A.  The breaker trip delay is the minimum available. The resistance of the generator field winding and its external series resistance is about 10 ohms. Thus a short circuit between the generator voltage input to the AG3SCR board and the connection to the field winding will occuif the generator voltage reaches 10 Ω * 30 A = 300 V.

The circuit breaker contains a shunt trip winding which allows it to be tripped by a signal from the AG3SCR board before the generator voltage reaches a damaging level.

The circuit breaker is reset manually- it cannot be manually tripped.

AG3SCR Control Board
The control board contains the following circuits:

• self-excitation
• low voltage power supply
• voltage and current regulation
• shorted PWM transistor protection
• loss of feedback protection
• diagnostics

Self-Excitation
In the bootstrap circuit of the AG3SCR board, the PWM transistor is bypassed by a normally on bootstrap transistor instead of the problematic NC contactor of the VR10 series of regulator cards.

Immediately after engine start, a control transistor turns on the bootstrap transistor in response to the initial generator voltage produced by the generator’s residual magnetism.  The generator then self-excites from field current passing through the bootstrap transistor. When the generator voltage reaches about 30 V, the control transistor turns off the bootstrap transistor and the PWM transistor takes over control of the field current. The current and voltage regulator circuits then acts to control the duty cycle of the PWM transistor to adjust the generator field current (about 10 A maximum) to obtain the generator voltage and current required for the 3-stage battery charge while supplying current to the varying loads (HVAC, lighting, etc.). The AG3SCR board also contains the essential circuitry that protects the connected loads from over-voltage.

Low Voltage Power Supply
The low voltage power supply provides +12 V and + 5 V control power and reference voltages.  A Zener pre-regulator is sourced from the generator voltage to provide a limited voltage input to the 12 V and 5 V precision regulators. The pre-regulator incorporates a positive temperature coefficient input resistor that allows the precision regulators to come into regulation during the bootstrap phase while avoiding high pre-regulator dissipation at the normal generator operating voltage.

Voltage and Current Regulation
Regulation is provided by separate voltage and current control loops. Current is limited to the commanded value when charging current exceeds the current setpoint. Similarly, voltage is limited when the generator voltage exceeds the voltage setpoint. The current command is adjusted by a board-mounted potentiometer tobetween 50 A and 100 A. Voltage is limited to the absorption level of 78 V after the bulk stagecurrent reaches the current setpoint. After the absorption/float stage transition, the voltage command is reduced to produce the 71 V float voltage.

Shorted PWM Transistor Protection
At high engine speed, the auxiliary generator will produce dangerously high voltage if the PWM transistor or the bootstrap transistor becomes shorted or if either transistor is turned on continuously due to a component failure. In this event, an over-voltage comparator trips the panel mounted circuit breaker if the auxiliary generator voltage exceeds the over-voltage trip threshold. This threshold is provisionally set at 115 V.

Loss of Voltage Feedback Protection
The generator voltage feedback signal is derived from the same rear panel conductor as the AG3SCR circuit board power.  An open circuit in this conductor removes power from the board thus causing the generator voltage to revert to the low residual voltage.

Loss of Current Feedback Protection
An open circuit signal connection to the current sensor causes the current signal input to the AG3SCR board to default to a higher than
normal level. This causes the PWM duty cycle to decrease to about 5%, effectively shutting down the generator.