AIR TRACTOR AT-502A, Секция обслуживания

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Maintenance Page 54 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
ELECTRICAL SYSTEM:
General
Each of the separate electrical circuits discussed in this section is accompanied by its particular wiring
schematic drawing. In most cases, the Air Tractor drawing number of a component is shown in parentheses
on the schematic drawing. The associated Air Tractor part number of a component is the drawing
number plus "-
1” or “-2” for left-hand and right -hand parts, respectively. In the event that the part is
symmetrical, without a “hand” designation, the suffix number is “-1”.
All troubleshooting of electrical systems is directed toward the identification of faulty components. The
most valuable tool for use in troubleshooting is the Volt /Ohmmeter.
The Voltmeter is used with electrical power “ON” for the circuit under examination. It is used to
determine that the proper voltage is available at the appliance or at any location in the circuit between the
airplane bus and the appliance. A socketed 28-volt bulb may be substituted for the Voltmeter, with one wire
lead connected to a wire or terminal while the other wire lead is connected to ground.
The Ohmmeter must be used with electrical power “OFF” for the circuit under examination. It is
primarily useful for establishing the continuity of wiring or ground contacts. Here, the resistance of the wiring
and ground circuits should read near “ZERO” Ohms. The Ohmmeter is of little value in evaluating appliances
such as motors, lamps, etc., since the desirable resistance of the appliance is unknown. The exception is
when the resistance is very high (infinite), indicating an open circuit and a failed component.
The simplest means of ascertaining a faulty component is substitution with a known functioning
component. This is practical if the substituted part is a switch or light bulb, or easily available part. Do not
use this method for the GCU or other delicate electronic parts as faults other than the electronic part will
usually damage the new part.
Starter/Generator
The wiring schematic for the Starter/Generator is shown in Drawing 60364. When the starter switch is
engaged, a small current is sent to the coil of the starter relay. This relay completes the circuit to provide
starting current to the starter.
The Starter/Generator, while acting as a generator, will produce 250 amps continuously.
Voltage Regulator
The wiring schematic for the Voltage Regulator is shown in Drawing 60364. This solid-state Voltage
Regulator controls the generator output at 27.5 volts. The Voltage Regulator is activated by closure of the
generator switch. When the generator output has risen to near 27.0 volts, a signal from the Voltage
Regulator activates the line contact relay, placing the generator “on line”. The line contact relay performs the
same function as the older reverse-
current relay in taking the generator “off line” when the generator output
voltage falls below residual battery voltage.
The Voltage Regulator uses logic, based on the status of the system, to control the regulator output.
This logic uses samples of voltages in the system to determine the current system condition. These include
voltages on both sides of the line contactor relay to determine if the relay is open or closed. Also, voltages at
the starter switch are monitored to determine the position of the starter switch.
The voltage output of the Voltage Regulator is pre-set to 27.5 volts. The voltage output may be set to
between 27 to 30 volts by adjusting the voltage-adjustment screw at the end of the case.