Cessna 170, Service Manual

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Cessna 170 Service Manual
Electrical System
6
If a generator is not correctly polarized the regulator and generator may be damaged.
VOLTAGE REGULATOR.
The voltage regulator is a vibrating contact type containing three relays. The
voltage and current limiting relays control the output of the generator according to the demand of the battery.
This control of the generator is accomplished by changing the amount of field current flowing through the
generator. The maximum current output
of the generator is controlled by the current limiting relay. If the
current output of the generator exceeds a preset amount the current limiting relay interrupts the generator
field circuit reducing the output by inserting a resistance into the field circuit. Similar conditions apply for
generator output voltage control. When the generator is not producing, such as in an idle condition it is
necessary to disconnect the battery from the generator. If the generator is not disconnected during no-output
conditions the armature appears as a path to ground for the battery voltage. To prevent this loss of battery
potential the cutout relay portion of the regulator disconnects the battery from the armature circuit when the
charging voltage drops below a specified level and current begins to flow in a reverse direction. The
operation of the voltage regulator controls the output of the generator with respect to certain preset
maximum levels, however the regulator must be informed of the batteries condition to taper the charge and
prevent overcharging. This information is supplied to the regulator by the battery counter electromotive force
on the regulator. The rate of charge is determined by the voltage-limiting relay. The higher the battery's
charge, the higher the charging voltage must be and the more effect the voltage limiter will have on the
charging rate. This reduces the charging rate as the battery comes up to charge.
PHYSICAL CHARACTERISTICS . A voltage regulator is designed for one purpose only and that is to
regulate or control the generator-battery circuit. When a battery charge condition is low the regulator will
automatically increase the charging rate until the battery becomes fully charged. As soon as the battery is
charged the regulator will automatically reduce the charging rate.
The regulator consists of three units, a
Cut-Out relay (also known as a circuit breaker or Reverse Current
Relay ), a Voltage Regulator Relay , and a Current regulator Relay . All three relays are nothing more than
magnetic switches that operate at high speed. See Appdx EL, Fig. 09.
The
Cut-Out Relay removes the generator from the battery circuit when the generator is rotating so slowly
(low engine RPM) that it cannot produce useful electricity. This prevents the Battery from discharging into
the generator circuits. The Cut-Out Relay may be identified as it is directly connected to the system via the
BATtery terminal on the regulator.
The
Current Regulator Relay controls the amperage output of the generator by connecting directly to the
generator armature thru the regulators ARMature terminal.
The
Voltage Regulator Relay controls the actual voltage of the generator-battery circuit by either
weakening or strengthening the current within generator field windings, thereby altering the magnetic field
surrounding the armature windings.
Both Current and Voltage regulator relays accomplish their work by means of resistances that automatically
are cut into or out of the generator circuits. When the battery is fully charged resistance is introduced to
reduce the charging rate of the generator so that the battery voltage is held within design limits.
REMOVAL AND REPLACEMENT OF VOLTAGE REGULATOR.
a. Disconnect the wiring from the voltage regulator terminals. CAUTION: When removing the battery
lead from the voltage regulator, tape the terminal on the end of the wire to prevent accidental short
circuits. GOOD OPERATING PRACTICE dictates that the battery ground strap be disconnected
PRIOR to any work involving removal of wiring.
b. Remove the three bolts securing the regulator to the firewall and remove the regulator.
c.
To replace the regulator, reverse this procedure and polarize the generator field when completed.
d. Be certain the voltage regulator base-plate is well grounded to the firewall. Usually this is
accomplished with a short ground strap across the shock mounts or directly to the case.
When replacing the generator or regulator, it is necessary to re-polarize the generator to establish proper
polarity. Connect a jumper momentarily between the ARMature and BATtery terminals of the regulator while
the master switch is on but before starting the engine. A momentary surge through the generator is enough
to correctly polarize it. Failure to properly polarize the generator/regulator may cause a failure to generate
electrical power condition.
IMPORTANT : Generators and Regulators must be matched in order to produce the rated current and to
avoid damage. While a high amperage generator may be operated with a low amperage regulator, such a
mismatch will result in the generator not producing maximum rated current. (The system will only produce
the maximum rated
regulator amperage.) However, the reverse condition…that of a high amperage rated
regulator connected to a low amperage rated generator will result in a damaged/burned generator due to an
overheat condition as the regulator attempts to demand more and more current from the lower rated