Maintenance and inspection of alternator systems is similar to that of dc systems. Check the exciter brushes for wear and surfacing. On most large aircraft with two or four alternator systems, each power panel has three signal lights, one connected to each phase of the power bus, so the lamp will light when the panel power is on. The individual buses throughout the airplane can be checked by operating equipment from that particular bus. Consult the manufacturer's instructions on operation of equipment for the method of testing each bus.

Alternator test stands are used for testing alternators and constant speed drives in a repair facility. They are capable of supplying power to constant speed drive units at input speeds varying from 2,400 rpm to 9,000 rpm.

A typical test stand motor uses 220/440 volt, 60 cycle, 3 phase power. Blowers for ventilation, oil coolers, and necessary meters and switches are integral parts of the test stand. Test circuits are supplied by a load bank. An ac motor generator set for ground testing is shown in figure 9-59.

A typical, portable, ac electrical system test set is an analyzer, consisting of a multirange ohmmeter, a multirange combination ac dc voltmeter, an ammeter with a clipon current transformer, a vibrating reed type frequency meter, and an unmounted continuity light.

A portable load bank unit furnishes a load similar to that on the airplane for testing alternators, either while mounted in the airplane or on the shop test stand. A complete unit consists of resistive and reactive loads controlled by selector switches and test meters mounted on a control panel. This load unit is compact and convenient, eliminating the difficulty of operating large loads on the airplane while testing and adjusting the alternators and control equipment.

Proper maintenance of an alternator requires that the unit be kept clean and that all electrical connections are tight and in good repair. If the alternator fails to build up voltage as designated by applicable manufacturer's technical instructions, test the voltmeter first by checking the voltages of other alternators, or by checking the voltage in the suspected alternator with another voltmeter and comparing the results. If the voltmeter is satisfactory, check the wiring, the brushes, and the drive unit for faults. If this inspection fails to reveal the trouble, the exciter may have lost its residual magnetism. Residual magnetism is restored to the exciter by flashing the field. Follow the applicable manufacturer's instructions when flashing the exciter field. If, after flashing the field, no voltage is indicated, replace the alternator, since it is probably faulty.

Clean the alternator exterior with an approved fluid; smooth a rough or pitted exciter commutator or slip ring with 000 sandpaper; then clean and polish with a clean, dry cloth. Check the brushes periodically for length and general condition. Consult the applicable manufacturer's instructions on the specific alternator to obtain information on the correct brushes.


Use the following table to assist in locating, diagnosing, and correcting alternator troubles:


TROUBLE: Voltmeter registers no voltage.

PROBABLE CAUSE: Voltmeter defective.
REMEDY: Remove and replace voltmeter.
PROBABLE CAUSE: Voltmeter regulator defective.
REMEDY: Replace regulator.
PROBABLE CAUSE: Defective exciter.
REMEDY: Replace alternator.


TROUBLE: Low voltage.

PROBABLE CAUSE: Improper regulator adjustment.
REMEDY: Adjust voltage regulator.


TROUBLE: Erratic meter indication.

PROBABLE CAUSE: Loose connections.
REMEDY: Tighten connections.
PROBABLE CAUSE: Defective meter.
REMEDY: Remove and replace meter.


TROUBLE: Voltage falls off after a period of operation.

PROBABLE CAUSE: Voltage regulator not warmed up before adjustment.
REMEDY: Readjust voltage regulator.