Operation Manual

ManualsBrandsPowerTech ManualsToolsPT20-EC

Insulation Test

Insulation resistance is a measure of the integrity of the

insulating materials that separate the electrical windings from

the generator’s steel core. This resistance can degrade over

time or be degraded by contaminants: dust, dirt, oil, grease,

and especially moisture. Most winding failures are due to a

breakdown in the insulation system. In many cases, low

insulation resistance is caused by moisture collected when

the generator is shutdown

Insulation resistance is measured with a megger (mega-ohm

meter). A megger measures insulation resistance by placing

500 volts between the winding and the frame of the

generator. Caution must be taken to remove all electronic

devices (regulators, diodes, surge protectors, capacitors,

protective relays, etc.) from the winding circuit before

checking the insulation. Winding insulation can be checked on

the main stator, main rotor, exciter stator, and exciter rotor.

Minimum resistance is 2 mega-ohms. If the winding

resistance is low it must be dried (see maintenance section)

or repaired.

DIODE TESTING

If the generator is close coupled to an engine, it may be

necessary to "bar over" the engine in order to gain access to

a given area of the rectifier assembly. NEVER use the

generator's fan as a fulcrum to accomplish this. Use the

engine manufacturer's recommended practice to manually

turn over the engine. To prevent possible injury to personnel,

and damage to the equipment, ensure that the engine

cannot start during this procedure.

Remove the two main rotor leads and the three exciter rotor

leads from the rectifier assembly (Figure 5). The rectifier

assembly is now electrically isolated from the generator. The

diodes remain mounted and the diode leads remain

connected to the terminal posts. Using an ohmmeter or a

battery light continuity tester, place one test probe on the

diode lead terminal post. In succession, touch the other test

probe to the lead screw hole in each heat sink. Reverse the

probes and repeat the procedure. You have now tested the

three diodes connected to this terminal post in both the

forward and reverse direction. Repeat the procedure using

the other diode terminal post.

FIGURE 4: DIODE POLARITY

When the positive test probe is connected to the diode's

anode and the negative test probe is connected to the diode's

cathode (forward biased), the diode will switch on and

conduct electricity (Figure 4). This is observed by a low

resistance reading when using an ohm meter or the lighting

of the bulb when using a battery light continuity tester.

Reversing the test leads (reverse biased) will result in the

diode switching off and no electricity will be conducted. The

results of these tests should indicate one of three conditions:

1. Good diode: Will have a much greater resistance in one

direction than the other. Typical reverse biased resistance

will be 30,000 ohms or greater, while forward biased

resistance will be less than 10 ohms. The battery light

tester will have the light "on" in one direction and "off" in

the other.

2. Shorted condition: Ohmmeter reading will be zero, or

very low in both directions. The continuity tester will have

the light "on" in both directions.

3. Open condition: Ohmmeter will have a maximum

(infinity) reading in both directions. Continuity tester light

will be off in both directions.

Diode failure after a 25 hour "run in" period is generally

traceable to external causes such as a lightning strike,

reverse current, line voltage spikes, etc. All 6 diodes are

essentially in the same circuit. When a diode is stressed to

failure, there is no easy method to determine remaining life in

the other diodes. To avoid possible continued failures, it is

recommended that the entire rectifier assembly be replaced

rather than replacing individual diodes.

SERVICE

GENERAL

The service procedures given in this section are those which

can reasonably be conducted on-site with a minimum

number of special tools and equipment. All service

procedures should be conducted by qualified maintenance

personnel. Replacement parts may be ordered through an

authorized service center or directly from the factory.

FIELD FLASHING

Restoring Residual Magnetism

(not applicable on PMG equipped generators)

To restore residual magnetism to the generator, connect a 12

volt battery to the exciter field while the generator using the

following procedure:

1. Shutdown the generator set. Remove the exciter field

leads F+ and F from the regulator.

Terminal End

Anode

(+)

Cathode

(-)

Stud End

Cathode

(-)

Anode

(+)

Forward

Reverse