Nidec 22765-01, Manual

Page: 205 / 227

Page 34
Main Rotor
1. Disconnect the generator field leads from the posi tive and
negative terminals of the rotating rectifier assembly.
2. Connect the positive and negative leads to one clamp of the
500-volt Megger, and connect the other clamp to the shaft.
3. Apply voltage from the Megger, and measure the resistance
reading after one minute. The reading must be a minimum
of 50 megohm. If it is not, refer to the cleaning or dry out
procedures. (See Table 3.)
4. Ground the field leads to the shaft after disconnecting the
Megger for a minimum of one minute. This will allow the
voltage build up to be properly discharged.
Main Stator
1. Disconnect power connections and all control appara tus from the
generator terminals.
2. Measure insulation resistance of each phase separately with the
two other phases shorted to the frame.
3. Use a Megger connected between the lead(s) of the phase to
be measured and generator frame. The minimum one-minute
insulation resistance must not be less than 50 megohm. (See
Table 3.)
4. Ground the leads to the frame after the one-minute Megger test.
This will allow the voltage build up to be properly discharged.
Dry Out Procedures
If the insulation resistance readings are below the recommended
minimum values specified previously, use one of the dry out
procedures described below. Select the procedure based on the size and
location of the unit, available equipment, and experience of personnel.
Before drying, remove the voltage regulator, and cover all inlet and
discharge openings. Provide an opening at the top of the machine,
preferably at the fan end, for moisture to evaporate.
Drying with External Heat:
Place heat lamps, space heaters (in
addition to the ones already supplied) or a steam pipe near the
windings. Monitor winding temperatures. Raise winding temperature
gradual ly at a rate of 50° F (28° C) per hour up to 200° F (93° C).
Measure insulation resistance at 1-hour intervals. Typically the
insulation resistance will slowly drop while the temperature is coming
up, and then gradual ly increase and level out.
Drying with AC Current in the Armature:
Short circuit the
generator terminals. Provide DC excitation to the brushless exciter
field winding. Insert a current transformer and an ammeter to read
full load current. Run the generator at rated speed. Apply excitation
to the exciter field until rated current is developed. Monitor winding
temperatures until they stabilize. Continue running until insulation
NOTE: Do not apply heat too rapidly. It
could damage the windings.
Winding
Rated
Voltage
(V)*
Insulation
Resistance Test
Direct Voltage
(V)
<1000 500
1000–2500 500–1000
2501–5000 1000–2500
5001–12000 2500–5000
>12000 5000–10000
* Rated line–line voltage for three-phase
AC machines, and line–ground voltage
for single-phase machines, and rated
direct voltage for DC machines or field
windings.
Table 3 DC Voltage to be Applied
During Insulation Resistance Tests