Check the insulation resistance of the windings of the exciter and auxiliary exciter (if equipped) periodically to determine the insulation condition of the insulation.
Testing a Diode
Diodes are durable, and do not require frequent tests. A failure in a diode may indicate a defect, or an increase in field current for the same load condition. Test the diode using the following procedure:
- Release the connections of all the diodes with the rotor winding of the exciter.
- With an ohmmeter, measure the resistance of each diode in both directions.
Note: When testing diodes, observe the polarity of the test terminals in regard to polarity of the diode. The polarity of the diode is indicated by an arrow on the frame.
The current conduction must occur only in the anode-cathode direction, or in the condition of direct polarization.
The diode is considered good when presenting low ohmic resistance (up to approximately 100 ohms) in the forward direction. The diode is considered good when presenting high resistance (approximately 1 megohm) in the opposite direction. Defective diodes have ohmic resistance of 0 ohms or greater than 1 megohm action in both measured directions. In most cases, the method with ohmmeter to test the diodes is sufficient to identify faults in the diodes. However, in some extreme cases applying the rated voltage of blocking and/or current flow for detecting fault in the diodes may be necessary. Due to the efforts required for these tests, in case of doubt, change the diodes.
Replacing a Diode
To access the diodes, contact your Cat dealer.
To replace the diodes, perform the following:
- Undo the connection of the six diodes of the exciter rotor.
- Release support for diodes of the exciter rotor and remove the support.
- Secure the support in a bench lathe with protection on the jaw, and remove all diodes.
- Install three new diodes of the same polarity (AND or CTD) in one of two connection bridges.
- Install the other connecting bridge, three new diodes of polarity opposite of the three diodes previously installed.
- Secure all diodes using a torque wrench, and the tightening torques in 2.
- Make the connections of the diodes with the exciter rotor winding
Note: Proper tightening torques must be used. Using improper torques may damage the diodes.
| Diode base thread (mm) | Torque key of torque meter (mm) | Tightening torque (mm) |
| M8 | 17 | 4 |
| Diode | Technical specification |
| AND | Diode thread M8 45A/1.200V AND |
| CTD | Diode thread M8 45A/1.200V CTD |
Testing a Varistor
The varistor is the device installed between the two bridges connecting the diodes and has the purpose of protecting diodes against overvoltage.
To test the conditions of operation of the varistor, an ohmmeter can be used.
The resistance of a varistor must be high (± 20,000 ohms).
If damage is noted in the varistor, or resistance is low, the varistor must be replaced.
Replacing a Varistor
To replace a varistor, always observe the following:
- Replace the damaged varistor with a new varistor identical to the original.
- To replace the varistor, loosen the screws that secure the bridges connecting the diodes.
- When removing the varistor, observe carefully how the components were mounted so that new varistor is installed the same way.
- Before mounting the new varistor, make sure that all contact surfaces of the components are clean, leveled, and smooth. Clean, level, smooth surfaces will ensure a perfect contact.
- Attach the new varistor, tightening the screws that hold the connecting bridges just enough to make a good electrical connection. Watch the filling with silicone in the compartment of the new capacitor.
Testing a Capacitor
The capacitor is the device installed between the two bridges connecting the diodes to protecting diodes against voltage surge.
To test the operating conditions of the capacitor, a multimeter can be used for measuring capacitance.
The capacitance measurement should be according to the specification for the component.
If damage is noted in the capacitor, or capacitance is low, the varistor must be replaced.
Replacing a Capacitor
To replace the capacitor, always observe the following
- Replace the damaged capacitor with a new one identical to the original
- To replace the capacitor, loosen the screws that secure the bridges connecting the diodes
- When removing the capacitor, observe carefully how the components were mounted so that the new capacitor is installed the same way
- Before mounting the new capacitor, make sure that the connection terminals and screws are clean, ensuring a perfect contact
- Fix the capacitor again tightening the terminals that attach to connecting bridges, enough to make a good electrical connection. Observe the filling with silicone in the compartment of the new capacitor