Usage:
Troubleshooting Guide
General
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Do not open or close the exciter with the engine-generator operating. When the generator is operating, the heat sinks and exciter parts are electrically charged. Components will be damaged if a metal contact is made between the generator frame and the heat sinks or exciter parts. |
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A large percentage of reported electrical faults result from undetected mechanical deficiencies. Take time to examine the entire installation carefully and methodically. Separate mechanical from electrical deficiencies.
A generator is essentially a constant speed device. RPM that is 5 to 10 percent above or below rated RPM can cause large variations in terminal voltage.
Generator heating is the result of line current. The higher the line current, the greater the heat.
A voltmeter and ammeter or kW meter will not necessarily indicate the KVA load on a generator because of the power factor of the load.
Before attempting to repair the generator, be familiar with the operating principles of the generator.
Recognize the apparent operational trouble and diagnose the trouble from the wiring diagram and troubleshooting guide.
When troubleshooting for voltage, a generator can usually be operated at a low RPM. Voltage and cycles are directly proportional to RPM. A six pole, 60 cycle, 230 volt, 1200 RPM generator operating at a low idle of 900 RPM produces 173 volts at 45 cycles.
NOTE: For information on the electrical and mechanical characteristics of SRCR generators, make reference to the section INDUSTRIAL DIVISION DATA SHEETS: No. 70.0 and TECHNICAL INFORMATION FILE (TIF).
Test Instruments
A Volt-Ohm-Milliameter having scales of approximately the following values:
AC Volts ... 0-12, 1-100, 0-300
DC Volts ... 0-50, 0-100, 0-150
Ohms ... x 1, x 10, x 100, x 1000
DC Amps ... 0-3
A Kelvin or a Wheatstone Bridge is used to measure the resistance of either the generator revolving field or the stator winding.
A 500 or 1000 Volt Megohmmeter is used to measure the generator stator revolving field insulation resistance.
A DC Ammeter with appropriate ampere shunt.
A tong-type Volt Ammeter with scales 0-600 volts and 0-600 amperes. Some generators are rated higher than 600 amperes, but these are normally connected with two or more conductors per phase. To measure line current for these generators, measure the current in each conductor per phase and add the individual conductor currents.
NOTE: See WARNING if generator is rated over 600 volts.
A direct reading Tachometer or a 3 or 5 second intergrating tachometer which is a relatively precise indicator, providing the drive does not slip. When precise frequency control is required, a direct comparison with utility power can be made.
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On generators rated above 600 volts, do not use direct reading test equipment to measure line voltage or line current (even through the instruments have high voltage scales). Potential and current transformers rated for applicable voltage must be used. DO NOT rely on test instrument insulation alone when working on power circuits above 600 volts. |
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Electrical instruments are not precisely accurate. Five percent difference in readings between two instruments is common. Multi-scale instruments can exhibit five percent difference between scales. Where uniform adjustments are necessary (as in parallel operation), use the same instrument for all voltage adjustments.
Many of the test procedures that follow designate the ohmmeter lead polarity (positive or negative) to use on the component being tested.
NOTE: Lead polarity markings of ohmmeter instruments are not to any standard. The lead marked in red, positive or "+" on some volt-ohm-milliammeters is at a positive potential when the instrument is used as an ohmmeter; on others, the lead marked in red, positive or "+" is a negative.
OHMMETER TERMINAL POLARITY CHECK
1. Selector switch. 2. Selector switch. 3. Mark to indicate polarity of ohmmeter leads.
Another volt-ohm milliammeter (multimeter) can be used with the test ohmmeter to determine the correct polarity of the leads.
Set one instrument selector switch (1) for DC volts and set the test ohmmeter selector switch (2) for ohms. Alternately connect the leads of both instruments until both instruments indicate a reading (at the same time).
The red, positive or "+" lead from the instrument with selector switch (1) set on DC volts is now connected to the test ohmmeter positive lead. Mark the ohmmeter to indicate the polarity of the ohmmeter leads (3). The correct position and negative leads on the test ohmmeter are now identified.
Temperature has a notable effect on electrical equipment. Resistance measurements made on a cold unit will be lower than the same measurement made at higher temperatures. Ohmmeters in general are quite inaccurate. Variations of up to 10 percent of published resistances can be expected if the total error of instrument method, temperature, and piece part tolerance happen to add in one direction.
A1 Regulator Assembly
C1 Surge Capacitor
CR1 Control Rectifier
CR2 Control Rectifier
CR3 Power Rectifier
CR4 Power Rectifier
CR5 Field Rectifier
CR6 Build-Up Diode
CR7 Surge Suppression Diode
CR8 Surge Suppression Diode
CR9 Blocking Diode
E1 Main Heat Sink
E2 Build-Up Heat Sink
E3 Auxiliary Heat Sink
F1 Fuse
F2 Fuse
FL1 Noise Suppressor Assembly
K1 Build-Up Relay
L1 Filter Choke
L2 Sensing Reactor
L3 Revolving Field
L4 Stator
R1 Regulator Gain Resistor
R2 Regulator Gain Potentiometer
R3 Voltage Droop Potentiometer
R4 Voltage Level Potentiometer
R5 Resistance Wire
R6 Surge Resistor
T1 Isolation Transformer
T2 Voltage Droop Transformer
T3 Regulator Transformer
TB1 Terminal Block
EXCITER AND REGULATOR ASSEMBLY
A1 Regulator Assembly
C1 Surge Capacitor
CR1 Control Rectifier
CR2 Control Rectifier
CR3 Power Rectifier
CR4 Power Rectifier
CR5 Field Rectifier
CR6 Build-Up Diode
CR7 Surge Suppression Diode
CR8 Surge Suppression Diode
CR9 Blocking Diode
E1 Main Heat Sink
E2 Build-Up Heat Sink
E3 Auxiliary Heat Sink
F1 Fuse
F2 Fuse
FL1 Noise Suppressor Assembly
K1 Build-Up Relay
L1 Filter Choke
L2 Sensing Reactor
L3 Revolving Field
L4 Stator
R1 Regulator Gain Resistor
R2 Regulator Gain Potentiometer
R3 Voltage Droop Potentiometer
R4 Voltage Level Potentiometer
R5 Resistance Wire
R6 Surge Resistor
T1 Isolation Transformer
T2 Voltage Droop Transformer
T3 Regulator Transformer
TB1 Terminal Block
SRCR GENERATOR WIRING DIAGRAM
Troubleshooting Charts
Initial Operating Procedure After Repairs
The engine should not be operated at rated RPM immediately after the cause for the fault condition has been repaired. The following procedure may prevent additional failures if the actual cause of the fault condition was not detected.
1. Remove build-up relay (K1) and disconnect wire (6) from regulator assembly (A1).
2. Start the engine and operate it at low RPM. The residual magnetism in the field should produce approximately 2 to 10 volts (generator output). Shorted controlled rectifiers (CR1 and CR2) will allow the field to produce higher output voltage.
3. Stop the engine and install build-up relay (K1).
4. Start the engine and operate it at low idle RPM. The output voltage will vary (up and down) when the points in the build-up relay are properly operating. If the output voltage is 2 to 10 volts and steady, either the build-up relay points are open or the controlled rectifier is open [or the field rectifier (CR5) is open]. If the build-up relay points remain closed, generator voltage will be higher and steady.
5. Stop the engine and reconnect wire (6) to regulator assembly (A1).
6. Start the engine and operate it at low RPM. The output voltage should be approximately 66% of generator rated voltage. If the output voltage is either too high or is not steady, the reference circuit may be open or the regulator assembly may be defective.
7. With the engine operating at low idle RPM and the generator producing a steady voltage (approximately 66% of rated voltage), slowly increase the RPM to rated RPM. Adjust the voltage level control to generator rated voltage. The generator is now ready to use.
Testing Power Rectifiers And Control Rectifiers
The power rectifiers and control rectifiers, used in the SRCR generator sets, are of the stud mounted type. Extra care must be taken in the installation, testing and replacement of these components.
SRCR RECTIFIERS
1. Build up diode CR6. 2. Control rectifier CR2. 3. Control rectifier CR1. 4. Power rectifier CR4. 5. Field Rectifier CR5. 6. Power rectifier CR3.
Failure of these units can result from:
- 1. Excess Current
- 2. Excess Voltage
- 3. Excess Heat
- 4. Incorrect Installation Torque
- 2. Excess Voltage
Rectifier failure normally is a short, rarely an open. However, their operating characteristics may change to the extent that performance in the excitation circuit is unsatisfactory. This type of rectifier fault can not be detected with ohmmeter checks.
CHECKING POWER RECTIFIER
Ohmmeter checks may be performed on these rectifiers, but only to determine open or shorted conditions. To perform these tests, the positive and negative leads of the ohmmeter are alternately connected to the anode and cathode. A shorted diode will indicate zero or equally low resistance in both directions, while an open diode will indicate an infinite resistance in both directions. A "good" rectifier will indicate a reverse resistance that is considerably higher than the forward resistance. Reverse resistance is 30,000 ohms or more. Forward resistance is approximately 10 ohms.
A "good" control rectifier will have both forward and reverse resistance of 30,000 ohms to 3,000,000 ohms, but must be approximately equal.
To test the gate circuit of a control rectifier, connect the positive and negative leads of the ohmmeter alternately to the gate and cathode leads. A shorted gate will indicate zero resistance in both directions and an open gate will indicate and infinite resistance in both directions. A "good" control rectifiers will have a forward (gate to cathode) and reverse (cathode to gate) resistance that are about equal and approximately 10 to 200 ohms.
The use of the mounting stud on these rectifiers accomplishes two purposes; electrical connection and heat (thermal) transfer to the heat sink which is the cooling media.
Be careful when mounting either a power rectifier or a control rectifier on the aluminum heat sink. The rectifier screw threads and mounting surfaces (rectifier and heat sink) must be clean. Apply a thin film of thermally conductive silicone grease to the mounting surfaces but not on the screw threads. 4L7464 Grease is a suitable conducting lubricant.
NOTE: A pounds-inch torque wrench (accurate in the listed torque range) should always be used when installing diode rectifiers and control rectifiers.
Voltage Regulator Assembly
A part in the Statically Regulated Controlled Rectifier Generator is the voltage regulator. It is a non-repairable epoxy sealed assembly. Failure of the assembly can usually be traced to a malfunction in the excitation or voltage sensing circuits.
A defective voltage regulator assembly (A1) can cause any of the following observed fault conditions:
- AC Voltage Too LowAC Voltage Too HighAC Voltage Unstable
NOTE: The detailed troubleshooting guide should be thoroughly reviewed before considering replacement of the voltage regulator.
Dynamic and static testing of the regulator is possible. For dynamic testing, a double pole single throw momentary close switch (DPST) must be installed in the excitation circuit. (See diagram). Install the switch with one wire in series between (F1) and wire 26, and the other wire in series between terminal TB1-5 and wire 39.
DPST switch connected in series in wires 26 and 39.
Start and operate the engine at low idle speed.
Momentarily close the switch and observe generator line voltage. If generated voltage is approximately the same percentage of rated voltage as low idle speed is of rated speed, the voltage regulator is functioning properly.
Example: An electric set rated for 230 volts at 1200 RPM (60 cycles) would generate only 172.5 volts at 900 RPM (45 cycles).
Increase engine speed to high idle and momentarily close the switch while watching line voltage. If line voltage is approximately equal to rated voltage, the regulator is functioning properly. If the fault condition persists, at either speed, the voltage regulator is probably defective.
As a final check, perform the static tests.
Isolate the voltage regulator assembly by removing the wires from terminals 1 through 12. Use an ohmmeter to measure the resistance between the terminals listed in the VOLTAGE REGULATOR TEST CHART.
NOTE: Make certain the positive and negative ohmmeter leads are known before testing the voltage regulator assembly circuits. See topic TEST INSTRUMENTS to verify the ohmmeter leads.
Flashing The Field
If the generator revolving field poles lose residual magnetism, the magnetism may be restored by "flashing" the revolving field windings with a direct current 24 volt source (battery).
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DO NOT OPEN OR CLOSE THE EXCITER WITH THE ENGINE-GENERATOR OPERATING. When the generator is operating, the heat sinks and exciter parts are electrically charged. Components will be damaged if metal contact is made between the generator frame and the heat sinks or exciter parts. |
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Static Flashing
With the engine stopped, disconnect revolving field (L3) wires from the noise suppressor (FL1). Wire C1 from terminal 5 and C2 from 7. Connect the positive "+" lead of the 24 volt source to wire C1. Momentarily apply (two or three times) the negative "-" lead of the source to wire C2.
NOTE: Correct polarity must be maintained (C1 is positive "+" and C2 is negative "-"). If the polarity of the field is reversed, the field rectifier will be damaged.
Dynamic Flashing
A more positive method of flashing the field can be effected by applying the DC source to the field while the engine is operating at low idle RPM.
With the engine stopped, connect an accurate AC voltmeter to terminals 3 and 22 on the noise suppressor (FL1). Connect the positive "+" lead of the 24 volt source to terminal 5 and the negative "-" lead to terminal 7. Start the engine and observe the voltmeter connected to terminals 3 and 22. When the voltmeter begins to show voltage carefully disconnect the DC source from terminals 5 and 7 (an OFF-ON switch in the DC source would be beneficial).