Testing And Adjusting

Usage:

Troubleshooting

General

Before attempting major adjustments or repairs on the SRCV Generator, the serviceman should have a thorough understanding of the operating principles of the generator, the applicable wiring diagrams, and he should verify the reported operating trouble.

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Generator Rated Above 600 Volts. Do not attempt to measure line voltage or line current on generator rated 600 volts with direct reading test metering equipment. Potential and current transformers rated for applicable voltages must be used, even though test instruments have high voltage scales. DO NOT rely on test instrument insulation alone when working on power circuits above 600 volts.

Successful troubleshooting can be accomplished in a minimum of time if you:

1. Recognize the apparent operational trouble.

2. Diagnose the trouble from the wiring diagram and troubleshooting guide.

3. Verify the diagnosed trouble by checking with appropriate test instruments.

4. Correct the trouble by adjustment or by replacing or repairing the defective components.

Keep these fundamentals in mind:

1. A generator is an energy converter. It can not delivery more power than the engine driving it.

2. A generator is essentially a constant speed device. Driving speeds 5 to 10 percent above or below rated speed can cause large variations in terminal voltage.

3. Electric motors are energy converters. Their power consumption depends on the load they drive. Automatically, they absorb more power if the connected load demands it. Power is a product of voltage and current. If the motor voltage is lower than normal, current input will increase proportionately.

4. Generator heating is the result of line current. The higher the line current the greater the heat.

5. Line currents do not necessarily indicate the power load on a generator because of the power factor of the load. Likewise, the line amperes will not necessarily be equal to the sum of the nameplate amperes of the connected loads since load components, such as motors, may not be fully loaded.

6. When load testing a generator with a resistance load bank or a water barrel, load amperes for rated kilowatts at rated volts are determined by multiplying nameplate amperes by nameplate power factor; for example, a generator rated 240 volts, 100 amperes, 0.8 power factor will be fully loaded by a resistance bank or water barrel when delivering 240 volts and 80 amperes.

7. Current will flow only through clean, solid connections. Vibration can and does loosen switchboard fasteners. Use depreciates switches and circuit breakers.

8. Electrical test (and panel) instruments are not precisely accurate. A 5 percent difference in readings between two instruments is common. Multi-scale instruments can exhibit a 5 percent difference between scales. Where uniform adjustments are necessary (as in parallel operation), use the same instrument for all voltage adjustments.

9. Temperature has a notable effect on electrical equipment. Resistance measurements made on a cold unit will be lower than the same measurement made at elevated 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.

10. Governor adjustments should never be hurried. Allow time for a governor to respond to slight adjustments.

11. A large percentage of reported electrical faults are the result of undetected mechanical deficiencies. Take time to examine the entire installation carefully and methodically, and separate mechanical deficiencies from electrical deficiencies.

Test Instruments

Most electrical troubleshooting can be accomplished with a volt-ohm-milliammeter having scales of approximately the following values:

AC Volts ... 0-50, 0-250

DC Volts ... 0-50, 0-250

Ohms ... 0-400, 0-50,000

DC Amperes ... 0-3

For flashing the generator field, a volt-amp tester with a heavy duty rheostat and a minimum of a 50 amp ammeter can be used.

If the generator revolving field amperes are to be measured, a DC ammeter with a 75 ammeter shunt is required. The volt-amp tester listed above may be adequate.

When measuring values of generator AC line voltage and line amperes, a tong-type volt-ammeter with scales 0-600 volts and 0-600 amperes may be used.

It should be noted that some of these generators are rated higher than 600 amperes, but these machines are normally connected with two or more conductors per phase. To measure line current for these generators, measure the current in one conductor per phase and multiply the reading by the number of conductors per phase.

Direct reading tachometers are seldom accurate enough for precise speed settings. A 3 or 5 second integrating tachometer 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 using lamps. Do not attempt this check with generators over 500 volts. When the lamps stay of uniform brilliance (no variation), generator frequency is precisely equal to that of the utility line. (Tachometers can be point calibrated by this arrangement at the synchronous engine speed, 900, 1200 or 1800 RPM at 60 cycles).

Troubleshooting Self-Regulated Constant Voltage Generators

I. No AC Voltage

A. Faulty Voltmeter

1. Check voltage with a meter know to be accurate.

2. Check fuses and connections in voltmeter circuit.

B. No excitation

1. Check for DC voltage at slip rings.

2. Check for DC voltage at the exciter lead terminal block. A₂ should be positive and A₁ should be negative. Refer to the exciter characteristic table for voltage values.

EXCITER LEAD TERMINAL BLOCK
1. Lead A₂. 2. Lead A₁.

3. Check for DC voltage at the exciter brushes.

C. Open circuit

1. Lift the slip ring brushes and check the generator field for continuity and resistance.

2. Check continuity of the leads from the slip ring brushes to the exciter brushes and fields.

3. Check continuity of the generator armature coils and connections, including the primary of the voltage droop transformer.

4. Check continuity of the commutating field coils and check continuity and resistance of the exciter field and exciter control field. See the tables for resistance values.

5. Check exciter armature for open coil.

6. An open circuited blocking rectifier will prevent exciter voltage build-up. Disconnect the No. 5 regulator wire (1). Connect one two-volt cell of a storage battery across the blocking rectifer with a low range ammeter in the circuit. A good rectifier will pass 1 to 2 amperes in the forward direction and less than 0.1 amperes in the reverse direction.

D. Grounded circuit

1. Disconnect the load lines and wires from the generator to the regulator terminals. If the generator neutral is connected to the frame or ground, open this connection. Test the generator armature insulation with a magneto or vibrator-type insulation tester. Generators rated 600 volts or less should test one megohm or more and 2400 volt generators should test three megohms or more.

NOTE: If neither type of insulation tester is available, the test for grounds may be made by measuring the resistance from any terminal to the frame with an ohmmeter.

CHECKING BLOCKING RECTIFIER
1. No. 5 regulator wire.

POLARIZING EXCITER FIELD
1. F₂ field lead. 2. No. 5 regulator terminal.

E. Exciter polarity reversed

1. Exciter reversed polarity will result in about 2 volt DC at terminals A₂-A₁ with a good blocking rectifier. Reversed polarity is indicated by A₂ negative and A₁ positive. With the generator not running, disconnect the F₂ field lead (1) from the exciter terminal block and connect it to the negative terminal of the battery. Touch a wire from the positive terminal of the battery to No. 5 regulator terminal (2). As much as 18 volts of storage battery may be required.

2. A short circuited blocking rectifier will not act to prevent a reverse flow of exciter field. Test the rectifier as described in IC 6. If 1 to 2 amperes can be passed through the rectifier in both directions, the rectifier is short circuited.

II. AC Voltage Too Low

A. Incorrect generator or regulator connections

1. Check the generator terminal connections against the high and low voltage connections show on the generator nameplate.

2. On 120-240 volt generators with the compensating current type regulator, check that the regulator leads 7, 8 and 9 are connected to the high voltage side of the terminal block when the 240 volt connection is used.

CONNECTION DIAGRAM FOR 120-240V GENERATOR (Compensating Current Transformer Type Regulator)
A. Voltage droop transformer. B. Voltage droop control switch. C. Saturable reactor. D. Full wave rectifier. E. Stator (alternator). F. Rotating field (alternator). G. Armature (exciter). H. Voltage level resistor (if used). I. Blocking rectifier. J. Voltage level rheostat. K. Exciter field. L. Exciter control field. M. Voltage change terminal block.

B. Faulty voltmeter

1. Check voltage with a meter know to be accurate.

2. Check fuses and connections in the voltmeter circuit.

C. Voltage level adjustment too low.

1. Increase the setting of the voltage level control.

D. Voltage droop adjustment too high

1. If voltage under load is too low and open circuit voltage is correct, decrease the setting of the voltage droop control. Make an adjustment to the open circuit voltage if necessary.

E. Engine speed too low

1. Check the engine speed with an accurate tachometer or frequency meter. Adjust the governor as required.

F. Exciter speed too low

1. Check for loose exciter belts.

2. When a 60 cycle generator is converted to operate at 50 cycles, the conversion includes changing one or both exciter drive pulleys to enable the exciter to operate at rated speed. CHECK the pulleys against those listed in the table of generator and exciter characteristics.

CHECKING BRUSH SPRING PRESSURE

G. Overload

1. Check the ampere load on each line against the generator rated amperes stamped on the nameplate. Reduce the load if overloading exits.

2. If the load is not balanced on a three phase generator, a light load on phase 2 with heavy loads on phases 1 and 3 may result in low voltage on phases 1 and 3. The regulator sensing circuit is connected to phase 2 so the regulator will act to maintain rated voltage on this phase. The heavily loaded phases will show lower voltage. Reconnect some of the loads from phases 1 and 3 to phase 2 in order to have better balance among the three phases.

H. Worn brushes or poor brush seating

1. Check exciter brushes for seating, length, tight connections, and binding in holders.

2. Check brush spring pressure. Pressure should be 1.5 to 2 psi (10 to 14 kPa) on brush face.

I. Faulty exciter coils

1. Check exciter coil resistances against the values shown in the table of exciter characteristics.

CHECKING VOLTAGE LEVEL RHEOSTAT
1. No. 5 field wire.

J. Faulty voltage level rheostat

1. Disconnect the No. 5 field wire (1) and check the operation of the rheostat as shown. The ohmmeter should show an even change when the rheostat control is moved from 100 to 0. The maximum resistance should be the value stamped on the rheostat.

K. Incorrect saturable reactor tap

1. Saturable reactor taps for self regulated constant voltage generators are listed in the table. If a tap lower than that listed is used, too much control field curwent will be obtained.

SATURABLE REACTOR TAPS (Compensating Current Transformer Type Regulator)

L. Faulty DC controlled reactor

1. The resistance of the two outer coils of the DC controlled reactor, when connected in series, is approximately 2 1/4 ohms. If the resistance is less, then too much control field current can flow. This will cause low AC voltage.

2. Test the AC winding of the DC controlled reactor as follows:

SATURABLE REACTOR TAPS (DC Controlled Reactor Type Regulator)

a. Connect a battery, rehostat and a 0 to 1 ampere DC ammeter across regulator terminals 8 and 9.

b. Adjust the rheostat so that .125 amperes flow in the center coil of the reactor.

c. Connect a low voltage AC source and a low range ammeter across regulator terminal 11 and the wire from the B terminal of the saturable reactor.

With the AC voltage adjusted to the value given in the table, the AC current should be within the range given in the table.

III. AC Voltage Too High

A. Incorrect generator or regulator connections

1. Check the generator terminal connections against the high and low voltage connections shown on the generator nameplate.

2. On 120-240 volt generators with the compensating current type regulator, check that the regulator leads 7, 8 and 9 are connected to the low voltage side of the terminal block when the 120 volt connection is used.

B. Faulty voltmeter

1. Check voltage with a meter known to be accurate.

2. Check fuses and connections in the voltmeter circuit.

C. Voltage level adjustment too high

1. Decrease setting of the voltage level control.

D. Voltage droop adjustment too low

1. If voltage under load is too high and open circuit voltage is correct, increase the setting of the voltage droop control. Readjust the voltage droop control. Readjust the open circuit voltage if necessary.

E. Engine speed too high

1. Check engine speed with an accurate tachometer or frequency meter. Adjust governor as required.

F. Exciter speed too high

1. When a 50 cycle generator is converted to operate at 60 cycle, the conversion includes one or both pulleys to enable the exciter to operate at rated speed. Check the pulleys against those listed in the table of exciter ratings.

G. Faulty full-wave rectifier

1. A faulty full-wave rectifier that will not furnish the required amount of DC current to the control field can cause high AC generator voltage. Test the rectifier as shown in the diagram.

CHECKING FULL-WAVE RECTIFIER

H. Open circuit in regulator and control field circuits

1. Disconnect wires from generator to regulator terminals 10 and 2. Check the continuity between these wires.

2. Disconnect regulator wires on terminals 10 and 1 and check continuity between them.

3. Disconnect regulator wires on terminals 1 and 2 and check continuity between them. Check at all points on the voltage droop switch.

4. Disconnect the exciter control field wires coming to regulator terminals 3 and 4. Check continuity between them.

I. Exciter reversed polarity

1. Using a DC voltmeter, check that exciter terminal A₂ is positive and A₁ is negative. If the polarity is reversed, stop the engine and flash the exciter field. (Refer to IE 1.)

2. Check the blocking rectifier (Refer to IC 6 and IE 2).

J. Control field reversed

1. Using a DC voltmeter, check that regulator terminal 3 is positive and terminal 4 is negative. If the polarity is reversed, reverse regulator wires 3 and 4 at the terminal strip.

CHECKING VOLTAGE DROOP RHEOSTAT
1. No. 7 wire.

K. Faulty voltage droop rheostat

1. Disconnect the No. 7 wire (1) from the regulator terminal and check the operation of the rheostat as shown. The ohmmeter should show an even change when the rheostat control is moved from 100 to 0. The maximum resistance should be the value stamped on the rheostat.

2. With the ohmmeter connected to the rheostat, move the arm from maximum to minimum resistance positions and observe the variation in meter reading.

L. Incorrect saturable reactor taps

1. Check the saturable reactor taps with the table. Reconnect to the proper taps if an incorrect connection has been made.

M. Leading power factor load

1. A leading power factor load will furnish excitation to the generator. This causes the generator voltage to tend to rise, and the regulator acts to reduce the exciter output in an effort to have only enough excitation at the generator to produce required voltage while supplying power to the connected load. If the load has an extremely leading power factor, there may be more excitation furnished to the generator than can be compensated for by the regulator, and the AC voltage will rise.

2. Leading power factor can be caused by an excessive amount of capacitors connected to the load, a lightly loaded synchronous motor, or an induction motor being driven by its load.

3. Leading power factor can be corrected by disconnecting capacitors, adding load or reducing the excitation of synchronous motors, and arranging the control of induction motors so that they do not generate back into the AC power source.

IV. AC Voltage Fluctuates

A. Engine speed fluctuates

1. Repair governor.

B. Load fluctuates

1. Fluctuating voltage cuased by changes in load is not a generator malfunction. The sequence of variation of different parts of the load should be checked from the viewpoint of reducing the number of simultaneous load changes.

C. Loose connection

1. Check that all connections are tight.

D. Incorrect exciter brush position

1. Mark the brush mounting assembly and bearing cap to indicate the initial brush position.

2. Loosen the setscrews (1) and (2) and move the brush mounting assembly against the direction of armature rotation the width of one commutator bar. Lock the setscrews.

CHANGING EXCITER BRUSH POSITION
1. Setscrew. 2. Setscrew.

NOTE: One setscrew locks in a hole drilled in the bearing cap. Care must be exercised that this screw does not slip back into its original location. If this does happen, disconnect the A₁ and A₂ leads at the brush mounting assembly, rotate the brushes one quarter turn, and reconnect the leads. This will place both setscrews on smooth surfaces.

Start the engine and observe the AC voltage. If it still fluctuates, move the brushes an additional small amount to minimize the fluctuation. Do not move the brushes so far that sparking becomes excessive.

3. A more exact method of determining the exciter brush neutral position is to check the exciter output for both directions of rotation. To perform this test, the exciter can be checked in place for normal direction of rotation. For reverse rotation, it can be dismounted, turned around, and mounted in some temporary manner so it can still be driven using the pulleys and belts furnished with the generator.

An alternate method is to take the exciter to a shop where there is some method of driving it at rated speed in both directions as well as loading it. The brush neutral position is determined in the following manner.

a. With the control field disconnected, operate the exciter at rated speed and rated output for at least one-half hour. See the table "Exciter and Generator Characteristics" for exciter rated volts and amperes.

b. Stop the exciter and inspect the brushes to be sure they are completely seated.

c. Start the exciter and clean the commutator with a brush seating stone.

d. Allow the exciter to operate at least one minute after cleaning the commutator.

e. Adjust the exciter field current until the exciter is operating at approximately rated output. The load can be resistance, lamp bulbs, or a water barrel.

f. Stop the exciter.

g. Start the exciter, but do not let it overspeed. Observe the output voltage and current.

h. Stop the exciter and reverse leads A₁ and A₂.

i. Start the exciter with rotation in the opposite direction to that used above.

j. Repeat Steps c, d, f, and g. Do not adjust the field current after the final adjustment of Step e. If the voltage and current differ more than 5 volts and 1/2 ampere from the values obtained in Step g, shift the exciter brushes against the direction of armature rotation to raise the voltage and with the direction of armature rotation to lower the voltage.

k. Repeat Steps h, i and j, three more times. In this test, the correct brush neutral position is indicated by obtaining the same output voltage and current for both forward and reverse rotation of the exciter. However, this is an ideal condition that rarely exists. The neutral position of the brushes may be considered to be adjusted satisfactorily if the difference between the output voltage and current for forward and reverse rotation of the exciter does not exceed 5 volts and 1/2ampere.

Two of the most important points in checking the neutral position of the exciter brushes are that the exciter voltage and current should be observed when the exciter is operating on the ascending portion of its saturation curve and that the brushes must be allowed to seat each time the rotation of the exciter is reversed. The first is taken care of in Step g and the second in Steps c and d.

E. Incorrect saturable reactor connection

1. Check the connections to the saturable reactor. Make reference to REGULATOR CONNECTIONS.

F. Excessive control field current

1. The table of generator and exciter characteristics shows the exciter control field current should be in the range of 0.7 to 0.9 amperes. If the control field current is greater than 0.9 amperes, add one or more sets of "E" and "I" laminations to the saturable reactor to bring the control field into the specified range.

G. Insufficient control field current

1. Remove one or more sets of "E" and "I" laminations from the saturable reactor to bring the control field current into the range of 0.7 to 0.9 amperes. When laminations are removed from a reactor, small wooden wedges should be inserted in the bobbin to keep the core tight.

TESTING SATURABLE REACTOR
A. Ammeter. B, H, L and M. Saturable reactor taps. V. Voltmeter.

H. Faulty saturable reactor

1. A complete test of the saturable reactor can be made as follows:

a. Connect a 110-120 volt AC source to the B and L taps of the reactor through a low range ammeter. Record the ammeter reading. Also record the voltages B-L, B-M, and B-H with the AC power connected to the B-L taps. Voltages and currents should be within 20% of the values in the table.

I. Excessive voltage droop adjustments

1. If the voltage droop control is adjusted too far toward maximum on generators with the DC controlled reactor type regulator, unstable generator voltage can result. This is evidenced by the voltage not returning to the original value upon successive stopping and starting of the engine or upon successive applications of load. To stabilize the voltage, the setting of the voltage droop control must be reduced.