What's In A Generator?

What's In A Generator?
Engine Burns Fuel

Creates:

- Rotating mechanical energy

- Electrical power
 What's In A Generator?
Main Components
 Prime Mover
 Alternator
 Controls
 Radiator/ Heat exchanger
 Base Rail
 Anti Vibration Mounts
 Coupling between alternator and prime mover for two bearing machine
 What's In A Generator?
The engine & the alternator are supported by the base rail through engine
& generator base mounting.
Base rails are made from standard commercial C/I (Cast Iron) channels or bend
plates/sheets. Entire Genset is mounted on the base rail. The base rail is the
foundation on which entire Genset performs.

Base rail also serves as the base for accessories, controls, cooling, oil & starting
systems & sometimes complete housing.

Base Rail & Mounting

Energy Flow Diagram - Genset
Energy Flow Diagram - Engine

There around 1000 components in engine, the losses arises due to friction,
inertia, cooling etc.
Energy Flow Diagram - Alternator
Genset Overview

Automatic Transfer Switch (ATS)

Prime mover
Generator

A standby generator system provides electricity when main electrical power is partially or
completely interrupted.

A typical standby generator system consists of:

A Genset
A Transfer Switch
Support equipment (fuel tank, cooling system, exhaust,
battery chargers, etc.)
Genset Overview
Genset Overview
Most Standby power systems include the hardware listed above. A few points
to clarify are:
1. Generator and Genset are common terms for the source of electrical power.
To the purist, any machine that produces Alternating Current (AC) is an
Alternator.
In this course, we will mostly use the term Alternator.
2. Transfer switches may be either manual or automatic. We will focus on
Automatic Transfer Switches (ATS). They perform the task of transferring
loads to emergency power and re-transferring loads back to commercial
(utility / mains) power.
3. Not all generating systems are used for standby power. There are many
instances where the genset is the only source of power. This type of
application is known as a Prime Power installation and probably wouldn‘t
include a transfer switch (because there is no other source of power to transfer
to).
4. Any conventional generator needs mechanical effort to spin the alternator.
The mechanical effort is supplied by a Prime Mover. The prime mover may be
in the form of a windmill (or wind turbine), hydro (in the form of a water
turbine), steam (via a steam turbine) or via an internal combustion engine. We
will focus on engine driven alternators in this class.
Genset Overview

A standby generator is used to supply electric power when the utility power
is interrupted.
A standby generator consists of an Automatic Transfer Switch (ATS), a
prime mover, and a generator.
The prime mover turns the rotor of the generator to generate electricity.
Basic Alternator Components

Rotor
Stator
(Coil windings and magnetic field)
Operation:
Relative motion between the rotor and the stator generates AC electricity by
cutting across magnetic lines of force.
Factors Determining Alternator Output:
Number of turns in the coil
Magnetic field strength
Number of poles
Rotation speed
Effect of Changing Speed:
The most pronounced (and immediate effect) is a change in frequency. As the
rate of change in lines of flux being cut (by the change in prime mover speed)
the voltage may rise or fall.
Number of Poles:
If you want to obtain AC voltage at a specific frequency from a generator, the
required shaft speed (rpm) is determined solely by the number of poles in the
generator. Using the formula below, one can easily determine the number of
poles in a machine if the rotational speed is known. Conversely, if the number
of poles is known, the rotational speed can be determined.
F= NP / 120
Where F is the Frequency, P is the number of poles and N is the rotational
speed.
Typically, the engine‘s output shaft is connected directly to the generator‘s input
shaft (1:1 speed ratio). The drive could also be geared up or down for a specific
application (just like your car‘s transmission.)
For most purposes, AC generators are favored over DC generators. However,
once the AC current is produced, it‘s very hard to change its frequency.
Adjusting AC Generator Output:
If AC generator speed has to remain constant, then how can we increase
or decrease the power output, as needed?
Since most practical generators use electromagnets to generate the
magnetic field, changing the current to the coils increases or decreases
the field strength, thereby increasing or decreasing the power output
without affecting the output frequency. This is usually done with a
Regulator.
Generator output ratings are given in Watts (or kilowatts-kW). Remember that
Watts are a measure of power – more useful than Volts or Amps alone because
for a given Wattage there are specific combinations of voltage and current. For
example, if you have a 5000 Watt home generator and you know that the
voltage is 240, then the formula A= W/V would give you 20.8 Amps.
One of the key principles to know about generators is that the Prime Mover
produces kW and the Alternator produces kVA. This means that the prime
mover needs to produce mechanical power for the alternator to produce volts
and amps. As a rule of thumb, for any genset the engine is the limiting factor
not the alternator. Under extreme loads, the engine (limitations of engine :
torque, speed, friction, noise, leakages, cooling, fuel quality, emission,
etc..)will usually stall before the alternator stops producing electricity.
Why Would You Need to Adjust the Generator’s Output?
To react to load changes. As more or less power is drawn by the attached loads,
the generator needs to supply more or less power to meet the demand (within
operational limits).

Typical Generator Construction

Rotor: The rotating portion of any piece of electrical equipment.
Stator: The stationary portion of any piece of electrical equipment.
Field: The part that generates a magnetic field.
Armature: The part that gets current induced in it.
Excitation: The process of getting electrons to flow in a conductor.
Exciter: The component that produces excitation (can be a rotor or stator).
Winding: Is a continuous piece of wire (usually copper) that is wound around a steel
assembly- there are stator and rotor windings.
In the world of alternators, there are two prevalent designs that are commonly
used: Rotating Armature and Rotating Field. Beyond that, there are two
excitation systems that are typically employed: Brush and Brushless.
We will discuss the rotating field- brushless design
In a rotating field design, the stator windings get current induced in them by a
rotating magnetic field. The rotor windings are fed a DC (through brushes and
slip rings) producing an electromagnet with north and south poles. The rotation of
N-S-N… poles of magnetism in the presence of the stator windings produces an
AC output. The current flow into the windings of the rotor is known as Excitation
Current. In most cases the excitation current is relatively small compared to the
amount of current that the output of the stator can produce and thus the wear on
the slip rings due to arcing is greatly reduced. The stator windings connect
directly to the load- eliminating any losses and maintenance issues that would
have been present with a rotating armature design. However, the slip
rings and brushes still remain and they are still a maintenance consideration.
The design shown above is a Rotating Field-Brushless design. In most cases this
design is known simply as a brushless system (the rotating field being implied).
In this design,
there are two stators- the Exciter Stator and a Main Stator.
There are two rotating components as well- the Exciter Rotor and the Main
Rotor.
The rotors are mounted on a common shaft.
The shaft is supported by a bearing on one end (near the exciter rotor) and by
the flywheel on the other end.
The drive disks fit into a recess machined into the flywheel where they are
bolted in place.
This arrangement is known as a Single Bearing alternator.
Realize that the flywheel, the bearing next to it and the crankshaft are all part of
the engine (or prime mover) and not part of the alternator.
Theory of Operation
At rest, when the rotor is not spinning, there is no current flow anywhere in the
alternator.
A small amount of residual magnetism remains in the exciter stator. This means
that even though the windings of the exciter stator have no current flowing
through them, there is still some magnetism present in the stator.
When in operation, the rotor (driven by the prime mover) turns. The exciter rotor
windings pass through the small residual magnetic field in the exciter stator
causing a current to be induced.
The current induced in the exciter rotor windings is AC (since the poles of the rotor
are moving past a fixed magnetic field). Similar in principal to the rotating field
design we discussed previously, the objective of this design is to create a consistent
N-S-N… relationship in the poles of the main rotor. If we were to feed the current
from the exciter rotor (AC) into the main rotor directly, we would have a
constantly changing polarity. This situation is undesirable. What we need is a
direct current (DC) to be fed into the main rotor windings. Lucky for us, there is a
means of converting AC to DC. By using a device called a Rectifier (also called a
Diode) we can provide direct current to the main rotor from the exciter stator. As
shown in detail ―A‖ of the diagram above, the Rotating Rectifiers are mounted to
the exciter rotor. There is a set of rectifiers that provides positive potential and a set
of rectifies that provides negative potential. The direct current in the main rotor
provides us with a rotating field that in turn causes an AC current to be induced in
the windings of the main stator.
As the prime mover approaches normal operating speed, the windings of the main
stator cut the lines of flux on the main stator with greater speed, resulting in higher a
voltage being produce in the stator. Some of this voltage is tapped and fed into the
AVR (Automatic Voltage Regulator). The AVR takes AC voltage from the main
stator, rectifies it and sends DC current to the exciter stator. This current enhances the
magnetic field (which was originally only a small residual amount) providing more
lines of flux that are cut by the exciter rotor. The result is more current being
delivered to the windings of the main rotor which in turn creates more lines of flux
that are cut by the main stator. This ―build up‖ continues up to the point where the
AVR starts to regulate. It does this by sensing the input (from the main stator) and
cutting back on the output (to the exciter stator) until a normal operating threshold is
achieved. The regulation process continues as long and the alternator is producing
power.
This type of system has no slip rings or brushes. Consequently, there are
none of the maintenance issues that would be present with a brush type
system.
In addition to being brushless, this system is known as a ―Self Excited‖
design. It gets its name from the fact that the excitation current is derived
from the output of the main stator, and not from an external source.
PMG Excitation Systems
When a load is applied to any generating system, there is a tendency for the
output voltage to drop in proportion with the size of the load. That is, the
greater the load the greater the voltage drop. The AVR we mentioned earlier
does a nice job of detecting these drops and it increases the excitation current
in an effort to compensate for the falling voltage. In many situations the self
excited system is perfectly adequate and no real problems exist when the
system is heavily loaded.
There are certain loads that do present difficulties to self excited systems. One
of the most difficult types of load for a genset to ―pick up‖ is a motor load.
Large motors present a nearly instantaneous demand for current when they
are connected. In cases where the motor size is large enough, the output
voltage of the alternator will drop to the point where the AVR cannot
compensate. Remember, the internal electronics of the AVR are powered by
the output voltage from the alternator. This voltage drop is so severe that the
AVR shuts off (because the input power to it isn‘t high enough to keep it
operating). Without the AVR there is no excitation current and the output from
the alternator falls to zero. This condition is known as a ―collapsed field‖.
Since the voltage drop was the reason the AVR shut off, wouldn‘t it be nice if the
AVR was powered by an external source (and was still able to sense the output
from the alternator)? That way, the AVR would keep working no matter what the
voltage output from the alternator was. Well, sure enough, that‘s exactly what the
PMG (Permanent Magnet Generator) does. It‘s a small, totally independent
generator whose sole function is to provide power to the AVR and the excitation
system. The AVR senses the voltage from the main stator while the PMG provides
power to the internal electronics and provides the actual excitation current. In this
application, the AVR senses and adjusts excitation (it regulates).
PMG excitation systems are standard on Cummins commercial gensets
(around 500kW and up) and are optional on smaller commercial units.
Because the PMG presents an extra cost, they are usually not found on
consumer equipment. The same can be said for brushless excitation
systems; they are commonly found on commercial generators, but not
normally found on consumer equipment. Why? Cost!
Prime Movers:
In most Cummins generators, gasoline and diesel engines are the prime movers
used to run the generator. The engine is one-half of a Cummins Genset. Both
gasoline and diesel engines contain an electrical system that needs to be
maintained and serviced.
How Do Internal Combustion Engines Work?
An internal combustion engine uses chemical energy as its input, and it produces
mechanical energy as its output.
Gas and diesel engines work in much the same way. The significant most
differences between the two are the type of ignition employed and the type of
fuel used.
Typical Engine Systems and Components:
An internal combustion engine consists of the following basic parts:
Cylinder
Valves
Piston
Connecting Rod
Camshaft
Flywheel
Spark Plug
Piston and Camshaft:
The piston is pushed downward by the burning gasoline and air mixture, the connecting
rod, which connects the piston to the camshaft, pivots as the piston moves, and
translates the piston‘s up and down motion into rotary motion. The camshaft is
connected to the flywheel and gears which regulate the speed and power of the rotary
drive motion.
Diesel Engines:
The diesel engine operates much like a gasoline engine in that it has an
operating cycle in which a combustible fuel and air mixture is burned to
produce a piston stroke.
Diesel engines are different from gasoline engines:
 Uses a different fuel – diesel oil, instead of gasoline.
 Uses heat of compression to ignite the fuel, not an electric spark plug.
 The fuel is injected into the cylinder at the end of the compression stroke.
 Preheating is required before starting the engine.
Diesel Fuel:
Diesel fuel is a less refined and heavier fuel that contains more sulphur
compounds than gasoline creating a dirtier and more polluting exhaust.
Improvements in the fuel and engine technology have reduced emissions
considerably.
How Does Diesel Ignition Occur?
With no spark plug to ignite the fuel-air mixture, how does diesel ignition take place?
Any time a gas (air in this case) is quickly compressed, it heats up as the molecules
are pressed together. Simply, the molecules hitting each other creates friction in the
same way that rubbing your hands together creates heat.
(Remember, when sprayed in a fine mist, gasoline or diesel fuel behaves as if it were a
gas.)
When the diesel fuel is sprayed into the cylinder, the heat of the compressing gas
causes the fuel to ignite and create the diesel engine‘s power stroke.
The gas heats up when it is compressed according to the Ideal Gas Law.
Controls:
A Genset‘s controls serve the following functions:
 Used by the operator to manually run the Genset.
 Allows the engine to monitor its own condition and adjust its own operation,
 accordingly.
 Provides communication between the engine and the generator, so that each knows
 what the other‘s status is and commands can be sent back and forth.
Built-in Genset Protection Devices
 Overspeed and underspeed
 Overvoltage and undervoltage
 Circuit breakers and fuses
 Emergency stop button

Integrated microprocessor-based (digital) controls allow the genset and transfer

switch to offer ―smart‖ functionality — accessing critical performance data and
communicating that data to each other as well as to other building management
systems. Digital controls use the power of computers to allow the genset
components to interact with each other. They provide more than the combination
of a good engine, alternator, controls and transfer switches.
The transfer switch monitors utility and emergency standby generator set power.
When utility power fails or is unsatisfactory, the control starts the generator set.
Most transfer switches have the ability to sense the quality of the power of the
generator as well as the utility. If the generator power is satisfactory, the ATS will
transfer to the generator. An automatic transfer switch immediately senses when
utility power has been interrupted, transferring the load to the generator.
The transfer panel will sense when utility power is restored. It automatically times
for user adjustable period to insure that the utility power is stable. If the power is
stable the ATS will retransfer back to utility power. After the load has been
retransferred, the ATS will keep the genset running for a cool down period. After
the cool down period has expired, the genset shuts down.
Transfer Switch Types:
Automatic or manual
Enclosed or Non-enclosed.
Open Transition or Closed Transition
Break-Before-Make Transfer Switch:
A Break-Before-Make transfer switch breaks contact with one source of
power before it makes contact with another. It prevents back feeding from
an emergency generator back into the utility line. During the time of the
power transfer the flow of electricity is interrupted.
Power Outage Occurs:
When utility power voltage falls to less than 85% of nominal, or fails entirely, the
standby power system will automatically go through a start sequence and connect to
a home. The transfer panel control constantly monitors the power quality from both
the utility source and the generator set. When the transfer panel control senses
unacceptable utility power, the control waits for 3 seconds and then sends a signal to
start the generator set engine. If the utility power returns before 3 seconds has
passed, the generator set engine will not be signaled to start. When the start signal is
received, the engine starts and
reaches the proper operating speed and AC power is available at the generator set.
The transfer panel control senses this, waits for 3 seconds and will then transfer
generator set power to the home through the transfer panel contractors. This
sequence of operations will usually occur in less than 10 seconds from the time the
power outage occurred to the time when generator set power is connected.
Utility Power Returns:
When utility power comes back on and returns to your home, the transfer
panel control senses this and will watch for acceptable voltage. After
checking for acceptable utility voltage for five minutes, the transfer panel
control will signal the transfer panel contractors to re-transfer the load back
to the utility source and disconnect the generator set source. At this point,
the generator set is "off-line" and will be operated automatically
another five minutes to properly cool down. After the cool down cycle, the
generator set will be turned automatically off and reset to standby mode.

Battery Charger:
Some Cummins transfer switches have a built-in battery charger that
automatically keeps the genset‘s battery charged up so that it will be able to
start the engine when needed.
SERVICE AND MAINTENANCE:
As part of routine maintenance procedures, periodic attention to winding condition
(particularly when generators have been idle for a long period) and bearings is
recommended.
WINDING CONDITION ASSESSMENT

The condition of the windings can be assessed by measurement of insulation

resistance [IR] between phase to phase, and phase to earth.
Measurement of winding insulation should be carried out : -
1. As part of a periodic maintenance plan.
2. After prolonged periods of shutdown.
3. When low insulation is suspected, e.g. damp or wet
windings.
Care should be taken when dealing with windings that are suspected of being
excessively damp or dirty.
The initial measurement of the [IR] Insulation Resistance should be established
using a low voltage (500V) megger type instrument.
If manually powered the handle should initially be turned slowly so that the full
test voltage will not be applied, and only applied for long enough to very quickly
assess the situation if low values are suspected or immediately indicated.
Full megger tests or any other form of high voltage test should not be applied
until the windings have been dried out and if necessary cleaned.
Procedure for Insulation Testing
Disconnect all electronic components, AVR, electronic protection equipment etc.
Ground the [RTD's] Resistance Temperature Detection devices if fitted. Short out
the diodes on the rotating diode assembly. Be aware of all components connected to
the system under test that could cause false readings or be damaged by the test
voltage. Carry out the insulation test in accordance with the ‗operating instructions
for the test equipment.
The measured value of insulation resistance for all windings to earth and phase to
phase should be compared with the guidance given above for the various 'life stages'
of a generator. The minimum acceptable value must be greater than 1.0 MΩ.
If low winding insulation is confirmed use one or more of the methods, given
below, for drying the winding should be carried out.
METHODS OF DRYING OUT GENERATORS
Cold Run
Consider a good condition generator that has not been run for some time, and has
been standing in damp, humid conditions. It is possible that simply running the gen
set unexcited – AVR terminals K1 K2 open circuit - for a period of say 10 minutes
will sufficiently dry the surface of the windings and raise the IR sufficiently, to
greater than 1.0 MΩ , and so allow the unit to be put into service.
Blown Air Drying
Remove the covers from all apertures to allow the escape of the water-laden air.
During drying, air must be able to flow freely through the generator in order to
carry off the moisture.
Direct hot air from two electrical fan heaters of around 1 – 3 kW into the generator
air inlet apertures. Ensure the heat source is at least 300mm away from the windings
to avoid over heating and damage to the insulation. Apply the heat and plot the
insulation value at half hourly intervals. The process is complete when the
parameters covered in the section entitled, ‗Typical Drying Out Curve‘, are met.
Remove the heaters, replace all covers and re-commission as appropriate. If the set
is not to be run immediately ensure that the anticondensation heaters are energised,
and retest prior to running
Once the Insulation Resistance is raised to an acceptable level - minimum value 1.0
MΩ − the dc supply may be removed and the exciter field leads ―X‖ and ―XX‖ re-
connected to their terminals on the AVR.
Rebuild the genset, replace all covers and re-commission as appropriate.
If the set is not to be run immediately ensure that the anti condensation heaters are
energised, and retest the generator prior to running.
It should be noted that as winding temperature increases, values of insulation
resistance may significantly reduce. Therefore, the reference values for insulation
resistance can only be established with windings at a temperature of
approximately 20°C.
TYPICAL DRYING OUT CURVE
Whichever method is used to dry out the generator the resistance should be measured
every half-hour and a curve plotted as shown in fig. below

1) Y axis = Resistance
2) X axis = Time
3) One Megohm limit

The illustration shows a typical curve for a machine that has absorbed a
considerable amount of moisture. The curve indicates a temporary increase in
resistance, a fall and then a gradual rise to a steady state. Point ‗A‘, the steady
state, must be greater than 1.0 MΩ. (If the windings are only slightly damp the
dotted portion of the curve may not appear).
BEARINGS (Supporting element for rotating part)
All bearings are supplied sealed for life and are, therefore, not regreasable.
The life of a bearing in service is subject to the working conditions and the
environment.
Long stationary periods in an environment where there is vibration can cause false
brinnelling which puts flats on the ball and grooves on the races. Very humid
atmospheres or wet conditions can emulsify the grease and cause corrosion. High
axial vibration from the engine or misalignment of the set will stress the bearing.
The bearing, in service, is affected by a variety of factors that together will
determine the bearing life.
If excessive heat, noise or vibration is detected, change the bearing as soon as
practicable. Failure to do so could result in bearing failure.
AIR FILTERS ( To filter the incoming air for combustion, air must be filtered
before it participates in combustion process, if unfiltered air is passed then it may
cause wear and tear of cylinder wall, piston etc…)
The frequency of filter maintenance will depend upon the severity of the site
conditions.
Regular inspection of the elements will be required to establish when cleaning is
necessary.
Remove the filter elements from the filter frames.
Immerse or flush the element with a suitable detergent until the element is clean.
Dry elements thoroughly before refitting.
FAULT FINDING
Before commencing any fault finding procedure examine all wiring for broken or
loose connections.
FAULT FINDING
UFRO (Under Frequency Roll Off)
AVR - FAULT FINDING
AVR - FAULT FINDING
TRANSFORMER CONTROL - FAULT FINDING
RESIDUAL VOLTAGE CHECK
With the generator set stationary remove AVR access cover and leads X and
XX from the AVR.
Start the set and measure voltage across AVR terminals 7-8 on SX460 AVR or
P2-P3 on AS440 or SX421 AVR.
Stop the set, and replace leads X and XX on the AVR terminals. If the
measured voltage was above 5V the generator should operate normally.
If the measured voltage was under 5V follow the procedure below.
Using a 12 volt d. c. battery as a supply clip leads from battery negative to AVR
terminal XX, and from battery positive through a diode to AVR terminal X. See
Fig. 10.
 A diode must be used as shown below to ensure the AVR is not damaged.

Restart the set and note output voltage from main stator, which should be
approximately nominal voltage, or voltage at AVR terminals 7 and 8 on SX460, P2-P3
on AS440 or SX421 which should be between 170 and 250 volts.
Stop the set and unclip battery supply from terminals X and XX. Restart the set. The
generator should now operate normally. If no voltage build-up is obtained it can be
assumed a fault exists in either the generator or the AVR circuits. Follow the
SEPARATE EXCITATION TEST PROCEDURE to check generator windings,
rotating diodes and AVR.
SEPARATE EXCITATION TEST PROCEDURE
The generator windings, diode assembly and AVR can be checked using the
appropriate following section
GENERATOR WINDINGS, ROTATING DIODES and PERMANENT
MAGNET GENERATOR (PMG) EXCITATION CONTROL TEST.
CHECKING PMG
Start the set and run at rated speed. Measure the voltages at the AVR terminals P2, P3 and P4.
These should be balanced and within the following ranges :-
50Hz generators - 170-180 volts
60Hz generators - 200-216 volts
Should the voltages be unbalanced stop the set, remove the PMG sheet metal cover from the
non drive end bracket and disconnect the multipin plug in the PMG output leads. Check leads
P2, P3, P4 for continuity. Check the PMG stator resistances between output leads. These should
be balanced and within +/-10% of 2.3 ohms. If resistances are unbalanced and/or incorrect the
PMG stator must be replaced. If the voltages are balanced but low and the PMG stator winding
resistances are correct - the PMG rotor must be replaced.
CHECKING GENERATOR WINDINGS AND ROTATING DIODES
This procedure is carried out with leads X and XX disconnected at the AVR or
transformer control rectifier bridge and using a 12 volt d.c. supply to leads X and XX.
Start the set and run at rated speed. Measure the voltages at the main output terminals
U, V and W. If voltages are balanced and within +/-10% of the generator nominal
voltage, refer to 7.5.1.1. Check voltages at AVR terminals 6, 7 and 8. These should be
balanced and between 170-250 volts. If voltages at main terminals are balanced but
voltage at 6, 7 and 8 are unbalanced, check continuity of leads 6, 7 and 8. Where an
isolating transformer is fitted (MX321 AVR) check transformer windings. If faulty the
transformer unit must be replaced.
BALANCED MAIN TERMINAL VOLTAGES
If all voltages are balanced within 1% at the main terminals, it can be assumed that all exciter
windings, main windings and main rotating diodes are in good order, and the fault is in the
AVR or transformer control. Refer to subsection 7.5.2 for test procedure.
If voltages are balanced but low, there is a fault in the main excitation windings or rotating
diode assembly. Proceed as follows to identify :-
Rectifier Diodes
The diodes on the main rectifier assembly can be checked with a multimeter. The flexible
leads connected to each diode should be disconnected at the terminal end, and the forward
and reverse resistance checked. A healthy diode will indicate a very high resistance (infinity)
in the reverse direction, and a low resistance in the forward direction. A faulty diode will give
a full deflection reading in both directions with the test meter
on the 10,000 ohms scale, or an infinity reading in both directions. On an electronic digital
meter a healthy diode will give a low reading in one direction, and a high reading in the other.
Replacement of Faulty Diodes
The rectifier assembly is split into two plates, the positive and negative, and the
main rotor is connected across these plates. Each plate carries 3 diodes, the
negative plate carrying negative biased diodes and the positive plate carrying
positive biased diodes. Care must be taken to ensure that the correct polarity
diodes are fitted to each respective plate. When fitting the diodes to the plates
they must be tight enough to ensure a good mechanical and electrical contact, but
should not be overtightened
Surge Suppressor
The surge suppressor is a metal-oxide varistor connected across the two rectifier plates
to prevent high transient reverse voltages in the field winding from damaging the
diodes. This device is not polarised and will show a virtually infinite reading in both
directions with an ordinary resistance meter. If defective this will be visible by
inspection, since it will normally fail to short circuit and show signs of disintegration.
Replace if faulty.
Main Excitation Windings
If after establishing and correcting any fault on the rectifier assembly the output is
still low when separately excited, then the main rotor, exciter stator and exciter
rotor winding resistances should be checked (see Resistance Charts), as the fault
must be in one of these windings. The exciter stator resistance is measured across
leads X and XX. The exciter rotor is connected to six studs which also carry the
diode lead terminals. The main rotor winding is connected across the two rectifier
plates. The respective leads must be disconnected before taking the readings.
Main Excitation Windings
If after establishing and correcting any fault on the rectifier assembly the output is
still low when separately excited, then the main rotor, exciter stator and exciter
rotor winding resistances should be checked (see Resistance Charts), as the fault
must be in one of these windings. The exciter stator resistance is measured across
leads X and XX. The exciter rotor is connected to six studs which also carry the
diode lead terminals. The main rotor winding is connected across the two rectifier
plates. The respective leads must be disconnected before taking the readings.
EXCITATION CONTROL TEST
AVR FUNCTION TEST
All types of AVR's can be tested with this procedure :
1. Remove exciter field leads X & XX (F1 & F2) from the AVR terminals X &
XX (F1 & F2).
2. Connect a 60W 240V household lamp to AVR terminals X & XX (F1 & F2).
3. Set the AVR VOLTS control potentiometer fully clockwise.
4. Connect a 12V, 1.0A DC supply to the exciter field leads X & XX (F1 & F2)
with X (F1) to the positive.
5. Start the generating set and run at rated speed.
6. Check that the generator output voltage is within +/-10% of rated voltage.
Voltages at AVR terminals 7-8 on SX460 AVR or P2-P3 on AS440 or SX421 AVR
should be between 170 and 250 volts.
If the generator output voltage is correct but the voltage on 7- 8 (or P2- P3) is low,
check auxiliary leads and connections to main terminals.
Voltages at P2, P3, P4 terminals on MX341 and MX321 should be as given in
7.5.1.
The lamp connected across X-XX should glow. In the case of the SX460, AS440
and SX421 AVRs the lamp should glow continuously. In the case of the MX341
and MX321 AVRs the lamp should glow for approximately 8 secs. and then turn
off. Failure to turn off indicates faulty protection circuit and the AVR should be
replaced. Turning the "VOLTS" control potentiometer fully anti-clockwise should
turn off the lamp with all AVR types.
Should the lamp fail to light the AVR is faulty and should be replaced.
TRANSFORMER CONTROL
The transformer rectifier unit can only be checked by continuity, resistance checks
and insulation resistance measurement.
Two phase transformer
Separate primary leads T1-T2-T3-T4 and secondary leads 10-11. Examine
windings for damage. Measure resistances across T1-T3 and T2-T4. These will be
a low value but should be balanced. Check that there is resistance in the order of 8
ohms between leads 10 and 11. Check insulation resistance of each winding
section to earth and to other winding sections. Low insulation resistance,
unbalanced primary resistance, open or short circuited winding sections, indicates
the transformer unit should be replaced.