Fault finding manual for self excited and separately excited generators - Stamford [2010, PDF]

SECTION 2 ELECTRICAL TERMINOLOGY AND RESISTANCES VOLTAGE AND CURRENT (AMPERES)

An AC Generator is designed to produce a voltage level suitable for the load to which it is connected. The control circuits are designed to automatically maintain this voltage level as the load is increased or decreased.
Sudden large changes in loading will produce temporary changes in the voltage. The control circuit is designed to recover to a stable condition as quickly as possible.
The current drawn from the AC Generator is determined by the amount of load connected to it. Current creates a temperature rise in the windings, hence the requirement for drawing air through the AC Generator by means of the fan. If the full load rated current is exceeded on any phase of the main stator windings, it will result in overheating in this winding. Similarly, any restriction in the flow of air through the machine will result in a rapid increase in the temperature of the windings.
Frequency (Hz) and Speed (RPM)
An AC Generator is a constant speed device, and should not be operated at speeds above 4% of the rated speed, or more than 1% below the rated speed.
Load changes will create temporary changes in the speed, but the engine must be capable of returning to the steady state condition within a few seconds.
The speed requirements for the AC Generator are determined by:-
(a) The frequency (Hz) requirement of the load
(b) The number of poles,( main rotor coils), in the generator
N (speed) X P (pairs of poles )
Frequency (HZ) = -eoisis'sj-
This can be shown more clearly in a chart: -
Frequency (Hz) Speed (R.P.M.) No. of Poles (rotor coils)
50 1500 4
60 1800 4
50 1000 6
60 1200 6
50 3000 2
60 3600 2
From this chart, a simple formula is produced to calculate the speed from the frequency, or vice versa.
4 pole machine 6 pole machine 2 pole machine
1 cycle (Hz) = 30 R.P.M. 1 cycle (Hz) = 20 R.P.M. 1 cycle (Hz) = 60 R.P.M.
Kilowatts (kW) kilo Volt Amperes (kVA) and Power Factors (pf.)
For an AC Generator to supply power for a load of 1kW, the prime mover (engine) driving the alternator must produce approximately 1.5 horsepower.
Kilowatts are calculated by the formula: -Volts x Amperes x Power Factor
kW =
1000
kVA (kilo Volt Amperes), are calculated by the formula:-Volts x Amperes
kVA =
1000
Both equations are multiplied by V3 (1.732) for a 3 phase machine.
Power Factor
The Power Factor (pf), is a measure of wasted current, which is a product of inductive loads such as motors, transformers, (magnetic circuits), and some forms of lighting.
The formula for calculating the Power Factor is:-kilowatts
pf
kVA
Unity Power Factor (pf 1)
Purely resistive load, i.e. heating, tungsten filament lighting, has a power factor of one, (pf 1), and contains very little Wattless (inductive) load, which is power factor zero, (pf 0).
An AC Generator will deliver continuously the rated full load current at any power factor between pf 1 (unity) and 0.8. However, the prime mover, (engine), is greatly affected by the power factor. At pf 1, the kVA and kW are equal; therefore the engine is supplying 20% more kW load at pf 1, than it is at pf 0.8. It is important, therefore, that this is taken into consideration, when approaching 75% to 100% load current of the Generator, with a power factor higher than 0.8.
5
Lagging Power Factors
Resistances - measuring component values
A Generator is designed to deliver the full load current at any power factor between unity and 0.8 lagging. Certain loads have a power factor lower than 0,8 lag, e.g. welding transformers; autotransformer, start motors, gas discharge lighting. A reduction in the full load (kVA), rating is required for a lagging pf lower than 0.8.
When fault finding it is necessary to measure the resistance values of components and windings, and compare them with known normal values, in order to identify a faulty winding. The normal resistances of the windings are given in the winding resistance charts, in the generator installation and maintenance handbooks, service and maintenance section.
Leading Power Factors
Capacitive load e.g. some fluorescent lighting, power factor correction capacitor banks, produce leading power factor current. The latter is required by the Electricity authorities to improve the customers lagging power factor. The capacitor bank size is measured in kVAr (reactive).
A purely Capacitive load can cause the Generator control system, (AVR), to loose control, creating voltage instability, and possible high voltage from the Generator.
This is due to the fact that, unlike most loads, which are pf 1, (unity) or lagging pf, a leading pf load current will cause the Generator excitation voltage to decrease, as the load current increases.
Eventually the control system will be unable to control the Generator excitation level, and voltage instability will occur.
The degree of instability is determined by the kVAr size of the capacitors, relative to the kVA size of the alternator.
Capacitive load can present a problem for mains failure (standby) Generators. When the mains electricity supply fails, all motor, (inductive), load is disconnected by the individual contactors. Subsequently, when the Generator is connected to the system, the load will mainly consist of lighting, and possibly the power factor correction capacitors. In this situation the AC Generator will see a very low, (leading), power factor, and may become unstable, and/or generate high voltage.
In order to prevent this situation, it is advisable to ensure that the power factor correction capacitors are switched OFF when the generator takes the initial load.
Further advice in this respect may be obtained from Newage-AVK SEG if required.
Resistance values above 10 ohms can be measured accurately with a multimeter. Between 0.5 and 5 ohms a multimeter has a limited accuracy, and other test methods may be adopted.
6V
DC
Multimeter on 10 Amp DC scale
Multimeter measuring DC Volts
Main Rotor Windings; (disconnected from rectifier Assembly)
Resistances between 0.5 And 5 Ohms
The resistance value of a winding such as a brushless main rotor will be between 0.5 and 3 ohms. A multimeter may not give an accurate enough reading at these levels. If a Wheatstone Bridge Resistance Meter is not available, an accurate measurement can be obtained by means of a battery supply, using a Multimeter in series on the 10 Amps D.C. range. Most Multimeters have this current range, or alternatively, a battery charging Ammeter could be used instead).
Using 6 volt battery cells the resistance of the winding can be calculated i.e.
V (vo lts) _ I (amps)
ohms (resistance)
The resultant can be compared with the correct value given in the resistance charts, and this method can be used for any resistance greater than 0.5 ohm.
Below this value the current in the circuit would drain the battery, and it is therefore impractical to use this method.
Very low resistance values (below 0.5 ohm)
Main stators and exciter rotors are included in this category.
These values can only be measured accurately with a special instrument, such as a Kelvin bridge test meter.
The test leads are equipped with special spiked probes, which penetrate the surface of the contact, ensuring accurate reading.
The generators main stator windings can also be tested by means of separately exciting the machine (see, Section 3, Test Method A), thus partly eliminating the need to have this specialised type of instrument when fault finding in the field.
6
Diode Testing
A Diode has two resistance values, forward and reverse,
These can be measured with a multimeter as shown in the diagram below.
An arrow printed on the diode body identifies the positive side of a diode.
The forward resistance is being measured in Fig. A with the positive meter lead connected to the forward side of the diode.
In Figure 'B' the meter leads have been reversed, and the reverse resistance is being measured.
Multimeter Set to Ohms (analogue) or
semiconductor (digital)
o -
Multimeter Set to Ohms (analogue) or
semiconductor
(digital)
Simple Alternative Diode Test Circuit
A good diode will light the bulb in only one direction. It should not light when test leads are reversed on the diode pin and base.
A faulty diode will light the bulb in both forward and reverse directions (short circuit diode), or no light in either direction, (open circuit diode).
If one or more diodes are found to be faulty, always change the complete set of diodes.
An electronic digital instrument will read true electron flow, hence the resistance polarity readings will be reverse to conventional current flow, i.e. forward and reverse readings will be reversed.
A Digital Multimeter usually has a semiconductor test scale on the selector switch, marked as shown
This measures true electron flow, and will give a forward, (indication reading only), or reverse (no reading) indication.
Using an analogue meter on resistance scale, the forward resistance varies considerably, depending on the internal impedance of the Multimeter, and the diode type.
A typical reading would be between 20 and 100 ohms.
The reverse resistance must be very much higher, usually in excess of 100k ohms, (100,000 ohms ).
A faulty diode will give a reading in both forward and reverse directions (short circuit), or no reading in either direction, (open circuit).

CONTENTS

FAULT FINDING MANUAL
SECTION 1
Recommended Metering and Test Instruments
SECTION 2
Electrical Terminology
SECTION 3
Fault Finding method ‘A’, for All Generators SECTION 4
Fault Finding method ‘B’, for Self-Excited Generators.
Automatic Voltage Regulator is powered from the Generator Output.
SECTION 5
Fault Finding method ‘B’, for Separately Excited Generators
Automatic Voltage Regulator is powered from the Permanent Magnet Generator.
SECTION 6
Parallel Operation and Fault Finding for All Generators
3
SECTION 1