zxc ® 28-Sep-2012 15:02

Руководство по поиску неисправностей в генераторах с самовозбуждением и незавысимым возбуждением / FAULT FINDING MANUAL For Self Excited and Separately Excited Generators

Year: 2010
Language: english
Author: STAMFORD Power Generation
Publisher: STAMFORD Power Generation
Format: PDF
Quality: OCR without errors
Number of pages: 29
To successfully carry out the various test procedures suggested in this manual, certain test instruments are essential. The following lists detail the basic requirements in this respect.
It should be noted that in addition to these instruments a comprehensive kit of tools is also essential. For fault finding purposes this need not include any specialised tools.
This instrument is for measuring the shaft speed of the alternator and should be capable of measuring speeds between 0 and 5000 revolutions per minute, (RPM).
An alternative to the tachometer is the frequency meter (see Section 2 on Frequency and Speed, for details). However the alternator must be generating its normal output voltage for this instrument to be accurate.
It is essential that all test instruments be regularly checked for safety, and any connection leads, probes or clips checked to ensure that they are suitable for the voltage levels being tested.
Never attempt to test a "LIVE" generator unless there is another competent person present who can switch off the power supply or shut down the engine in an emergency.
Never expose "LIVE" connections unless you have created a safe working area around you. Make sure you have made all other persons in the immediate area fully aware of what you are doing.
Do not attempt to carry out tests on medium or high voltage generators without using specialised instruments and probes, with suitable protection equipment and procedures for grounding (earthing) the output terminals.
Item 3 - Megger (Insulation test meter)
This instrument generates a voltage of 500V or 1000V, and is used to measure the resistance value of the insulation to earth (ground). It may be an electronic push button type, or a hand cranked generator type.
Item 4 - Clip-On Ammeter (Clampmeter)
Used to measure A C current, it consists of a pair of callipers, which are clamped around the conductor, and by means of a transformer action, gives an indication of the amperes flowing in the conductor. Useful ranges to have on this meter are:-
A C Amps 0-10-50-100-250-500-1000
Item 5 - Kelvin Bridge - low resistance meter
This instrument is used to measure resistance values below 1.0 ohm. They are bulky, and expensive, but are the only means of accurately measuring very low resistances, such as main stator and exciter rotor windings.
However, there are other methods of testing low resistance windings, and these are included in the various test procedures, i.e. Test Method A (Section 3). This section will enable the main generator windings to be tested while running the generator at normal speed without load.


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 =
kVA (kilo Volt Amperes), are calculated by the formula:-Volts x Amperes
kVA =
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
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.
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.
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.
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
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).


Recommended Metering and Test Instruments
Electrical Terminology
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.
Fault Finding method ‘B’, for Separately Excited Generators
Automatic Voltage Regulator is powered from the Permanent Magnet Generator.
Parallel Operation and Fault Finding for All Generators
Rating: 4.9 / 5 (Votes: 27)

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