Electrical Specification of Generators

it is very difficult to reply in this discussion.

One thing you should do is ask the manufacturer of each generators..Of course you should be aware of your load capacity if is it enough for your generator and the frequency.

Your doubt creates another doubt..... That is whether you are going to buy a Generator or only a stator.....? Stator alone will not rule the Generator or Generator not only has stator. Better you ask for a full...!

The 3 replies so far are all on the money.

The best starting point in my experience is to play what I call "little boy lost" - much as you have done in coming to this forum. Ask three or more suppliers about their product and why it is any good, or why you should prefer their product or the like and listen very closely to the answers. That will give you additional information on what other questions to ask.

One big trap for young players is the type of load that is being driven - the rating for an alternator for driving induction motors (one on one) has to be much larger usually for proper starting. You used the word "generator" so I presume you are talking DC. For DC the issues are more about voltage stability etc.

The stator is one part of a system, but for comparing like with like the issues are more about material choices, tolerances, ratings and quality. Any differences in pole numbers and configuration that produce an advantage in the stator may produce a negative as part of the system. My exprience does not run to comment on that sort of detail.

Good luck.

every brands of generator has its own specifications, so surely it has differences, but basically every stators of those brands has the same functions. there are some things that you should be aware: 1. the type of generator you're going to buy, it, could be DC, synchronous, or induction one. 2. the system that the generator works in, whether the single phase or the three phase system. 3. the speed of the generator. it can be determined from the number of pole and the frequency it works on. 4. the insulation class, it is used to know the maximum temperature the generator can endure. 5. the voltage and the current rating. all of those criteria can be seen in the name plate of the generator. and of course, the budget you spend to buy the generator also be the part of consideration.

How can I compare between stators of two brands of generators? and what are the main technical (Electrical)issues that I have to be aware of if I want to buy 500 KVA generator.

Dear amnajjar

You must compare them apple to apple, with following the recognized standard (depend which standard are used or your required),

Following list were taken from NEMA MG-1 standard, please compare the generator performance to the list items test and measurement. Sorry I cant attached this standard in this field.

Good Luck

Rgds

Siswanto

MG 1-2003

Section IV PERFORMANCE STANDARDS APPLYING TO ALL MACHINES

Part 32_SYNCHRONOUS GENERATORS (EXCLUSIVE OF GENERATORS

COVERED BY ANSI STANDARDS C50.12, C50.13, C50.14,

AND C50.15 ABOVE 5000 kVA) RATINGS

32.0 SCOPE ...............................................................................................................................32-1

32.1 BASIS OF RATING..............................................................................................................32-1

32.2 KILOVOLT-AMPERE (KVA) AND (KW) RATINGS ............................................................32-1

32.3 SPEED RATINGS................................................................................................................32-1

32.4 VOLTAGE RATINGS...........................................................................................................32-3

32.4.1 Broad Voltage Ratings, Volts............................................................................................32-3

32.4.2 Discrete Voltage Ratings, Volts .......................................................................................32-3

32.5 FREQUENCIES...................................................................................................................32-3

32.6 TEMPERATURE RISE .......................................................................................................32-3

32.7 MAXIMUM MOMENTARY OVERLOADS............................................................................32-4

32.8 OVERLOAD CAPABILITY ..................................................................................................32-5

32.9 OCCASIONAL EXCESS CURRENT...................................................................................32-5

32.10 MAXIMUM DEVIATION FACTOR ....................................................................................32-5

32.11 TELEPHONE INFLUENCE FACTOR (TIF) ......................................................................32-5

32.12 EFFICIENCY......................................................................................................................32-6

32.13 SHORT-CIRCUIT REQUIREMENTS ...............................................................................32-7

32.14 CONTINUOUS CURRENT UNBALANCE.........................................................................32-8

32.15 OPERATION WITH NON-LINEAR OR ASYMMETRIC LOADS ......................................32-8

32.16 OVERSPEEDS .................................................................................................................32-8

32.17 VARIATION FROM RATED VOLTAGE.............................................................................32-9

32.17.1 Broad Voltage Range ....................................................................................................32-9

32.17.2 Discrete Voltage ............................................................................................................32-9

32.18 SYNCHRONOUS GENERATOR VOLTAGE REGULATION

(VOLTAGE DIP)..........................................................................................................................32-9

32.18.1 General .........................................................................................................................32-9

32.18.2 Definitions .....................................................................................................................32-9

32.18.3 Voltage Recorder Performance ....................................................................................32-11

32.18.4 Examples ......................................................................................................................32-11

32.18.5 Motor Starting Loads ....................................................................................................32-11

32.19 PERFORMANCE SPECIFICATION FORMS...................................................................32-14

32.19.1 Slip-ring Synchronous Generators ...............................................................................32-14

32.19.2 Brushless Synchronous Generators.............................................................................32-15

32.20 ROUTINE FACTORY TESTS...........................................................................................32-16

32.20.1 Generators Not Completely Assembled in the Factory ................................................32-16

32.20.2 Generators Completely Assembled in the Factory........................................................32-16

32.21 HIGH-POTENTIAL TESTS...............................................................................................32-16

32.21.1 Safety Precautions and Test Procedures .....................................................................32-16

32.21.2 Test Voltage_Armature Windings .................................................................................32-16

32.21.3 Test Voltage_Field Windings, Generators with Slip Rings ...........................................32-16

32.21.4 Test Voltage_Assembled Brushless Generator Field

Winding and Exciter Armature Winding.....................................................................................32-16

32.21.5 Test Voltage_Brushless Exciter Field Winding ...........................................................32-17

32.22 MACHINE SOUND SYNCHRONOUS (GENERATORS) ...............................................32-17

32.22.1 Sound Quality ..............................................................................................................32-17

32.22.2 Sound Measurement ....................................................................................................32-17

32.23 VIBRATION......................................................................................................................32-17

MANUFACTURING DATA..........................................................................................................32-18

32.24 NAMEPLATE MARKING .................................................................................................32-18

32.25 SHAFT EXTENSION KEY................................................................................................32-19

32.26 GENERATOR TERMINAL................................................................................................32-19

32.27 EMBEDDED TEMPERATURE DETECTORS..................................................................32-19

APPLICATION DATA..................................................................................................................32-20

32.29 PARALLEL OPERATION .................................................................................................32-20

32.30 CALCULATION OF NATURAL FREQUENCY.................................................................32-20

32.31 TORSIONAL VIBRATION.................................................................................................32-20

32.32 MACHINES OPERATING ON AN UNGROUNDED SYSTEM.........................................32-20

32.33 SERVICE CONDITIONS...................................................................................................32-20

32.33.1 General ........................................................................................................................32-20

32.33.2 Usual Service Conditions..............................................................................................32-21

32.33.3 Unusual Service Conditions..........................................................................................32-21

32.34 NEUTRAL GROUNDING..................................................................................................32-22

32.35 STAND-BY GENERATOR................................................................................................32-22

32.36 GROUNDING MEANS FOR FIELD WIRING ..................................................................32-22

NEMA MG-1 .......Cont'd

32.0 SCOPE

The standards in this Part 32 of Section IV cover synchronous generators of the revolving-field type at speeds and in ratings covered by Tables 32-1 and 32-2.

32.1 BASIS OF RATING

Synchronous generators shall be rated on a continuous duty basis, and the rating shall be expressed in kilovoltamperes available at the terminals at 0.8-power-factor lagging (overexcited). The corresponding kilowatts shall also be stated. General purpose synchronous generators may have a standby continuous rating in accordance with 32.35.

32.2 KILOVOLT-AMPERE (KVA) AND (KW) RATINGS

The ratings for 60- and 50-hertz, 0.8-power-factor lagging (overexcited) synchronous generators shall be as shown in Table 32-1.

32.3 SPEED RATINGS

Speed ratings shall be as shown in Table 32-2.

32.4.3 Excitation Voltages

The excitation voltages for field windings shall be 62-1/2, 125, 250, 375, and 500 volts direct current.

These excitation voltages do not apply to generators of the brushless type with direct-connected exciters.

NOTE_It is not practical to design all KVA ratings of generators for all of the excitation voltages.

32.5 FREQUENCIES

Frequencies shall be 50 and 60 hertz.

32.6 TEMPERATURE RISE

The observable temperature rise under rated-load conditions of each of the various parts of thesynchronous generator, above the temperature of the cooling air, shall not exceed the values given in Table 32-3. The temperature of the cooling air is the temperature of the external air as it enters the ventilating openings of the machine, and the temperature rises given in the table are based on a maximum temperature of 40°C for this external air. Temperatures shall be determined in accordance with IEEE Std 115.

Temperature rises in Table 32-3 are based upon generators rated on a continuous duty basis. Synchronous generators may be rated on a stand-by duty basis (see 32.35). In such cases, it is recommended that temperature rises not exceed those in Table 32-3 by more than 25 C under continuous operation at the standby rating.

Temperature rises given in Table 32-3 are based upon a reference ambient temperature of 40 C. However, it is recognized that synchronous generators may be required to operate at an ambient temperature higher than 40 C. For successful operation of generators in ambient temperatures higher than 40 C, the temperature rises of the generators given in Table 32-3 shall be reduced by the number of degrees that the ambient temperature exceeds 40⁰C.

(Exception: for totally enclosed water-air cooled machines, the temperature of the cooling air is the temperature of the air leaving the coolers. Totally enclosed water-air cooled machines are normally designed for the maximum cooling water temperature encountered at the location where each machine is to be installed. With a cooling water temperature not exceeding that for which the machine is designed: a. On machines designed for cooling water temperature from 5 C to 30 C _ the temperature of the air leaving the coolers shall not exceed 40 C.

b. On machines designed for higher cooling water temperatures _ the temperature of the air leaving the coolers shall be permitted to exceed 40 C provided the temperature rises of the machine parts are then limited to values less than those given in Table 32-3 by the number of degrees that the temperature of the air leaving the coolers exceeds 40 C

32.6.1 For machines which operate under prevailing barometric pressure and which are designed not to exceed the specified temperature rise at altitudes from 3300 feet (1000 meters) to 13000 feet (4000 meters), the temperature rises, as checked by tests at low altitudes, shall be less than those listed in the foregoing table by 1 percent of the specified temperature rise for each 330 feet (100 meters) of altitude in excess of 3300 feet (1000 meters).

32.7 MAXIMUM MOMENTARY OVERLOADS

Synchronous generators shall be capable of carrying a 1-minute overload with the field set for normal rated load excitation in accordance with the following:

It is recognized that the voltage and power factor will differ from the rated load values when generators are subjected to this overload condition. Also, since the heating effect in the machine winding varies approximately as the product of the square of the current and the time for which this current is being carried, the overload condition will result in increased temperatures and a reduction in insulation life. The generator should therefore not be subjected to this extreme condition for more than a few times in its life.

It is assumed that this excess capacity is required only to coordinate the generator with the control and protective devices.

32.8 OVERLOAD CAPABILITY

General-purpose synchronous generators and their exciters (if provided) shall be suitable for operation at a generator overload of 10 percent for 2 hours out of any consecutive 24 hours of operation. When operated at any load greater than rated load the temperature rise will increase and may exceed the temperature rises specified in Table 32-3.

32.9 OCCASIONAL EXCESS CURRENT

Generators shall be capable of withstanding a current equal to 1.5 times the rated current for not less than 30 seconds when the generator is initially at normal operating temperature.

32.10 MAXIMUM DEVIATION FACTOR

The deviation factor of the open-circuit line-to-line terminal voltage of synchronous generators shall not exceed 0.1.

32.11 TELEPHONE INFLUENCE FACTOR (TIF)

The telephone influence factor of a synchronous generator is the measure of the possible effect of harmonics in the generator voltage wave on telephone circuits.

32.11.1 The balanced telephone influence factor (TIF) based on the weighting factors given in 32.11.3 shall not exceed the following values:

32.11.2 The residual component telephone influence factor based on the weighting factors given in

32.11.3 shall not exceed the following values. The residual component applies only to those generators having voltage ratings of 2000 volts and higher.

32.11.3 The single-frequency telephone influence weighting factors (TIFf), according to the 1960 single frequency weighting are as listed in Table 32-4.

32.11.4 The telephone influence factor shall be measured in accordance with IEEE Std 115.

TIF shall be measured at the generator terminals on open circuit at rated voltage and frequency.

32.12 EFFICIENCY

Efficiency and losses shall be determined in accordance with IEEE Std 115. The efficiency shall be determined at rated conditions.

The following losses shall be included in determining the efficiency:

a. I2 R loss of armature

b. I2 R loss of field

c. Core loss

d. Stray-load loss

e. Friction and windage loss

f. Exciter loss if exciter is supplied with and driven from shaft of machine

Power required for auxiliary items, such as external pumps or fans, that are necessary for the operation of the generator shall be stated separately. In determining I2R losses at all loads, the resistance of each winding shall be corrected to a temperature equal to an ambient temperature of 25°C plus the observed rated-load temperature rise measured by resistance. When the rated-load temperature rise has not been measured, the resistance of the winding shall be corrected to the following temperature:

If the rated temperature rise is specified as that of a lower class of insulation system, the temperature for resistance correction shall be that of the lower insulation class.

In the case of generators which are furnished with thrust bearings, only that portion of the thrust bearing loss produced by the generator itself shall be included in the friction and windage loss for efficiency calculation. Alternatively, a calculated value of efficiency, including bearing loss due to external thrust load, shall be permitted to be specified.

In the case of generators which are furnished with less than a full set of bearings, the efficiency may be determined by testing with shop test bearings. Friction and windage losses which are representative of the actual installation shall be determined by (1) calculation or (2) experience with shop test bearings and shall be included in the efficiency calculations.

32.13 SHORT-CIRCUIT REQUIREMENTS

A synchronous generator shall be capable of withstanding, without damage, a 30-second, three-phase short circuit at its terminals when operating at rated kVA and power factor, at 5-percent over-voltage, with fixed excitation. The generator shall also be capable of withstanding, without damage, at its terminals any other short circuit of 30 seconds or less provided: a. The machine phase currents under fault conditions are such that the negative-phase-sequence current, (I2), expressed in per unit of stator current at rated kVA, and the duration of the fault in seconds, t, are limited to values which give an integrated product, (I2)2t, equal to or less than

1. 40 for salient-pole machines

2. 30 for air-cooled cylindrical rotor machines

b. The maximum phase current is limited by external means to a value which does not exceed the maximum phase current obtained from the three-phase fault.

NOTE_Generators subjected to faults between the preceding values of (I2)2t and 200 percent of these values may suffer varying degrees of damage; for faults in excess of 200 percent of these limits, serious damage should be expected.

32.13.1 With the voltage regulator in service, the allowable duration, t, of the short circuit shall be

determined from the following equation in situations where the regulator is designed to provide ceiling voltage continuously during a short circuit:

Where:

Nominal field voltage is the voltage across the generator field winding at rated load condition.

32.14 CONTINUOUS CURRENT UNBALANCE

A synchronous generator shall be capable of withstanding, without damage, the effects of a continuous current unbalance corresponding to a negative-phase sequence current I2 of the following values, providing the rated kVA is not exceeded and the maximum current does not exceed 105 percent of rated current in any phase. (Negative-phase-sequence current is expressed as a percentage of rated stator current.)

These values also express the negative-phase-sequence current capability at reduced generator kVA capabilities, as a percentage of the stator current corresponding to the reduced capability.

32.15 OPERATION WITH NON-LINEAR OR ASYMMETRIC LOADS

Non-linear loads result in a distortion of the current from a pure sinewave shape when sinusoidal voltage is applied. A synchronous generator shall be capable of withstanding, without damage, the effects of continuous operation at rated load on such a circuit provided the instantaneous value of the current does not differ from the instantaneous value of the fundamental current by more than 5 percent of the amplitude of the fundamental, and when neither the negative-sequence nor zero-sequence component of current exceeds 5 percent of the positive-sequence component when any unbalance between phases is present.

The foregoing levels of current distortion may result in generator output voltage distortion levels beyond user limits.

32.16 OVERSPEEDS

Synchronous generators and their exciters (if provided) shall be so constructed that, in an emergency not to exceed 2 minutes, they will withstand without mechanical damage overspeeds above synchronous speed in accordance with the following:

32.17 VARIATION FROM RATED VOLTAGE

32.17.1 Broad Voltage Range

Synchronous generators shall be capable of delivering rated output (kVA) at rated frequency and power factor, at any voltage within the broad range (see 32.4) in accordance with the standards of performance established in this Part 32.

32.17.2 Discrete Voltage

Synchronous generators shall be capable of delivering rated output (kVA) at rated frequency and power factor, at any voltage not more than 5 percent above or below rated voltage but not necessarily in accordance with the standards of performance established for operation at rated voltage (see 32.4).

32.18 SYNCHRONOUS GENERATOR VOLTAGE REGULATION (VOLTAGE DIP)

32.18.1 General

When a synchronous generator is subjected to a sudden load change there will be a resultant timevarying change in terminal voltage. One function of the exciter-regulator system is to detect this change in terminal voltage and to vary the field excitation as required to restore the terminal voltage. The maximum transient deviation in output voltage that occurs is a function of (1) the magnitude, power factor, and rate of change of the applied load; (2) the magnitude, power factor, and current versus voltage characteristic of any initial load; (3) the response time and voltage forcing capability of the exciterregulator system; and (4) the prime mover speed versus time following the sudden load change.

Transient voltage performance is therefore a system performance criterion involving the generator, exciter, regulator, and prime mover and cannot be established based on generator data alone. The scope of this section is only the generator and exciter-regulator system. Performance of the prime mover, its governor, and associated controls are outside the scope of NEMA standards.

In selecting or applying synchronous generators, the maximum transient voltage deviation (voltage dip) following a sudden increase in load is often specified or requested. When requested by the purchaser, the generator manufacturer should furnish expected transient voltage regulation, assuming either of the following criteria applies:

a. Generator, exciter, and regulator furnished as integrated package by the generator manufacturer

b. Complete data defining the transient performance of the regulator (and exciter if applicable) is made available to the generator manufacturer

When furnishing expected transient voltage regulation, the following conditions should be assumed unless otherwise specified:

a. Constant speed (rated)

b. Generator, exciter, regulator initially operating at no load, rated voltage, starting from ambient temperature

c. Application of a constant impedance linear load as specified

32.18.2 Definitions

See Figure 32-1.

32.18.2.1 Transient Voltage Regulation

Transient voltage regulation is the maximum voltage deviation that occurs as the result of a sudden load change.

NOTE_Transient voltage regulation may be voltage rise or a voltage dip and is normally expressed as a percent of rated voltage.

32.18.2.2 Voltage Dip

Voltage dip is the transient voltage regulation that occurs as the result of a sudden increase in load. (See

Figure 32-1.)

NOTE_Voltage dip is normally expressed as a percent of rated voltage.

32.18.2.3 Transient Voltage Overshoot

Transient voltage overshoot is the maximum voltage overshoot above rated voltage that occurs as a result of the response of the exciter-regulator system to a sudden increase in load. (See Figure 32-1.)

NOTE_Transient voltage overshoot is normally expressed as a percent of rated voltage.

32.18.2.4 Steady-state Voltage Regulation

Steady-state voltage regulation is the settled or steady-state voltage deviation or excursion that occurs as the result of a load change after all transients due to the load change have decayed to zero. (See Figure 32-1.)

NOTE_Steady-state voltage regulation is normally expressed as a percent of rated voltage for any load between no load and rated load with the range of unity (1.0) to rated power factor.

32.18.2.5 Recovery Voltage

Recovery voltage is the maximum obtainable voltage for a specified load condition.

NOTE_Recovery voltage is normally expressed as a percent of rated voltage. For loads in excess of rated, recovery voltage is limited by saturation and field forcing capability.

32.18.2.6 Recovery Time

Recovery time is the time interval required for the output voltage to return to a specified condition following a specified sudden load change. (See Figure 32-1.)

32.18.3 Voltage Recorder Performance

The voltage recorder used for making measurements shall meet the following specifications:

a. Response time < 1 millisecond

b. Sensitivity > 1 percent per millimeter

NOTES

1_When peak-to-peak recording instruments are used, readings of the steady-state terminal voltage before and after load application should be made with an rms-indicating instrument in order to determine minimum transient voltage (see Figure 32-2).

2_See IEEE Std 115 for care in calibration of oscillograph.

32.18.4 Examples

A strip chart of output voltage as a function of time demonstrates the transient performance of the generator, exciter. and regulator system to sudden changes in load. The entire voltage envelope should be recorded to determine the performance characteristics.

An example of a voltage recorder strip chart is illustrated in Figure 32-2. The labeled charts and sample calculations should be used as a guide in determining generator-exciter-regulator performance when subjected to a sudden load change.

32.18.5 Motor Starting Loads

The following test procedure and presentation of data is recommended for evaluating the motor startingcapability of a synchronous generator, exciter, and regulator system.

32.18.5.1 Load Simulation

a. Constant impedance (non-saturable reactive load)

b. Power factor < 0.3 lagging

NOTE_The current drawn by the simulated motor starting load should be corrected by the following ratio whenever the generator terminal voltage fails to return to rated voltage:

This value of current and rated terminal voltage should be used to determine the actual kVA load applied.

32.18.5.2 Temperature

The test should be conducted with the generator and excitation system initially at ambient temperature.

32.18.5.3 Presentation of Data

Transient voltage regulation performance curves should be identified as "Voltage Dip" (in percent of rated voltage) versus "kVA Load" (see Figure 32-3).

The performance characteristics will vary considerably for broad voltage range generators (see 32.4.1) when operating over the broad voltage adjust range. (See Figure 32-3.) Therefore, the percent voltage dip versus kVA load curve provided for broad voltage range generators should show the performance at the extreme ends of the operating range; i.e 208/416V and 240/480V. For discrete voltage generators, the percent voltage dip versus kVA load curve should show the performance at the discrete rated voltage(s).

Unless otherwise noted, the percent voltage dip versus kVA load curve should provide a voltage recovery to at least 90 percent of rated voltage. If the recovery voltage is less than 90 percent of rated voltage, a point on the voltage dip curve beyond which the voltage will not recover to 90 percent of voltage should be identified or a separate voltage recovery versus kVA load curve should be provided.

In the absence of manufacturers' published information, the value of voltage dip may be estimated from machine constants, subject to the conditions set forth in 32.18.1 and the following:

a. Voltage regulator response time < 17 milliseconds

b. Excitation system ceiling voltage* > 1.5

c. Rated field voltage

Data estimated in accordance with the above calculation should be identified as _Calculated Voltage

Dip._

* See IEEE Std 421

Figure 32-3

PERFORMANCE CURVES (PF < 0.3) (STEP LOADING)

32.20 ROUTINE FACTORY TESTS

32.20.1 Generators Not Completely Assembled in the Factory

The following tests shall be made on all generators (and exciters if provided) which are not completely assembled in the factory, including those furnished without a shaft or a complete set of bearings, or neither:

a. Resistance of armature and field windings

b. Polarity of field coils

c. High-potential test in accordance with 32.21

32.20.2 Generators Completely Assembled in the Factory

The following tests shall be made on generators (and exciters, if provided) which are completely assembled in the factory and furnished with a shaft and a complete set of bearings:

a. Resistance of armature and field windings

b. If brushless exciter is not provided, check generator field current at no load with normal voltage and frequency on the generator. On generators having brushless excitation systems, check instead the exciter field current at no load with normal voltage and frequency on the generator.

c. High-potential test in accordance with 32.21

32.21 HIGH-POTENTIAL TESTS

32.21.1 Safety Precautions and Test Procedures

See 3.1.

32.21.2 Test Voltage_Armature Windings

The test voltage for all generators shall be an alternating voltage whose effective value is 1000 volts plus twice the rated voltage of the machine but in no case less than 1500 volts.

A direct instead of an alternating voltage is sometimes used for high-potential tests on primary windings of machines. In such cases, a test voltage equal to 1.7 times the alternating-current test voltage (effective value) as given in 32.21.2 and 32.21.3 is recommended. Following a direct-voltage highpotential test, the tested winding should be thoroughly grounded. The insulation rating of the winding and the test level of the voltage applied determine the period of time required to dissipate the charge and, in many cases, the ground should be maintained for several hours to dissipate the charge to avoid hazard to personnel.

32.21.3 Test Voltage_Field Windings, Generators with Slip Rings

The test voltage for all generators with slip rings shall be an alternating voltage whose effective value

is as follows:

a. Rated excitation voltage < 500 volts direct current_ten times the rated excitation voltage but in no case less than 1500 volts

b. Rated excitation voltage > 500 volts direct current_4000 volts plus twice the rated excitation voltage

32.21.4 Test Voltage_Assembled Brushless Generator Field Winding and Exciter Armature Winding

The test voltage for all assembled brushless generator field windings and exciter armature windings shall be an alternating voltage whose effective value is as follows:

a. Rated excitation voltage < 500 volts direct current_ten times the rated excitation voltage but in no case less than 1500 volts

b. Rated excitation voltage > 500 volts direct current_4000 volts plus twice the rated excitation voltage

The brushless circuit components (diodes, thyristors, etc.) on an assembled brushless exciter and synchronous machine field wiring shall be short-circuited (not grounded) during the test.

32.21.5 Test Voltage_Brushless Exciter Field Winding

The test voltage for all brushless exciter field windings shall be an alternating voltage whose effective value is as follows:

a. Rated excitation voltage < 500 volts direct current_ten times the rated excitation voltage but in no case less than 1500 volts

b. Rated excitation voltage > 500 volts direct current_4000 volts plus twice the rated excitation voltage

c. Exciters with alternating-current excited stators (fields) shall be tested at 1000 volts plus twice the rated alternating-current voltage of the stator, but in no case less than 1500V

32.22 MACHINE SOUND SYNCHRONOUS (GENERATORS)

32.22.1 Sound Quality

Sound quality, the distribution of effective sound intensities as a function of frequency, affects the acceptability of the sound.

A measurement of total sound does not completely define sound acceptability because machines with the same overall decibel sound level may have a different sound quality. It may be necessary, in some cases, to describe sound profile in more detail, including octave band values.

32.22.2 Sound Measurement

Machine sound should be measured in accordance with IEEE Std 85 in overall sound power levels using the A-weighting network and stated in decibels (reference = 10^-12 watts).

Generator sound tests should be taken at rated voltage no load. The generator should be isolated from other sound sources.

Sound power values are related to the sound source and are not affected by environmental conditions.

They are calculated from test data taken under prescribed conditions and the values can be repeated.

Field measurements are measured in sound pressure. Measurements of sound pressure levels of generators installed in the field can be correlated to sound power levels using corrections to environmental conditions as outlined in NEMA Standards Publication No. MG 3.

32.23 VIBRATION

See Part 7 for evaluation of vibration for two-bearing generators. Vibration limits and test methods for single-bearing machines are by agreement between the user and the manufacturer.

MANUFACTURING DATA

32.24 NAMEPLATE MARKING

The following information shall be provided. The items need not all be on the same plate. For abbreviations, see 1.78.

a. Manufacturer_s type and frame designation

b. Kilovolt-ampere output

c. Power factor

d. Time rating

e. Temperature rise1

f. Rated speed in rpm

g. Voltage

h. Rated current in amperes per terminal

i. Number of phases

j. Frequency

k. Rated field current2

l. Rated excitation voltage2

Additional information that may be included on the nameplate:

a. Enclosure or IP code

b. Manufacturer_s name, mark, or logo

c. Manufacturer_s plant location

d. Serial number or date of manufacture

e. Applicable rating and performance standards

f. Connection diagram located near or inside the terminal box, if more than 3 leads

g. Maximum momentary overspeed

h. Maximum ambient if greater than 40 C

i. Maximum water temperature for water-air-cooled machines if greater than 25 C

j. Minimum ambient if other than that in 32.33.2.a

k. Altitude if greater than 3300ft (1000m)

l. Approximate weight

m. Direction of rotation for unidirectional machines, by an arrow

32.25 SHAFT EXTENSION KEY

When the machine shaft extension is provided with a keyway it should be provided with a full key.

32.26 GENERATOR TERMINAL

32.26.1 When generators covered by this Part are provided with terminal housings for wire-to-wire connections, the housings shall have the following dimensions and usable volumes:

1 As an alternate marking, this item shall be permitted to be replaced by the following:

a. Maximum ambient temperature for which the generator is designed (see 32.6).

b. Insulation system designation (if armature and field use different classes of insulation systems, both insulation systems shall be given, that for the armature being given first).

2 Applies to exciter in case of brushless machine.

Terminal housings containing surge capacitors, surge arrestors, current transformers, or potential transformers require individual consideration.

32.26.2 For generators rated above 600 volts, accessory leads shall terminate in a terminal box or boxes separate from the generator terminal housing. As an exception, current and potential transformers located in the generator terminal housing shall be permitted to have their secondary connections terminated in the generator terminal housing if separated from the generator leads by a suitable physical barrier to prevent accidental contact.

32.26.3 For generators rated 601 volts and higher, the termination of leads of accessory items normally operating at a voltage of 50 volts (rms) or less shall be separated from leads of higher voltage by a suitable physical barrier to prevent accidental contact, or shall be terminated in a separate box.

32.27 EMBEDDED TEMPERATURE DETECTORS

See 20.28.

32.29 PARALLEL OPERATION

Many of the factors which affect the parallel operation of generators are contained in the prime mover, and the characteristics of the equipment connected to the system with which the generator is to operate in parallel also impose conditions which should be taken into account in parallel operation:

When requested, the generator manufacturer should furnish the following and any other information as may be required, in determining the system requirements for successful parallel operation.

a. Synchronizing torque coefficient Pr_unless otherwise specified the value of Pr should correspond to a pulsation frequency of one-half the rpm (see 21.36);

b. Wk2 of the generator rotor.

c. Generator third harmonic line-neutral voltage at no load.

32.30 CALCULATION OF NATURAL FREQUENCY

See 21.36.

32.31 TORSIONAL VIBRATION

Excessive torsional vibration may result in overstressed shafts, couplings, and other rotating parts. Torsional vibration is difficult to determine and measure, and it is recommended that torsional stresses be investigated when generators are to be driven by prime movers producing periodic torque pulsations.

While the factors which affect torsional vibration are primarily contained in the design of the prime mover, the design of the generator rotor should also be considered. When requested, the generator manufacturer should furnish the Wk2 and weight of the generator rotor, and any other information, such as the stiffness of the spider, as may be required to make a successful design of the combined unit.

Before the generator spider and such part of the shaft as may be furnished by the generator manufacturer are manufactured, the final drawings of the same should be submitted for approval insofar as their design affects torsional vibration.

32.32 MACHINES OPERATING ON AN UNGROUNDED SYSTEM

Alternating-current machines are intended for continuous operation with the neutral at or near ground potential. Operation on ungrounded systems with one line at ground potential should be done only for infrequent periods of short duration, for example as required for normal fault clearance. If it is intended to operate the machine continuously or for prolonged periods in such conditions, a special machine with a level of insulation suitable for such operation is required. The generator manufacturer should be consulted before selecting a generator for such an application.

Auxiliary equipment connected to the generator may not be suitable for use on an ungrounded system and should be evaluated independently.

32.33 SERVICE CONDITIONS

32.33.1 General

Generators should be properly selected with respect to their service conditions, usual or unusual, both of which involve the environmental conditions to which the machine is subjected and the operating conditions. Machines conforming to this Part 32 are designed for operation in accordance with their ratings under usual service conditions. Some machines may also be capable of operating in accordance with their ratings under one or more unusual service conditions. Definite-purpose or special-purpose machines may be required for some unusual conditions.

Service conditions, other than those specified as usual, may involve some degree of hazard. The additional hazard depends upon the degree of departure from usual operating conditions and the severity of the environment to which the machine is exposed. The additional hazard results from such things as overheating, mechanical failure, abnormal deterioration of the insulation system, corrosion, fire, and explosion.

Although experience of the user may often be the best guide, the manufacturer of the driving equipment and the generator manufacturer should be consulted for further information regarding any unusual service conditions which increase the mechanical or thermal duty on the machine and , as a result, increase the chances for failure and consequent hazard. This further information should be considered by the user, his consultants, or others most familiar with the details of the application involved when making the final decision.

32.33.2 Usual Service Conditions

Usual service conditions include the following:

a. Exposure to an ambient temperature in the range of -15 C to 40 C or, when water cooling is used, an ambient temperature range of 5 C (to prevent freezing of water) to 40 C, except for machines rated less than 600 watts and all machines other than water cooled having commutator or sleeve bearings for which the minimum ambient temperature is 0 C

b. An altitude not exceeding 3300 feet (1000 meters)

c. A location or supplementary enclosure, if any, such that there is no serious interference with the ventilation of the generator

d. Installation on a rigid mounting surface

32.33.3 Unusual Service Conditions

The manufacturer should be consulted if any unusual service conditions exist which may affect the construction or operation of the generator. Among such conditions are:

a. Exposure to:

1. Combustible, explosive, abrasive, or conducting dusts

2. Lint or very dirty operating conditions where the accumulation of dirt will interfere with normal ventilation

3. Chemical fumes, flammable or explosive gases

4. Nuclear radiation

5. Steam, salt-laden air, or oil vapor

6. Damp or very dry locations, radiant heat, vermin infestation, or atmospheres conducive to the growth of fungus

7. Abnormal shock or vibration from external sources

8. Abnormal axial or side thrust imposed on the generator shaft

b. Operation where:

1. There is excessive departure from rated voltage (see 32.17)

2. Low noise levels are required

3. Generator neutral will be solidly grounded (see 32.34)

c. Operation at speeds other than rated speed

d. Operation in a poorly ventilated room, in a pit, or in an inclined position

e. Operation where subjected to:

1. Torsional vibration (see 32.31)

2. Out-of-phase paralleling

3. Excessive unbalanced load

4. Excessive current distortion (see 32.15)

5. Excessive non-linear loads (see 32.15)

f. Applications where generators are belt, chain or gear driven

32.34 NEUTRAL GROUNDING

For safety of personnel and to reduce over-voltages to ground, the generator neutral is often either grounded solidly or grounded through a resistor or reactor. When the neutral is grounded through a resistor or reactor properly selected in accordance with established power systems practices, there are no special considerations required in the generator design or selection unless the generator is to be operated in parallel with other power supplies. The neutral of a generator should not be solidly grounded unless the generator has been specifically designed for such operation. With the neutral solidly grounded, the maximum line-to-ground fault current may be excessive (see 32.13), and in parallel systems excessive circulating harmonic currents may be present in the neutrals.

32.35 STAND-BY GENERATOR

Synchronous generators may at times be assigned a standby rating where the application is an emergency back-up power source and is not the prime power supply. Under such conditions, temperature rises up to 25°C above those for continuous-duty operation may occur per 32.6. Operation at these stand-by temperature rise values causes the generator insulation to age thermally at about four to eight times the rate that occurs at the continuous-duty temperature rise values, i.e., operating 1 hour at stand-by temperature rise values is approximately equivalent to operating 4 to 8 hours at continuous-duty temperature rise values.

32.36 GROUNDING MEANS FOR FIELD WIRING

When generators are provided with terminal housings for wire-to-wire connections or fixed terminal connections, a means for attachment of an equipment grounding conductor termination shall be provided inside, or adjacent with accessibility from, the terminal housing. Unless its intended use is obvious, it shall be suitably identified. The termination shall be suitable for the attachment and equivalent fault current ampacity of a copper grounding conductor as shown in Table 32-5. A screw, stud, or bolt intended for the termination of a grounding conductor shall be not smaller than shown in Table 32-5. For generator full-load currents in excess of 30 amperes ac or 45 amperes dc, external tooth lockwashers, serrated screw heads, or the equivalent shall not be furnished for a screw, bolt, or stud intended as a grounding conductor termination.

When a generator is provided with a grounding terminal, this terminal shall be the solderless type and shall be on a part of the machine not normally disassembled during operation or servicing. When a terminal housing mounting screw, stud, or bolt is used to secure the grounding conductor to the main terminal housing, there shall be at least one other equivalent securing means for attachment of the terminal housing to the machine frame.

Thank you so much Siswanto

this is good article . read this one for commins generato and perkins genertaor types

https://bit.ly/2GpS5pD

Since both are 500kva,since u know what yr sytem or load require,select the one that matches yr load requirement better as well as the area to install the machine and yr average temperature so that the machine would witstand during full load operation at that temperature.

Voltage is important,if load rating is 400/415,select 415v rating.(2)select one with higher kw output power. (3)select one with higher power factor. (4)select one that matches frequency of yr loads. (5)select one rated for continous duty,bcos it can work for longer periods. (6)select one with higher amperage rating. (7)good speed of rotation,eg,1500rpm. (7)good ambient temperature,eg 40degree centigrade(8)suitable insulation class,eg-some use class H.

Patrick Whowha

Assuming you want to compare two generators of different makes(NOT stator).Check for Efficiency, Insulation class,Temp Rise and ambient for whcih it is designed.AVR type.Arrangement to access the RRA during maintenace.many times you will find its almost same.So best is to go for a reputed brand like Stamford,Leroy,Marathan.Hope this is useful