Motor Starting Notes

8/4/2019 Motor Starting Notes

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Technical

TopicsVolume V Issue 1

Starting Large Motors from an Onan Engine-GeneratorSet

Larry BeyTechnical Marketing Specialist

Jim IversonManager, Technical Sales

Publication No. 900-0286A

1997 by Onan

Synopsis:

A common problem in sizing engine-generators is starting large motors (large relative to thegenerator set capacity). Typical problems include holding coils dropping out or chattering of themotor starter, and stalling of the motor due to insufficient torque for acceleration. The transientperformance of an engine-generator set when starting large motors is a function of the completegenerator set system, including the engine power available, the generator and its excitationsystem, and the energy stored in the rotating inertia of the generator set. Proper sizing for asuccessful start requires consideration of all these factors, more than just the generator onlydata.


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Starting Large Motors from an Onan Engine-Generator

A common problem in sizing engine-generators is starting large motors (large relative to thegenerator set capacity). Large voltage and frequency deviations may occur if the generator setis not sized properly. Typical problems include holding coils dropping out or chattering of themotor starter, and stalling of the motor due to insufficient torque for acceleration. Otherconnected loads may be more sensitive to voltage and frequency deviations than the motor ormotor starter. The rate of change in generator frequency can also be a problem for somestatic uninterruptible power supplies. If the load on the generator set consists of a single largemotor, particularly motors requiring high starting torque, a number of problems may occur if thegenerator set does not have sufficient capacity. These problems may include sustained lowvoltage operation, extended load acceleration times, tripping of circuit breakers or motorprotective devices, engine-generator protection shutdowns, etc.

The engine generator set needs to be sized for both starting and running the loads, and it isvery common for the load starting kW or kVA requirements to be the dominant factor in sizing.This is particularly true when a large percentage of the connected load is motors. Unlike atypical utility source, an engine-generator set is a limited power (kW) source, basically

determined by the available engine horsepower. The transient performance of an engine-generator set when starting large motors is a function of the complete generator set system,including the engine power available, the generator and its excitation system, and the energystored in the rotating inertia of the generator set. Proper sizing for a successful start requiresconsideration of all these factors, more than just the generator only data. Be careful not to usegenerator manufacturers data for transient voltage dip which does not take into account thecomplete system, e.g., the engine performance. Onans Gensize 96 uses a database ofrepresentative test data on complete engine-generator set models for selection and predictionof transient performance.

Motor Starting CharacteristicsInduction motors have starting characteristics as shown in Figure 1. During starting the motor

draws approximately six times its full load current. This current causes a significant dip inoutput voltage from a high impedance source like an engine generator set until the motorreaches nearly full speed. Initially the starting power factor of the motor is very low whichmeans that the engine power required is also low, but the power required by the motor peaksat about 80% of rated speed. The low power factor at motor standstill means that the correctcalculation of generator locked rotor kVA capacity would use vector addition. The locked rotorkVA of the motor would be added vectorially to the running load kVA vector. However, sincesimple arithmetic addition will always give a larger result than vector addition, it will error on theconservative side. For a successful start the motor must develop greater torque than requiredby the load. The difference in torque between the motor and the load determines the rate ofacceleration.

Premium efficiency motors (Design E) have considerably higher starting currents than typicalDesign B, C , and D motors. For example, a 15 HP premium efficiency motor has a lockedrotor kVA approximately 50% higher than a standard motor. The higher starting current mustbe taken into account to maintain specified transient voltage dip and recovery voltage of atleast 90% of rated voltage during acceleration. If premium efficiency motors are powered bythe generator set, there can be a significant increase in generator set size required to maintainvoltage. The increase in size may be offset somewhat by the use of reduced voltage startingmethods.


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Figure One. Induction Motor Starting Characteristics - High inrush current causes the generator voltage to dip.Real power required from the engine peaks at about 80% of motor speed.

Voltage DipWhen starting a motor across the line with a generator set, the motor represents a lowimpedance load while at locked rotor or stalled condition, causing a high inrush current,typically six times the rated running current. The high inrush current causes the generatorvoltage to drop. This voltage dip is comprised of two main components, the transient voltage

dip and the recovery voltage dip. A maximum transient voltage dip of 30% is important tomaintain the holding force on coils and a recovery voltage of 90% is important to providesufficient torque to accelerate the motor and its load. It is the RMS (root-mean-square) oreffective voltage that provides the holding force and torque during starting. Onans Gensize96 sizing software will not select an engine-generator set with less than 90% recovery voltage.The software does permit the user to adjust the transient voltage dip to the requirements of theconnected load equipment.


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Transient voltage dip may also have two components, the instantaneous voltage dip andpossibly a roll off voltage dip. The instantaneous dip is a solely a function of the relativereactances of the generator and motor, and occurs instantaneously upon connecting the motorto the generator output. Because the instantaneous component of the transient voltage dip isthe product of the motor starting current times the subtransient reactance of the generator (Imsx Xd), the benefit of a low reactance generator is less instantaneous voltage dip. Theinstantaneous voltage dip is predicted on curves published by the generator manufacturer.

These voltage dip curves give an approximation of what might be expected for theinstantaneous dip assuming frequency is constant, that is, with unlimited kilowatts availablefrom the driver such as a test stand dynamometer fed by the utility. With actual engine-drivenequipment, if the engine slows down due to a heavy starting kilowatt requirement, the transientvoltage dip would include an additional dip as the torque matching characteristic of the voltageregulator rolls off excitation to help the engine recover speed. This is shown in Figure Two.

Figure Two. Transient Voltage Dip - Transient voltage dip includes a reactance drop and may or may not include avoltage roll-off depending on the engine speed. Following the transient voltage dip the regulator excitation systemforces the field to recover voltage to rated.


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Following the transient voltage dip, the generator excitation system detects the low voltage andresponds by increasing excitation to recover to rated voltage. The excitation level depends onthe field forcing capability of the excitation system. Field current cannot be changedinstantaneously in response to load change. The regulator, exciter field, and main field all havetime constants. The voltage regulator has a relatively fast response. The main field has asignificantly slower response than the exciter field because it is many times larger. Fieldforcing is designed into all excitation system components to optimize recovery time. It must be

enough to minimize recovery time, but not so much as to lead to instability (overshoot) orovercome the engine (which is a limited source of power). See Figure Two. Moderngenerators typically have the ability to force field current at about three times normal full loadfield current. Generators with permanent magnent generator (PMG) excitation have aconstant excitation power source that is independent of the generator output. Onanrecommends the use of PMG excitation for starting heavy motor loads, because a low voltagecondition on the generator output would not affect the excitation power available for forcing thefield.

At the same time the motor begins to accelerate to rated speed, assuming that the motordevelops enough torque. For induction motors, motor torque is directly proportional to thesquare of the applied voltage. The rate at which the motor accelerates to rated speed is afunction of the difference between the torque the motor develops and the torque requirementsof the load. In order to avoid problems with excessive acceleration time or possibly stalling themotor, it is important for the generator to recover to rated voltage as quickly as possible. Themanner in which the generator voltage recovers is a function of several factors including therelative sizes of the generator, the motor, the kilowatt capacity of the engine, and thegenerator excitation forcing capability. If the excitation system responds too fast and too stiffthe generator can actually overload the engine when starting large motors. Depending on theseverity of the load, several cycles to a few seconds later, the generator recovers to ratedvoltage.

The maximum locked rotor kVA capability of a generator, while still maintaining 90% of rated

voltage is derived from its sustained overload characteristic, shown in Figure Three. Thecharacteristic is graphed based on short circuit testing done at near zero power factor. Sincethe motors locked rotor kVA is expressed based on full voltage starting it is necessary tocorrect the generator kVA at 90% voltage to kVA at 100% voltage. To do this graphically a lineis drawn from zero through the overload characteristic curve at 90% voltage. The line isextended to the intersection at 100% voltage. From that intersection a vertical line is droppeddown to read the maximum motor starting kVA that corresponds to the motors locked rotorkVA. A typical kVA overload capability is 2.5 to 3.0 times the generators rating.


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0

10

20

30

40

50

60

70

80

90

100

kVA or Amperes PU

%

RatedV

olts

kVA at

100% Voltage

3.01.0 2.0

Figure Three. Generator Overload Characteristic - Maximum motor starting kVA at 90% recovery voltage correctedto motor nameplate voltage (100%).

Real Power and Frequency DipIf I can start it, I can run it. Thats the way a senior engineer once summed up sizing anengine-generator set for a motor load. He was making the point that real engine power is whatstarts and accelerates motors, and that this starting kW (SkW) requirement often determineswhat size the engine-generator needs to be. The starting kW requirement of the motor can beas high as 3 times its running kW requirement. It is particularly important when starting largemotors that the engine has sufficient power (SkW) to meet this demand, or the motor may notstart successfully. Any engine preload subtracts from the available engine power. If the engine

cannot develop sufficient power, the motor being started will rotate at a low speed determinedby the balance between the power developed by the engine and the power required by themotor. The stored energy in the rotating inertia of the engine generator set can overcome themaximum power required by motors that are small compared to the engine, but on largermotors the engine may slow significantly during the momentary overload or a generator setprotective device may operate. Onan generator sets include a torque matching voltageregulator function which senses the slowing down of an overloaded engine and rolls offexcitation power to reduce output voltage enough to allow the engine to recover. Since poweris proportional to the square of the voltage, a small reduction in voltage will assist the engine inpicking up the kilowatt load.

Motor Moment of InertiaThe moment of inertia for a rotating mass is its resistance to acceleration. To start the motorand its load rotating this inertia must be overcome by an accelerating torque which translatesto engine power (SkW). The load connected to the motor shaft has its moment of inertia andin practical situations for specific equipment this may or may not be available information.Fortunately, for the purpose of sizing the engine-generator set, or more specifically todetermine the engine power needed to start and accelerate a rotating motor load, the motorload moment of inertia need only be broadly categorized as low or high inertia. High inertialoads are characterized by high breakaway torque requiring prolonged acceleration timesand/or pulsating or unbalanced loads. Low inertia loads are characterized by low starting


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torque at standstill, increasing as motor speed increases resulting in high acceleration to ratedspeed. High inertia loads are harder to start than low inertia loads.

Examples of high inertia loads include:

Single and multi-cylinder pumps

Single and multi-cylinder compressors

Crushers

Hydraulic Elevators without unloading valves

Examples of low inertia loads include:

Fans, Centrifugal and Blower

Rotary Compressors

Pumps, Rotary and Centrifugal

Motor-Generator Elevators

Note: Pumps starting into high head pressure, large diameter fans or fans starting into high

restriction should be classified as high inertia loads.

If the motors are driving low inertia loads with low starting torque requirements, Gensize willmultiply SkW by a factor of 0.6 because these loads accelerate so quickly that the enginesinertia provides sufficient energy to maintain near rated speed until the motor achieves ratedspeed and running kilowatts.

Power Factor CorrectionLoads that include power factor correction or filters for power quality improvement should notbe applied to a generator set operating at light load levels. The capacitive elements of theseloads can cause the voltage of a generator set to rise uncontrollably at light load levels. Powerfactor correction capacitors for large motors should either be switched with the motor or be

connected to the utility side of the transfer switch, not the load side.

Regenerative LoadsLoads such as elevators, cranes and hoists often rely on the capability of the source to absorbpower during certain sequences of operation, typically for braking purposes. Since the utility isessentially an infinite power source serving diverse loads, this is not a problem when operatingfrom utility power. A generator set, however, is a limited power source and has limitedcapability to absorb power, especially if no other loads are connected. The regenerative powercapability of a generator set is information that is available from the manufacturer. A typicalregenerative capability is about 15% of the generator set rating.

If the regenerative power of the load exceeds the capacity of the generator set, the generatorset may overspeed and shutdown on overspeed protection. The overspeed limit of a generatorset rated at 1800 RPM is usually about 2100 +/- 50 RPM. Applications that are most likely tobe a problem are where an elevator is the major load on the generator set. Generally, theregeneration problem can be solved by adding loads which can absorb the regenerativepower. For example, transfer the lighting load to the generator first before transferring theelevator.


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Running Surge Voltage DipOnce all the loads on the generator set have been started, some of those loads may cycle ONand OFF when called on by an automatic control, such as an air conditioner or an elevator. Ifthere are motors that cycle, a running surge voltage dip should be calculated. The worst casecondition would be when all cyclical loads start simultaneously with the rest of the connectedload running. Taking cyclical loads into account could significantly increase the size of the

required generator set depending on the required transient performance and could invalidatethe process of placing loads in a step starting sequence. The worst case calculation forrunning surge voltage dip starts with the total preload in running kW and kVA and adds thetotal starting kW and kVA of all of the cyclical loads. Onans Gensize 96 asks the user tospecify a maximum running surge voltage dip and to select which loads cycle ON and OFF.With this information the software will calculate the running surge voltage dip and select anappropriate generator set.

Reduced Voltage Starting Methods

Although voltage dip often causes various problems, a controlled reduction in voltage at the

motor terminals can be beneficial when it is used to reduce the starting kVA of a motor inapplications where the reduced motor torque is acceptable. It is important to determine that anacceptable level of motor torque is achieved. If not, loads will be slow to accelerate or may failto reach full speed and motor damage can result. Reducing motor starting kVA can reduce thesize of the generator set required, lessen the voltage dip and provide a softer start for themotor loads. All of the following reduced voltage starting methods are included in the Gensize96 program.

A Comparison of Motor Starting Methods:Table 1 compares the effects of full voltage, autotransformer and resistor starting on a 50horsepower, Code G motor. As can be seen, autotransformer starting requires less motor

starting capacity from the generator set.

TABLE 1. REDUCED VOLTAGE MOTOR STARTING COMPARISON

TYPE OF STARTER

AUTOTRANSFORMER RESISTOR FULL VOLTAGE

% of applied voltage(tap)

65 42 100

% of full voltage 42 42 100

Starting kVA 124 124 295

Starting kW 47 106 110

Run kVA 46 46 46

Run kW 42 42 42

Full Voltage Motor Starting

Full voltage, acrosstheline starting is typical unless it is necessary to reduce motor startingkVA because of the limited capacity of the generator set or to limit voltage dip during motor


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starting. There is no limit to the HP, size, voltage or type of motor. This method is mostcommon because of its simplicity, reliability and initial cost. Note on the kVA and torque curvesthat starting kVA remains fairly constant until the motor almost reaches full speed. Also notethat kW peaks at about 300 percent of rated near 80 percent of synchronous speed.

Autotransformer Motor Starting, Open Transition

The autotransformer is in the circuit only during starting to reduce voltage to the motor. The

opening of the circuit during transition can cause severe transients. Open transition switchingof reduced voltage starters should be avoided in generator set applications, especially whenthe motors are not brought up to full speed at the time of transition. The reason for this is thatthe motor slows down and is then out of synchronization during the switching transition. Theresult is similar to paralleling generator sets out of phase. The kVA drawn immediately afterswitching can exceed starting kVA. Also note that the starting power factor is lower when anautotransformer is used.

Autotransformer Motor Starting, Closed Transition

The circuit is not interrupted during starting. During transfer, part of the autotransformerwinding remains in the circuit as a series reactor with the motor windings. Closed transition is

preferred over open transition because of less electrical disturbance. The switching, however,is more expensive and complex due to three elements. It is the most commonly used reducedvoltage starting method for large motors with low load torque requirements, such as MG sets,sewage lift pumps and chillers. The principle advantage is more torque per current than withother reduced voltage starting methods. Operation can be automatic and/or remote. Also notethat the starting power factor is lower when an autotransformer is used.

100

200

300

400

500

600

20 40 60 80 100

TYPICAL TORQUE AND KVA CURVES FORSQUIRREL CAGE INDUCTION MOTORS

KVA

TORQUE

SPEED (% SYNCHRONOUS)

kVA&TORQUE(%F.

L.)

MOTOR

1

2

3

START: CLOSE 1-2-3RUN: NO CHANGE

MOTOR STARTING DIAGRAM

LINE

100

200

300

400

500

600

20 40 60 80 100


KVA


kVA&TORQ

UE(%F.L.)

TORQUE

MOTOR

1 2

3

45

6

78

START: CLOSE 2-3-5-6-7RUN: OPEN 2-3-5-6-7; CLOSE 1-4-8


LIN

E


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Reactor Motor Starting, Closed Transition

Reactor starting has the advantage of simplicity and closed transition, but results in lowerstarting torque per kVA than with autotransformer starting. Relative torque, however, improvesas the motor accelerates. Reactor starting is generally not used except for large, highvoltageor highcurrent motors. The reactors must be sized by HP and voltage and may have limitedavailability. Typically, reactor starting costs more than autotransformer starting for smaller

motors, but is simpler and less expensive for larger motors. Starting power factor isexceptionally low. Reactor starting allows a smooth start with almost no observabledisturbance on transition and is well suited for applications such as centrifugal pumps or fans.

Resistor Motor Starting, Closed Transition

Resistor starting is occasionally used for smaller motors where several steps of starting arerequired and no opening of motor circuits between steps is allowed. Also available as astepless transition starter which provides a smoother start. Resistor starting is usually the leastexpensive with smaller motors. Loads accelerate faster because the voltage increases with adecrease in current. Resistor starting has a high starting power factor, so it may actuallyincrease the size of the generator set.

100

200

300

400

500

600

20 40 60 80 100


KVA

TORQUE


kVA&TORQU

E(%

MOTOR

START: CLOSE 6-7-2-3-4TRANSFER: OPEN 6-7RUN: CLOSE 1-5


1

2

3

5

6

74

I

100

200

300

400

500

600

20 40 60 80 100


KVA

SPEED % SYNCHRONOUS

k

VA&TORQUE(%F.L.)

TORQUE

MOTOR

START: CLOSE 1-2-3RUN: CLOSE 4-5-6


1

2

3

5

6

4LINE


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StarDelta Motor Starting, Open Transition

StarDelta starting requires no autotransformer, reactor or resistor. The motor starts starconnected and runs deltaconnected. This starting method is becoming more popular wherelow starting torques are acceptable. It has the following disadvantages:

1. Open transition. Closed transition is available at extra cost.

2. Low torque.

3. No advantage when the motor is powered by a generator set unless the motor reachessynchronous speed before switching. In applications where the motor does not reachsynchronous speed, the generator set must be sized to meet the surge.

Part Winding Motor Starting, Closed Transition

Part winding starting is less expensive because it requires no autotransformer, reactor orresistor and uses simple switching. Available in two or more starting steps depending on size,speed and voltage of motor. Automatically provides closed transition. First, one winding isconnected to the line; after a time interval, the second winding is paralleled with the first.Starting torque is low and is fixed by the motor manufacturer. The purpose of part winding is

not to reduce starting current but to provide starting current in smaller increments. There is noadvantage to this method if the motor is powered by a generator set unless the motor canreach synchronous speed before transition to the line.

100

200

300

400

500

600

20 40 60 80 100


TORQUE


kVA&T

ORQUE(%F.L.)

KVAMOTOR

START: CLOSE 1-2-3SECOND STEP: CLOSE 4-5-6THIRD STEP: CLOSE 7-8-9


1

2

3

5

6

4 7

8

9

LINE

100

200

300

400

500

600

20 40 60 80 100

TYPICAL TORQUE AND KVA CURVES FORSQUIRREL CAGE INDUCTION

KVA


kV

A

&

T

O

R

Q

UE

(%

F.

L.)

TORQUE

MOTOR

1

2

3

START: CLOSE 1-2-3-4-5-6RUN: OPEN 4-5-6; CLOSE 7-8-9


4 5 67

8

9

INE


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Wound Rotor Motor Starting

A wound rotor motor can have the same starting torque as a squirrel cage motor but with lesscurrent. It differs from squirrel cage motors only in the rotor. A squirrel cage motor has shortcircuit bars, whereas a wound rotor motor has windings, usually threephase. Starting current,torque and speed characteristics can be changed by connecting the proper amount of externalresistance into the rotor. Usually, wound rotor motors are adjusted so that the starting kVA is

about 1.5 times running kVA. This is the easiest type of motor for a generator set to start.

Solid-State StartersSolid-state starters can adjust the starting torque, acceleration ramp time, and current limit fora controlled acceleration of the mechanical load while starting motors. For the purpose ofsizing the generator set, the current limit adjustment reduces the inrush current and may beused to reduce the starting kW and kVA requirement on the generator. The range of availablecurrent limit settings are typically from 150 to 600% of full load current. A 600% current limitsetting would be sized just the same as across-the-line starting. A 300% current limit settingwould reduce SkW and SkVA by 50%. Use of the current limit setting also reduces the motor

torque available to the load.

The sequence of operation of a solid-state starter is shown in Figure 4. From standstill thestarter applies an initial voltage to provide sufficient torque to begin to rotate the motor.Typically this voltage (torque) is adjustable from 0 to 100%, with a typical setting of 60% or atthe minimum effective starting torque. The acceleration ramp time is adjusted to control thetime it takes the starter to reach current limit. A typical range of ramp time adjustment is from 0to 60 seconds. The setting is based on the desired time to accelerate the mechanical load.The current limit setting caps the inrush current until the motor comes up to full speed. From agenerator sizing perspective, an extended acceleration ramp time and a low current limit

100

200

300

400

500

600

20 40 60 80 100


KVA


kVA&T

ORQUE(%F.L.)

TORQUE

MOTOR

1

2

3

START: CLOSE 1-2-3RUN: CLOSE 4-5-6


4

5

6

LINE

100

200

300

400

500

600

20 40 60 80 100

MOTORROTOR

1

2

3

START: CLOSE 1-2-3

STEP #1: CLOSE 4-5STEP #2: cLOSE 6-7RUN: CLOSE 4-5-6


LINE

4

5

6

7

8

9

RESISTORS

TYPICAL TORQUE AND KVA CURVES FORWOUND ROTOR MOTORS


kVA&TORQUE(%F.L.)

TORQUE

KVA


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setting (if also appropriate for the motor and the mechanical load) would have the minimumimpact on voltage and frequency excursions.

Figure 4. Solid-state Starter Sequence of Operation - The initial torque, acceleration time, and current limit may beadjusted to provide a smooth start for the load.

Solid-state starters use SCRs and are a non-linear load that will cause voltage distortion. Tocompensate for this distortion the generator capacity should be increased. The recommendedgenerator capacity required is two times the running kilowatt load, except where an automaticbypass is used. If the solid-state starter is equipped with an automatic bypass, the SCRs areonly in the circuit during starting. Once the motor is running the bypass contactor closes andshunts the SCRs. In this case, the voltage distortion may be ignored during starting and therewould be no additional generator capacity required. Gensize 96, Version 2.0, includes aselection for automatic bypass with solid-state starters.

Variable Frequency Drives

All variable frequency drives are current limiting and reduce both starting kW and kVA. Thecurrent drawn by these drives is non-linear harmonic currents, which cause a distorted voltagedrop across the reactance of the generator. Since VFDs are non-linear an additionalgenerator capacity sizing factor needs to be used to keep voltage distortion to a reasonablelevel, 15% total harmonic distortion or less. For six-pulse VFDs a typical generator sizingfactor would be two times the running kW of the drive. The increase in size of the generatorfor the non-linear current will more than offset any reduction in starting kW and kVA. If theVFD is the pulse width modulated type, or includes an input filter to limit current distortion to

less than 10%, then the additional sizing factor may be reduced to 1.4 times the running kW ofthe drive. Gensize 96, Version 2.0, includes a selection for PWM drives with input filters.

Sizing examples using Gensize 96:

Sizing the engine-generator set may be done with manual calculations using a worksheetavailable in Onan Application manual T-030 or with GenSize 96 software. The basic processis the same. It is always best to use actual data if known, otherwise one advantage of GenSize

ACCELERA

TIONR

AMP

INITIAL TORQUE

CURRENT LIMIT

RUN CURRENT

CURRENT

TIME


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96 is that much of the required information on typical load characteristics is available asdefault information.

One commonly used approach is to assume that all connected loads will be started in a singlestep. This assumption will result in the most conservative (largest) generator set selection.Unless something has been done, such as multiple transfer switches with staggered timedelays or a step load controller, then a single step load should be used for sizing purposes.

However, once all of the loads have been brought up on line with the generator set, the loadequipment may be frequently stopped and started by automatic controls. In such cases, thegenerator set will have to be sized to start the largest motor last, with all other connected loadson line.

Following are five sizing examples. The first three examples have been selected to show theeffect on recommended generator set size of transient voltage dip specifications andautotransformer reduced voltage starting. The last two examples have been selected to showthe effect of multiple steps compared to a single step when starting several large motors.

Example 1. 200 HP motor across-the-line with 10% transient voltage dip. Refer to Figures Five

and Six. In this example the specification for no more than 10% transient voltage dip whenstarting a large (200 HP) motor across the line results in a selection of a 1100 kW generatorset with an oversized 80 degree C rated rise generator. The running load on this generatorset is only 14% of rated standby kilowatts which is well below the 30% recommendedminimum. This would not be a good selection. If used this diesel engine would probablyexperience problems with wet stacking, carboning, oil diluted with fuel, etc.

Figure Five. 200 HP Motor Load Characteristics - The motor is started across the line and is low inertia.


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Figure Six. Example #1 Generator Set Selection - With 10% transient voltage dip a 1100 kW generator would berequired.


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Example 2: Figure Seven. A 200 HP motor across-the-line with 30% transient voltage dip. Inthis example starting the same motor as in the first example, but allowing the transient voltagedip to be 30% maximum (29% estimated actual), the generator set selection goes down to a200 kW set with a 125 degree C rated rise generator running at 81% of rated load. This wouldbe a reasonable selection.

Figure Seven. Example #2 Generator Set Selection - With 30% transient voltage dip allowed, a 200 kW generatorset is selected.


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Example 3: Figures Eight and Nine. 200 HP motor with autotransformer start and 30%transient voltage dip. In this example use of the autotransformer reduced voltage starting atthe 65% tap further reduces the generator set selection to a 175 kW with a 125 degree C ratedrise generator. The estimated actual transient voltage dip is down to 18% and the runningload is up to 93% of rated. If no future load increases are expected, this would also be areasonable selection.

Figure Eight. 200 HP Motor Load Characteristics - The motor is started with autotransformer starter at 65% tap andis low inertia.


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Figure Nine. Example #3 Generator Set Selection - With autotransformer starter, a 175 kW generator set isselected.


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Example 4: Figure Ten. Three 200 HP high inertia motors started across-the-line in a singlestep with 30% transient voltage dip allowed. Refer to Figure Ten. In this example a selectionis made for a 800 kW standby rated generator set running at 61% of rated load.

Figure Ten. Three 200 HP motors started in a single step - The starting kilowatts (SkW) required results inselection of an 800 kW standby generator set.


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Example 5: Figure Eleven and Twelve. Three 200 HP motors in three steps. A step startsequence is accomplished using three automatic transfer switches with staggered time delays,or other step sequence controller. Use of three steps reduces the generator set selection to a500 kW model.

Figure Eleven. Three 200 Hp motors started in three steps - Using three step starting sequence reduces the size ofthe generator set from 800 kW to 500kW standby.


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The running load of 488 kW, which conservatively assumes that the motors are loaded

to nameplate rating, equals about 97% of the generator standby rating. The generatorset is rated to carry this load in applications within the standby rating definition. Sincethe margin, if any, between motor nameplate power and the actual load on the motor is

seldom actually known, a more definite margin may be added by stepping up a size tothe 600 kW model. Gensize 96 allows the user to select models as shown in Figure

Twelve. With a nameplate running kW of 488, the 600 kW generator set would run at81% of rating or less.

Figure Twelve. Example #5 Generator Set Selection - Stepped up a size to the 600 kW model.


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Gensize 96 Glossary of Terms

GkW - Generator capacity provided to compensate for non-linear distortion, usually equal toRkW times a factor for the type of non-linear load.

Max SkVA - Maximum Surge Kilovolt-amperes, the highest kilovolt-ampere load in any loadstep.

Max SkVA Req. - Maximum Surge Kilovolt-amperes Required, the highest total of any surgekVA load added to the previously running kilovolt-ampere load.

Max SkW - Maximum Surge Kilowatts, the largest block of kilowatt load in any load step.

Max SkW Req. - Maximum Surge Kilowatts Required, the highest total of any surge kW loadadded to the previously running kilowatt load.

RAmps - The total running amperes for a step.

Reduced Max SkVA Req. - The Max SkVA Req. times a multiplier to compensate for reducedvoltage. Must be less than or equal to the generator set Max kVA.

Reduced Max SkW Req. - The Max SkW Req. times a multiplier to compensate for reducedvoltage. Must be less than or equal to the generator set Site rated Max SkW.

RkVA - Running Kilovolt-amperes, the running load.

RkW - Running Kilowatts, the running load.

RPF - Running power factor is the load power factor when the load is running at rated steadystate conditions.

RSkVA - The running surge kilovolt-amperes, the total surge kVA of all cyclic loads, added toany welder loads, and medical imaging loads.

RSkVA Req. - The running surge kilovolt-amperes required, the highest total of any runningsurge kW load added to the previously running kilowatt load.

RSkW - The running surge kilowatts, the total surge kW of all cyclic loads, added to anywelder loads, and medical imaging loads.

RSkW Req. - The running surge kilowatts required, the highest total of any running surge kWload added to the previously running kilowatt load.

SPF - Starting power factor is the power factor of the load at the time it is initially energized or

started. This is a particularly important parameter for motor loads which exhibit a low powerfactor during locked rotor conditions, causing a high inrush kVA (SkVA) during starting.

SummarySizing an engine-generator set for starting large motors requires an understanding of motorcharacteristics, starting methods, and the transient performance of the engine-generatorduring motor starting. Because the engine-generator set is a limited power source both in kWand kVA, voltage and frequency excursions will occur when starting motors. A properly sizedengine generator set will keep the voltage and frequency excursions within reasonable limitswithout oversizing the machine. Use Gensize 96 to make the sizing problem easier and theresulting recommendation more accurate.


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About the Authors

Jim IversonManager, Technical Sales has been with Onan since 1976. He has a Masters in EngineeringScience, Bachelors in Electrical Engineering. Jim spent 7 years at General Electric and 2years at Electric Machinery before joining Onan. While at Onan, he managed Transfer SwitchDesign, Systems Engineering and Switchgear and Controls. Current responsibilities are toprovide technical direction to the Commercial Marketing and Sales Department, participate indomestic codes and standards development, sales and service training, technical input for andreview of all published literature, published papers on relevant industry topics.

Lawrence A. BeySenior Technical Marketing Specialist and a 1978 University of Minnesota Graduate. Larryhas been an Onan employee from 1978 to the present, spending 7 years in Engineering, and11 years in Marketing and Sales. He represents Onan on Technical Committees of theNational Fire Protection Association for NFPA 110 Emergency and Standby Power Systemsand NFPA 99 Essential Electrical Systems for Health Care Facilities. Larry is active withNational electrical Manufacturers Association (NEMA) on Motor and Generator Subcommittee

and the Automatic Transfer Switch Equipment Subcommittee. He is a member of theInternational Association of Electrical Inspectors.


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Motor Starting Notes

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