SIZING GENERATORS FOR MOTOR STARTING KOHLER

Sizing Generators For Motor Starting Kohler-Free PDF

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BASIC CHARACTERISTICS OF MOTOR LOADS The motor starting kVA can be determined by. the motor s nameplate The National Electrical,Motor loads cause difficulty because a motor. Manufacturers Association NEMA sets design,draws high current when started at full voltage. standards for motors and has established a,Starting current is typically six times a motor s. NEMA code letter designation for classifying,rated full load current and this inrush current. motors according to the ratio of locked rotor,stays high until the motor reaches about 75.
kVAs LRKVAs per horsepower These code,percent of rated speed When a motor is started. letters range from A to V covering motors with an, on normal utility power the high inrush current will. LRKVA per horsepower ratio of 3 14 or less to a, cause only a small voltage dip because the utility. ratio of 22 4 LRKVA per horsepower or more,is a more robust voltage source However when. See Figure 1,a motor is started on generator power the high.
inrush currents measured in kilovolt amperes or For example a 50 hp Code F motor requires 279 5. KVAs can result in a large voltage dip LRKVA per horsepower upon starting. that can inhibit the motor from reaching its 50 hp x 5 59 LRKVA per hp 279 5 LRKVA hp. operating speed LRKVA is also known as starting kVA or SKVA. Small motors have a higher NEMA code letter and,The challenge then is to size the generator. correspondingly higher LRKVA per horsepower,to handle the motor starting load but also to. requirement than large motors Typical motor sizes,minimize the impact on the other connected loads. and codes are shown in Figure 2,that may be affected by voltage dips or frequency. dips Therefore when sizing a generator it is, critical to accurately predict voltage dips and to VOLTAGE DIP.
understand how much excess starting capability, is available in the motor and what amount of The kVA requirements of a motor running at full. voltage dip can be allowed The most common load and rated speed are normally less than one. methodology for sizing generators for motor kVA per horsepower With the possible exception. starting focuses on understanding allowable of small motors it would be overly conservative. instantaneous voltage dips as the primary criteria to size a generator set simply by matching the. However there is one manufacturer that considers alternator s kVA to the motor s kVA This would. allowable sustained voltage dips as the primary typically result in a generator with more than. criteria for motor starting load twice the capacity necessary However due to the. dynamic interaction of the system components, kVA per Horsepower several characteristics combine to make this. Letter Designation,with Locked Rotor,approach impractical. B 3 15 3 54 The first characteristic is power factor Three. C 3 55 3 99 phase generator sets are usually rated in kVA at. D 4 0 4 49 0 8 power factor Starting power factors of motors. E 4 5 4 99 vary from 0 3 to 0 5 and increase towards unity as. F 5 0 5 59 the motor accelerates and its kVA demand drops. G 5 6 6 29 With a 0 4 power factor load a typical generator is. H 6 3 7 00 capable of producing nearly twice its continuous. J 7 1 7 99 rated kVA for the time required to accelerate a. K 8 0 8 99 motor to the speed at which its kVA requirement. L 9 0 9 99 drops sharply The generator engine will not stall. M 10 0 11 10 even though it is being asked to supply more than. N 11 2 12 49 its rated kVA because low power factor loads. P 12 5 13 99 see Figure 3 do not require as much horsepower. R 14 0 15 99 as higher power factor loads This generator. S 16 0 17 99 characteristic allows satisfactory motor starting. T 18 0 19 99 results with a generator half the size predicted by. U 20 0 22 39 the conservative approach which matches the. V 22 4 and up generator 0 8 power factor kVA rating to the. Figure 1 Locked Rotor Indicating Code Letters motor LRKVA rating. Source 2006 NEMA,2 Sizing Gensets for Motor Starting. POWER SYSTEMS TOPICS 103, VOLTAGE DIP determining what voltage dip will be acceptable.
for a particular load when considering its effect,The other characteristic that can substantially. on all components in the system some of,reduce the size of the generator needed for a. which may have unknown transient acceptance,particular motor starting load is voltage dip. specifications A voltage dip can affect motors,Values for motor LRKVA are based on full voltage. themselves in addition to other loads on the,starting In practice there is always a voltage.
system For example excessive voltage dip can,dip when a motor is started on generator power. cause control relays or magnetically held motor,and there is even a small dip when a motor is. starting contactors to drop out or ultimately,started on utility power When the voltage drops. cause the motor to not start at all If the relays,inrush current is also proportionally reduced so. or contactors drop out the load is removed,that starting kVA is reduced as the square of the.
from the generator causing voltage to rise and,voltage dip A 30 percent voltage dip reduces. the cycle to repeat rapidly This can damage,starting kVA by about 50 percent 0 7 kilovolts x. contactors if allowed to continue Most control,0 7 amps 0 49 kVA. relays and motor starting contactors will tolerate. Locked Rotor a 35 percent voltage dip However there are. Size Code exceptions Some relays or contactors will start to. 1 2 HP L or M 9 11 chatter if subjected to a voltage dip as little as 20. 3 HP K 8 9,percent Likewise other voltage sensitive loads. need to be accounted for e g UPS systems,5 HP J 7 8.
medical equipment HID lighting in any generator, 7 5 10 HP H 6 7 sizing exercise To ensure satisfactory operation. 15 HP and up G 5 6 6 3 on a given standby power system consult. Figure 2 Typical Code Letters for Various HP Motors the voltage frequency limitations of control. Source 2006 NEMA,components from the manufacturers or suppliers. At least for the first few cycles the voltage Voltage dips also reduce the torque a motor can. dip is determined by the size of the load supply to its load A common NEMA Design B. i e the motor s LRKVA and the reactance of motor will develop 150 percent of rated full load. the alternator which is somewhat proportional torque during starting Torque is proportional to. to the total mass of copper and iron present in the kVA delivered to the motor so a 30 percent. the alternator The issue in sizing a generator is voltage dip that reduces kVA to 49 percent also. reduces torque to 49 percent of its rating If the,motor starts unloaded as most fans centrifugal. pumps and motors used with elevators do this,torque reduction produces no problem other. than a somewhat longer acceleration time Other,types of loads such as positive displacement.
pumps may require more torque than the,8 motor can develop at reduced voltage which. AT prevents the motor from reaching full speed,KV Additional consequences could include tripping. H of breakers or overheating of the motor To ensure. IN 4 P proper motor starting in these applications it is. E A necessary to compare the torque curves of the,GIN pump and the motor at reduced voltage. AMPS OR KVA,0 4 Power Factor 50 HP Factor,Figure 3 Engine Power Required. Figure 3 Engine,forPower Required,Low Power for Load.
Low Power Factor Load,Source Kohler Power Systems,Sizing Gensets for Motor Starting 3. MOTOR STARTERS CAN REDUCE VOLTAGE DIP,LINE CURRENT OF FULL LOAD. Full Voltage Starting,The high inrush current and high starting torque. associated with full voltage starting of motors,on utility power may create problems with the. equipment driven by the motor or the voltage, dips may raise objections from the electric utility Autotransformer Starting on 65 Tap.
To circumvent these issues many facilities use,various types of motor starters for their motors. Some of these devices also benefit motor starting 100. Full Load Current,when running on generator power often allowing 0. a smaller generator to be utilized MOTOR SPEED Full Load. Reduced voltage starters Most reduced Figure 5 Autotransformer Starting. Source Kohler Power Systems, voltage starters connect the load to the power respectively To avoid reducing starting torque. source in two or more steps The starters may be to unacceptable levels use either the 80 or 65. either open or closed transition starters but percent taps. only closed transition reduced voltage starters, are helpful when running on generator power As Solid state soft start starters This type of. seen in Figure 4 open transition starters create starter is most popular and provides exceptional. an unacceptable spike in kVA demand when operating flexibility It is a form of reduced voltage. switching between steps occurs starter that utilizes silicon controlled rectifiers. SCRs to increase voltage at a predetermined, Part winding starters Part winding starters are rate Limits on the starting current can also be.
used with motors that have two identical windings adjusted to increase system performance A note. intended to be connected in parallel These of caution Any performance prediction made at. windings can be energized in sequence to provide a specific value will change when the settings. reduced starting current and torque Since part are changed in the field Also since solid state. winding starters are inherently closed transition starters utilize nonlinear SCRs they can cause. starters the maximum inrush current occurs at voltage distortion during motor starting that must. the moment the first winding is energized and the be considered. maximum inrush kVA load on a generator set will, be reduced to 60 70 percent of normal Wye delta starters Some motors have six leads. that allow them to be connected in either wye or, Autotransformer starters This type of starter delta configurations By connecting the motor. provides reduced voltage at the motor terminals winding in the wye configuration and using a. from a tapped 3 phase autotransformer and voltage source corresponding to the delta rating. generally gives the best results with generators starting current and torque are reduced to 33. See Figure 5 Taps on the transformer provide percent of the delta connected values Use only. selection of 80 65 or 50 percent of initial line with closed transition starters however. voltage to the motor terminals Starting torque is See Figure 6. reduced by the voltage squared to give 64 42 or,25 percent of the full voltage value. Closed Transition Open Transition,Line Circuit,Line Circuit. Figure 4 Closed Transition vs Open Transition Starters. Figure 4 Closed Transition vs Open Transition Starters. Source Kohler Power Systems,4 Sizing Gensets for Motor Starting.
POWER SYSTEMS TOPICS 103,Nonlinear loads Electronic motor starters and. LINE CURRENT OF FULL LOAD, Full Voltage Starting Variable Frequency Drives VFDs are nonlinear. loads VFDs are not actually a motor starting,problem but a potential harmonic distortion. problem Check the generator manufacturer s,sizing program to see if it predicts the voltage. Wye Delta Starting, Closed Transition harmonics present when running nonlinear.
loads This can be very useful for analyzing,100 harmonic concerns. Full Load Current,0 MOTOR LOADING,MOTOR SPEED Full Load. There are two types of motor loading high,FigureStarting. Figure 6 Wye Delta 7 Wye Delta Starting breakaway torque loads i e unloaded motors. Source Kohler Power Systems,and high inertia loads i e loaded motors. FACTORS AFFECTING REAL WORLD Unloaded motors These tend to reach. MOTOR STARTING operating speed quickly A few examples of. unloaded motors include the following centrifugal,Generator frequency dip The generator s.
blowers rotary and centrifugal pumps starting,engine cannot be ignored in motor starting due. under low head pressure clutched motors and,to the high horsepower demanded when a large. fan motors starting with low restriction To avoid,motor is started When the engine slows under. improper operation of magnetic motor starters,load frequency dips this in turn increases the. and other equipment running on the generator,alternator voltage dip The amount of impact on.
never exceed a 35 percent instantaneous,engine rpm during motor starting is dependent. voltage dip,on the performance characteristics of a given. configuration of engine and alternator These Loaded motors These tend to take longer to. factors are taken into consideration when accelerate and recover to full voltage due to the. running the sizing software based on a maximum initial loaded condition of the motor With loaded. allowable voltage and frequency dip motors there is a more significant relationship. between recovery voltage during motor starting, Voltage regulator and excitation system and the generator s ability to accelerate the. response time Thorough testing has revealed motor to full speed and rated voltage A few. that in addition to the transient reactance of the examples of loaded motors include the following. alternator voltage regulators and exciters affect rock crushers elevators conveyors single. voltage dip and recovery A fast responding multicylinder compressors and submersible pumps. Solutions Center deals with motor loads Armed with this understanding power system specifiers will be able to select the most cost effective and reliable generator for motor starting applications A Practical Guide to Understanding how Motor Starting Loads Affect Generator Performance SIZING GENERATORS FOR MOTOR STARTING 2 Sizing Gensets for Motor Starting BASIC CHARACTERISTICS OF MOTOR

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