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  Power Matching Following the selection of a centrifugal pump that is properly “performance” matched to the systems requirements, it is necessary to select a driver that is properly “power” matched to the pump requirements.Drivers are machines that take energy in one form electrical, chemical, etc.! and convert it to a different form resulting in power, which can e used to drive other machines. #ny rotating shaft driver can e used to power a centrifugal pump, provided that the output shaft is rotating the correct direction and the speed of the shaft is suita le for the pump. $he drivers most commonly used to power centrifugal pumps are electric motors and internal com ustion engines. %ach kind has its own particular output and shaft speed characteristics and limitations which must e taken into consideration when matching a  pump and driver.&asically speaking, the power load against the driver must e within the continuous duty rating load capa ility of the driver at the operating conditions for the pumping system. 'hen the est estimation of power load including all power losses! has een made then a driver with suita le power and speed characteristics can then e power matched to the  pumping system requirements. #s discussed earlier, the formula to determine the pump  power requirements &(P! is as follows) BHP = (GPM X TDH X SG) / (3960 X PUMP EFFICIENCY) Depending upon the pump type, there may e power transmission losses, which must e added to the asic hydraulic load of the pump to o tain the total power input requirement.*ome of these types include elt+drive or chain+drive power transmission systems, vertical hollow shaft motor, and right angle gear drives. $ypically, losses will e etween- + /- for elt or chain drive units, varia le for 0(* motors depending on the diameter of the shaft and it1s length, and appro2imately 3- for right angle gear drives.Proper power matching can make the difference etween a “good” pump installation or a “ ad” pump installation. Electric Mtr!  'here satisfactory electrical power is economically availa le at the pump site, it is common practice to drive the pump with an electric motor squirrel cage induction type!.  'hen electric current amperes! flows through the wire coils stator windings! inside the motor, very strong magnetic fields are produced. $he magnetic forces attract the rotor of the motor, producing a twisting effect called torque!, which is com ined with the rotational speed of the motor to produce horsepower &(P!.*quirrel cage induction motors are essentially constant speed machines. $he 4PM of the motor shaft is closely related to the rotational speed of the magnetic field inside the motor. $he rotational speed is determined y the num er of poles coils of wire in the stator of the motor! and the frequency of the alternating current #5! electrical  power. $his is called the synchronous speed of the magnetic field and is shown in the following formula. PM = (#$0) X (%) / (&) 4PM 6 4evolutions per minute. $his is the synchronous speed of the magnetic field.f!+ Frequency of the alternating current #5! power e2pressed in cycles or hert7 per    second.p!+ $he num er of poles in the motor stator.$he squirrel cage rotor spins at a speed slightly less than the field synchronous speed. $his speed difference is called “slip”, and it will range etween 3- + - at full+rated  power output for the kinds of motors used to drive centrifugal pumps. $he motor will  produce only the amount of power demanded y the load. 'hen spinning without any load no power demand!, there is very little slip and the electric power consumption is very low. 'hen the load power demand is increased the slip increases, which increases the motor power output and the electric power input.$he wire coils inside the motor, which generate the magnetic field that causes the motor to turn, also act like electric heaters, converting some of the electrical energy coming in from the power system into heat. $he amount of heat produced is related to the amperes  passing through the wire coils, which is proportional to the load placed against the motor.%lectric motors have a thermal limit a ma2imum internal operating temperature!, that is determined y the class of insulation used. Modern insulation systems will tolerate very high continuous operating temperatures. 8f the temperature limit is e2ceeded, however, the insulation will fail leading to the urning out of the motor windings. $hat is why every motor has a ma2imum continuous current ampere! limit. 8n a three+phase motor, this is the ma2imum current through any leg, and is not the average current.8t is standard practice in motor manufacturing to show a “full load amps” F9#! rating onthe motor nameplate. F9# is the amperage rating at the motor nameplate horsepower rating. 'hen operating at this load, the motor efficiency and power factor are usually at their ma2imum. 8t is at this point that the motor prefers to operate. 8f the load is other than full load, oth the efficiency and power factor will change. $his can su stantially change the over+all efficiency of the pumping system.  *quirrel cage induction motors are usually designed to have e2tra thermal capacity. $his  permits continuous operation, without thermal failure, at a higher rating than “full load”. $his e2tra capacity is called “service factor” *F! and is stamped on the motor nameplate.$he amperage rating on the motor nameplate for this e2tra capacity is called “service factor amps” *F#! or “ma2 amps”. E'&le* + #00HP tr, SF = #-#., c &erte & t ##.HP 8t is normal practice in the pump industry to use, at least, some of the service factor to getthe most power possi le from a given motor. 8t should e understood that the amperage demand at . service factor load is - greater than the full load amps, and the resulting higher internal temperature will reduce the life of the motor insulation. 8n some cases, this is an accepta le loss. $he insulation life reduction will vary ased on the actual operating temperature and the insulation materials used in the motor. # rule of thum in the industry is for every / / 5 increase, the insulation life will e cut in half.'ater can conduct electricity and is also somewhat corrosive. $herefore, the electrical windings and internal parts of an electric motor must e protected from water or other liquids.$he most common enclosure for motors used on centrifugal pumps is “open drip proof” :DP!. 8t is suita le for indoor use and normal outdoor use where there is little danger of water directly entering the motor. $hese motors are cooled y air lown through the inside of the motor y integral fans. $he air enters the motor through ventilation openings that are protected from downward falling water. 8n vertical motors, used on vertical pumps, this type of enclosure is called “weather protected 6 $ype ” 'P+!.  ;ormally, these types of motors will have a . service factor.8f the operating environment is wet and there is su stantial risk of water entering the motor, all openings into the motor are closed, and the risk of water entering the motor under normal conditions is eliminated. For very small motors this enclosure system is called “totally enclosed non+ventilated” $%;0!. 9arger motors have an e2ternal fan to   low air over the outside of the motor frame and are called “totally enclosed fan cooled” $%F5!. ;ormally, these types of motors will have a ./ service factor. $hey can e ordered with a . service factor. 5aution must e advised on these motors ecause catalog performance curves for centrifugal pumps are frequently ased on impeller diameters that will load the motor to the . service factor. 'hen using the motors with a ./ service factor, the impeller diameter must e reduced to prevent serious damage to the motor.#nother motor enclosure, called “e2plosion proof”, will sometimes e required for operation in e2plosive atmospheres. $he e2plosion proof enclosure is like the $%;0 and$%F5 enclosures with additional provisions to prevent the entry of e2plosive gas into the motor or the escape of sparks into the surrounding atmosphere. $hese motors are usually made with a ./ service factor and can e very e2pensive, as well as difficult to o tain.  $he same caution applies to e2plosion proof motors as with $%;0 and $%F5 motors regarding motor loading and impeller diameters.$he following information is needed to properly select a motor. $he speed 4PM! of the motor shaft should match the rated operating speed of the pump. $he motor must e compati le with the power system voltage, phase, and frequency. $he motor enclosure selected should provide proper protection when operating in the pump installation site environment. $he greatest load that the pump can demand under any possi le operating condition must not e2ceed the service factor rated output of the motor. :n close+coupled centrifugal pumps the motor frame si7e is needed as well. 8f it is economically feasi le, the pump and motor set should e selected to load the motor as close as possi le to the nameplate rated horsepower. $his will allow long insulation life, the est motor efficiency, and the est power factor.%lectric motors commonly used as pump drivers are usually cooled y air, which is circulated y an integral fan, through the windings or over the motor frame. 8f the air density pounds of air per cu ic foot! is decreased, the amount of air flowing through the motor will have reduced a ility to remove heat from the windings. $his could lead to the winding temperature ecoming too high. 8f the operating altitude will e a ove 3,3// feet /// meters!, the am ient temperature could e2ceed /< ° F </ ° 5!. $he motor manufacturer should advise you whether the insulation system of the motor tolerates the higher operating temperature or if the motor needs to e de+rated.#fter installation, with the pump supplying the system demands satisfactorily, measurements and calculations can e made to verify motor loading for the installation. $he preferred method of measurement uses a 'attmeter, which shows the input electrical power (P 8; ! in 'atts or =ilowatts!. $he following formula can e used for the calculations. HP IN  = 1tt! / 26 r 4il5tt! / 0-26 $he power input to the pump which is the power output from the motor! can e determined y multiplying the electrical input horsepower times the motor efficiency. $he motor efficiency can e o tained from the motor nameplate or from the motor manufacturer. 8n the a sence of etter information, the appro2imate motor efficiencies listed in many engineering manuals will allow you to come up with a good estimate of the power output for a motor operating near the normal, full+load rating.:n three+phase motors, it is important to verify that the electrical load is eing shared  properly etween the windings of the motor. $he ma2imum allowa le deviation of the current amps! in any leg, from the average of the three legs, is -. #lso, the current in the highest leg must not e2ceed the *ervice Factor amperage rating listed on the motor nameplate.
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