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What is a Variable Frequency Drive? How does a VFD Work? Many utilities are now offering rebates for the installation of VFD's or retro-fitting existing equipment with variable frequency drives. ontact your local utility or search VFD rebate. What is a VFD? By: Dave Polka You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torue, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor
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  What is a Variable Frequency Drive?How does a VFD Work? Many utilities are now offering rebates for the installation of VFD's or retro-fitting existing equipment with variable frequency drives. ontact your local utility or search VFD rebate. What is a VFD? By: Dave Polka You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torue, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and!like their DC counterparts!control speed, torue, and horsepower. Application As An Example et#s take a $rief look at a drive application. %n &ig. ', you can see a simple application with a fi(ed speed fan using a motor starter. You could replace the )!phase motor starter with *aria$le &reuency Drive +*&D to operate the fan at varia$le speed. -ince you can operate the fan at any speed $elow its ma(imum, you can vary airflow $y controlling the motor speed instead of the air outlet damper. Figure 1, Fixed Speed Fan Application A drive can control two main elements of a )!phase induction motor: speed and torue. o understand how a drive controls these two elements, we will take a short review of AC induction motors. &ig. / shows the construction of an induction motor. he two $asic parts of the motor, the rotor and stator, work through magnetic interaction. A motor contains pole pairs. hese are iron pieces in the stator, wound in a specific pattern to provide a north to south magnetic field. Figure 2, Basic Induction Motor ConstructionFigure 3, Operating Principles of Induction Motor  0ith one pole pair isolated in a motor, the rotor +shaft rotates at a specific speed: the $ase speed. he num$er of poles and the freuency applied determine this speed +&ig. 1. his formula includes an effect called 2slip.2 -lip is the difference $etween the rotor speed and the rotating magnetic field in the stator. 0hen a magnetic field passes through the conductors of the rotor, the rotor takes on  magnetic fields of its own. hese rotor magnetic fields will try to catch up to the rotating fields of the stator. 3owever, it never does !! this difference is slip. hink of slip as the distance $etween the greyhounds and the hare they are chasing around the track. As long as they don#t catch up to the hare, they will continue to revolve around the track. -lip is what allows a motor to turn. Motor Slip: -haft -peed 4'/5 6 &P! -lip-lip for 789A B 9otor 4 ) to ; of Base -peed which is '<55 =P9 at &ull oad & 4 &reuency applied to the motorP 4 7um$er of motor poles Example: -haft -peed 4'/5 6 >5 3?1! -lip Figure 4, Induction Motor Slip Calculation 0e can conveniently ad@ust the speed of a motor $y changing the freuency applied to the motor. You could ad@ust motor speed $y ad@usting the num$er of poles, $ut this is a physical change to the motor. %t would reuire rewinding, and result in a step change to the speed. -o, for convenience, cost!efficiency, and precision, we change the freuency. &ig.  shows the torue!developing characteristic of every motor: the *olts per 3ert? ratio +*3?. 0e change this ratio to change motor torue. An induction motor connected to a 1>5*, >5 3? source has a ratio of .>. As long as this ratio stays in proportion, the motor will develop rated torue. A drive provides many different freuency outputs. At any given freuency output of the drive, you get a new torue curve. Figure 5, olts! ert# $atio How Drive Changes Motor Speed ust how does a drive provide the freuency and voltage output necessary to change the speed of a motor hat#s what we#ll look at ne(t. &ig. > shows a $asic P09 drive. All P09 drives contain these main parts, with su$tle differences in hardware and software components. Figure %, Basic P&M 'ri(e Co)ponents Although some drives accept single!phase input power, we#ll focus on the )!phase drive. But to simplify illustrations, the waveforms in the following drive figures show only one phase of input and output. he input section of the drive is the converter. %t contains si( diodes, arranged in an electrical $ridge. hese diodes convert AC power to DC power. he ne(t section!the DC $us section!sees a fi(ed DC voltage. he DC Bus section filters and smoothes out the waveform. he diodes actually reconstruct the negative halves of the waveform onto thepositive half. %n a 1>5* unit, you#d measure an average DC $us voltage of a$out >5* to ><5*. You can calculate this as line voltage times '.1'1. he inductor +  and the capacitor +C work together to filter out any AC component of the DC waveform. he smoother theDC waveform, the cleaner the output waveform from the drive.  he DC $us feeds the final section of the drive: the inverter. As the name implies, this section inverts the DC voltage $ack to AC. But, it does so in a varia$le voltage and freuency output. 3ow does it do this hat depends on what kind of power devices your drive uses. %f you have many -C= +-ilicon Controlled =ectifier!$ased drives in your facility, see the -ide$ar. Bipolar ransistor technology $egan superceding -C=s in drives in the mid!'E5s. %n the early 'EE5s, those gave way to using %nsulated Fate Bipolar ransistor +%FB technology, which will form the $asis for our discussion. Switching !s With #$s oday#s inverters use %nsulated Fate Bipolar ransistors +%FBs to switch the DC $us on and off at specific intervals. %n doing so, the inverter actually creates a varia$le AC voltage and freuency output. As shown in &ig. , the output of the drive doesn#t provide an e(act replica of the AC input sine waveform. %nstead, it provides voltage pulses that are at a constant magnitude. Figure *, 'ri(e Output &a(efor) he drive#s control $oard signals the power device#s control circuits to turn 2on2 the waveform positive half or negative half of the power device. his alternating of positive and negative switches recreates the ) phase output. he longer the power device remains on, the higher the output voltage. he less time the power device is on, the lower the output voltage +shown in &ig.<. Conversely, the longer thepower device is off, the lower the output freuency. Figure +, 'ri(e Output &a(efor) Co)ponents he speed at which power devices switch on and off is the carrier freuency, also known as the switch freuency. he higher the switch freuency, the more resolution each P09 pulse contains. ypical switch freuencies are ),555 to 1,555 times per second +)G3? to 1G3?. +0ith an older, -C=!$ased drive, switch freuencies are /5 to 55 times per second. As you can imagine, the higher the switch freuency, the smoother the output waveform and the higher the resolution. 3owever, higher switch freuencies decrease the efficiency of the drive $ecause of increased heat in the power devices. Shrin%ing cost and si&e Drives vary in the comple(ity of their designs, $ut the designs continue to improve. Drives come in smaller packages with each generation. he trend is similar to that of the personal computer. 9ore features, $etter performance, and lower cost with successive generations. Hnlike computers, however, drives have dramatically improved in their relia$ility and ease of use. And also unlike computers, the typical drive of today doesn#t spew gratuitous harmonics into your distri$ution system!nor does it affect your power factor. Drives are increasingly $ecoming 2plug and play.2 As electronic power components improve in relia$ility and decrease in si?e, the cost and si?e of *&Ds will continue to decrease. 0hile all that is going on, their performance and ease of use will only get $etter. Side'ar: What i( )o! have SC*s? 0ith the large installed $ase of -C=s, you might want to know how these operate. An -C= +srcinally referred to as a thyristor contains a control element called a gate. he gate acts as the 2turn!on2 switch that allows the device to fully conduct voltage. he device conductsvoltage until the polarity of the device reverses!and then it automatically 2turns off.2 -pecial circuitry, usually reuiring another circuit $oard and associated wiring, controls this switching. he -C=#s output depends on how soon in the control cycle that gate turns on. he %FB output also depends the length of time the gateis on. 3owever, it can turn off anytime in the control cycle, providing a more precise output waveform. %FBs also reuire a control circuitconnected to the gate, $ut this circuitry is less comple( and doesn#t reuire a reversal of polarity. hus, you would approach trou$leshooting differently if you have an -C=!$ased drive.
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