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.Although the first cost for variable inlet vaneswhen factory installed is lower than for a variable speed motor, inletvanes are not suitable for retrofit applications.Fan Speed ControlFan speed control is the most effective method for controlling fanperformance, but it is also the most expensive.Each centrifugal blowerwheel has a characteristic curve for each discrete fan speed.The perfor-mance  curve for a fan running at different speeds is plotted as a fam-ily of curves, with a curve for each increment of fan speeds.A change in fan speed will result in a corresponding change in theair volume delivered, static pressure developed, and power required.Methods available for controlling fan speed include: mechanical speedchanging mechanism which uses mechanical methods to vary the motoroutput shaft speed from a constant speed motor, and electric current in-verters which modify the electric current characteristics to cause stan-dard constant-speed motors to operate at variable speed.The lattermethod is more popular in HVAC systems.Mechanical speed control.This system consists of a variable-speedbelt drive plus a speed controller.A sheave with a spring-loaded vari-58 HVAC CONTROLS OPERATION & MAINTENANCEable pitch diameter mechanism is mounted on the fan motor drive shaftand is connected with a specific duty v-belt drive to a fixed pitch sheaveon a countershaft which connects to the fan shaft with standard v-beltsand sheaves.Upon receiving a signal from the controller, the variable drive basemoves closer to or further from the fixed drive base.This allows thefaces of the variable pitch diameter sheave to move against their com-pensating springs; they move closer together to increase diameter of thevariable pitch diameter sheave when the drive bases are close together,and force the sheave faces further apart and decrease the pitch diameteras the drives move further apart.As the pitch diameter of the motorsheave increases, the speed of the fan sheave increases: as the pitchdiameter decreases, the fan sheave speed decreases.Electric current inversion.This system uses an inverter, which is asolid-state device used to control motor speed.The incoming electriccurrent is processed through an inverter to change the electrical currentcharacteristics so that a standard constant-speed induction motor willoperate at any speed down to about 25% of the nominal motor RPM.Inverters use one of three basic technologies variable voltage in-put (VVI), current source input (CSI), or pulse width modulation(PWM).Each inverter system uses three common components: 1) a rec-tifier to convert alternating current to direct current, 2) a regulator toreceive the automatic control system speed input signal and establishthe appropriate dc voltage, and 3) an inverter which processes the dcsignal to generate a proportioned voltage-to-frequency waveform.Eachof the three inverters uses different techniques to modify the electricalcurrent for frequency and voltage.Variable Voltage Input InverterThis inverter, shown by schematic in Figure 2-16, has 3-phase linepower as the primary input into the phase controlled rectifier whichcontrols the phase angle.The rectifier receives a motor speed referencesignal from the regulator, which causes the rectifier to vary its output bycontrolling the firing angle of the silicon controlled rectifier (SCR) orthyristor banks.The pulsing dc is filtered and then inverted to an ac waveform bysix power switching devices within the inverter.The frequency of thewave is generated on the basis of the speed referenced signal receivedHVAC EQUIPMENT-TO-CONTROL INTERACTIONS 59from the regulator section.The resulting voltage and current waveformsare as shown in Figure 2-17.The resulting voltage and current wave-forms are proportioned in volts and frequency to produce the requiredspeed-to-torque relationship to suit the driven motor and load.When it is desirable to produce minimum line noise and a powerfactor near unity, the phase controlled rectifier diagrammed in Figure 2-16 can be replaced with a diode rectifier/chopper module where voltageis controlled in a transistorized chopper section by converting the dccurrent to a truncated waveform of controlled amplitude or voltage, asdiagrammed in Figure 2-18.The diode/chopper module may also beused on CSI and PWM type inverters.Current Source Input InverterThe current source input inverter (CSI), or adjustable current in-verter, controls current, not voltage.The adjustable current waveform isshown in Figure 2-19 [ Pobierz całość w formacie PDF ]