Carrier 48FK Controls Operation And Troubleshooting

Carrier 48FK Controls Operation And Troubleshooting Manual pdf 38 page image

Supply Fan Control with IGV — In most VAV units,the supply fan static pressure is controlled by inlet guide vanes.The inlet guide vanes operate independently from the micro-processor. The supply static pressure is controlled by a dif-ferential pressure switch. If the unit is equipped with a re-turn fan, building pressure is controlled by another differentialpressure switch.For example, assume that set point on supply fan differ-ential switch is 1.9 in. wg. If pressure in supply duct goesabove 1.9 in. wg, switch will make to the normally opencontact and energize inlet guide vane motor to drive inletguide vanes to a more closed position, thus reducing airflowand lowereing duct pressure. Once set point pressure is reached,switch will open and deenergize inlet guide vane motor. Ifpressure in supply duct is below 1.9 in. wg, the switch willmake to the normally closed contact and energize inlet guidevane motor to drive inlet guide vane to a more open posi-tion; increasing airflow and raising duct pressure. Once again,once desired pressure has been reached, switch will open anddeenergize inlet guide vane motor. How far above or belowthe set point setting the switch goes before energizing de-pends on setting of null span (null span is pressure changethat can be made without contacts opening or closing). Ifnull span is at maximum position, pressure will vary from0.17 in. wg to 0.31 in. wg depending on set point (if set pointis at minimum setting, null span will be 0.17 in. wg, whileif it is at maximum position, the null span will be0.31 in. wg) before switch acts. If null span is adjusted to aminimum setting, duct pressure will vary from 0.06 in. wgto 0.11 in. wg (again depending on switch set point) beforeswitch acts. Setting null span to minimum position will re-sult in a smaller pressure fluctuation than if it is set at maxi-mum position.Supply Fan Control with VFD — When equippedwith the VFD option, the supply fan static pressure is con-trolled by modulating the fan wheel speed. The VFD oper-ates independently from the microprocessor. A duct pressuretransducer monitors duct static pressure. The transducer out-put (4 to 20 mA) is directed into the VFD. The VFD adjustssupply fan motor speed (which changes wheel speed) as mea-sured duct pressure varies from set point as established atthe VFD. The VFD will modulate fan speed until the ductpressure set point is achieved.NOTE: The VFD will always provide the proper phase se-quence to the supply fan motor. This motor will operate inproper rotation regardless of the phase sequence to the unit.If, upon start-up, the outdoor fans operate backwards but theindoor fan operates in the correct direction, reverse any twoleads to the main terminal block. All fans will then operatein the correct direction.Modulating Power Exhaust (Option or Acces-sory except FY,JY units) — The power exhaust as-sembly consists of two parallel and independent belt-driveforward curve fans. The fans, motors, and drives are locatedover the return air opening of the unit, in a plenum beneaththe outside air intake plenum. The fans discharge air hori-zontally out the back of the unit through individual baro-metric backdraft dampers with hoods. (See Fig. 44 and 45.)Operation is interlocked with economizer operation. Sheetmetal installation is shown in Fig. 46 and 47.Fan no. 1 is equipped with a variable position dischargedamper located in the outlet of the fan housing. This damperis controlled by an actuator (PEDM), based on signals fromthe building pressure differential pressure switch (DPS). Avail-able range on the DPS is −0.50 to +0.50 in. wg, adjustable.Building pressure is sensed by a pick-up (field-supplied and-installed) located in the occupied space.Operation of the modulating power exhaust is a combi-nation modulating/staged control, with fan no. 1 providingmodulating control from 0 to 100%, and fan no. 2 being stagedOn/Off according to damper position on fan no. 1.As the economizer actuator opens past 17% open, auxil-iary switch DMS1 closes, energizing fan contactor PEC1.Fan motor no. 1 starts and runs.Capacity of fan no. 1 is controlled by the position of theoutlet damper. As building pressure increases above set point,the DPS will close its contact and drive the power exhaustdamper motor (PEDM) open until set point is achieved. DPSthen opens its control contacts and PEDM maintains currentposition.When space demand moves PEDM to 90% of full-openposition, auxiliary switch PEDMS closes, energizing fan con-tactor PEC2. Fan motor no. 2 starts and runs. Increased ex-haust airflow will lower space pressure, causing DPS to drivePEDM back towards its closed position, until the set pointis achieved.If space pressure decreases until PEDM position is re-duced to 10% of open position, PEDMS will open, deener-gizing fan contactor PEC2 and shutting off fan no. 2.Unit Staging — Compressor loading and unloading se-quences are shown in Table 19.38