Carrier 30XA Operation Instructions Manual

Carrier 30XA Operation Instructions Manual Manual pdf 27 page image

277.6.2 - ResetReset.means.the.active.setpoint.is.modified.so.that.less.machine.capacity is required. In the cooling mode the setpoint is increased,whereas.in.the.heating.mode.it.is.decreased..This.modification.is.in general a reaction to a drop in the load.The reset can be based on the following parameters:• OAT that gives the measure of the load trends for thebuilding• Return water temperature(ΔT provides the average building load)• Space temperature (EMM option)• Dedicated 4-20 mA inputThe.reset.source.and.the.reset.parameters.can.be.configured.in.the Main menu (RESETCFG – Reset Configuration). Inresponse to a drop in the reset source, the cooling setpoint isnormally reset upwards to optimise unit performance.The amount of reset is determined by linear interpolationbased on the following parameters:• A reference at which reset is zero (no reset value)• A reference at which reset is maximum (full reset value)• The maximum reset value20 Reset based on OAT 250 Reset based on delta T 34 Reset based on analog input 20no_reset selection full_resetLegend: A: Maximum reset valueB: Reference for zero resetC: Reference for maximum resetD: Building load7.7 - Capacity limitationThe Touch Pilot control system allows for the constant controlof the unit capacity by setting its maximum allowable capacity.The main control system enables to limit the unit capacityusing one of the external orders:• By means of user-controlled volt-free contacts. Unitswithout the energy management module have onecontact. Units with the energy management modulepermit three capacity limitation levels (see alsosection 3.9.4). The unit capacity can never exceed thelimit setpoint activated by these contacts. The limitsetpoints.can.be.modified.in.the.SETPOINT.menu.• By lag limit set by the master unit (master/slave assembly).• By night mode limitation control. The demand limitvalue in the night mode is selectable if the value isbelow the selected limit. A limit value of 100% meansthat the unit can use all capacity stages.In certain conditions, the unit power consumption can exceedthe capacity limitation threshold to protect the compressors.7.8 - Current limitationCurrent limitation is used via the demand limit function. Ifthe current limitation is active (Current Limit Select in theGEN_CONF menu), the control calculates the sum ofcompressors current to obtain the total compressor current.If this value exceeds the pre-defined limit, the controlcommands a reduction of the compressor load, until it isbelow the limit again. Before loading a capacity stage, thecontrol estimates the future total compressor current andensures that it does not exceed the limit.The current limit is based on two parameters:• The current limit that corresponds to 100% capacity(CurrentLimit at 100%, GEN_CONF – GeneralConfiguration)• The active demand limit determined either by thedemand limit contact (see also section 3.9.4) or by thenetwork (Active Demand Limit Val, GENUNIT –General Parameters)Chiller current limit is displayed in the GENUNIT menu.Current limitation is disabled if the unit operates in themaster/slave mode, the unit is controlled by a SystemManager or the night mode is active.7.9 - Capacity controlThis function adjusts the capacity using the compressor slidevalve to keep the water exchanger temperature at its setpoint.The control system continuously takes account of thetemperature error with respect to the setpoint, the rate ofchange in this error and the difference between entering andleaving water temperatures in order to determine the optimalmoment at which to add or withdraw capacity.Compressors are started and stopped in a sequence designedto equalise the number of start-ups (value weighted by theiroperating time). For more information about compressorssequence, see Balanced loading sequence and Staged loadingsequence in section 7.13.7.10 - Night modeNight.mode.allows.users.to.configure.the.unit.to.operate.with.specific.parameters.in.a.specific.time.period..During.the.night.period, the unit capacity is limited and the number ofoperating fans is reduced.The.night.period.is.defined.by.a.start.time.and.an.end.time.that are the same for each day of the week. The Night modesettings.or.the.maximum.capacity.value.can.be.configured..via the Configuration menu (GEN_CONF – GeneralConfiguration)..Only logged-in users can modify Night Mode settings (seealso section 4.3.3).