User guide

14 DYN BRAK (DYNAMICBRAKE)
 This parameter enables the dynamic braking circuit. Set this parameter to ON only if the optional
dynamicbrakingcircuitboardandresistorsareinstalled.
 Dynamicbraking isusedin applicationswherehigh-inertia loadsneedto bedeceleratedquickly. When
thisis attempted,themotorregeneratesvoltagebackinto thedrive, causingtheDCbusvoltage torise,
eventually resulting in a HI VOLTS fault. With the dynamic braking option, the DC bus voltage is
monitored,andwhenitreachesacertainlevel,atransistorisswitchedonthatconnectsanexternalresistor
bankacrosstheDCbus.Thisallowstheregeneratedenergyfromthemotortobedissipatedthroughthe
resistorsasheat,whichkeepstheDCbusvoltagebelowthetriplevel.
16 CURRENT (CURRENTLIMIT)
 Thisparametersetsthemaximumallowableoutputcurrentofthedrive,whichalsodeterminesthetorque
capabilityofthemotor.Formostapplications,CURRENTisleftatthemaximumsetting,whichis150%
or180%(ofthedrive’soutputcurrentrating),dependingonwhethertheinputvoltagetothedriveislow
orhigh(seeParameter0-LINEVOLTS).RegardlessoftheCURRENTsetting,thedriveiscapableof
deliveringamaximumof150%currentforoneminute,and180%currentforapproximately30seconds,
beforetrippingintoanOVERLOADfault.SeeParameter17–MOTOROL.
The drive will enter current limit when the load demands more current than the drive can deliver, which
resultsinalossofsynchronizationbetweenthedriveandthemotor.Tocorrectthiscondition,thedrive
will enter FREQUENCY FOLDBACK, which commands the drive to decelerate in order to reduce the
output current andregainsynchronizationwiththemotor. When the overcurrent condition passes, the
drivewillreturntonormaloperationandacceleratebacktothespeedsetpoint.However,ifFREQUENCY
FOLDBACK cannot correct the condition and the drive remains in current limit for too long, it will trip
intoanOVERLOADfault.Ifthedriveenterscurrentlimitwhileaccelerating,thetimerequiredtoreach
thespeedsetpointwillbelongerthanthetimeprogrammedintoACCEL(Parameter8).
17 MOTOR OL (MOTOROVERLOAD)
The MICRO Series is UL approved for solid state motor overload protection. Therefore, a separate thermal
overloadrelay isnotrequired forsingle motorapplications.TheMOTOROVERLOAD circuitis used
to protect the motor from overheating due to excessive current draw. The trip time for the MOTOR
OVERLOAD setting is based on what is known as an “inverse I2t” function. This function allows the
drive to deliver 150% of the rated output current for one minute, and even higher current levels for shorter
periodsoftime.Oncetheoverloadcircuit“timesout”,thedrivewilltripintoanOVERLOADfault.
 TheMOTOROVERLOADshouldbesettoavaluewhichisequaltotheratio(inpercentage)ofthemotor
full load current rating to the drive output current rating. This will result in an overload capacity of 150%
of the MOTOR current rating for one minute. If this parameter is set to 100%, the motor will be allowed
to draw 150% of the DRIVE output current rating for one minute. This distinction is important in cases
where the motor full load current rating is significantly less than the drive output current rating, such as
applicationswherethedriveisoversizedtomeettorquerequirements.
Example 1: A 5 Hp, 480 Vac drive is operating a 3 HP motor with a full load current rating of 4.8 amps.
Divide the motor current rating by the drive output current rating: 4.8 / 7.6 = 63%. Entering this value
will allow continuous operation at 4.8 amps, and will also allow the motor to draw 7.2 amps (150% of 4.8
amps)foroneminute.Ifthesettingisleftat100%,themotorcoulddraw11.4amps(150%of7.6amps)
for one minute before tripping the drive.
The MC Series drive has two options for thermal overload protection. One depends on the speed of the
drive,andtheotherdoesnot.Thediagrambelowillustratesthedifferencebetween“speedcompensated”
and“non-compensated”thermaloverloadprotection.
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