Danfoss VLT Decentral Drive FCD 302 Design Manual

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Danfoss VLT Decentral Drive FCD 302 Design Manual Manual pdf 27 page image

NOTEWhen the holding brake is included in a safety chain:A frequency converter cannot provide a safe control of amechanical brake. A redundancy circuitry for the brakecontrol must be included in the total installation.2.7.2 Dynamic BrakingDynamic Brake established by:• Resistor brake: A brake IGBT keep the overvoltageunder a certain threshold by directing the brakeenergy from the motor to the connected brakeresistor (2-10 Brake Function=[1]).• AC brake: The brake energy is distributed in themotor by changing the loss conditions in themotor. The AC brake function cannot be used inapplications with high cycling frequency sincethis will overheat the motor (par. 2-10 BrakeFunction=[2]).• DC brake: An over-modulated DC current addedto the AC current works as an eddy current brake(≠0 s ).2.7.3 Selection of Brake ResistorTo handle higher demands by generatoric braking, a brakeresistor is necessary. Using a brake resistor ensures that theenergy is absorbed in the brake resistor and not in thefrequency converter. For more information see the BrakeResistor Design Guide, MG90O.If the amount of kinetic energy transferred to the resistorin each braking period is not known, the average powercan be calculated based on the cycle time and brakingtime also called intermittent duty cycle. The resistorintermittent duty cycle is an indication of the duty cycle atwhich the resistor is active. Illustration 2.25 shows a typicalbraking cycle.NOTEMotor suppliers often use S5 when stating the permissibleload which is an expression of intermittent duty cycle.The intermittent duty cycle for the resistor is calculated asfollows:Duty cycle=tb/TT=cycle time in stb is the braking time in s (of the cycle time)ta tc tb to ta tc tb to ta130BA167.10LoadTimeSpeedIllustration 2.25 Dynamic Braking Cycle TimeCycle time [s] Braking duty cycle at 100%torqueBraking duty cycle at over torque(150/160%)3x380-480 VPK37-P75K 120 Continuous 40%P90K-P160 600 Continuous 10%P200-P800 600 40% 10%Table 2.7 Braking at High overload torque levelBrake resistors have a duty cycle of 5%, 10% and 40%. If a10% duty cycle is applied, the brake resistors are able toabsorb brake power for 10% of the cycle time. Theremaining 90% of the cycle time will be used ondissipating excess heat.NOTEEnsure the resistor is designed to handle the requiredbraking time.The maximum permissible load on the brake resistor isstated as a peak power at a given intermittent duty cycleand can be calculated as:Rbr Ω =Udc2PpeakwherePpeak=Pmotor x Mbr [%]xηmotorxηVLT[W]Product Overview VLT® Decentral Drive FCD 30226 MG04H102 - VLT® is a registered Danfoss trademark22