Danfoss VLT AutomationDrive FC 300 Design Manual

Also see for VLT AutomationDrive FC 300: Instruction manual Operating instructions Design guide Service manual Operating instructions

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Danfoss VLT AutomationDrive FC 300 Design Manual Manual pdf 47 page image

FC 300 Design GuideIntroduction to FC 300Danfoss recommends the brake resistance RREC,i.e. one that guarantees that the frequencyconverter is able to brake at the highestbraking torque (M br ) of 160%.η motor is typically at 0.90, while ηVLT istypically at 0.98.For 200 V, 500 V, and 600 V frequency converters,R REC at 160% braking torque is written as:NB!:The resistor brake circuit resistance selected should not be higher than that recommended byDanfoss. If a brake resistor with a higher ohmic value is selected, the 160% braking torque maynot be achieved because there is a risk that the frequency converter cuts out for safety reasons.NB!:If a short circuit in the brake transistor occurs, power dissipation in the brake resistor is onlyprevented by using a mains switch or contactor to disconnect the mains for the frequencyconverter. (The contactor can be controlled by the frequency converter)." Control with Brake FunctionThe brake is to limit the voltage in the intermediate circuit when the motor acts as a generator.This occurs, for example, when the load drives the motor and the power accumulates on the DClink. The brake is built up as a chopper circuit with the connection of an external brake resistor.Placing the brake resistor externally offers the following advantages:- The brake resistor can be selected on the basis of the application in question.- The brake energy can be dissipated outside the control panel, i.e. where the energy can be utilized.- The electronics of the frequency converter will not be overheated if the brake resistor is overloaded.The brake is protected against short-circuiting of the brake resistor, and the brake transistor is monitored toensure that short-circuiting of the transistor is detected. A relay/digital output can be used for protectingthe brake resistor against overloading in connection with a fault in the frequency converter.In addition, the brake makes it possible to read out the momentary power and the mean power forthe latest 120 seconds. The brake can also monitor the power energizing and make sure it doesnot exceed a limit selected in par. 2-12. In par. 2-13, select the function to carry out when thepower transmitted to the brake resistor exceeds the limit set in par. 2-12.NB!:Monitoring the brake power is not a safety function; a thermal switch is required for thatpurpose. The brake resistor circuit is not earth leakage protected.Over voltage control (OVC) (exclusive brake resistor) can be selected as an alternative brake functionin par. 2-17. This function is active for all units. The function ensures that a trip can be avoided if theDC link voltage increases. This is done by increasing the output frequency to limit the voltage fromthe DC link. It is a very useful function, e.g. if the ramp-down time is too short since tripping of thefrequency converter is avoided. In this situation the ramp-down time is extended.47MG.33.B4.02 - VLT is a registered Danfoss trademark