Danfoss VT2800 Series Design Manual

Contents

VLT ® 2800 Series■ Dynamic brakingWith the VLT 2800 the dynamic braking quality in anapplication can be improved in two ways, either withthe aid of brake resistors or AC braking.Danfoss offers a complete range of brake resistorsfor all VLT 2800 frequency converters.It is the job of the brake resistor to apply aload to the intermediate circuit during braking,thereby ensuring that the brake power can beabsorbed by the brake resistor.Without a brake resistor, the intermediate circuitvoltage of the frequency converter would go on rising,until cutting out for protection. The advantage ofusing a brake resistor is that you can brake quicklywith large loads, e.g. on a conveyor belt.Danfoss has chosen a solution in which the brakeresistor is not integrated into the frequency converter.This gives the user the following advantages:- The resistor’s cycle time can be selected as required.- The heat generated during braking can bediverted outside the panel cabinet, where theenergy can possibly be utilised.- No overheating of the electronic components,even if the brake resistor is overloaded.AC braking is an integrated function that is usedfor applications in which there is a need for limiteddynamic braking. The AC braking function makesit possible to reduce the brake power in the motorinstead of in a brake resistor. The function is intendedfor applications where the required braking torqueis less than 50% of rated torque. AC braking isselected in par. 400 Brake function.NB!:The AC brake cannot be used if therequired braking torque is more than 50%of rated braking torque. In such instancesa brake resistor must be used.■ Brake SetupThe figure shows a brake Setup with afrequency converter.In the following paragraphs, expressions andacronyms are used about brake Setups thatcan be seen from the figure.■ Calculation of brake resistanceThe following example and formula only applyto VLT 2800 Series.To ensure that the frequency converter does not cutout for safety reasons when the motor brakes, theresistance value is selected on the basis of the peakbraking effect and the intermediate circuit voltage:It can be seen that the brake resistance dependson the intermediate circuit voltage (UDC).With frequency converters that have a mains voltageof 3 x 380 - 480 Volt, the brake will be active at770 Volt (UDC); if the frequency converter has amains voltage of 3 x 200 - 240 Volt, the brakewill be active at 385 Volt (UDC).You can also choose to use the brake resistancerecommended by Danfoss (RREC ). This is aguarantee that the frequency converter is able tobrake at the highest braking torque (M BR). Therecommended brake resistance can be seen fromthe ordering table for brake resistors.RREC calculated as:NB!:Remember to check that the brake resistancecan manage a voltage of 850 Volt or 430 Volt,if Danfoss brake resistors are not being used.MG.28.E9.02 - VLT is a registered Danfoss trademark24