Danfoss VLT AQUA Drive FC 202 Design Manual

Also see for VLT AQUA Drive FC 202: Service guide Installation guide Installation guide Operating guide Installation guide

Contents

Use the brake resistance Rrec, to ensure that the frequencyconverter is able to brake at the highest braking torque(Mbr(%)) of 160%. The formula can be written as:Rrec Ω =Udc2x 100Pmotor x Mbr %x ηVLT x ηmotorηmotor is typically at 0.90ηVLT is typically at 0.98When a higher brake resistor resistance is selected, 160%/150%/110% braking torque cannot be obtained, and thereis a risk that the frequency converter cuts out of DC-Linkovervoltage for protection.For braking at lower torque, for example 80% torque, it ispossible to install a brake resistor with lower power rating.Calculate size using the formula for calculating Rrec.Frequency converter D and F enclosure sizes contain morethan one brake chopper. Use a brake resistor for eachchopper for those enclosure sizes.The VLT® Brake Resistor MCE 101 Design Guide contains themost up-to-date selection data, and describes thecalculation steps in more detail, including:• Calculation of braking power• Calculation of brake resistor peak power• Calculation of brake resistor average power• Braking of inertia3.4.13 Brake Resistor CablingEMC (twisted cables/shielding)To meet the specified EMC performance of the frequencyconverter, use screened cables/wires. If unscreened wiresare used, it is recommended to twist the wires to reducethe electrical noise from the wires between the brakeresistor and the frequency converter.For enhanced EMC performance, use a metal screen.3.4.14 Brake Resistor and Brake IGBTBrake resistor power monitorIn addition, the brake power monitor function makes itpossible to read out the momentary power and the meanpower for a selected time period. The brake can alsomonitor the power energising and make sure it does notexceed a limit selected in 2-12 Brake Power Limit (kW). In2-13 Brake Power Monitoring, select the function to carryout when the power transmitted to the brake resistorexceeds the limit set in 2-12 Brake Power Limit (kW).NOTICEMonitoring the brake power does not fulfil a safetyfunction. The brake resistor circuit is not ground leakageprotected.The brake is protected against short-circuiting of the brakeresistor, and the brake transistor is monitored to ensurethat short-circuiting of the transistor is detected. Use arelay or digital output to protect the brake resistor againstoverloading in the event of a fault in the frequencyconverter.Overvoltage control (OVC) can be selected as an alternativebrake function in 2-17 Over-voltage Control. If the DC-linkvoltage increases, this function is active for all units. Thefunction ensures that a trip can be avoided. This is doneby increasing the output frequency to limit the voltagefrom the DC link. It is a useful function, e.g. if the ramp-down time is too short since tripping of the frequencyconverter is avoided. In this situation the ramp-down timeis extended.3.4.15 Energy EfficiencyEfficiency of the frequency converterThe load on the frequency converter has little effect on itsefficiency.This also means that the frequency converter efficiencydoes not change when other U/f characteristics areselected. However, the U/f characteristics do influence theefficiency of the motor.The efficiency declines a little when the switchingfrequency is set to a value above 5 kHz. The efficiency isalso slightly reduced when the motor cable is longer than30 m.Efficiency calculationCalculate the efficiency of the frequency converter atdifferent loads based on Illustration 3.19. Multiply the factorin this graph with the specific efficiency factor listed inchapter 7.1 Electrical Data.1.00.990.980.970.960.950.930.920% 50% 100% 200%0.94Relative Efficiency130BB252.111.01150%% Speed100% load 75% load 50% load 25% loadIllustration 3.19 Typical Efficiency CurvesSystem Integration Design GuideMG20N602 Danfoss A/S © 09/2014 All rights reserved. 593 3