Danfoss VLT 2800 Design Manual

Also see for VLT 2800: Operating instructions Manual Instruction manual Quick guide Installation instructions

Contents

Danfoss VLT 2800 Design Manual Manual pdf 132 page image

The function ensures that the frequency converterautomatically adjusts the switching frequency between fsw,min and fsw, max (parameter 411 Switching frequency), asshown in Illustration 5.2.Illustration 5.2 Switching Frequencies vs. TemperatureWhen an LC filter is used, the minimum switchingfrequency is 4.5 kHz.5.1.10 Derating for Air PressureProtective Extra Low Voltage (PELV) requirements stated inIEC 61800-5-1 are not fulfilled at altitudes above 2000 m(6562 ft.). Contact Danfoss for further information.Below 1000 m derating is not necessary.Above 1000 m, derate the ambient temperature (TAMB) ormaximum output current (IMAX) in accordance with thediagram in Illustration 5.3:1. Derating of output current versus altitude atTAMB=maximum 45 °C.2. Derating of max. TAMB versus altitude at 100%output current.Illustration 5.3 Derating for Air Pressure5.1.11 Derating for Running at Low SpeedWhen a motor is connected to a frequency converter, it isnecessary to ensure adequate cooling of the motor. At lowRPM values, the motor fan is not able to supply anadequate volume of cooling air. This problem occurs whenthe load torque is constant (for example, with a conveyorbelt) across the full regulating range. The reduced amountof ventilation determines the permissible torque incontinuous operation. If the motor is to run continuouslyat an RPM lower than half the rated value, extra cooling airmust be supplied to the motor. Instead of providing extracooling, reduce the motor load ratio by selecting a largermotor. However, the design of the frequency converterputs limits on the size of motors that can be connected tothe frequency converter.5.1.12 Derating for Long Motor CablesThe frequency converter has been tested using a 75 munscreened/unarmoured cable and a 25 m screened/armoured cable and has been designed to work using amotor cable with a rated cross-section. If a cable with alarger cross-section is required, reduce the output currentby 5% for each step the cable cross-section is increased.(Increased cable cross-section leads to increasedcapacitance to ground, and thus to an increased groundleakage current).5.1.13 Derating for High SwitchingFrequencyA higher switching frequency (to be set in parameter 411Switching frequency) leads to higher losses in theelectronics of the frequency converter.VLT 2800 has a pulse pattern in which it is possible to setthe switching frequency from 3.0- 10.0/14.0 kHz.The frequency converter automatically derates the ratedoutput current IVLT,N, when the switching frequencyexceeds 4.5 kHz.In both cases, the reduction is carried out linearly, down to60% of IVLT,N.All about VLT 2800 Design Guide130 Danfoss A/S © Rev. May/2014 All rights reserved. MG27E40255