Danfoss VLT 2800 Design Manual

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Danfoss VLT 2800 Design Manual Manual pdf 9 page image

1.4.2 VLT 2800 Control PrincipleA frequency converter is an electronic unit which is able toinfinitely variably control the RPM of an AC motor. Thefrequency converter controls the motor speed byconverting the regular voltage and frequency from mains,e.g. 400 V/50 Hz, into variable magnitudes. Today, thefrequency converter controlled AC motor is a natural partof all types of automated plants.The frequency converter has an inverter control systemcalled VVC (Voltage Vector Control). VVC controls aninduction motor by energising it with a variable frequencyand a voltage suitable for it. If the motor load changes, sodoes its energising and speed. That is why the motorcurrent is measured on an ongoing basis, and a motormodel is used to calculate the actual voltage requirementand slip of the motor.1.4.3 Programmable Inputs and Outputs in4 Set-upsIn the frequency converter, it is possible to program thedifferent control inputs and signal outputs and to select 4different user-defined set-ups for most parameters.Program the required functions on the control panel or viaserial communication.1.4.4 Mains ProtectionThe frequency converter is protected against the transientsthat occur on the mains sometimes, e.g. if coupling with aphase compensation system, or if fuses blow whenlightning strikes.Rated motor voltage and full torque can be maintaineddown to approx. 10% undervoltage in the mains supply.As all 400 V units in the VLT 2800 Series have intermediatecircuit coils, there is only a low amount of harmonic mainssupply interference. This gives a good power factor (lowerpeak current), which reduces the load on the mains instal-lation.1.4.5 Frequency Converter ProtectionThe current measurement in the intermediate circuitconstitutes perfect protection of the frequency in casethere is a short-circuit or a ground fault on the motorconnection.Constant monitoring of the intermediate circuit currentenables switching on the motor output, e.g. with acontactor.Efficient monitoring of the mains supply means that theunit stops if a phase drop-out occurs. In this way, theinverter and the condensers in the intermediate circuit arenot overloaded, which would dramatically reduce theservice life of the frequency converter. The frequencyconverter offers temperature protection as standard. Ifthere is a thermal overload, this function cuts out theinverter.1.4.6 Reliable Galvanic IsolationIn the frequency converter, all digital inputs/outputs,analog inputs/outputs and the terminals for serialcommunication are supplied from or in connection withcircuits that comply with PELV requirements. PELV is alsocomplied with in relation to relay terminals, so that theycan be connected to the mains potential.For further information, see chapter 5.1.1 Galvanic Isolation(PELV).1.4.7 Advanced Motor ProtectionThe frequency converter has integral electronic motorprotection. The frequency converter calculates the motortemperature on the basis of current, frequency and time.As opposed to traditional, bimetallic protection, electronicprotection takes account of reduced cooling at lowfrequencies because of reduced fan speed (motors withinternal fan). This function cannot protect the individualmotors when motors are connected in parallel. Thermalmotor protection can be compared to a protective motorswitch, CTI.See chapter 5.1.1 Galvanic Isolation (PELV) for furtherinformation.WARNINGIf motors are connected in parallel, individual motors stillhave the risk of overheating. To protect the frequencyconverter from overheating, install a thermistor andconnect it to the thermistor input (digital input) of thefrequency converter. See chapter 4.2.2 Termisk motorbe-skyttelse -parameter 128 for further information.1.5 Approvals and CertificationsThe frequency converter complies with UL508C thermalmemory retention requirements. For more information,refer to chapter 4.2.2 Termisk motorbeskyttelse -parameter128.Introduction to VLT 2800 Design GuideMG27E402 Danfoss A/S © Rev. May/2014 All rights reserved. 71 1