Danfoss VLT FCD 311 Design Manual

Also see for VLT FCD 303: Operating instructions

Contents

Danfoss VLT FCD 311 Design Manual Manual pdf 119 page image

VLT® FCD SeriesEarth faultThe IGBT module cuts out within 100 s in case ofan earth fault on one of the motor terminals U, V, W(96, 97, 98), although depending on impedance andmotor frequency.Output connectionMotor terminals U, V, W (96, 97, 98) for the motorcan be connected/disconnected as often as required.There is no way that the frequency converter can bedestroyed by connecting/disconnecting the motor ter-minals. However, fault messages may appear.Motor-generatedovervoltageThe voltage in the intermediate circuit is increasedwhen the motor acts as a generator. To protect thefrequency converter the IGBT module is discon-nected when a specific voltage level is reached.Motor-generated overvoltage can occur in two in-stances:1. The load drives the motor, i.e. the load generatesenergy.2. During deceleration (ramp-down) if the moment ofinertia is high, the load is low and the ramp-downtime too short for the energy to be dissipated asa loss in the frequency converter, the motor andthe unit. The control unit attempts to correct theramp if possible.The fault can be removed by connecting a brake re-sistor, if the frequency converter has an integratedbrake module. If the frequency converter does nothave an integrated brake module an AC brake canbe used, see parameter 400 Brake function.See the section entitled Brake resistors.Static overloadingWhen the frequency converter is overloaded (currentlimit in parameter 221 Current limit I LIM is reached),the control reduces the output frequency in anattempt to reduce the load. If the overloading is ex-treme, there might be an output current that causesthe frequency converter to trip after approx. 1.5 sec.See parameter 409 Trip delay overcurrent, ILIM.An extreme overload will cause the switching fre-quency to be derated to 3000 Hz. dU/dt on motorWhen a transistor in the inverter is opened, the volt-age across the motor terminals will rise by a voltage/time ratio (dU/dt) determined by:- the motor cable (type, cross-section, induction,capacity, length and screened/armoured/un-screened/unarmoured)- the mains voltageSelf-induction in the motor cable leads to an over-swing U PEAK of the output voltage each time atransistor in the inverter is opened. After U PEAK theoutput voltage will stabilise at a level determined bythe voltage in the intermediate circuit. U PEAK anddU/dt influence the lifetime of the motor, especiallymotors without phase insulation paper in the coils. Ifthe motor cable is short (a few metres), the over-shoot U PEAK is low, while the dU/dt is high. If themotor cable length is increased, U PEAK is increased,while dU/dt decreases. Switching on the inputThe waiting time between switching the mains volt-age on terminals 91, 92 and 93 must be min. 30 sec. Acoustic noiseThe acoustic noise from the frequency convertercomes from two sources:1. DC intermediate circuit coils.2. Inverter.Below are the typical values measured at a distanceof 1 m from the unit at full load:FCD 303-330 3 x 400 V: 52 dB(A). Derating for ambient temperatureThe ambient temperature (T AMB,MAX) is the maxi-mum temperature allowed. The average (T AMB,AVG )measured over 24 hours, must be at least 5 Clower. If the frequency converter operates at temper-atures above 40 C, a derating of the rated outputcurrent is necessary . Temperature-dependent switch frequencyThis function ensures the highest possible switch fre-quency without the frequency converter becomingthermally overloaded. The internal temperature isthe actual expression of the degree to which theMG.04.A1.02 - VLT is a registered Danfoss trade mark 119All about FCD 300