Danfoss VLT Decentral Drive FCD 302 Design Manual

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Danfoss VLT Decentral Drive FCD 302 Design Manual Manual pdf 46 page image

2.7.2.5 Control with Brake FunctionThe brake is protected against short-circuiting of the brakeresistor, and the brake transistor is monitored to ensurethat short-circuiting of the transistor is detected. A relay/digital output can be used for protecting the brake resistoragainst overloading in connection with a fault in thefrequency converter.In addition, the brake makes it possible to readout themomentary power and the mean power for the latest120 s. The brake can also monitor the energizing powerand make sure that it does not exceed a limit selected inparameter 2-12 Brake Power Limit (kW). Inparameter 2-13 Brake Power Monitoring, select the functionto carry out when the power transmitted to the brakeresistor exceeds the limit set in parameter 2-12 Brake PowerLimit (kW).NOTICEMonitoring the brake power is not a safety function; athermal switch is required for that purpose. The brakeresistor circuit is not ground leakage protected.Overvoltage control (OVC) (exclusive brake resistor) can beselected as an alternative brake function inparameter 2-17 Over-voltage Control. This function is activefor all units. The function ensures that a trip can beavoided if the DC-link voltage increases. This is done byincreasing the output frequency to limit the voltage fromthe DC link. It is a very useful function to avoidunnecessary tripping of the frequency converter, forexample when the ramp-down time is too short. In thissituation, the ramp-down time is extended.NOTICEOVC cannot be activated when running a PM motor(when parameter 1-10 Motor Construction is set to [1] PMnon-salient SPM).2.7.2.6 Brake Resistor CablingEMC (twisted cable/shielding)To reduce the electrical noise from the wires between thebrake resistor and the frequency converter, the wires mustbe twisted.For enhanced EMC performance, use a metal shield.2.8 Safe Torque OffTo run STO, additional wiring for the frequency converter isrequired. Refer to VLT® Frequency Converters Safe Torque OffOperating Guide for further information.2.9 EMC2.9.1 General Aspects of EMC EmissionsBurst transient is usually conducted at frequencies in therange 150 kHz to 30 MHz. Airborne interference from thefrequency converter system in the range 30 MHz to 1 GHzis generated from the inverter, motor cable, and the motor.Capacitive currents in the motor cable coupled with a highdU/dt from the motor voltage generate leakage currents.The use of a shielded motor cable increases the leakagecurrent (see Illustration 2.38) because shielded cables havehigher capacitance to ground than unshielded cables. Ifthe leakage current is not filtered, it causes greaterinterference on the mains in the radio frequency rangebelow approximately 5 MHz. Since the leakage current (I1)is carried back to the unit through the shield (I3), there isonly a small electro-magnetic field (I4) from the shieldedmotor cable.The shield reduces the radiated interference but increasesthe low-frequency interference on the mains. Connect themotor cable shield to the frequency converter and motorenclosures. Use integrated shield clamps to avoid twistedshield ends (pigtails). Twisted shield ends increase theshield impedance at higher frequencies, which reduces theshield effect and increases the leakage current (I4).When a shielded cable is used for fieldbus relay, controlcable, signal interface, or brake, ensure that the shield ismounted on the enclosure at both ends. In somesituations, however, it is necessary to break the shield toavoid current loops.Product Overview and Functi... VLT® Decentral Drive FCD 30244 Danfoss A/S © 05/2018 All rights reserved. MG04H30222