Danfoss VLT Micro Drive FC 51, 0.75-2.2 kW, 3 x 380-480 V

Overview

The Danfoss FC 51, sold as the VLT® Micro Drive FC 51, is a compact frequency converter from Danfoss for controlling three-phase induction motors from 0.18 to 22 kW. This page covers the 0.75, 1.5 and 2.2 kW sizes for three-phase 380 to 480 V supplies, the ratings most often found on small pumps, fans and conveyors on 400 and 415 V networks. The FC 51 earned a wide installed base thanks to its small footprint, simple local control panel and complete I/O, and many machines and building services packages in the region still run on it. Danfoss now lists the FC 51 among its legacy VLT drives and offers the iC2-Micro as its successor, so this page also helps engineers maintaining existing equipment.

Seven Star LLC supplies the Danfoss FC 51, its accessories and successor drives to maintenance teams, OEMs and contractors in Muscat, across the Sultanate of Oman and the GCC. We help identify the correct order number from the label of an installed unit, advise on direct replacement or migration, and provide engineering support in Muscat.

Operating Principle

As general engineering background, a frequency converter rectifies the AC supply into a DC intermediate circuit and then switches that DC voltage through an inverter bridge to create a three-phase output of variable voltage and frequency. Motor speed follows the output frequency, so pumps and fans can be run at the speed the process requires, which saves energy compared with throttling valves or dampers. The Danfoss FC 51 output voltage ranges from 0 to 100% of the supply voltage, with an output frequency of 0 to 200 Hz in VVC+ mode or 0 to 400 Hz in U/f mode, and ramp times from 0.05 to 3600 s.

The Danfoss design guide lists the control functions built into the Danfoss FC 51. A PI controller maintains the desired speed, pressure or temperature by adjusting the output frequency to match the varying load, so a pressure transmitter can be wired directly to the drive for constant pressure pumping. Automatic Motor Tuning determines the electrical parameters of the connected motor at standstill, slip compensation keeps speed almost constant as load changes, an electronic thermal relay estimates motor temperature, and Smart Logic Control executes user-defined actions when user-defined events become true. Two parameter set-ups can be stored and switched.

Engineering Design of the Danfoss FC 51

In the 380 to 480 V range the 0.75 kW Danfoss FC 51 is built in frame M1, measuring 150 mm high (205 mm including the de-coupling plate), 70 mm wide and 148 mm deep, with a weight of 1.1 kg. The 1.5 and 2.2 kW sizes use frame M2 at 176 mm high (230 mm with de-coupling plate), 75 mm wide, 168 mm deep and 1.6 kg. The enclosure is IP20, with IP21 and NEMA Type 1 kits available, and IP20 units can be mounted side by side with 100 mm clearance above and below for cooling. The type code allows versions with or without an RFI filter (class A1/B), with a brake chopper from 1.5 kW upward, with or without the numerical local control panel or the panel with potentiometer, and with coated PCBs.

The control card of each Danfoss FC 51 drive provides five programmable digital inputs (PNP or NPN, 0 to 24 V DC, with terminal 33 usable as a pulse input up to 5 kHz), two analog inputs (terminal 53 for 0 to 10 V or 0/4 to 20 mA, terminal 60 for 0/4 to 20 mA), one programmable 0/4 to 20 mA analog output, one programmable form C relay rated 250 V AC, 2 A resistive, a 24 V DC supply and a 10 V DC reference. RS-485 serial communication on terminals 68 and 69 supports the Danfoss FC protocol and Modbus RTU. All control circuits are galvanically isolated from the supply voltage (PELV). Built-in protections include electronic thermal motor protection, heatsink temperature monitoring, protection against short circuits and earth faults on the motor terminals, and supervision of missing motor or mains phases and DC link voltage.

Key Features

Key features of the Danfoss FC 51 in the 0.75 to 2.2 kW, 380 to 480 V sizes, from the Danfoss design guide, are:

• Normal overload capability of 150% for 1 minute.
• Efficiency of 96.0 to 97.2% typical (up to 98.0% best case) at rated load.
• Built-in PI controller, Automatic Motor Tuning, slip compensation and Smart Logic Control.
• RS-485 with Danfoss FC protocol and Modbus RTU as standard.
• Compact M1 and M2 frames, 70 and 75 mm wide, for side-by-side mounting.
• Optional RFI filter class A1/B and brake chopper from 1.5 kW.
• Coated PCB option for aggressive environments (class 3C3).
• Galvanically isolated control I/O with two parameter set-ups.

Industrial Applications

The VLT Micro Drive FC 51 was designed for simple speed control tasks in machine building, building services and light industry. Typical uses of the 0.75 to 2.2 kW Danfoss FC 51 sizes are booster and circulation pumps with PI pressure control, supply and extract fans, cooling tower and condenser fans, conveyors and roller tables, mixers and agitators, packaging and food machines, and small compressors. The constant torque characteristic suits conveyors and displacement pumps, while variable torque operation on centrifugal pumps and fans brings the largest energy savings. Because so many Danfoss FC 51 units are installed, spare drives are often needed to keep existing machines running without rewiring.

In Oman and the GCC the Danfoss FC 51 is installed in HVAC systems, water pumping stations, irrigation, food processing plants, workshops and utility packages in Muscat, Sohar, Nizwa and Salalah. High ambient temperatures in the region require attention to derating: the design guide states a maximum ambient of 40 °C at full output, with operation at 50 °C possible using one motor size smaller than nominal. Seven Star LLC supplies the Danfoss FC 51 and can also offer a migration path to newer Danfoss drives.

See the Danfoss iC2-Micro for the successor model.

Technical Specifications

The following Danfoss FC 51 data for 3 x 380 to 480 V supply come from Danfoss Design Guide MG02K202; see the guide for fuses, derating and installation details.

How to Order

To order the Danfoss FC 51, send Seven Star LLC the order number or full type code from the drive label, or the motor power and supply voltage, and we will prepare a formal quotation and advise whether a direct replacement or a successor drive is the better choice. The Danfoss design guide is available as a download on this page. Seven Star LLC supplies Danfoss drives in Oman and the GCC with engineering support in Muscat. For larger or application-specific Danfoss drives see Danfoss VLT AutomationDrive FC 302 Variable Frequency Drive, Danfoss VLT HVAC Drive FC 102 Variable Frequency Drive and Danfoss VLT AQUA Drive FC 202 Variable Frequency Drive.