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AC Variable Speed Drive – Control Techniques UNI3401
The UNI3401 is an AC variable speed drive from the Unidrive Classic series by Control Techniques. It is a medium-frame (frame 3) three-phase inverter for 400 V AC supply, designed to control induction and permanent-magnet synchronous motors in CNC machine tools, industrial automation, and motion control applications. The drive supports five operating modes: open-loop V/Hz, open-loop vector, closed-loop vector (with encoder feedback), servo (brushless PM motors), and regenerative. It is not a CPU board or standalone I/O module, but a fully integrated power conversion and digital regulation drive. Option expansion slots are provided for additional I/O, fieldbus modules, and supplementary encoder interfaces.
The Unidrive drive is the core of the axis and spindle motion system in a CNC machine: it receives the speed or torque reference from the numerical control and continuously adjusts output frequency and voltage to drive the motor with high accuracy. A drive failure immediately stops the controlled axis or spindle, rendering the machine unusable; replacing or repairing the drive becomes the critical path to restoring production.
Control Techniques is a British manufacturer of drives and servo systems founded in 1968 in Newtown (Wales, UK), acquired by Emerson Electric in 1994 and absorbed into the Nidec Corporation group in 2017. The Unidrive Classic series is legacy / discontinued: superseded by the Unidrive SP series and subsequently by Unidrive M, it is now available only as used, surplus, or through specialist repair centres.
Main Technical Specifications
| Type | AC variable speed drive – Unidrive Classic series, frame 3 |
| Part number | UNI3401 |
| Brand | Control Techniques (Nidec group – formerly Emerson Electric) |
| Known application | CNC machine tools, industrial automation, motion control systems (1990s–2000s) |
| Function | Speed and torque control of AC induction and PM synchronous motors |
| Input voltage | 380–480 V AC three-phase ±10%, 50/60 Hz |
| Rated output current | ~25–40 A |
| Motor power | ~11–18 kW at 400 V |
| Operating modes | Open-loop V/Hz, open/closed-loop vector (encoder), servo (PM), regenerative |
| Interfaces / connectors | RS-485 (CT protocol), option expansion slots (I/O, fieldbus, encoder), control terminal strips |
| Protection rating | IP20 (panel mounting) |
| Weight | ~10–18 kg |
| Dimensions | ~250 × 500 × 220 mm W×H×D |
| Production status | Legacy / discontinued — used/surplus/repair only |