BRN120T5A-3S

BRN120T5A-3S in the aircraft AC generator system

BRN120T5A-3S is an aircraft voltage regulator unit used in three phase alternating current power generation systems. It works in the generator control channel and keeps the generator phase voltage within the required limits during continuous operation. The unit is not a generator and does not distribute power by itself. It controls the regulation process, mainly through the generator excitation circuit and voltage feedback line. In aircraft documentation, this regulator is usually listed together with the AC generator, mounting frame, damping frame, generator control wiring, protection units and the aircraft power distribution system.

Voltage regulation function of BRN120T5A-3S

The main task of BRN120T5A-3S is automatic stabilization of generator voltage when load or generator speed changes. The regulator receives power from the DC supply and also works with three phase AC inputs at aviation frequencies. In operation it maintains the rated phase voltage used by onboard consumers. The unit has limited static regulation error and allows voltage adjustment with a trimming resistor. This is important during maintenance, because the regulated voltage has to be checked not only at the regulator itself, but also at the generator output and aircraft bus under the conditions specified in the maintenance manual.

Internal layout of the BRN120T5A-3S unit

The regulator is built as a removable cassette installed on a mounting frame. In typical installations it may be used with frames such as RM-355G or similar aircraft mounting and damping assemblies. The internal circuit includes a measuring section that senses voltage deviation, a phase shifting section, pulse forming circuits and an output stage that controls generator excitation. The unit also includes generator excitation relay functions. On the front side, a trimming resistor is used for fine voltage setting. This layout makes the unit part of the active generator control system rather than a passive monitoring device.

BRN120T5A-3S application with aircraft generators

BRN120T5A-3S is used in aircraft electrical systems with three phase AC generators. It is associated with generator systems such as the GN30NZh412 brushless AC generator and is found in helicopter electrical equipment, including Mi-8 and Mi-17 family applications, such as Mi-171E configurations. The exact installation depends on the aircraft wiring diagram and approved parts catalog. Some systems use separate channel operation, so the installed regulator must be checked by designation, connector layout and aircraft documentation. A similar regulator from another generator channel may require confirmation before installation.

Installation and checking of BRN120T5A-3S

During troubleshooting, BRN120T5A-3S should be checked together with the generator, excitation circuit, AC bus voltage, DC supply line, connectors, grounding points and related protection equipment. Voltage instability is not always caused by the regulator alone. Generator condition, poor contact in the wiring or incorrect adjustment can give similar symptoms. Before ordering a replacement, the marking on the unit should be compared with the removed part, aircraft logbook and manufacturer documentation. The product is also known under alternative designations BRN120T5A-3S and БРН120Т5А-3С.

Product Type: Voltage regulator unit
Function: Regulation and stabilization of three phase generator voltage
Application / Where Used: Aircraft electrical systems with three phase generators
System / Installation Area: Aircraft three phase AC power generation and distribution systems
Compatible Aircraft / Equipment: Aircraft generator systems using BRN120T5A-3S regulator configuration; GN30NZh412 brushless AC generator applications; Mi-8 / Mi-17 family helicopter electrical systems, including Mi-171E configurations
Operating Voltage: 24 to 29.7 V DC
AC Supply Voltage: 22 to 29 V per phase, three phase, 800 Hz
Rated Adjustable Voltage: 117 V per phase, three phase, 400 Hz
Input Current, DC: Not more than 0.5 A
Input Current, AC 400 Hz: Not more than 0.04 A
Input Current, AC 800 Hz: Not more than 1.3 A
Power Losses: Not more than 20 W
Static Voltage Regulation Error: Not more than ±2 V
Voltage Adjustment Range: Phase voltage variation +5 V with trimming resistor −4 V
Operating Temperature Range: −60 °C to +60 °C
Operating Mode: Continuous
Weight: Not more than 1.9 kg
Construction Type: Removable cassette for installation on aircraft mounting / damping frame
Condition: As applicable
Technical Documentation: Manufacturer passport and technical documentation available upon request

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