The MR-J3-40B is Mitsubishi Electric’s 400 W MELSERVO-J3 series servo amplifier for 200 V single-phase or three-phase AC power supply with SSCNET III optical fibre network interface — a legacy servo amplifier from Mitsubishi’s third-generation MELSERVO platform providing 400 W continuous servo control at 3.0 A for HC-KFS43 and HC-MFS43 series servo motors in active multi-axis SSCNET III servo systems managed by Q172DSCPU or Q173DSCPU motion controllers. The MR-J3-40B is available through Atlantech Drives — quote within 24-48 hours, worldwide express shipping, 12-month warranty.
What Is the MR-J3-40B?
The MR-J3-40B is a MELSERVO-J3 series servo amplifier providing 400 W (0.4 kW) continuous output power via SSCNET III optical fibre — the 400 W SSCNET III network-commanded variant in the MR-J3-B sub-series, compatible with Mitsubishi HC-KFS43, HC-MFS43, and equivalent 400 W servo motors with the MR-J3’s 17-bit multi-turn absolute encoder. The «B» suffix designates SSCNET III network command interface — the MR-J3-40B receives all position and speed commands exclusively via SSCNET III plastic optical fibre at 0.888 ms communication cycle from a Q172DSCPU or Q173DSCPU MELSEC-Q motion controller, with no pulse train input. As the SSCNET III counterpart to the MR-J3-40A pulse train variant, the MR-J3-40B provides tighter multi-axis synchronisation for 400 W servo axes in coordinated SSCNET III motion systems — robot wrist axes, assembly machine transfer axes, and precision inspection positioning axes where network-commanded servo response enables sub-millisecond position update rates. The MR-J3-40B shares the same 40 mm compact housing as the MR-J3-20B (200 W SSCNET III), maintaining dimensional consistency in multi-axis SSCNET III panels where 200 W and 400 W axes are installed side by side. Like all MR-J3-B amplifiers, the MR-J3-40B does not include a built-in STO safety function and is specified for direct maintenance replacement in active MR-J3 SSCNET III servo systems.
Key Technical Specifications
- Model: MR-J3-40B
- Rated Output Power: 400 W (0.4 kW)
- Power Supply: 200–240 VAC single-phase or three-phase, 50/60 Hz
- Continuous Output Current: 3.0 A
- Maximum Output Current: 9.0 A (3× continuous peak)
- Compatible Servo Motors: HC-KFS43, HC-MFS43, HG-KR43 (J4 motor, J3 compat. mode)
- Command Interface: SSCNET III optical fibre (10 Mbps)
- Communication Cycle: 0.888 ms
- Encoder Interface: Serial 17-bit absolute encoder (131,072 pulses/rev)
- Safety Function: None built-in (external safety contactor required)
- Regenerative Brake: Built-in
- Communication: USB (MR Configurator2), RS-422 (cascade)
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 40 mm × 150 mm × 138 mm
- Weight: Approx. 0.8 kg
Compatibility & System Integration
The MR-J3-40B operates exclusively on SSCNET III chains managed by MELSEC-Q motion controllers — Q172DSCPU (up to 32 axes), Q173DSCPU (up to 96 axes), Q170MCPU (up to 8 axes), and Q170MSCPU. The MR-J3-40B is compatible with all other MR-J3-B amplifiers of different power ratings (MR-J3-20B, MR-J3-100B, MR-J3-200B, etc.) on the same SSCNET III chain — all MR-J3-B variants use identical SSCNET III communication, enabling mixed 200 W and 400 W axes in the same coordinated motion system. MR-J4-B SSCNET III/H amplifiers cannot be mixed with MR-J3-40B on the same SSCNET chain — the two protocols are physically identical (same POF connector and cable) but electrically incompatible, and each chain must use a single protocol generation. For parameter configuration, MR Configurator2 connects via USB and provides full MR-J3-40B parameter access alongside MR-J4 amplifiers in the same software session — facilitating parameter management in facilities operating mixed J3/J4 servo generations. Current-generation HG-KR43 motors (MR-J4 standard, 22-bit encoder) operate with the MR-J3-40B in J3 motor compatibility mode (parameter PA22 = 1), enabling motor-only upgrade from HC-KFS43 to HG-KR43 while retaining the MR-J3-40B amplifier — the encoder cable must be replaced with an HG-KR-compatible cable as the HC-KFS43 and HG-KR43 encoder connector physical types differ. For Q172DSCPU motion systems where the MR-J3-40B must coexist with a Q02UCPU sequence CPU in a multi-CPU configuration, the MR-J3-40B receives motion commands from the Q172DSCPU’s SSCNET III output while the Q02UCPU manages machine sequencing — the two CPUs exchange data via the multi-CPU shared device memory without PLC instruction overhead.
Troubleshooting Guide
For MR-J3-40B amplifiers generating AL.E3 (SSCNET III communication error) after previously reliable operation, inspect the SSCNET III fibre cable connections at both CN1A (input from controller or previous amplifier) and CN1B (output to next amplifier) first — the plastic optical fibre push-pull connectors can partially disengage from vibration in installations near vibrating machine structures, causing intermittent communication loss that manifests as AL.E3 at random intervals correlated with machine vibration events. Press both fibre connectors firmly to confirmed engagement and verify the connector locking mechanism is fully activated. If AL.E3 persists after connector verification, substitute the SSCNET III cable between the MR-J3-40B and the preceding device (controller or previous amplifier) with a known-good cable — POF cable can develop internal micro-cracks in the fibre core at tight bend points that increase optical attenuation above the J3 receiver threshold without visible external cable damage. For MR-J3-40B amplifiers with AL.50 (overload) occurring at loads previously executed without alarm, verify whether the alarm occurs consistently at the same point in the motion cycle — AL.50 triggered at the same point each cycle suggests a consistent load peak (mechanical interference, friction increase, or inertia change) at that position rather than gradual degradation. Monitor the MR-J3-40B’s load ratio display in the motion controller’s servo monitor screen — if load ratio exceeds 90% at the problem point in the cycle, the mechanical system has changed (increased friction, additional load, mechanical binding) rather than the amplifier failing. A change in mechanical condition that increased friction by 10–15% can push a previously marginal servo axis across the overload trip threshold without any change in the amplifier itself.
Frequently Asked Questions
Q: Can the MR-J3-40B be replaced with an MR-J4-40B if the Q172DSCPU motion controller is also upgraded to SSCNET III/H simultaneously?
A: Yes. Upgrading from MR-J3-40B to MR-J4-40B requires simultaneous motion controller upgrade (Q172DSCPU SSCNET III → Q172DSCPU with SSCNET III/H firmware or RD77MS) and replacement of the SSCNET cables (same physical POF cable type — no cable change required as both J3 and J4 use standard POF). The MR-J4-40B provides 22-bit encoder resolution, built-in STO, AVSC II, and 0.444 ms minimum communication cycle — significant performance improvements over the MR-J3-40B. The HC-KFS43 motor can be retained during the amplifier upgrade using PA22 = 1 compatibility mode on the MR-J4-40B, or replaced simultaneously with an HG-KR43 for full 22-bit encoder benefit.
Q: In a SSCNET III chain with both MR-J3-20B (200 W) and MR-J3-40B (400 W) amplifiers, does the position in the chain affect communication performance?
A: No. In a SSCNET III chain, all amplifiers regardless of power rating or model receive and transmit the SSCNET III signal at the same 10 Mbps rate with the same 0.888 ms cycle time — there is no performance difference based on amplifier position in the chain or power rating. The Q172DSCPU motion controller assigns axis numbers sequentially based on physical chain order during system initialisation, and all axes receive position commands at the same communication cycle update rate.
Q: What is the expected spare unit sourcing lead time for MR-J3-40B in 2025-2026, and should spare units be stockpiled?
A: The MR-J3-40B is discontinued from production and available from aftermarket specialist distributors. For facilities with multiple MR-J3-40B units in critical production SSCNET III systems, maintaining at least one spare unit per five installed units is recommended — emergency aftermarket sourcing of a specific J3 servo amplifier model can require 2–8 weeks lead time from specialist distributors, and the production downtime cost of an unprotected critical axis failure typically exceeds the cost of a spare unit by a substantial margin.
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