The MR-J3-350B is Mitsubishi Electric’s 3.5 kW 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 product from Mitsubishi’s third-generation MELSERVO platform providing 3.5 kW continuous servo control for HC-SFS352 and HC-RFS353 series servo motors in active multi-axis SSCNET III servo systems where direct maintenance replacement is required. The MR-J3-350B is available through Atlantech Drives — quote within 24-48 hours, worldwide express shipping, 12-month warranty.
What Is the MR-J3-350B?
The MR-J3-350B is a MELSERVO-J3 series servo amplifier providing 3.5 kW (3,500 W) continuous output power via SSCNET III optical fibre, compatible with Mitsubishi HC-SFS352, HC-RFS353, and equivalent 3.5 kW servo motors with the MR-J3’s 17-bit multi-turn absolute encoder. The MR-J3-350B occupies the same mid-range position in the MR-J3-B series as the MR-J4-350B in the current generation — filling the gap between 2 kW and 5 kW for applications where 2 kW is insufficient and 5 kW would be oversized. The housing width is 85 mm, consistent with the MR-J4-350B’s housing, making the MR-J3-350B and MR-J4-350B physically interchangeable in terms of panel mounting space — a practical consideration when planning an MR-J3-to-MR-J4 migration where existing panel cutouts and DIN rail positions are to be reused. The MR-J3-350B does not include a built-in STO function, relying on external safety contactors for safety compliance, and uses the SSCNET III 10 Mbps communication standard incompatible with current SSCNET III/H controllers.
Key Technical Specifications
- Model: MR-J3-350B
- Rated Output Power: 3.5 kW (3,500 W)
- Power Supply: 200–240 VAC single-phase or three-phase, 50/60 Hz
- Continuous Output Current: 17.0 A
- Maximum Output Current: 51.0 A (3× continuous peak)
- Compatible Servo Motors: HC-SFS352, HC-RFS353, HG-SR352 (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, external MR-RB series resistor connectable
- Communication: USB (MR Configurator2)
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 85 mm × 200 mm × 192 mm
- Weight: Approx. 2.8 kg
Architecture & Design Overview
The MR-J3-350B implements the MELSERVO-J3 three-loop digital servo control architecture — position loop, speed loop, and current loop at 2 kHz bandwidth — with the 3.5 kW power stage providing the higher continuous torque capability for medium-to-high load axis applications. The SSCNET III communication processor receives position command data from the Q172DSCPU or Q173DSCPU motion controller at each 0.888 ms cycle and feeds the position data to the position loop in synchronisation with the motion controller’s trajectory planner. The MR-J3-350B’s Advanced Vibration Suppression Control (first generation, AVSC) implements a single notch filter for mechanical resonance suppression, compared to the dual notch filter and automated resonance detection of the MR-J4-350B’s AVSC II — the single notch is sufficient for most applications with one dominant mechanical resonance frequency but may require manual frequency identification and tuning rather than the automated machine analyser function of the MR-J4 generation. The MR-J3-350B’s auto-tuning function (one-touch tuning via MR Configurator2) operates on the same principle as the MR-J4 auto-tuning — the amplifier performs a series of controlled acceleration cycles and identifies the load inertia ratio, setting the servo gains accordingly — but the J3 auto-tuning algorithm is less aggressive in its gain setting than the MR-J4, typically requiring more tuning iterations to reach the same servo bandwidth. The built-in regenerative resistor handles light-duty regenerative energy; external MR-RB series resistors are recommended for applications with frequent high-speed deceleration cycles where the built-in resistor capacity may be exceeded.
Troubleshooting Guide
For MR-J3-350B amplifiers generating AL.50 (overload) alarms in normally operating machines, the first diagnostic step is to verify the amplifier’s thermal condition using MR Configurator2’s diagnostic monitor — elevated operating temperature above 45°C suggests cooling fan degradation or inadequate panel ventilation. A second common cause of AL.50 in older MR-J3-350B units is DC bus capacitor degradation — aging capacitors with increased equivalent series resistance cause the DC bus voltage to sag more during motor acceleration, reducing the available torque headroom and causing the thermal model to accumulate load faster. If AL.50 occurs consistently during acceleration at loads within the 3.5 kW rating, DC bus capacitor degradation should be suspected and the amplifier should be serviced or replaced. For AL.E3 (SSCNET III communication error) alarms, inspect both the CN1A input fibre and the CN1B output fibre for physical damage and connector contamination — a single degraded fibre connection on an MR-J3-350B causes the amplifier to report AL.E3 and may also disrupt communication to all downstream amplifiers in the SSCNET chain if the CN1B output is affected. For AL.16 (encoder error) intermittent alarms without encoder cable damage, verify the encoder battery voltage in MR Configurator2 — a battery approaching depletion can cause sporadic AL.16 as the encoder’s internal memory retention becomes marginal.
Frequently Asked Questions
Q: Can the MR-J3-350B be directly replaced by the MR-J4-350B without replacing the servo motor?
A: Yes, provided the motion controller is also upgraded to SSCNET III/H simultaneously. The HC-SFS352 and HC-RFS353 servo motors used with the MR-J3-350B can continue operating with the MR-J4-350B in motor compatibility mode (PA22 = 1 in the MR-J4 parameter settings). The motor’s 17-bit encoder is read by the MR-J4-350B in J3 motor compatibility mode, providing the same position feedback as in the original J3 configuration while the amplifier benefits from the MR-J4’s improved control algorithms. Upgrading to the full 22-bit encoder resolution requires replacing the motor simultaneously with the amplifier.
Q: In a 5-axis machine with MR-J3-350B amplifiers on the primary axes, can the motion controller be upgraded to iQ-R without simultaneously replacing all amplifiers?
A: No. The MELSEC iQ-R RD77MS motion module uses SSCNET III/H protocol, which cannot communicate with MR-J3-350B amplifiers using SSCNET III. Migrating the motion controller to iQ-R requires replacing all MR-J3 amplifiers in the SSCNET chain with MR-J4 amplifiers simultaneously. The amplifier-first migration path (replacing amplifiers before the controller) is also not possible — MR-J4 amplifiers require a SSCNET III/H controller. The migration must be executed as a complete simultaneous replacement of controller and all amplifiers during a planned production shutdown.
Q: What is the MR-J3-350B’s equivalent model in Mitsubishi’s current servo product line?
A: The direct current-generation equivalent is the MR-J4-350B (3.5 kW, 200 V, SSCNET III/H). For facilities adopting the MELSEC iQ-R platform as part of their migration, the MR-J4-350B-RJ (3.5 kW with integrated safety sub-unit) or the equivalent iQ-R generation servo amplifier should be evaluated depending on the safety compliance requirements of the machinery directive assessment for the specific machine.
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