Mitsubishi HG-KN73BJ-S100 Retrofit AC Servo for MELSERVO-J4

Mitsubishi HG-KN73BJ-S100 Retrofit-Ready AC Servo Motor for MELSERVO-J4 Control Systems

The Mitsubishi Electric HG-KN73BJ-S100 is a 750W AC servo motor engineered for seamless compatibility with the MELSERVO-J4 series drive platform. As legacy servo systems age and original components reach end-of-life, the HG-KN73BJ-S100 has become a critical retrofit solution for engineers tasked with modernizing aging production lines, upgrading control cabinets, and restoring discontinued servo axes without full system replacement. Whether you are replacing a failed unit on a running line or executing a planned migration from an older J2S or J3 servo system, this motor delivers the performance, encoder resolution, and drive compatibility required for a smooth, low-risk transition.

Rated at 750W continuous output with a 17-bit absolute encoder, the HG-KN73BJ-S100 integrates directly with the MR-J4-70B and MR-J4-70A servo amplifiers via the standard CN2 encoder connector. The motor’s compact flange dimensions and shaft specifications are consistent with the HG-KN series mechanical envelope, allowing direct mechanical substitution in most existing servo mounts without bracket modification. For retrofit projects involving older MELSERVO-J2S or J3 axes, engineers should verify shaft diameter, key slot orientation, and flange bolt pattern against the legacy motor datasheet before installation.

Upgrade Compatibility Table

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Retrofit Planning for Existing Automation Systems

Successful integration of the HG-KN73BJ-S100 into an existing control system begins well before the motor arrives on site. Retrofit planning must account for the full servo axis architecture: the MR-J4-70B amplifier, the MR-J4W2-22B or similar multi-axis unit if applicable, the Q172DSCPU or Q173DSCPU motion controller, and the SSCNET III/H fiber optic network linking each axis. Engineers should document the existing parameter file from MR Configurator2 before any hardware swap, preserving gain settings, electronic gear ratios, home position offsets, and software travel limits.

Power supply capacity is a frequent oversight in retrofit projects. The 750W motor draws peak current during acceleration that can stress an undersized MR-J4-DU075B power regeneration unit or shared DC bus. Confirm that the existing MR-BT-6V1SET battery backup for absolute encoder retention is installed and charged before powering down the axis — loss of absolute position data will require a full home return sequence that may not be feasible on a running production line.

Terminal wiring adaptation is often required when replacing older HG-KN73J units with the HG-KN73BJ-S100. The encoder cable assembly — typically a MR-EKCBL□M-H or equivalent — must be inspected for pin compatibility with the 20-pin CN2 connector. Power wiring to the motor’s MS connector should be verified against the amplifier’s U, V, W, and PE terminal assignments. Any discrepancy in phase sequence will cause the motor to run in reverse or trigger an overcurrent fault at startup.

For systems migrating from MELSERVO-J3 to J4, the MR-J3-J4-DU conversion unit is not applicable to this motor class; a full amplifier replacement to MR-J4-70B is required. The motion program stored in the Q172DSCPU controller typically requires no modification if axis numbering and SSCNET station addresses are preserved. However, HMI screens built on GOT2000 series terminals (GT2710-VTBA or similar) should be reviewed for any servo status display elements that reference J3-specific alarm codes, as J4 alarm structures differ in numbering and description.

I/O expansion modules such as the QX41 digital input or QY41P digital output units connected to the main Q-series CPU rack are unaffected by the servo motor swap, but any interlock logic tied to servo ready signals, torque limit outputs, or in-position flags should be verified in the ladder program before resuming automatic operation. Communication links via CC-Link IE Field or MELSEC iQ-R network modules should remain stable across the motor replacement, provided the amplifier station address is unchanged.

Downtime Control During System Migration

Minimizing unplanned downtime during a servo motor replacement requires a structured pre-swap checklist and a clear rollback plan. Before removing the HG-KN73BJ-S100’s predecessor, capture a full parameter backup from the MR-J4 amplifier using MR Configurator2 and save the Q-series motion program to a CF card or USB memory. Label all encoder and power connectors with axis identification tags to prevent cross-connection errors during reinstallation.

The physical swap of the motor — disconnecting the MS power connector, releasing the encoder cable, unbolting the flange, and remounting the new unit — typically takes 20 to 40 minutes for an experienced technician on an accessible servo mount. The more time-critical steps are the post-installation checks: verifying absolute encoder battery voltage, confirming the encoder cable seating, restoring the parameter file, and executing a low-speed JOG run before returning the axis to automatic mode.

To protect original program logic, do not modify any motion parameters during the swap unless a gain re-tuning is explicitly planned. The HG-KN73BJ-S100’s rotor inertia is consistent with the standard HG-KN73 series, so existing gain settings from the predecessor motor will typically produce stable operation without re-tuning. If vibration or hunting is observed after the swap, use MR Configurator2’s one-touch tuning function to re-optimize gains with the actual load attached, rather than manually adjusting parameters under time pressure.

For critical production axes where any downtime is unacceptable, consider staging the replacement during a scheduled maintenance window and pre-wiring the new motor to a spare amplifier for bench verification before the live swap. Pre-shipment functional testing performed on every HG-KN73BJ-S100 unit shipped from our facility confirms encoder communication, winding resistance balance, and no-load rotation — reducing the risk of a DOA unit extending your downtime window.

Retrofit Support FAQ

Q: Is the HG-KN73BJ-S100 a direct drop-in replacement for the HG-KN73J?
A: Yes, with one important distinction: the -S100 suffix indicates the addition of an oil seal on the output shaft, providing protection against lubricant ingress in splash or vertical-mount environments. The electrical interface, encoder type, flange dimensions, and amplifier compatibility are identical to the standard HG-KN73J. If your application does not require the oil seal, both variants are functionally interchangeable with the MR-J4-70B amplifier.

Q: What commissioning steps are required after installing this motor?
A: After mechanical installation and cable connection, restore the parameter file from your MR Configurator2 backup. Power on the amplifier and confirm no encoder communication alarms (AL.16 or AL.20). If the absolute position is lost, perform a home return sequence per your machine’s homing routine. Execute a low-speed JOG run in both directions to confirm correct rotation direction and no mechanical interference. Finally, run one full automatic cycle at reduced speed before returning to production rate.

Q: Can this motor be used with a MELSERVO-J3 amplifier?
A: No. The HG-KN73BJ-S100 uses a 17-bit absolute encoder with a CN2 connector pinout designed for the MR-J4 amplifier series. It is not electrically compatible with MR-J3 amplifiers. If you are migrating from a J3 system, the amplifier must also be upgraded to MR-J4-70B or MR-J4-70A. Contact our technical team for a complete J3-to-J4 migration component list.

Q: What does the 12-month warranty cover, and is pre-shipment testing performed?
A: Every HG-KN73BJ-S100 unit undergoes pre-shipment functional testing covering encoder signal integrity, winding insulation resistance, and no-load rotation verification. The 12-month warranty covers manufacturing defects in materials and workmanship from the date of shipment. It does not cover damage resulting from incorrect wiring, parameter misconfiguration, mechanical overload, or environmental exposure beyond the motor’s IP rating. Warranty claims are supported with a replacement-first policy to minimize your downtime.

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