Mitsubishi MR-J3 to MR-J4 Replacement: Can You Keep the Existing Motor and Cables?

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Yes—some MR-J3 motors can remain in service with an MR-J4 amplifier, and some existing cables can be reused. The decision depends on the exact motor, amplifier model, amplifier software version and cable arrangement. Matching the power rating is not enough.

Mitsubishi’s transition handbook includes supported combinations for driving selected HF, HC and HA motors with MR-J4 amplifiers. It also gives software-version requirements, so “MR-J4 compatible” is an incomplete purchasing specification. Mitsubishi Electric transition handbook, Part 8, Section 5

For a machine with difficult cable routing or a motor buried inside a mechanism, retaining those parts can make the replacement much less disruptive. Start by establishing what can stay before ordering what must change.

Check the motor against the amplifier’s software version

These selected 200 V-class combinations illustrate why the software version belongs on the enquiry:

Existing motor	MR-J4-A amplifier and minimum software	MR-J4-B amplifier and minimum software
HF-KP43	MR-J4-40A — A8	MR-J4-40B — A8
HF-SP102	MR-J4-100A — A8	MR-J4-100B — A8
HF-JP53	MR-J4-60A — A8	MR-J4-60B — D4

The listed B-model requirements apply in both J4 mode and J3 compatibility mode. These are selected combinations, not a complete replacement table; they do not cover MR-J4-B-RJ020.

The HF-JP53 example is useful: its A-model and B-model requirements differ. An enquiry specifying only the amplifier’s part number could miss that distinction.

Ask the supplier to confirm the software version of the actual unit offered. Keep that confirmation with the quotation. If the motor has a brake, gearbox or special suffix, provide the complete nameplate reference rather than shortening it to the base model.

For an unlisted combination, obtain confirmation from Mitsubishi or the machine’s responsible engineer. A similar connector or matching rated output does not establish compatibility.

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J3 compatibility mode answers a different question

J3 compatibility mode allows supported MR-J4-B-family amplifiers to operate with an existing SSCNET III controller. Mitsubishi also describes systems containing both MR-J3 and MR-J4 amplifiers, subject to the mode’s functional limitations. MELSERVO-J4 integration guidance

That addresses the controller connection. Motor support still requires its own check.

On the replacement worksheet, keep two separate entries:

Controller compatibility: controller model, network and intended operating mode.
Motor compatibility: complete motor reference, proposed amplifier and required software version.

Separating them prevents a statement such as “it works in J3 mode” from being mistaken for approval of the entire axis.

There is another detail when sourcing a previously configured unit. Mitsubishi states that an MR-J4(W)-B operating in J3 compatibility mode needs the mode-selection application when changing to J4 operation with an SSCNET III/H controller. Establish its configuration history before commissioning.

Review each cable separately

“Keep the existing cables” should become a list of individual decisions. A cable assembly may be reusable even where its amplifier-end connection needs attention.

Mitsubishi’s cable comparison lists reusable encoder, motor-power, brake and SSCNET cable options. It also identifies amplifier power connectors that must be replaced with the appropriate supplied connectors.

Use the following as a practical inspection record:

Cable or connection	Information to record before approving reuse
Motor power	Cable reference, conductor size, motor-end connection and amplifier-end termination
Encoder	Complete cable reference, both connector ends, junctions and any thermal-sensor wiring
Electromagnetic brake	Brake-equipped motor reference, cable arrangement and brake circuit
SSCNET optical cable	Cable reference, route, length and connector condition
Control wiring	Existing signal assignments and the replacement’s corresponding connections

Photograph both ends before dismantling. Where a cable reference is missing, record that as an unresolved item rather than identifying the assembly by appearance.

Compatibility and condition are separate questions. A correctly specified cable with a damaged jacket, loose connector or uncertain repair history still needs assessment. Conversely, a sound cable should not be replaced merely because the amplifier series has changed.

Replacing the motor can change the cable decision

Mitsubishi describes shared power, encoder and brake cable options between several older motor families and their HG replacements. However, its guidance specifically identifies different encoder cables for 11–55 kW HG-JR motors compared with HF-JP motors. Check the exact motor pair and applicable cable table. Mitsubishi motor and cable compatibility notes

This matters when a project changes scope. Approval to retain a cable with the existing motor should be revisited if the order later includes a new motor.

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Make the quotation describe the intended repair

A useful request gives the supplier enough information to distinguish an amplifier-only replacement from an axis upgrade.

Include:

Photographs of the complete amplifier and motor nameplates.
Controller model and current control interface.
Cable references and photographs of both ends.
Whether the motor, brake, gearbox and existing wiring must remain.
Available parameter backups and the proposed replacement’s software requirement.

Ask for the reply to identify the offered amplifier, its software version and any required connectors or accessories. “Equivalent replacement” leaves too much room for different interpretations.

For a parts enquiry, send this information through Kwoco’s contact page. State explicitly that you intend to retain the existing motor and cables so the quotation can be reviewed against that requirement.

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Frequently Asked Questions

Will a retained motor gain 22-bit feedback just because the amplifier is MR-J4?

No. Installing a different amplifier does not change the encoder fitted to the motor. Operating mode also matters: Mitsubishi specifies 18-bit resolution for an HG motor with MR-J4-B in J3 compatibility mode, compared with 22-bit in J4 mode.

If the motor runs normally, is retaining it always the better choice?

Not necessarily. Consider its condition, the work needed to access it later and the machine’s remaining service requirements. Retaining a verified, serviceable motor may suit an urgent repair; a planned shutdown may justify reviewing the whole axis. Make that decision before purchasing, so the cable and commissioning scope stays clear.

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Leave time for configuration and acceptance checks

Mitsubishi provides parameter-conversion tools: MR Configurator2 for applicable MR-J3-A conversions, and MT Works2 support for applicable B-series migration. Conversion helps transfer settings, but the finished machine still needs verification.

Keep the original backup unchanged. Save the replacement configuration separately and document any adjustments.

Electrical work requires qualified personnel following the applicable isolation, discharge and voltage-verification procedures. Mitsubishi also cautions that the motor’s electromagnetic brake is intended for holding, not ordinary stopping.

Before the shutdown, agree on an acceptance record covering direction, commanded travel, home position, operating limits, brake operation and machine stop functions. Include the production conditions under which the repair will be accepted.

After commissioning, archive the verified amplifier software version, final settings and retained cable references. That gives the next maintenance team a documented combination to work from.

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Mitsubishi MR-J3 to MR-J4 Replacement: Can You Keep the Existing Motor and Cables?

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