MR-J4-A load monitoring: effective, peak and regenerative ratios are not interchangeable

A servo’s “load percentage” can describe different physical quantities. A relatively low effective load ratio and a higher peak load ratio are not automatically contradictory. A high regenerative load ratio does not mean high motor output torque either. This article uses Mitsubishi Electric’s MR-J4-_A_(-RJ)/MR-J4-03A6(-RJ) Servo Amplifier Instruction Manual, SH(NA)030107ENG-V (November 2025), and the status display in §4.5.3. The diagnostic interpretation is limited to rotary servo motors. Do not transfer it directly to MR-J4-B, MR-J4W, MR-J5, positioning-mode-specific displays, or linear/direct drive motors.

1. Identify the symbol before comparing percentages

Display	Reference and timing defined by the manual	What it cannot replace
J: effective load ratio	Continuous effective load current; effective value over the preceding 15 seconds, with rated current as 100%	Neither an instantaneous peak nor a motor-temperature reading
b: peak load ratio	Maximum generated torque; highest value over the preceding 15 seconds, with rated current as 100% in the table	Neither the peak duration nor an all-time maximum retained since power-on
L: regenerative load ratio	Regenerative power as a percentage of permissible regenerative power	Not motor output load; this entry does not specify the same 15-second window as J and b
T: instantaneous torque	Torque generated in real time, with rated torque as 100%	Not the effective or peak value over the preceding 15 seconds

These definitions come from §4.5.3(3), printed page 4-27. In particular, do not silently rewrite b as “percentage of rated torque”: the table first describes maximum generated torque and then explicitly states a rated-current reference. Preserve the original display name, symbol and unit in maintenance records rather than converting the monitor value to an unverified shaft torque.

2. The 15-second window changes what a screenshot means

J and b look back over 15 seconds, whereas T reflects the current state. After an axis has stopped, b may therefore still reflect a peak before the stop. A later change does not establish that a fault has been removed. Conversely, a low reading collected afterward does not prove that load was low at an earlier abnormal event.

Before comparing records, check acquisition time, program phase, travel direction, load conditions and the interval covered. A brief peak and a sustained load affect the two indicators differently. However, J and b alone cannot reconstruct where the peak occurred or how long it lasted. Use existing speed, torque and alarm records on a common time base; do not deliberately reproduce an overload to collect data.

3. 100% is a reference, not a universal alarm criterion

Section 10.1 (10-1 to 10-2) relates AL.50 to the electronic thermal protection curve and AL.51 to sustained maximum current, and distinguishes protection curves by motor combination. Consequently, b exceeding 100% is not automatically AL.50, and J below 100% does not guarantee that operation may continue. The applicable combination, duration and operating conditions also matter.

Rated references do not replace permitted operating conditions. Section 2.6 (2-7) gives effective and regenerative load-ratio restrictions for altitudes above 1000 m and up to 2000 m, and also notes close-mounting conditions. The permissible value must match the actual model, installation and environment. This article sets no universal safe percentage and recommends no protection-parameter changes.

4. Route three common observations to the right evidence

Observation in existing records	Evidence to check next	What remains unproven
b rises markedly while J changes little	Acceleration/deceleration phase, speed and torque around the peak, with matching time windows	This alone proves neither mechanical seizure nor amplifier failure
J remains elevated under comparable conditions	Duty cycle, load conditions and complete alarms, matched to the motor’s protection curve	The value alone cannot identify a bearing, brake or power component as faulty
L rises without high J	Existing regenerative-configuration documents, duty cycle and deceleration records	Neither resistor failure nor the need for a higher-power resistor is established

This is an editorial method for organizing evidence, not a manufacturer’s field case. Regenerative selection in §11.2.2 (from 11-10) analyzes energy through the motion cycle and distinguishes rough horizontal-axis calculation from precise selection. Qualified personnel must base selection or replacement on the complete configuration and calculation, not infer resistor specifications from one L screenshot.

5. Make the handover usable for further diagnosis

Record full amplifier and motor models, control mode, display name/symbol/unit, acquisition time, comparable operating conditions, and full alarms including detail codes. Separate observations, interpretations supported by the manual, and unresolved questions. If readings come from different times or only post-stop screenshots remain, state that limitation rather than inventing test results.

This article interprets existing records available safely. It does not require live cabinet measurements, touching terminals, changing regenerative configuration or repeated reset-and-run attempts. A stopped axis or low displayed load is not proof of maintenance isolation. Qualified personnel must follow the machine’s approved isolation and residual-energy verification procedure for wiring, inspection or disassembly, together with Safety Instructions A-2.

Source: MR-J4-_A_(-RJ)/MR-J4-03A6(-RJ) Servo Amplifier Instruction Manual, SH(NA)030107ENG-V (November 2025, English original). §4.5.3(3), 4-27; §10.1, 10-1–10-2; §2.6, 2-7; §11.2.2, from 11-10; Safety Instructions A-2. All references use printed page numbers. The diagnostic routing and recording method are editorial interpretations, not a manufacturer-prescribed field test.