Mitsubishi HC-MFS73 (HCMFS73) — 750W AC Servo Motor, Straight Shaft, No Brake, 3000 rpm, MELSERVO J2-Super Series

Product Overview

Part Number: HC-MFS73

Also Searched As: HCMFS73, HC MFS 73, HC-MFS-73

Series: Mitsubishi MELSERVO HC-MFS (J2-Super Generation)

Classification: Ultra-Low Inertia AC Brushless Servo Motor — 750 W, 200V class, 3000 rpm, Straight Shaft, No Brake

Starting Point: The Ultra-Low Inertia Philosophy

Most servo motors in the MELSERVO range — the HC-SF, HC-SFS, and HC-KFS families — are described as medium-inertia or low-inertia designs. The HC-MFS73 belongs to a different category entirely: ultra-low inertia. The distinction is not cosmetic. It is a deliberate design trade-off that defines where this motor belongs and why nothing else quite substitutes for it in the applications it serves.

Ultra-low inertia motors are built with rotors that are physically small in diameter and light in mass. This keeps the rotor's moment of inertia as low as physically achievable at a given power level. The consequence is that the motor itself contributes very little to the total inertia that the amplifier must accelerate and decelerate during each move. When the load inertia reflected to the motor shaft is also modest — which is typical in the compact, fast-cycling machines this series targets — the axis response is exceptionally fast.

At 750W and 3,000 rpm, the HC-MFS73 is the largest motor in the HC-MFS ultra-low inertia range. It pairs the J2-Super generation's 17-bit absolute encoder at 131,072 ppr with that ultra-low inertia rotor design — delivering the encoder precision of the premium HC-KFS and HC-SFS series in a motor optimised for high-frequency short-move duty cycles. Straight shaft, no brake, 80 × 80 mm flange, IP55 protection.

Technical Specifications

The Ultra-Low Inertia Advantage at 750W

There is a specific duty cycle that defines the market for ultra-low inertia servo motors, and understanding it explains why the HC-MFS73 exists at 750W rather than simply scaling up a smaller ultra-low inertia motor.

Imagine an axis that must complete hundreds of short positioning moves per minute. Each move is a few millimetres to a few tens of millimetres. The dwell time at each position is measured in fractions of a second. Cycle time is the machine's throughput constraint. Making each move faster — shorter acceleration ramp, quicker settling, shorter deceleration — directly increases the number of parts the machine produces per hour.

Two factors determine how fast an axis can complete a move at a given torque level: the total inertia being accelerated and decelerated, and the torque available to do it. Reducing rotor inertia reduces the total inertia the available torque must overcome. For a given peak torque — 7.2 Nm in the HC-MFS73's case — a lower inertia axis reaches target speed sooner and decelerates to rest sooner. The move takes less time. The machine cycles faster.

This is why ultra-low inertia motors command a design premium for high-cycle-rate applications. The 7.2 Nm peak from the HC-MFS73 is the same peak torque as the HC-KFS73 — a low-inertia motor at the same power level. The difference is the rotor inertia. Where the HC-KFS73's rotor contributes more to the total axis inertia, the HC-MFS73's ultra-low rotor inertia puts a larger fraction of that 7.2 Nm peak to work accelerating the load rather than the motor itself.

At 750W, this motor serves the applications that have outgrown the 400W HC-MFS43 — where the load is slightly heavier, the move slightly longer, or the sustained torque requirement slightly higher — but still runs on the same ultra-low inertia design philosophy.

Comparing HC-MFS73 and HC-MF73: Generation Matters

The HC-MFS73 has a direct predecessor in the first-generation MR-J2 platform: the HC-MF73. Both are 750W, 3000 rpm, ultra-low inertia motors on an 80 × 80 mm flange. Their torque figures are identical. But these are not interchangeable motors:

The encoder resolution jump — from the J2 generation's encoder to the 17-bit 131,072 ppr unit in the HC-MFS73 — is the most significant performance difference. That eight-fold resolution increase means finer velocity feedback, smoother operation at low speeds, better following error performance during rapid moves, and the absolute position capability that prevents homing routines on every startup.

The IP rating improvement from IP44 to IP55 means the HC-MFS73 resists directed water jets from any direction, not just splashes — a useful upgrade for machines in environments with any moisture exposure.

Amplifier compatibility is what makes the generation distinction non-negotiable: the HC-MF73 cannot connect to an MR-J2S-70 amplifier, and the HC-MFS73 cannot connect to a first-generation MR-J2-70. Match the motor to the amplifier generation on the machine being serviced.

No Brake: The Right Specification for High-Cycle Horizontal Axes

The HC-MFS73 has no electromagnetic brake. Position at rest is maintained by the MR-J2S-70 amplifier's servo lock — closed position loop, continuous encoder feedback at 131,072 counts per revolution, corrective current keeping zero following error at every dwell position between moves.

For the application category ultra-low inertia motors serve — horizontal assembly machine axes, pick-and-place mechanisms, XY positioning stages, and high-speed feeding and transfer mechanisms — servo lock is entirely adequate and the no-brake configuration is the natural choice. These axes are horizontal. The loads are light. No gravitational force acts along the shaft rotation direction at rest. Adding a brake on these axes would add a 24V DC circuit, a relay, surge suppression, MBR interlock logic, and periodic brake inspection — for no functional return whatsoever.

There is also a subtle dynamic argument for the no-brake variant on high-cycle axes. Every time a brake engages and releases, it adds latency to the start and end of each move while the MBR relay operates. On an axis completing hundreds of moves per minute, that latency across thousands of cycles per hour adds up. The no-brake motor eliminates this entirely — the axis moves as soon as the servo command arrives, and stops exactly when the trajectory commands it to.

The calculation changes on vertical or inclined axes where the load would move under gravity at servo-off. For those applications, the HC-MFS73B (straight shaft, spring-applied brake) is the correct motor. The spring-applied design holds the axis mechanically the moment 24V is removed from the coil — fail-safe behaviour that servo lock cannot provide. On a machine with a mix of horizontal and vertical ultra-low inertia axes, specifying the HC-MFS73 on horizontal ones and the HC-MFS73B on vertical ones produces the correct result on both.

17-Bit Encoder on an Ultra-Low Inertia Motor

High-cycle-rate ultra-low inertia axes might seem like the applications least likely to need 131,072 encoder counts per revolution. If the moves are short and fast, does the fine angular resolution really matter?

It matters in ways that are not immediately obvious from the basic description of the application.

Velocity feedback during rapid acceleration. At the moment of peak torque — 7.2 Nm driving the axis from rest to maximum speed in the shortest possible time — the shaft is accelerating rapidly. A coarser encoder delivers fewer position updates per unit time during this phase, making the velocity computation noisier and the torque control less precise exactly when precision matters most. The 17-bit encoder's fine count rate keeps the velocity feedback clean throughout the acceleration ramp, allowing higher bandwidth servo gain without instability.

Settling time at position. After the rapid move, the axis must settle to the commanded position quickly and stay there. Finer encoder resolution gives the position loop a more precise view of the shaft angle and tighter control over small position corrections during settling. The difference in settling time between a fine-resolution absolute encoder and a coarser incremental encoder is measurable on high-performance axes.

Position knowledge through power cycling. Assembly machines and handling equipment frequently cycle power between production shifts, during maintenance windows, and after any alarm stop. The 17-bit absolute encoder retains the exact shaft angle and multi-turn count through all power-off periods, backed by the A6BAT battery in the MR-J2S-70 amplifier. On a machine with many ultra-low inertia axes, eliminating the homing routine for all of them on every startup — rather than homing each one sequentially — saves real time and simplifies the startup sequence.

Battery location. The A6BAT is in the MR-J2S-70 amplifier, not in the motor. Replace it at the first low-battery alarm. Allowing full depletion resets the multi-turn counter and requires reference-return on the next startup.

Compatible Amplifiers

The HC-MFS73 requires the MR-J2S-70 amplifier — the 750W capacity J2-Super platform. Three interface variants:

MR-J2S-70A accepts pulse-train position commands from CNC controllers and PLCs, plus analog speed and torque references. All control modes are available. RS-232C connects to MR Configurator for commissioning and tuning. For high-cycle assembly machine axes, pick-and-place machine drives, and any application where the command source is an external controller, this is the standard choice.

MR-J2S-70B connects to Mitsubishi A-series and Q-series motion controllers via SSCNET fiber-optic serial bus. For machines where the ultra-low inertia axes must coordinate with other axes under a motion controller — a robot arm where all joints are servo-driven and must move in defined geometric relationships — the SSCNET bus provides the real-time axis coupling that pulse and analog interfaces cannot achieve.

MR-J2S-70CP provides built-in single-axis positioning with up to 31 stored point-table positions, activated by digital I/O or CC-Link command. For standalone indexed axes that do not require real-time coordination with other axes, the CP eliminates the cost of a dedicated motion controller.

Compatibility notes. The HC-MFS73 is not compatible with the first-generation MR-J2-70 amplifier, which cannot decode the 17-bit J2-Super encoder protocol. For machines running original MR-J2-70 hardware, the correct motor is the HC-MF73 (same mechanical specification, J2-generation encoder). Not compatible with MR-J3 or MR-J4 amplifiers without a renewal adapter kit.

HC-MFS Ultra-Low Inertia Range: Where the 73 Sits

The HC-MFS73 is the largest motor in the HC-MFS ultra-low inertia J2-Super family — the top of the range. It shares the 80 × 80 mm flange with the HC-MFS43 directly below it. The consistent 3:1 peak-to-continuous torque ratio runs across all five capacity steps.

Every capacity in the HC-MFS range is available in multiple shaft and brake configurations: straight shaft no brake (HC-MFS73), straight shaft with brake (HC-MFS73B), keyed shaft (HC-MFS73K), and keyed shaft with brake (HC-MFS73BK). All use the MR-J2S-70 amplifier at this capacity. Reduction gear variants (G1 flange type, G2 shaft type, G5 and G7 precision types) are also available — the MR-J2S-70 amplifier pairing remains unchanged regardless of gear configuration.

Typical Applications

High-cycle pick-and-place machine drives. Primary positioning axes on electronic component assembly pick-and-place equipment running hundreds of placements per minute. The ultra-low inertia allows rapid short-distance moves between component positions; the 17-bit absolute encoder ensures precise placement coordinates without homing on startup; the 750W capacity handles the slightly heavier gantry structures and wider travel ranges of medium-format assembly machines.

Semiconductor and flat panel display handling axes. Wafer transfer arm drives, substrate positioning axes, and cassette handler mechanisms on semiconductor processing and flat panel manufacturing equipment. These mechanisms move light loads through precise trajectories at high cycle rates in clean indoor environments — exactly the operating profile the HC-MFS73 was designed for.

Robot joint drives requiring high responsiveness. Secondary joint axes on small SCARA robots, elbow drives on lightweight articulated arms, and traverse axes on Cartesian gantry robots. Ultra-low inertia enables the high bandwidth position control needed for smooth trajectory following at robot-speed update rates; the 750W capacity handles joint loads that exceed what the HC-MFS43 can sustain.

Laser cutting and marking head positioning. Lightweight cutting head positioning axes on small-to-medium laser processing machines where the combined head mass requires 750W capacity but the rapid scan-and-position duty cycle demands ultra-low inertia acceleration performance. The 17-bit encoder supports the fine position increments used during slow-speed marking passes.

High-speed packaging machine feed and registration axes. Film pull drives, register correction axes, and product spacing mechanisms on packaging lines where the axis must respond quickly to register mark corrections and maintain synchronisation with the line speed through a continuous stream of small velocity adjustments. Ultra-low inertia enables the fast torque response these registration control loops need.

Frequently Asked Questions

Q1: What is the difference between the HC-MFS73 and the HC-KFS73?

Both are 750W, 3000 rpm J2-Super motors on an 80 × 80 mm flange with 17-bit absolute encoders and IP55 protection, and both produce 2.4 Nm continuous and 7.2 Nm peak. The difference is the rotor inertia class. The HC-MFS73 is ultra-low inertia — a rotor designed with minimum mass to maximise acceleration responsiveness at the cost of somewhat lower sustained torque capacity per unit rotor volume. The HC-KFS73 is low inertia — a slightly different design trade-off. Both use the MR-J2S-70 amplifier. Choose the HC-MFS73 for high-cycle-rate light-load axes where acceleration speed is the performance driver. Choose the HC-KFS73 for axes where lower inertia mismatch ratio or marginally higher sustained torque capacity at a given frame size is the priority.

Q2: Can the HC-MFS73 be used with a first-generation MR-J2-70 amplifier?

No. The HC-MFS73 carries the 17-bit J2-Super encoder protocol, which the first-generation MR-J2-70 amplifier cannot read. Connecting this motor to an MR-J2-70 will produce an encoder communication fault immediately on startup. For machines running original MR-J2-70 hardware, the correct motor is the HC-MF73 — same mechanical frame, same torque ratings, J2-generation encoder, compatible with MR-J2-70.

Q3: Is the IP55 rating adequate for environments near machining operations?

IP55 provides complete protection against dust ingress (dust-tight) and protection against directed water jets from any angle. This is adequate for most industrial indoor environments outside the cutting zone of machine tools — general assembly, electronics manufacturing, packaging, and handling equipment. For axes physically inside a machining centre's cutting zone where coolant spray directly reaches the motor body, IP65 is preferred and motors from the HC-KFS or HC-SFS range should be considered. IP55 is a significant improvement over the HC-MF73's IP44, making the HC-MFS73 more suitable for environments with occasional moisture exposure.

Q4: Where is the absolute encoder backup battery?

The Mitsubishi A6BAT lithium cell is inside the MR-J2S-70 servo amplifier, not in the motor. It maintains the 17-bit multi-turn absolute counter through all power-off periods. Replace it at the first low-battery alarm from the amplifier. If fully depleted, the absolute position counter resets and a reference-return cycle is required before the axis can resume production. On assembly machine applications where homing requires a defined clearance sequence, preventing this through timely battery replacement is straightforward.

Q5: Is the HC-MFS73 still available, and what is the current-generation equivalent?

The HC-MFS73 is discontinued by Mitsubishi but remains available through industrial automation surplus dealers and Mitsubishi servo specialist suppliers as new old stock and tested refurbished units. For machines committed to J2-Super hardware, this sourcing path is practical. For new machine designs or full platform upgrades, the current-generation equivalent is the HG-MR73 (MR-J4 series, 750W, 3000 rpm, straight shaft, 22-bit absolute encoder at 4,194,304 ppr, 80 × 80 mm flange, IP65) paired with an MR-J4-70 amplifier. The HG-MR73 improves on the HC-MFS73 with higher encoder resolution, better protection rating, and current-generation servo platform performance — but both motor and amplifier must be replaced together as the encoder protocols are incompatible.