Mitsubishi HC-SFS301B (HCSFS301B) — 3kW AC Servo Motor, Straight Shaft + Brake, 1000 rpm, MELSERVO-J2S Series

Product Identification

Part Number: HC-SFS301B

Also Searched As: HCSFS301B, HC-SFS-301B

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

Motor Type: AC Brushless Servo Motor — Straight Shaft with Electromagnetic Brake, 1000 rpm

Condition: New In Box, Factory Sealed

The Torque Story: 3kW at 1000 rpm

Most servo motors optimise for speed. The HC-SFS301B is built around a different priority: sustained torque at low shaft speed. Three kilowatts of output routed through a 1,000 rpm winding produces 28.6 Nm of continuous rated torque — double what a 3kW motor rated at 2,000 rpm would deliver at its shaft. No gearbox required to achieve that figure. No intermediate reduction stage adding backlash, maintenance intervals, or mechanical complexity to the drive train.

That is the core reason engineers specify this motor. Where the application demands substantial torque continuously at slow shaft speed — directly coupled turntable drives, slow pallet transfer mechanisms, winding axes running under constant tension load — the 1,000 rpm, high-torque profile solves the problem without additional power transmission hardware between the motor and the load.

The 85.9 Nm peak provides the acceleration headroom. Getting a high-inertia load moving from rest to rated speed and stopping it cleanly requires significantly more torque than sustaining a steady-state velocity. That three-to-one peak-to-rated ratio is what allows the HC-SFS301B to drive heavy loads through sharp start-stop cycles while remaining within its continuous thermal rating across the working portion of each cycle.

The "B" designates the electromagnetic brake variant. The motor ships with a spring-applied brake fitted and a straight shaft for friction-clamp coupling interfaces. This is the configuration for applications that combine a need for fail-safe mechanical hold with a plain shaft coupling design.

Technical Specifications

J2-Super Platform: What the 17-Bit Encoder Changes

The HC-SFS family is the J2-Super successor to the original HC-SF range. The physical dimensions are the same; the encoder is not. The HC-SFS301B carries a 17-bit serial absolute encoder — 131,072 positions per revolution — compared to the 14-bit device (16,384 ppr) in the HC-SF301B. The step between them is not marginal: 131,072 versus 16,384 is an eight-fold resolution increase.

At 1,000 rpm, high encoder resolution earns its value in a specific way. Low-speed servo axes are the hardest to run smoothly from a velocity control standpoint. The amplifier computes velocity from consecutive encoder position samples taken at a fixed sample interval. With 16,384 positions per revolution, each sample covers a relatively coarse angular step at low speed, and the velocity estimate contains more granularity noise. With 131,072 positions available, the same calculation resolves much finer angular increments, giving the speed loop a cleaner velocity signal to work with. The result is noticeably smoother rotation at low feed rates — less ripple in the velocity waveform, better torque uniformity at the shaft, and improved surface finish in applications like grinding or slow contouring cuts.

The absolute function stores multi-turn position in the encoder and retains it through power loss, backed by the A6BAT lithium battery housed inside the MR-J2S amplifier. From the machine's perspective, a power interruption is invisible to the position system. Restart after any power event — planned shutdown, e-stop recovery, alarm reset — proceeds without a reference return cycle. The axis simply comes up in the last known absolute position.

One important distinction from the HC-SF301B: the HC-SFS301B's 17-bit encoder requires the MR-J2S amplifier platform. It is not backward-compatible with first-generation MR-J2 amplifiers. If the machine currently runs MR-J2 (without the S suffix) hardware, the HC-SF301B is the correct motor to source. If the amplifiers are MR-J2S, the HC-SFS301B is the precise match — and the better-performing one.

The Electromagnetic Brake at This Capacity

Spring-applied brakes become more critical as continuous torque rises. At 28.6 Nm rated, an axis carrying a high-torque load — a loaded rotary table, a heavy part suspended on a Z-axis — applies that torque in whatever direction gravity or load imbalance dictates the instant servo current drops to zero. On a machine without brake interlock, the first e-stop or power event could allow significant uncontrolled movement before the dynamic brake in the amplifier brings the shaft to rest.

The HC-SFS301B's spring-applied brake removes that uncertainty. When 24V DC is present, the coil holds the brake disc clear and the shaft turns freely. Remove the 24V — by software command during a normal shutdown sequence, by relay dropout on e-stop, or by loss of control power — and the spring immediately engages the disc. The shaft is held mechanically, regardless of amplifier state.

Three things to get right in the installation:

The brake must engage only after the axis has stopped. The MR-J2S amplifier provides the MBR signal specifically for this purpose — it delays brake engagement until the motor has decelerated below the threshold speed. Wiring the brake coil directly to an e-stop contact, without the MBR interlock, risks engaging the spring against a rotating shaft. At 85.9 Nm peak motor torque, the collision between spring-clamped disc and spinning rotor is abrupt and will shorten brake life substantially.

Surge suppression is mandatory. The brake coil is an inductive load. Switching 24V DC to an unprotected coil produces a large inductive voltage spike at turn-off. The surge absorber — a snubber or freewheeling diode — must be installed directly across the brake coil terminals, as close to the coil as possible. Omitting it risks damage to the relay or the amplifier's digital output driving the brake circuit.

For vertical axes, Mitsubishi's published guidance places the recommended maximum static unbalanced torque at or below 70% of the motor's rated torque — approximately 20 Nm at the shaft for the HC-SFS301B. Loads producing higher unbalanced torque should be supplemented with mechanical counterbalancing rather than relying on the servo system and brake alone.

Straight Shaft: Coupling Design and Installation

The HC-SFS301B's straight shaft calls for friction-clamp or split-clamp coupling hubs. The coupling must be selected and fitted for the peak torque figure — 85.9 Nm — not just the 28.6 Nm continuous rating. A hub rated for the continuous torque but marginal under the peak will eventually develop micro-slip, introducing positional errors that accumulate subtly over multiple machine cycles before they become visible as part dimension shifts.

Correct hub installation on a straight shaft follows a specific procedure in Mitsubishi's servo motor manual: use the threaded hole at the shaft end to draw the hub on axially — a drawbolt, washer, and nut seated against the hub face — rather than pressing or hammering the hub directly. Shaft impact loads from hammering travel directly back through the motor to the encoder disc. Even if no immediate fault occurs, shock damage to the encoder mechanism can cause intermittent feedback errors or early encoder failure that only becomes apparent after the machine has been in production for some time.

For applications where a more positive shaft-to-hub connection is preferred, the HC-SFS301BK is the keyed-shaft version of the same motor — identical in every specification, with a machined keyway added. Both variants mount on the same 176 × 176 mm flange and pair with the same MR-J2S-350 amplifiers.

Compatible Amplifiers — MR-J2S-350 Class

The HC-SFS301B requires a MR-J2S-350 amplifier. Three standard variants are available:

• MR-J2S-350A — General-purpose interface with analog/pulse command. Supports position, speed, and torque control modes. The standard choice for CNC and PLC-driven systems using step/direction or analog velocity command.
• MR-J2S-350B — SSCNET fiber-optic bus interface, designed for coordinated multi-axis systems through Mitsubishi motion controllers (A-series, Q-series). Position commands are delivered over the serial network rather than by pulse train.
• MR-J2S-350CP — Built-in positioning function. Point table data stored in the amplifier itself, with CC-Link or I/O interface. Suited for standalone positioning without a dedicated motion controller.

All three variants support the 17-bit encoder protocol and are rated for the motor's continuous current demands at 3kW. The HC-SFS301B is not compatible with first-generation MR-J2-350 amplifiers, which cannot read the J2S 17-bit encoder format, nor with MR-J3 or MR-J4 amplifiers, which use a different physical and protocol interface entirely.

Application Context

Low-speed rotary table and 4th-axis drives. Large pallet-style rotary tables on machining centres handle substantial workpiece and fixture mass. At 28.6 Nm continuous, the HC-SFS301B drives a direct-coupled rotary table through sustained contouring cuts and sharp indexing cycles without running near its thermal ceiling. The absolute encoder handles the demanding position accuracy required for angular positioning in multi-face machining, and the brake holds the table during clamping and unclamping sequences.

HMC pallet shuttle and transfer drives. Pallet transfer systems on horizontal machining centres cycle repeatedly under heavy load. The stop-and-hold requirement at each station — often while clamping and unclamping occurs — is where the fail-safe brake pays for itself. The combination of high continuous torque and mechanical hold makes the HC-SFS301B a natural fit for drives in this application category.

Tension-control winding axes. Material winders and unwinds running in torque control mode need a motor that can sustain the required torque at low and variable shaft speeds across a wide roll diameter range. A 1,000 rpm motor in torque control on a winding axis stays within a sensible speed range without demanding unusual electronic gear ratio settings, and the 17-bit encoder provides fine-grained torque regulation feedback at any speed within the operating window.

Slow conveyor and transfer indexing. Drive shafts for chain conveyors and belt transfer systems running at low surface speeds under loaded conditions benefit from the HC-SFS301B's torque density. Where a gearmotor would otherwise be specified, a direct-drive servo at 1,000 rpm with sufficient continuous torque simplifies the drive train and adds the programmable speed, torque, and position control capabilities that servo systems provide.

Z-axis gravity-loaded drives on heavy-duty machines. Vertical axes carrying heavy spindle assemblies on large-format machines need the combination of adequate torque and reliable mechanical hold. The HC-SFS301B's 28.6 Nm continuous torque — at 1,000 rpm without reduction gearing — provides direct-drive options for some vertical axis configurations where a higher-speed motor would require significant gear reduction to achieve equivalent torque at the ballscrew input shaft.

HC-SFS 1000 rpm Range — Capacity Reference

All models in this family use the 17-bit serial absolute encoder, IP65 protection, oil-sealed shaft, 200V AC class supply, and MR-J2S-350 amplifier compatibility. The HC-SFS301B sits at the top of the 1,000 rpm range with the highest continuous torque in this sub-family.

New In Box, Factory Sealed

Factory sealed means original Mitsubishi packaging, shaft-end protective cap in place, all connector ports covered, internal foam packing undisturbed, and no prior thermal or mechanical history to account for. For a machine currently waiting for this part to resume production, in-stock new-in-box is the most reliable path back to operation — known condition, one-year warranty, no variables from previous installation or repair.

At a weight consistent with this motor frame size and brake assembly, the HC-SFS301B ships securely packed for transport. Stored under appropriate conditions — cool, dry, away from vibration — factory-sealed stock maintains full specification for several years. Beyond five years of storage, a brief slow-speed run of the shaft before commissioning helps redistribute bearing grease.

Frequently Asked Questions

Q1: Which amplifiers are compatible with the HC-SFS301B?

The HC-SFS301B requires a MR-J2S-350 class amplifier from the MELSERVO-J2S (J2-Super) platform. The three standard variants are the MR-J2S-350A (general-purpose analog/pulse command), MR-J2S-350B (SSCNET fiber-optic bus for motion controllers), and MR-J2S-350CP (built-in positioning with CC-Link interface). All support the motor's 17-bit serial encoder. The HC-SFS301B is not compatible with original MR-J2-350 amplifiers or with MR-J3 / MR-J4 amplifiers.

Q2: What is the difference between the HC-SFS301B and the HC-SF301B?

They are mechanically interchangeable — same flange, same torque output, same shaft type, same brake configuration, same rated speed. The difference is the encoder and amplifier generation. The HC-SF301B (J2 series) uses a 14-bit encoder (16,384 ppr) and works with both MR-J2 and MR-J2S amplifiers. The HC-SFS301B (J2S series) uses a 17-bit encoder (131,072 ppr) and requires MR-J2S amplifiers. If the machine runs MR-J2 (first-generation) amplifiers, the HC-SF301B is the correct motor to source.

Q3: The HC-SFS301B is 3kW, but it has 28.6 Nm rated torque. How is that possible?

Torque and speed are inversely related at the same power level. At 1,000 rpm with 3kW output, the rated continuous torque is approximately 28.6 Nm. Run the same 3kW through a 2,000 rpm motor and the continuous torque falls to roughly 14.3 Nm. The HC-SFS301B is specifically designed for low-speed, high-torque applications where the axis load demands sustained torque without the need for reduction gearing between the motor and the driven mechanism.

Q4: Does the 17-bit encoder need a battery, and where is the battery located?

Yes. The absolute function of the 17-bit encoder relies on battery backup to retain multi-turn position data through power-off periods. The battery — a Mitsubishi A6BAT lithium cell — is installed in the MR-J2S servo amplifier, not in the motor body. When healthy, the absolute position is retained through any power interruption and the axis does not need a homing cycle on restart. Replace the A6BAT promptly when the amplifier displays a low-battery warning alarm, before full discharge causes loss of absolute position.

Q5: Is the HC-SFS301B suitable for a vertical gravity-loaded axis without additional counterbalancing?

It can be — provided the static unbalanced torque at the motor shaft does not exceed approximately 70% of the rated torque, which for this motor is roughly 20 Nm. This figure comes from Mitsubishi's published guidance for servo motor vertical axis applications. Loads producing unbalanced torque above that threshold should have supplemental mechanical counterbalancing (pneumatic cylinder, counterweight) rather than relying entirely on the servo torque and brake to hold the axis. For loads within the 20 Nm limit, the HC-SFS301B with its spring-applied brake provides reliable hold on servo-off.