Mitsubishi HC-SFS702B (HCSFS702B) — 7kW AC Servo Motor with Electromagnetic Brake, MELSERVO-J2S Series

Product Identification

Part Number: HC-SFS702B

Also Searched As: HCSFS702B, HC-SFS-702B

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

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

Condition: New In Box — One Year Warranty — In Stock

Overview

The Mitsubishi HC-SFS702B is the largest standard motor in the 2,000 rpm HC-SFS family — a 7kW medium-inertia AC brushless servo motor from the MELSERVO-J2S (J2-Super) platform, fitted with a spring-applied electromagnetic brake and a straight shaft. Rated at 33.4 Nm continuous and 100 Nm peak, it is the motor that gets specified when an axis genuinely needs that capacity: heavy Z-columns, large pallet shuttles, high-mass rotary tables, and industrial handling systems where modest mid-range servo motors simply run out of torque margin.

Two features define this exact variant. The electromagnetic brake is the fail-safe holding device — spring-applied, 24V DC release — that keeps a powered-off axis stationary regardless of what happens upstream in the control. The straight shaft makes it the right pick for friction-clamp couplings, split-clamp hubs, and precision bellows coupling designs where a plain shaft OD is the correct interface. For applications needing a keyway, the HC-SFS702BK is the parallel variant; everything else between the two models is identical.

Inside, the J2S generation encoder puts this motor in a different class from earlier HC-SF motors at the same capacity. The 17-bit serial absolute encoder — 131,072 positions per revolution — feeds a feedback stream that the MR-J2S amplifier uses to run its speed loop at 550 Hz bandwidth or higher. The result is an axis that holds position tightly under cutting load and responds quickly to trajectory commands, even when driving large, high-inertia loads.

Technical Specifications

Why 7kW — Understanding the Torque Numbers

There is a meaningful gap between 5kW and 7kW in real machine tool terms, and it shows up most clearly in two situations: heavy axis acceleration and sustained cutting at high feed rates.

At 33.4 Nm continuous, the HC-SFS702B can sustain that torque indefinitely under thermal steady-state, within the amplifier's current rating. The 100 Nm peak — exactly three times the continuous figure — is available for acceleration bursts. When a machine needs to move a 200 kg Z-slide to a new Z height in a rapid traverse, the acceleration torque determines how fast that move can happen. The 100 Nm peak gives the amplifier the headroom to demand maximum acceleration torque for the duration of the move, then settle to the lower torque required to hold position once the axis arrives.

The 10 kVA power facility capacity reflects what this motor asks of the electrical supply side. Panel design, cable sizing, fusing, and regenerative resistor capacity all need to be sized to this figure. The MR-J2S-700 amplifier handles regenerative energy — the energy returned to the DC bus when a large inertia load decelerates — and the panel design should account for the regenerative heat dissipation requirements, particularly on axes that decelerate from high speed frequently.

The Straight Shaft: Correct Coupling Selection at 7kW

The HC-SFS702B uses a plain straight shaft. At this capacity level, coupling selection and installation quality matter more than at smaller shaft sizes — 33.4 Nm of sustained torque through an improperly clamped coupling hub will eventually cause slip, and on a 7kW axis, slip events tend to damage both the coupling and the load mechanism before the fault registers.

For ballscrew-driven axes, a disc coupling or bellows coupling with a properly sized split-clamp hub is the standard approach. The clamp force must be sufficient to transmit the peak torque (100 Nm) without any relative motion between shaft and hub under worst-case conditions — typically a sudden reversal during active machining, or a deceleration from maximum rapid traverse. Coupling manufacturers publish maximum transmittable torque values for their products; always verify the peak, not just the rated continuous torque, when selecting a coupling for this motor.

One installation note that applies specifically to straight-shaft motors in large-frame sizes: radial shaft loads from misaligned couplings are proportionally more damaging on a heavier motor than on a small one, because the bearing span does not scale linearly with motor size. Careful alignment — both angular and parallel offset — during commissioning extends bearing life considerably. The IP65 oil seal at the shaft through-point is a secondary protection; it is not a substitute for proper alignment.

Electromagnetic Brake — Engineering Rationale at This Scale

Spring-applied brakes have a logic that becomes more important as motor size increases: the larger the motor, the greater the consequence of an unbraked axis losing servo control. A 7kW servo motor driving a Z-column carries the full weight of the machine's spindle head — on a substantial VMC, that assembly can represent several hundred kilograms of steel positioned above the workpiece and fixture. Servo torque drops to zero in milliseconds when the amplifier trips. Without a mechanical hold, the column follows gravity.

The HC-SFS702B's brake changes that equation. The spring engages the moment 24V DC is removed — by design during a normal shutdown sequence, or by necessity during an e-stop, alarm trip, or power loss. The column holds. The workpiece is protected. The machine can be recovered under controlled conditions rather than under emergency response.

For machine designers and integrators building around this motor, three points apply:

First, the brake's rated static holding torque is matched to the motor's capacity — but it is designed to hold a stopped load, not to arrest a moving one. The amplifier's deceleration routine must bring the axis to rest before the brake is allowed to engage. Using the MR-J2S amplifier's MBR signal (electromagnetic brake interlock output) to time the brake relay achieves this correctly. Hard-wiring the brake directly to an e-stop contact — without the MBR interlock — risks engaging the brake against a moving 7kW load, which accelerates brake wear dramatically.

Second, at 35A rated current and 7kW capacity, the motor power cables carry significant current. Brake coil wiring should be routed separately from power conductors where possible, and the surge absorber across the brake coil terminals is non-negotiable at this power level.

Third, for vertical axes with significant unbalanced load, Mitsubishi's guidance places the recommended static unbalanced torque at no more than 70% of the motor's rated torque — roughly 23 Nm at the motor shaft for this model. Above that threshold, counterbalancing through a pneumatic or hydraulic cylinder should supplement the servo and brake system rather than relying on motor torque and brake hold alone.

17-Bit Encoder — What It Delivers in a 7kW System

The J2-Super generation encoder runs at 17 bits — 131,072 distinct positions per motor revolution. Compared to the 14-bit (16,384 ppr) encoder in the older HC-SF702B, that is an eight-fold increase in resolution.

At 7kW, the practical benefit shows up in two areas. Velocity estimation accuracy is the first: the amplifier computes velocity from consecutive encoder position samples. More positions per revolution means finer velocity measurement per sample interval, which gives the speed loop a cleaner signal and allows it to operate at higher bandwidth without instability. The second benefit is low-speed torque smoothness. At very low feed rates — slow contouring moves, thread cutting on a lathe, hob-following on a gear machine — the encoder's resolution determines how finely the amplifier can regulate velocity. Coarse encoder data leads to velocity ripple, which appears as surface finish variation. The 17-bit encoder reduces that ripple substantially compared to older encoder generations.

The absolute function keeps working through any power interruption, sustained by the A6BAT battery in the MR-J2S amplifier. Machine restart after any power event — planned maintenance shutdown, e-stop recovery, alarm reset — does not require a homing cycle as long as the battery is healthy.

Typical Applications

VMC Z-axis on large-format machining centres. The Z-column carrying a full-size spindle head is the most common application for a brake-equipped 7kW servo. It needs the capacity for rapid traverse acceleration and sustained milling torque, the brake for safe rest position, and the absolute encoder for clean restart after any power event.

HMC W-axis and pallet drive systems. Horizontal machining centre W-axis positioning for facing heads and pallet shuttle drives for large fixture transfer both use high-capacity servo drives. The pallet shuttle must start and stop heavy loads repeatedly at production cycle rates; the 100 Nm peak handles the acceleration torque, and the brake holds pallet position at each station.

Large CNC lathe turret and sub-spindle feed axes. High-capacity turning centres with multi-position servo turrets need decisive, fast indexing under the weight of multiple tools. The 7kW capacity handles large turrets that a mid-range motor would struggle with.

Industrial gantry loaders and overhead handling systems. Gantry-style part transfer systems with significant vertical travel and heavy end-effectors use brake-equipped servo drives on the Z-axis for the same fail-safe reasons as machine tool Z-columns. The straight shaft suits the coupling designs common in gantry drive systems.

Rotary tables and 4th-axis drive. Large-format rotary tables with workpieces that demand sustained torque during contouring — impeller machining, aerospace structural parts — use high-capacity servo drives with direct encoder feedback.

HC-SFS Series — 2000 rpm Range at a Glance

Each capacity point is available in straight/keyed shaft and with/without brake combinations. The HC-SFS702B is the straight-shaft-with-brake variant at the 7kW top of this range. All models share the 17-bit serial absolute encoder, 200V AC class input, IP65 protection, and oil-sealed shaft.

New In Box, One Year Warranty — In Stock

Factory-sealed new-in-box means original Mitsubishi packaging, all protective covers intact, never powered or installed. One year warranty covers the motor against defects from delivery. For a machine currently waiting on a servo motor to resume production, in-stock new-in-box is the fastest and most reliable path back to operation — no repair lead time, no reconditioned-part variables, no history to account for.

At 32 kg, the HC-SFS702B ships in a protective carton with inner foam support. The shaft end cap and all connector ports remain covered until installation. Bearing grease is factory-fresh; no pre-installation service is required for motors going directly into service within normal storage periods.

Frequently Asked Questions

Q1: Which amplifiers are compatible with the HC-SFS702B?

The HC-SFS702B requires a MR-J2S-700 class amplifier. The three main options are the MR-J2S-700A (general-purpose analog/pulse command), the MR-J2S-700B (SSCNET fiber-optic bus, for motion controller systems), and the MR-J2S-700CP (built-in positioning function). All three support the 17-bit serial encoder and the 35A rated current of this motor. The HC-SFS702B is not compatible with original MR-J2 amplifiers (which cannot read the 17-bit J2S encoder) or with MR-J3 / MR-J4 amplifiers without dedicated conversion hardware.

Q2: What is the difference between the HC-SFS702B and the HC-SFS702BK?

The two models are mechanically and electrically identical in every respect except the shaft. The HC-SFS702B has a plain straight shaft for use with friction-clamp style couplings. The HC-SFS702BK has a machined keyway, providing a positive key-and-keyway torque connection to the driven component. Both share the same 7kW output, 33.4 Nm rated torque, 100 Nm peak, 17-bit encoder, brake, IP65 rating, and MR-J2S-700 amplifier compatibility. Choose based solely on your coupling design.

Q3: How does the electromagnetic brake work, and what is the correct wiring practice?

The brake is spring-applied and fail-safe: 24V DC energises the coil and holds the brake disc clear, allowing free shaft rotation. Remove the 24V — by design, fault, or power loss — and the spring immediately clamps the shaft. It is a holding device, not a stopping brake. Always use the MR-J2S amplifier's MBR (brake interlock) output to control the brake relay, ensuring the brake engages only after the motor has decelerated to a stop. Fit a surge absorber directly across the brake coil terminals to suppress inductive voltage spikes at switch-off.

Q4: Does the 17-bit encoder require a battery, and what battery does it use?

Yes. The 17-bit serial absolute encoder retains position data during power-off through battery backup. The battery is the Mitsubishi A6BAT lithium cell, installed inside the MR-J2S servo amplifier — not in the motor or encoder body. When the A6BAT is healthy, the axis retains exact absolute position through any power interruption and restarts without a homing cycle. When the amplifier displays a low-battery warning alarm, replace the A6BAT during the next planned maintenance window — depleting the battery fully results in loss of absolute position, which requires a reference return cycle before the machine can resume production.

Q5: How does the HC-SFS702B compare to the older HC-SF702B at the same 7kW capacity?

Both are 7kW, 2,000 rpm, 33.4 Nm motors with straight shafts and electromagnetic brakes, and they share the same 176 × 176 mm flange — meaning they are mechanically interchangeable at the mounting point. The critical difference is the encoder generation: the HC-SF702B (J2-era) uses a 14-bit encoder at 16,384 ppr and pairs with MR-J2 or MR-J2S amplifiers. The HC-SFS702B (J2S-era) uses a 17-bit encoder at 131,072 ppr and requires MR-J2S amplifiers. If your machine already runs MR-J2S amplifiers, the HC-SFS702B is the correct and higher-performance choice. If it runs original MR-J2 amplifiers, the amplifier must be upgraded at the same time, or the HC-SF702B is the right motor to source.