Mitsubishi HC-SF702B (HCSF702B) — 7kW AC Servo Motor with Electromagnetic Brake, MELSERVO J2 Series

Product Identification

Part Number: HC-SF702B

Also Searched As: HCSF702B, HC-SF-702B

Series: Mitsubishi MELSERVO HC-SF (J2 Generation)

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

Condition: New In Box, Factory Sealed

About This Motor

Seven kilowatts. Straight shaft. Electromagnetic brake. Factory sealed.

The Mitsubishi HC-SF702B is the top-capacity model in the standard 2,000 rpm HC-SF family — a medium-inertia AC brushless servo motor from Mitsubishi's MELSERVO J2 platform that was widely deployed across heavy-duty machine tool axes, large-format transfer systems, and high-capacity industrial motion control applications. At 33.4 Nm continuous rated torque and a 100 Nm peak, it is built for axes where genuine load capacity matters, not just positioning finesse.

The "B" suffix means this variant carries a spring-applied electromagnetic brake — the fail-safe holding device required whenever a powered-off axis cannot be left to its own devices. Cut the 24V DC supply to the brake coil, and the spring engages instantly. No relay timing, no software interlock, no dependency on amplifier output. The axis is held mechanically regardless of what happens upstream in the control chain.

This is a discontinued product available as factory-sealed new-in-box stock. For machines that were built on the MELSERVO J2 platform and are still in production service, this is the only condition that provides a genuine like-for-like original replacement — the same encoder type, the same connector scheme, the same amplifier compatibility, all as delivered from Mitsubishi's factory.

Technical Specifications

The HC-SF Series at 7kW — Built for the Toughest Axes

Within the HC-SF family's 2,000 rpm range, the 702B sits at the top of the production lineup. Getting to 7kW from the 5kW HC-SF502B means more copper, larger magnetic circuit, and a physically heavier motor body — this is not a motor you pick up with one hand. The power facility capacity requirement of 10 kVA reflects a machine electrical design that must provision accordingly: panel sizing, cable cross-section, breaker ratings, and regenerative capacity all scale up compared to the mid-range models.

What that capacity brings in return is torque headroom. The 3:1 peak-to-rated ratio — 100 Nm instantaneous against 33.4 Nm continuous — gives the axis real acceleration authority when moving heavy loads. In machining, that translates to fast rapids on a Z-column carrying a large spindle assembly, quick turret index cycles on a heavy-duty turning centre, or rapid traverse on a pallet axis where the fixture and workpiece add significant inertia. The motor accelerates the load hard when it needs to, then settles to sustained torque for the actual work cycle.

The 176 × 176 mm flange is standard across the upper HC-SF range. Machines designed around the 3.5kW or 5kW HC-SF motors in this frame size can accept the 702B without any mounting modification — a relevant fact when an axis has been chronically undersized and needs to be upgraded without machine redesign.

Straight Shaft, Electromagnetic Brake — The Specifics

Why straight shaft? The HC-SF702B uses a plain straight shaft suitable for friction-clamp couplings — split-clamp hubs, compression collars, and similar designs that transmit torque purely through clamping force on the shaft OD. At 7kW, this works well for ballscrew drive systems where the coupling hub is properly sized and the clamping force is correctly specified. For applications requiring a key — rack-and-pinion drives, chain sprockets, or timing pulleys under high pre-tension — the HC-SF702BK (keyed shaft variant) is the correct alternative. The two models are otherwise identical.

What the brake does, and what it doesn't do. The spring-applied brake in the HC-SF702B is a position-holding device, not a stopping brake. Mitsubishi is clear on this in their documentation: the brake is designed to hold a stationary axis, not to arrest a moving one. Under normal operation, 24V DC flows through the brake coil and holds the disc clear of the friction surface; the shaft rotates freely under amplifier control. Deceleration is handled entirely by the servo system. Only after the motor has been brought to a stop — by the amplifier, under closed-loop control — should the brake be engaged.

Applying a brake to a motor that is still carrying significant torque shortens the brake's service life dramatically and creates mechanical shock in the drivetrain. At 7kW, that shock is not trivial. The MR-J2S amplifier's MBR (electromagnetic brake interlock) output provides the correct timing signal: use it to control the brake relay, ensuring engagement happens only after the deceleration sequence completes.

Surge protection is not optional. The brake coil is an inductive load. At the moment of brake engagement or release, the collapsing magnetic field generates a voltage spike that can damage relay contacts and adjacent amplifier I/O circuits over time. A surge absorber wired directly across the brake coil terminals absorbs that spike. This is standard wiring practice and should not be omitted during installation.

Absolute Encoder — What the 14-Bit Device Provides

The HC-SF702B carries a 14-bit absolute encoder delivering 16,384 positions per revolution. The "absolute" designation means position is known the moment the control powers up — the axis does not need to perform a reference return or homing cycle as long as the encoder's battery backup has been maintained.

That battery lives in the servo amplifier, not the motor. On MR-J2 and MR-J2S amplifiers, the encoder backup battery is the A6BAT lithium cell. Deplete that battery — by leaving the amplifier without power for an extended period with an old or missing cell — and the absolute position memory is lost. The first power-up after that event will require a manual reference return before the machine can resume normal operation. Regular battery checks at scheduled maintenance intervals prevent that outcome.

The 14-bit resolution (16,384 ppr) is lower than the 17-bit encoder (131,072 ppr) found in the later HC-SFS generation, but it is not a meaningful limitation for the duty cycles this motor serves. At 7kW, the axes are typically ballscrew or rack-and-pinion driven with gear ratios that multiply the encoder resolution at the load — fine enough for the feed and positioning accuracy demanded by general-purpose machine tool work.

Compatible Amplifiers and J2 Platform Context

The HC-SF702B was designed for the MELSERVO J2 platform and its amplifier family. The MR-J2 series amplifiers at the 700 capacity class are the original pairing. The later MR-J2S-700A and MR-J2S-700B amplifiers — the J2-Super generation — retained backward compatibility with HC-SF series motors, meaning a machine that has been upgraded to J2S amplifiers can continue running HC-SF702B motors without motor replacement. This made the J2S upgrade commercially practical: new amplifiers, existing motors, no mechanical disturbance to the machine.

What the J2S amplifier brings to an HC-SF702B is improved performance at the amplifier level — faster CPU, higher speed-loop bandwidth, better adaptive tuning — without changing the motor itself. The 14-bit encoder still feeds back 16,384 ppr; the J2S amplifier processes that feedback more quickly and with tighter algorithms than the original J2 unit. Whether that improvement is perceptible in a given application depends on the axis requirements, but the compatibility itself is well established.

The HC-SF702B is not compatible with MR-J3 or MR-J4 amplifiers without dedicated conversion hardware. The J3 and J4 series use different encoder protocols and physical connector schemes. A machine platform change from J2/J2S to J4 involves motor replacement, not just amplifier swaps.

Where This Motor Is Typically Installed

Heavy VMC Z-axis. The Z-column on a large vertical machining centre carries the full weight of the spindle head — on a substantial machine, that can be 300–600 kg of steel and motor. The HC-SF702B provides the torque for rapid traverse and controlled feed at that mass level, and the brake holds the column stationary whenever the servo is de-energised.

Horizontal machining centre pallet drive and W-axis. HMC pallet transfer systems move heavy fixtures across long travel paths. The W-axis for facing head depth positioning and the pallet shuttle drive both benefit from a high-capacity servo with reliable brake hold at each transfer station.

Large CNC lathe turret. High-capacity turning centres with large-diameter turrets require decisive index cycles under load. The 100 Nm peak torque drives rapid, accurate indexing even when the turret carries multiple heavy tools.

Press back-gauge and beam positioning. Large press brake back-gauge systems use servo drives to position a heavy stop beam at precise depths. Between press cycles, the beam position must hold exactly — the electromagnetic brake provides that hold as a fail-safe against any servo fault or power interruption during the pressing cycle.

Rotary transfer machines and pallet indexers. Heavy-duty rotary indexing tables and linear transfer systems used in high-volume machining cells need servo drives that can move large payloads quickly and hold position reliably between stations.

HC-SF 2000 rpm Family — Capacity Reference

All models in this table share the J2-generation 14-bit absolute encoder, 200V AC class input, 2,000 rpm rated speed, IP65 protection, and oil-sealed shaft. The 352, 502, and 702 models use the 176 × 176 mm large-frame flange.

New In Box, Factory Sealed

Factory sealed means exactly that: original Mitsubishi carton and inner packaging intact, shaft end cap in place, connector ports covered, inner foam cradle undisturbed. The motor has never been powered, never had a coupling pressed onto its shaft, and carries no thermal or mechanical history from any prior installation.

For a machine currently down waiting for a replacement, new-in-box eliminates the variables that come with repaired or refurbished units — replaced bearings of unknown specification, re-potted encoder assemblies, rewound stators. What ships is what Mitsubishi built.

For long-term storage: cool, dry, and vibration-free conditions preserve the motor's full specification for several years. Beyond five years of storage, slow manual shaft rotation at intervals helps redistribute bearing grease before installation, as per Mitsubishi's own storage guidance.

Frequently Asked Questions

Q1: Which servo amplifiers are compatible with the HC-SF702B?

The HC-SF702B works with MR-J2 series amplifiers at the 700 capacity class, and also with the later MR-J2S-700A (general-purpose analog/pulse interface) and MR-J2S-700B (SSCNET fiber-optic bus). The J2S amplifiers retained backward compatibility with the J2-generation HC-SF motor encoder, so machines upgraded from MR-J2 to MR-J2S drives can continue using HC-SF702B motors without changing the motor. The HC-SF702B is not compatible with MR-J3 or MR-J4 amplifiers.

Q2: What is the difference between the HC-SF702B and the HC-SFS702B?

Both are 7kW, 2,000 rpm, 33.4 Nm motors with electromagnetic brakes and straight shafts — and they share the same 176 × 176 mm flange, making them mechanically interchangeable at the mounting point. The key difference is the encoder generation: the HC-SF702B uses a 14-bit absolute encoder (16,384 ppr) and is a J2-generation motor, while the HC-SFS702B uses a 17-bit encoder (131,072 ppr) from the J2S generation. The HC-SF702B can pair with MR-J2 or MR-J2S amplifiers; the HC-SFS702B requires MR-J2S amplifiers. If your machine runs original MR-J2 amplifiers, the HC-SF702B is the correct motor.

Q3: How does the electromagnetic brake engage, and how should it be wired?

The brake is spring-applied and fail-safe: 24V DC holds it open; remove the voltage and the spring engages the brake disc, locking the shaft. It is a holding brake — not designed to stop a moving motor. Always use the servo amplifier's MBR (brake interlock) output to control the brake relay, which ensures the brake engages only after the axis has decelerated to a stop. Fit a surge absorber directly across the brake coil terminals to suppress inductive voltage spikes at switch-off. Skipping the surge absorber risks progressive damage to relay contacts and amplifier I/O over time.

Q4: Does the 14-bit absolute encoder require a battery?

Yes. The 14-bit serial absolute encoder requires battery backup to retain position memory during power-off. The battery — an A6BAT lithium cell — is housed in the MR-J2 or MR-J2S servo amplifier, not in the motor body. When the battery is good, the axis retains its absolute position through any power interruption and requires no homing cycle on restart. Check battery condition at regular maintenance intervals; the amplifier will display a low-battery alarm before the cell is fully depleted, giving a scheduled replacement window rather than forcing an emergency response.

Q5: Can the HC-SF702B be used on a vertical axis, and what load limitation applies?

Yes — the brake makes it suitable for vertical axes. Mitsubishi's guidance recommends sizing the axis so that the static unbalanced torque does not exceed 70% of the motor's rated torque (approximately 23.4 Nm at the motor shaft for this model). For a vertical axis with a large unbalanced load approaching or exceeding that threshold, additional mechanical counterbalancing — a hydraulic or pneumatic cylinder counterbalance — should be used alongside the servo drive rather than relying on motor torque and the brake alone.