The HG-KR73 is Mitsubishi Electric’s 750 W ultra-low-inertia small-capacity servo motor from the HG-KR series for the MELSERVO-J4 and MELSERVO-J3 servo amplifier platforms, providing 2.39 N·m rated torque at 3,000 rpm with a 22-bit multi-turn absolute encoder as standard — the 750 W model in the HG-KR ultra-low-inertia series, designed for high-speed, high-cycle precision positioning applications in electronic assembly equipment, semiconductor handling, high-speed pick-and-place machines, and inspection automation where minimising rotor inertia to achieve the fastest possible acceleration and deceleration times is the primary motor selection criterion. In stock at Atlantech Drives. Request your quote within 24-48 hours with worldwide shipping and 12-month warranty.
What Is the HG-KR73?
The HG-KR73 is a Mitsubishi Electric MELSERVO HG-KR series servo motor providing 750 W (0.75 kW) continuous rated output at 3,000 rpm rated speed, with a peak torque of 7.16 N·m (3× rated torque). The «KR» designation identifies the ultra-low-inertia small-capacity motor family — the «K» indicating the ultra-low-inertia classification and «R» indicating rectangular flange mounting, consistent with the HG-SR and HG-RR series flange convention. At 0.239 × 10⁻⁴ kg·m² rotor inertia, the HG-KR73 has approximately 58% lower rotor inertia than the equivalent-power HG-SR73 (0.410 × 10⁻⁴ kg·m²) — the defining characteristic of the KR series that enables substantially faster acceleration to target speed compared to the SR series at the same power level and the same servo amplifier peak torque capability. This ultra-low inertia comes at a cost: a higher load-to-motor inertia ratio for any given load, making servo tuning more challenging for high-inertia loads and requiring careful inertia ratio verification during servo sizing. The HG-KR73 is compatible with MR-J4-70A (pulse train) and MR-J4-70B (SSCNET III/H) servo amplifiers. In J3 motor compatibility mode (PA22 = 1), the HG-KR73 is also compatible with MR-J3-70A and MR-J3-70B amplifiers, enabling motor-only upgrade from legacy HC-KFS73 motors.
Key Technical Specifications
- Model: HG-KR73
- Rated Output Power: 750 W (0.75 kW)
- Rated Torque: 2.39 N·m
- Rated Speed: 3,000 rpm
- Maximum Speed: 6,000 rpm
- Peak Torque: 7.16 N·m (3× rated)
- Rotor Inertia: 0.239 × 10⁻⁴ kg·m² (ultra-low — 42% lower than HG-SR73)
- Encoder: 22-bit multi-turn absolute (262,144 pulses/rev)
- Power Supply Voltage: 200 VAC (via compatible servo amplifier)
- Insulation Class: F (155°C maximum winding temperature)
- Protection Rating: IP65 (shaft seal and connector side)
- Mounting: Square flange, IEC standard
- Operating Temperature: 0°C to 40°C (ambient)
- Weight: Approx. 1.5 kg
Series Comparison & Selection Guide
At 750 W, Mitsubishi offers three HG-series motors with different inertia characteristics: the HG-KR73 (ultra-low inertia, 0.239 × 10⁻⁴ kg·m²), the HG-SR73 (low inertia, 0.410 × 10⁻⁴ kg·m²), and the HG-RR703 (medium inertia, higher rotor inertia for better load matching). The HG-KR73 is selected when the application’s primary requirement is minimum cycle time — the ultra-low rotor inertia enables the servo amplifier’s peak torque (7.16 N·m = 3× rated) to accelerate the motor rotor itself in the minimum possible time, maximising the available torque for load acceleration during the first portion of the acceleration profile. In high-speed pick-and-place machines where the arm mass is very small relative to the motor frame, the HG-KR73’s ultra-low inertia provides a load inertia ratio close to 1:1 — the ideal ratio for maximum servo bandwidth and fastest settling. The HG-SR73 is selected when the load inertia is moderate (inertia ratio 5:1 to 30:1) and the slightly higher rotor inertia provides better mechanical damping without sacrificing excessive acceleration response. The HG-RR703 medium inertia is selected for high-load applications where inertia ratio management is the primary concern. When the calculated load inertia ratio with the HG-KR73 exceeds 30:1 for the specific mechanical system, the HG-SR73 should be evaluated — a 30:1 inertia ratio with the HG-SR73 is mechanically equivalent to approximately 52:1 with the HG-KR73, well outside the recommended servo tuning range.
Installation Advice
The HG-KR73’s ultra-low rotor inertia requires careful servo tuning attention during commissioning — the auto-tuning algorithm in MR Configurator2 will identify a potentially high load-to-motor inertia ratio (even with moderate mechanical loads), and the resulting servo gains may cause oscillation on the first test move if the response level is set too aggressively. Always start auto-tuning at response level 3 or 4 for HG-KR series motors and increase incrementally, monitoring the position error waveform in MR Configurator2 for oscillation after each deceleration. For HG-KR73 motors on direct-coupled ball screw axes, verify the coupling alignment precisely before servo commissioning — the ultra-low inertia HG-KR73 is more sensitive to coupling misalignment forces than the HG-SR73, as the additional radial load from misalignment represents a larger fraction of the motor’s rated radial load capacity relative to the motor’s smaller frame. The HG-KR73’s IP65 shaft seal is critical for coolant environment installations — the shaft seal’s IP65 rating is only maintained when the shaft is not subjected to axial loads beyond the specified maximum (typically 49 N axial at rated speed). Verify that the coupling or load connection method does not impose axial preload on the HG-KR73 shaft — press-fit couplings without correct axial relief can impose sustained axial loads that degrade the shaft seal and allow coolant ingress to the bearing housing over time.
Frequently Asked Questions
Q: At 750 W and the same rated torque as the HG-SR73, why would the HG-KR73 provide faster cycle times than the HG-SR73?
A: Both motors provide 2.39 N·m rated torque and 7.16 N·m peak torque. The difference is that the HG-KR73’s lower rotor inertia (0.239 versus 0.410 × 10⁻⁴ kg·m²) means more of the servo amplifier’s peak torque is available for accelerating the mechanical load — a larger fraction of the peak torque is consumed by rotor inertia acceleration in the higher-inertia HG-SR73. At equal peak torque and equal load inertia, the HG-KR73 accelerates faster because its lower rotor inertia results in higher total system angular acceleration for the same torque input.
Q: Can the HG-KR73 replace an HC-KFS73 in an MR-J3 servo system without changing the amplifier?
A: Yes, in J3 motor compatibility mode. Set MR-J3-70A or MR-J3-70B parameter PA22 = 1 to enable J4 motor support, then configure the motor code for the HG-KR73 in MR Configurator2. Replace the HC-KFS73 encoder cable with an HG-KR-compatible cable (MR-J3ENCBL series). Re-run auto-tuning after motor replacement as the HG-KR73’s rotor inertia differs slightly from the HC-KFS73, requiring gain recalibration.
Q: What is the HG-KR73’s maximum speed compared to the HG-SR73, and does it differ?
A: Both the HG-KR73 and HG-SR73 share the same 6,000 rpm maximum speed at 750 W — unlike the HG-SR302 (2,000 rpm rated) where the lower rated speed winding limits maximum speed. At 750 W, the KR and SR series both reach 6,000 rpm maximum, with the HG-KR73’s advantage being purely in acceleration response from its lower rotor inertia rather than any difference in rated or maximum speed.
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