Torque motor

01

Read continuous and peak torque with the cooling

HIWIN's torque motor catalogue describes its TM-2 motors as wear-free direct drives without a reduction mechanism, with a large hollow shaft, low cogging torque from a high pole count, and built-in temperature sensors. For the TM-2-1A-LA0, it lists continuous torque of 90 N·m and stall torque of 74 N·m water-cooled, peak torque of 150 N·m for 1 s, a torque constant of 9.53 N·m/A, 22 poles, rotor inertia of 0.0033 kg·m² and a maximum DC bus of 750 V. Specifications carry a ±10% tolerance. [1]

Editorial calculation: peak torque is 150 ÷ 90 ≈ 1.67 times continuous, and only for 1 s, so size the motor on the RMS torque of the duty cycle and use peak torque for short acceleration. Continuous torque here assumes water cooling; ask what the rating is with your cooling. [1]

Give the supplier the load inertia and friction, motion profile (angles, times, dwell), required continuous and peak torque, speed, accuracy, cooling available, hollow-bore and mounting size, feedback (encoder type and resolution), drive and DC bus, and environment.

02

Check heat, speed and temperature protection

The same datasheet gives a motor constant of 2.57 N·m/√W at 25 °C, water-cooled thermal resistance of 0.072 K/W, resistance of 9.2 Ω line to line at 25 °C, maximum speed of 437 rpm at continuous torque and 268 rpm at peak torque, and temperature sensors PTC100, PTC130 and Pt1000. [1]

Editorial calculation: copper loss at continuous torque is about (90 ÷ 2.57)² ≈ 1,230 W, and with 0.072 K/W the winding runs about 1,230 × 0.072 ≈ 88 K above the coolant. [1] Because Km is quoted at 25 °C and winding resistance rises with temperature, the real loss at operating temperature is higher; ask suppliers for torque and loss at operating temperature.

Connect the temperature sensors to the drive so it can protect the winding, and confirm that the required speed is available at the torque you need: in the example, speed falls from 437 rpm at continuous torque to 268 rpm at peak. [1]

03

Check bearing loads, runout and encoder on a housed direct-drive motor

IAI's catalogue describes its DD motor as a housed rotary actuator driving a rotary table without belt or reduction gears. Its DD-H18 high-torque type lists 600 W, a rated torque of 25 N·m and a maximum instantaneous torque of 75 N·m, an allowable load inertia of 1.8 kg·m² against a rotor inertia of 0.0106 kg·m², speeds of 1 to 1,440 deg/s, a ø20.5 mm hollow bore and 0 to 40 °C operation. The rated torque applies only when the motor sits on a mounting surface that dissipates heat like a 400 × 400 × 10 mm aluminium plate. [4]

IAI bases these load ratings on a life of five years at 8 hours a day at rated speed with smooth operation without shock, and asks to be consulted where a 20-bit unit would exceed the allowable dynamic moment or thrust load. [4]

Editorial calculation: peak torque is 75 ÷ 25 = 3 times rated, against 150 ÷ 90 ≈ 1.67 for 1 s in the water-cooled frameless example above; the load inertia allowance is 1.8 ÷ 0.0106 ≈ 170 times the rotor inertia; 1,440 deg/s is 240 rpm; and five years at 8 hours a day is about 14,600 operating hours. [4] [1]

For a housed direct-drive motor, give the supplier the load inertia, axial load, overturning moment and runout you need, the mounting orientation and the heat path of the mounting surface.

04

Read frameless datasheets: test conditions, cooling and sizes

ETEL's torque motor brochure describes a torque motor as a frameless motor with no housing, bearings or feedback device, which the machine builder selects or buys as an assembly, and says sizing is based on torque rather than power: peak torque is the maximum the motor produces, continuous torque what it can supply continuously, and the duty cycle decides which one governs. It associates light duty cycles with high short peaks with air-cooled series and rising continuous torque with liquid cooling, and states a standard range of 140 to 1,290 mm outside diameter and 38 to 42,900 N·m peak torque. [2]

Tecnotion states all values ±10%, a PTC 1 kΩ / KTY83-122 temperature sensor, a maximum allowable speed of 16,500 rpm for the series, and that the continuous values apply only with the mounting surface at 20 °C and the motor at maximum continuous current. [3]

Editorial calculation: with Km² = 0.127 N²·m²/W, copper loss at 5.4 N·m is 5.4² / 0.127 ≈ 230 W at 25 °C coils; the stated 300 W at 100 °C coils reflects the higher resistance when hot. 300 W × 0.25 °C/W = 75 °C rise, which from a 20 °C mounting surface gives about 95 °C, close to the 100 °C coil condition. Peak torque is 6.7 / 5.4 ≈ 1.24 times continuous here, against 150 / 90 ≈ 1.67 in the water-cooled example above, so compare ratios only at the same stated conditions. [3] [1]

Ask for each torque figure with its test condition (coil temperature, temperature rise rate, cooling and mounting surface temperature), the thermal resistance basis, and the sensor type.

05

Rotary tables, frameless kits and reducers alongside the motors

HIWIN lists torque motors and rotary tables with drives; CSK lists DDR torque motor series with linear motors; Mosrac lists frameless and housed direct-drive rotary motors with encoders and joint modules; Honpine lists frameless torque and DD motors with harmonic and RV reducers.

06

Stator, magnet rotor, sensors, bearing and encoder

The bearing and housing row separates the two forms on this page: a frameless motor comes without them, while a housed motor such as IAI's DD-H18 carries its own runout and load ratings. [2] [4]

07

Torque, speed, bus voltage and bore: quoting a direct-drive axis

Ask for every torque value together with its condition; the rated torque row here sets a water-cooled figure beside one stated at 100 °C coil temperature. [1] [3]

08

Send the load, duty and cooling data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Load: [inertia; friction; external torque]
Motion: [angles; times; dwell; RMS and peak torque]
Speed: [maximum rpm at torque]
Cooling: [water; air; coolant temperature]
Feedback: [encoder; resolution; accuracy]
Mechanics: [bore; outside diameter; frameless or housed]

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