What is a torque motor?

A torque motor is an electric motor designed to deliver substantial rotational torque, often directly to the load. Torque motors are established industrial products used in rotary tables, robot joints, winding mechanisms and precision machine axes.

Every rotary motor produces torque, but high output torque alone does not make a motor a torque motor. A conventional motor can deliver high torque through a gearbox. A torque motor is designed to produce the required moment within its intended speed range. Low-speed direct drive is common, although specialised high-speed designs also exist.

A simple way to understand torque

Torque is the turning effect of a force about an axis. Apply a 10 N force perpendicular to a 20 cm wrench, and the resulting torque is T = F × r = 10 × 0.20 = 2 N·m. Force is measured in newtons, lever-arm length in metres and torque in newton-metres. When converting torque units, keep force and turning moment distinct.

Torque can exist without rotation. A resisting door handle still experiences torque when you try to turn it. Similarly, a suitable motor can produce torque with its shaft stationary. The permitted amount and duration depend on the motor’s thermal limits.

Construction and operating principle

Common industrial torque motors generate torque through interaction between current in the stator windings and the rotor’s magnetic field. The stator and rotor are therefore the main electromagnetic components. Many modern designs are permanent-magnet synchronous motors; a large diameter and short axial length can suit direct-drive integration.

A frameless version is supplied as stator and rotor components for installation in the machine. With a frameless motor, bearings, housing, position feedback and cooling become part of the machine design. Torque motors are not all frameless: housed and complete motor assemblies are also available.

Torque motors also exist in brushed DC motor form. The term “torque motor” does not by itself specify AC, DC or brushless construction. Identify the technology, connections and compatible drive from the product documentation.

Direct drive versus a geared motor

In direct drive, the motor turns the load without a speed-reducing transmission. A geared arrangement runs the motor faster while reducing output speed and increasing output torque. For example, a motor delivering 3 N·m through a 20:1 gearbox at 90% transmission efficiency produces approximately 54 N·m at the output, which turns at one-twentieth of the motor speed.

The gearbox creates no energy: output speed falls and transmission losses occur. Direct drive can eliminate gear backlash and remove transmission components. However, load inertia, bearing behaviour and structural flexibility still matter. Removing a gearbox does not remove every mechanical limitation.

How do torque motors relate to servo control?

A torque motor can operate as the actuator in a servo motor system with suitable drive electronics and feedback. Torque motor describes the motor design; torque control describes how the system is commanded. A conventional servomotor can also run in torque mode without becoming a dedicated torque motor.

Speed control and torque control have different objectives. Speed control aims to maintain commanded speed; torque control aims to deliver commanded torque. A torque command does not guarantee a particular shaft speed. If the resisting load falls, the motor may accelerate, so appropriate speed limits and protection are necessary.

Torque and power are different quantities

The relationship between torque and power is P = T × 2π × n / 60, with T in N·m, n in rpm and P in watts. For example, 100 N·m at 60 rpm produces approximately 628 W of mechanical output. At zero speed, the same applied torque produces zero mechanical output power.

Zero shaft speed does not mean zero electrical consumption. A motor using current to maintain torque can still generate winding losses and heat. Selection therefore requires more than rated power: torque, speed, operating duration and cooling must be considered together.

Continuous, peak and standstill torque

Among the different torque ratings, continuous torque is available continuously under specified mounting and cooling conditions. Peak torque is generally available for a limited period. Standstill torque describes capability at zero speed; continuous and peak standstill ratings may be listed separately.

A motor rated for 10 N·m continuously and 30 N·m peak cannot automatically deliver 20 N·m continuously. Check permitted duration, repetition rate, initial temperature, drive current and cooling. Configure motor overload protection and temperature feedback as specified by the manufacturer.

The speed range matters too. A torque curve shows the torque available at different speeds. A motor suitable at low speed may not sustain the same torque at higher speed. Maximum torque and maximum speed should not be assumed available simultaneously.

Does an encoder measure torque?

Usually not. The encoder in a feedback system provides position and speed information. A suitable absolute encoder reports position for position control. The drive generally calculates or estimates motor torque using current measurements and its motor model.

Direct shaft torque measurement may require a torque sensor. Estimated electromagnetic torque and torque measured at the load can differ because of acceleration, friction and other losses. A current-based estimate is not equivalent to direct torque measurement in every operating condition.

What information is needed for selection?

When selecting a motor for its torque capability, establish the load’s running torque first. Then account for acceleration torque, friction, target speed, holding periods and cycle frequency. Maintaining constant speed and accelerating the load rapidly are different requirements.

Check motor-drive current and voltage compatibility, feedback interfaces, temperature limits, cooling, shaft loads and mounting conditions. Compare alternatives such as a stepper motor or geared servo against the same load and duty cycle. The largest catalogue torque figure does not identify the best system.

Typical applications

Torque motors are used in CNC rotary tables, robot joints, printing and packaging machinery, optical pointing systems and specialised winding applications. Direct drive can be attractive where transmission backlash and component count need to be reduced.

Precision is only part of the decision. High continuous torque also brings cooling and total system cost into consideration. Where a load must remain safely supported, active motor holding torque is not a substitute for a safety brake that functions after power loss.

Common commissioning mistakes

Sudden torque increases can overstress connections or move a load unexpectedly. Set suitable acceleration and deceleration ramps; where supported, an S-curve can provide smoother motion. Increasing the drive’s current limit does not automatically increase the motor’s continuous torque rating.

Frameless motor installation affects performance. Air gap, concentricity, rotor retention, bearings and heat transfer must meet the manufacturer’s requirements. Correct drive selection cannot compensate for incorrect mechanical installation.