What affects stepper motor torque?

Hello everyone. A stepper motor rated for 2 N·m holding torque does not deliver 2 N·m at every speed. Available running torque depends on speed, phase current, supply voltage, the driver and the mechanical load. Selecting a motor means checking more than its label: you need to know how much torque is available at the actual operating speed.

The stepper motor operating principle relies on winding currents creating a magnetic field that moves the rotor between positions. Torque is the resulting turning effect at the shaft, expressed in newton metres (N·m). The stepper motor FAQ provides further background on the basic terms.

Holding, starting and running torque are different

Holding torque: The maximum static torque an energised motor can resist at standstill under the specified current conditions. A driver's standby current reduction can lower it. This energised holding effect is lost when power is removed.

Pull-in torque: The torque–speed boundary within which the motor can start and stop without an acceleration ramp, for a given load and inertia. Greater load inertia makes an immediate start more difficult. In speed and position control, starting speed and target speed therefore need separate consideration.

Pull-out torque: The maximum load torque a running motor can sustain at a particular speed without losing synchronism. The manufacturer's speed–torque curve describes this boundary. Allow an application-specific torque margin for changing loads rather than operating directly on the limit.

Detent torque: The magnetic resistance to rotation felt with power removed from a permanent-magnet or hybrid stepper motor. It is usually much lower than holding torque and is ideally absent in a pure variable-reluctance motor. It should not be confused with torque multiplication through a gearbox.

Why does torque fall as speed increases?

At higher speeds, less time is available for winding current to reach its target. Winding inductance opposes rapid current changes, while back EMF from the rotating motor makes it harder to establish the required current. Available torque can consequently fall at high speed. The stepper driver's operating method has a direct effect on this behaviour.

The same motor can have different curves with different drivers and supply voltages. Check the curve's test voltage, phase current and drive mode. A curve measured under different conditions is not a guaranteed performance curve for your system.

Current, voltage and temperature

Phase current: Setting current too low reduces torque. Increasing it does not provide unlimited torque: magnetic saturation and heating impose limits. Follow the motor's allowable current rating and check the driver's RMS and peak current definitions. Identical numerical settings on different drivers may represent different winding currents.

Supply voltage: With a current-regulated driver, a suitably higher voltage can help winding current rise faster, particularly at higher speeds. Supply voltage selection must stay within the driver's recommended operating range and allow headroom for voltage excursions. Winding voltage and driver supply voltage are different specifications. Do not apply the higher supply voltage directly to a winding.

Power supply and temperature: A stepper motor power supply whose voltage sags under load can limit high-speed performance. Checking motor and driver temperatures after extended operation is part of thermal management. Use the manufacturer's temperature limits rather than a universal case-temperature rule.

Does 1/16 microstepping divide torque by sixteen?

No. When comparing microstepping and other drive modes, distinguish incremental torque for one microstep from total running torque. A smaller commanded step produces a smaller change in the restoring torque; this does not mean that 1/16 microstepping reduces total motor torque to one sixteenth.

Microstepping can smooth motion and reduce vibration. However, friction may prevent the rotor from moving by exactly one microstep for every command. Increasing command resolution does not improve actual positioning accuracy by the same factor. More pulses alone do not guarantee more accurate mechanical movement.

Worked example: Why a 2 N·m rating may not be enough

These figures describe an illustrative application. With all quantities referred to the motor shaft, assume a load torque of 0.35 N·m, friction torque of 0.05 N·m, total inertia of 0.002 kg·m² and angular acceleration of 100 rad/s².

Acceleration torque = J × α = 0.002 × 100 = 0.20 N·m
Required motor torque = 0.35 + 0.05 + 0.20 = 0.60 N·m

If the curve gives only 0.55 N·m at a speed reached during this acceleration, the motor is overloaded despite its 2 N·m holding torque. Reducing acceleration, changing the driver and supply combination, or selecting another motor may be necessary. With a gear mechanism, account for ratio, efficiency and the resulting motor speed together: a gearbox does not create free torque.

A practical approach to lost steps

Check the mechanism first: A binding slide, excessive belt tension or damaged bearing can increase the load. Raising current before completing maintenance and mechanical checks may conceal the underlying problem. A slipping belt or coupling can also resemble motor step loss.

Separate the operating conditions: A problem occurring only at startup points towards acceleration and inertia; one at high speed warrants checking the curve and supply; one within a narrow speed band suggests investigating resonance. These are starting clues, not a diagnosis. Common stepper system problems can have several electrical or mechanical causes.

Verify the command signals: Check pulse width, direction setup timing and interference on the STEP/DIR control signals. In an open-loop system, commanded pulse count is not proof of actual movement. An encoder-equipped stepper motor with a suitable driver can monitor position error, but feedback does not remove the motor's physical torque limit.

Technical references: Oriental Motor — speed and torque; Analog Devices — microstepping; Texas Instruments — winding current at high speed. Refer to the specific motor and driver manuals for device limits.