Servo and stepper motors

In short:Use a stepper for low-cost, low-to-medium speed positioning where torque at speed and efficiency matter less; use a servo where you need torque across a wide speed range, high dynamics or continuous duty. Size either one from the torque-speed curve, RMS torque and inertia ratio, with margin.

Where servo and stepper motors fit

Housed servo and stepper motors are the default for positioning, indexing and speed control on production machines. They drive screws, belts, gearboxes and rotary tables, and they are where most OEM engineers start before considering frameless motors or direct-drive linear motors.

The basic choice is simple. A stepper is the lowest-cost way to position a load at low to moderate speed. A servo costs more and needs tuning, but gives torque across a much wider speed range, peak torque in reserve and closed-loop confirmation of position. Closed-loop steppers and integrated servos fill the gap between the two.

Motor types compared

Type	Feedback	Typical strengths	Limitations
AC servo (mains-fed)	Absolute or incremental encoder, resolver	Wide speed range, high peak torque, continuous duty	Cost, tuning, separate drive and cables
Low-voltage DC servo	Encoder	24–96 VDC safety and battery systems, AGVs, robots	Higher current for the same power
Integrated servo	Encoder, drive on motor	Less cabling and cabinet space	Shared heat, lower power range
Hybrid stepper (open loop)	None	Low cost, high low-speed torque, simple	Torque falls with speed, can stall unnoticed, runs hot
Closed-loop stepper	Encoder	No lost steps, cooler, faster settling	Top speed still limited
Integrated stepper	Optional encoder	Compact, fieldbus or pulse control on the motor	Limited to smaller frames

AC servo motors

Mains-fed brushless servo motors from 50 W to several kW cover most machine axes. Ranges are usually offered in low, medium and high inertia versions. Low inertia suits fast indexing on stiff mechanics; higher inertia tolerates larger loads and less rigid couplings. Examples include Kinco SMK, Siemens SIMOTICS S-1FK2, Allen-Bradley Kinetix VPL and SANMOTION R2. Food and pharmaceutical machines use stainless washdown motors such as Kinetix VPH or SIMOTICS S-1FS2.

Low-voltage DC and integrated servo motors

Battery-powered AGVs, cobots and distributed machines often run at 24–60 VDC. Low-voltage servo ranges such as Leadshine ELVM or Kinco SMC low-voltage suit them. Integrated motor-plus-drive units such as Leadshine iSV2 and Kinco iSMK take the drive out of the cabinet and connect over CANopen, Modbus or pulse/direction.

Hybrid and closed-loop steppers

Two-phase hybrid steppers with a 1.8° step are the workhorse, for example the Leadshine CM series or SANMOTION F2. Five-phase versions such as SANMOTION F5 use a finer 0.72° step for lower vibration. Closed-loop motors such as Leadshine CS-M add an encoder; integrated versions such as Novanta Liberty MDrive put drive and control on the motor.

Servo vs stepper: what actually differs

Parker’s comparison puts numbers on the trade-offs (Parker):

• Torque at speed. In Parker’s example, a stepper’s torque had fallen by almost half at 1,200 rpm and its usable top speed was about 1,200–3,000 rpm. The servo held near-constant torque to about 3,600 rpm, with typical top speeds of 5,000–7,500 rpm.
• Peak torque. Servos can deliver about three times rated torque for short periods. A stepper’s holding torque is its ceiling.
• Heat and efficiency. Open-loop steppers draw full current whether the load needs it or not, so they run hot. Servos draw only what the load requires.
• Resonance. Steppers have mid-range resonance bands, typically between about 60 and 300 rpm, which microstepping and anti-resonance features help with.
• Stall behaviour. An open-loop stepper that stalls loses position silently. An encoder, on a closed-loop stepper or a servo, detects it.

The servo vs stepper guide covers the decision in more depth.

How to size a servo or stepper motor

1. Build the move profile

For each segment (accelerate, constant speed, decelerate, dwell), work out the torque: load torque (friction, gravity, process force reflected through the mechanism) plus acceleration torque, T = J_total × α. Include motor rotor inertia, coupling, gearbox and load reflected to the motor shaft.

2. Check peak torque against the curve

Plot each torque and speed point on the motor’s torque-speed curve. For a servo, the peak (usually during acceleration) must fall inside the intermittent zone at that speed. For a stepper, every point must sit below the pull-out torque curve for your drive voltage and current, with margin. Higher supply voltage generally extends a stepper’s torque to higher speeds.

3. Check RMS torque for heating

RMS torque is the constant torque that would heat the motor the same as the real cycle:

T_rms = √[(Ta²·ta + Tc²·tc + Td²·td + Th²·th) / t_total]

It must sit inside the servo’s continuous zone (Motion Control Tips). Include holding torque during dwell on vertical axes.

4. Apply margin

Oriental Motor gives reference safety factors of about 2 for 2- and 5-phase steppers, and 1.5–2 on servo rated and RMS torque (Oriental Motor). It also suggests running steppers at 50% duty or less for temperature reasons.

5. Check the inertia ratio

Load-to-motor inertia ratio affects tuning, overshoot and settling. Most sizing guides aim for 10:1 or less (Motion Control Tips). Oriental Motor publishes a reference limit of 30:1 for its steppers and up to 50 times rotor inertia for its servo motors. Stiff, direct-coupled mechanics tolerate higher ratios than belts or long shafts.

A gearbox divides reflected inertia by the square of the ratio. Worked example: a 20 kg·cm² load on a 0.5 kg·cm² motor is 40:1 direct. Through a 3:1 gearbox the load reflects as 20 / 9 ≈ 2.2 kg·cm², giving a ratio of about 4.4:1, and the motor needs a third of the torque. The gearbox adds its own inertia and backlash, so see gearboxes for the trade-offs.

6. Choose feedback

Incremental encoders are cheapest but need homing after power-up. Absolute encoders (single- or multi-turn) know position at switch-on, which matters on robots and axes that cannot be homed safely. Resolvers suit high-temperature or high-shock duty. The absolute vs incremental encoder guide explains the options.

7. Check the frame and mounting

NEMA frame sizes (NEMA ICS 16) define the stepper faceplate in tenths of an inch: NEMA 17 ≈ 42 mm, NEMA 23 ≈ 57 mm, NEMA 34 ≈ 86 mm. Servo motors are usually specified by metric flange size (40, 60, 80, 130 mm and so on). Neither standard fixes shaft diameter, pilot or length across manufacturers, so check the drawing before you swap brands.

Common mistakes

• Sizing a stepper on holding torque. Holding torque is a standstill figure; what matters is torque at your running speed.
• Ignoring the drive. Stepper torque curves depend on drive voltage and current. Servo peak torque is limited by the drive’s peak current.
• Leaving out reflected inertia. Couplings, pulleys and gearboxes often add more inertia than expected.
• Only checking the peak. A motor that passes the peak check can still overheat on RMS.
• No brake on vertical axes. Neither motor type holds a load safely without power.
• Overlooking cable length and EMC. Long motor cables on servo drives may need filters or chokes; check the drive manual.

What to send an engineer for a quote

• Move profile: distance, time or speed, acceleration, dwell, cycles per minute
• Load: mass or inertia, mechanism (screw lead, pulley diameter, gear ratio), friction and process forces, orientation
• Accuracy, repeatability and settling time required
• Supply voltage (mains AC or DC bus) and preferred drive or controller
• Control interface: EtherCAT, CANopen, PROFINET, Modbus, pulse/direction or analogue
• Feedback needs: absolute or incremental, multi-turn, safety functions such as STO
• Environment: IP rating, temperature, washdown, vibration
• Mounting constraints: frame size, shaft, brake, connector orientation

Add these to the spec builder below and our supply network will propose matched motor and drive combinations; see also servo drives and motion controllers.

Guides

Where servo and stepper motors are used

What we need to know to specify servo and stepper motors

These are the questions an applications engineer works through first. Approximate answers are fine: ranges narrow the options quickly, and we confirm the details with you.

• Continuous torque: Under 1 Nm, 1–5 Nm, 5–20 Nm, Over 20 Nm
• Speed: Under 500 rpm, 500–3,000 rpm, Over 3,000 rpm
• Supply: 24–48 V DC, 230 V AC, 400 V AC
• Feedback: Open loop (stepper), Incremental encoder, Absolute encoder, Resolver
• Frame size: NEMA 17/23, NEMA 34/42, 60–80 mm, 100 mm and above

We also ask about the environment (washdown, outdoor, subsea, ATEX, vacuum, shock and EMC), annual quantities and timescale, because these decide whether a standard, modified or custom design is the best fit.

151 series compared

Series from 43 manufacturers, grouped by type. Figures are the highest published for each series. Open a series for the full specification and, for brands outside our supply network, the alternatives that meet or exceed it.

AC servo motor (54)

DC servo motor (21)

Hybrid stepper motor (21)

Integrated servo motor (11)

Smart servo actuator (10)

Integrated stepper motor (9)

Coreless DC motor (7)

Stainless servo motor (5)

Closed-loop stepper motor (4)

Drone outrunner motor (2)

Slotless BLDC motor (1)

AC induction motor (1)

Geared PMDC motor (1)

Brushed DC motor (1)

Brushed DC servo motor (1)

Servo wheel drive (1)

Servo motors (1)

Summarised from manufacturers' published data. Always confirm against the current datasheet; an engineer will check the fit before recommending anything.

Brands

Frequently asked questions

What is the main difference between a servo and a stepper motor?

A stepper moves in fixed steps and usually runs open loop: it produces high torque at low speed, but that torque falls as speed rises and it draws full current even when lightly loaded. A servo runs closed loop from an encoder, delivers near-constant torque up to rated speed, can give about three times rated torque for short peaks, and only draws the current the load needs.

What is a closed-loop stepper and when should I use one?

A closed-loop stepper adds an encoder to a hybrid stepper so the drive can detect and correct lost steps and, on many drives, reduce current when the load is light. It runs cooler and settles faster than an open-loop stepper while keeping the stepper's strong low-speed torque and simple tuning. It still has the stepper's limited top speed, so it suits short, frequent moves rather than high-speed axes.

What inertia ratio should a servo motor have?

Most sizing guides aim for a load-to-motor inertia ratio of 10:1 or less, and highly dynamic axes may need 5:1 or lower. Modern drives can control higher ratios on stiff mechanics, and some manufacturers publish limits of 30 to 100 times rotor inertia for particular motors. A gearbox reduces reflected load inertia by the square of the ratio, which is usually the simplest fix.

What does NEMA 17, 23 or 34 mean?

NEMA frame sizes describe the motor's square faceplate in tenths of an inch: NEMA 17 is about 1.7 in (roughly 42 mm), NEMA 23 about 2.3 in (57 mm) and NEMA 34 about 3.4 in (86 mm). The numbers fix the mounting face, not torque, length or shaft details, which vary between manufacturers. Always check the drawing for shaft diameter, pilot and hole pattern.

Is an integrated servo motor worth it?

Integrated servo motors put the drive, and often a controller, on the back of the motor. They save cabinet space and cabling, which helps on AGVs, conveyors and distributed machines running from 24 to 48 VDC. The trade-offs are that motor and drive heat share one housing, a failure means replacing both together, and power ranges are usually lower than for separate motor and drive.

How much torque margin should I allow?

Oriental Motor recommends a safety factor of about 2 for open-loop steppers and 1.5 to 2 for servo rated and RMS torque. Parker suggests about 50% extra torque for open-loop steppers to cover mid-range resonance. Steppers need more margin because a stall loses position without warning; servos have peak torque in reserve and report faults.

Sources

1. Comparison of motor technologies: servo, stepper, stepper with encoder (Parker)
2. Selecting a motor: selection points (Oriental Motor)
3. How do I determine what gear ratio I need for my servo system? (Motion Control Tips)
4. How to calculate continuous and peak torque values for servo applications (Motion Control Tips)
5. NEMA ICS 16-2001 Motion/Position Control Motors, Controls and Feedback Devices (ANSI webstore)

Figures are summarised from the sources above and from manufacturers' published data on the date shown. Always confirm against the current datasheet for your exact part.