Choosing a Motor Technology

The motor families

Dozens of motor types exist, but a motion axis usually comes down to five: the brushed DC motor, the brushless (servo) motor, the stepper, the AC induction motor and, for special jobs, the linear motor or voice coil. They differ in how the torque is produced, how they are controlled, what they cost and where they run out of steam. A motor is never chosen alone: the drive, feedback and transmission that go with it decide whether it works (see Motion Control and Actuators).

Motor	How torque is made	Control	Feedback	Relative cost
Brushed DC	Brushes and a commutator switch the current in the rotor windings	Vary the voltage (PWM): the simplest	Optional	Low
Brushless DC (BLDC)	Electronics switch the stator phases according to rotor position	Electronic commutation from Hall sensors	Hall sensors at least	Medium
Servo (permanent-magnet synchronous)	Sinusoidal currents set by an encoder or resolver	Servo drive with position, velocity and current loops	Encoder or resolver, always	High
Stepper	Many magnetic poles: each pulse moves the rotor one step	Step and direction pulses to a driver	None (open loop) or an encoder	Low to medium
Induction (AC)	A rotating stator field induces current and torque in the rotor	Mains direct, or a variable-frequency drive	None; optional for vector control	Low per kW
Linear motor, voice coil	The same principles, unrolled to give a straight force	Servo drive	Linear scale	High

Torque-speed curves

The torque-speed curve is the most useful picture of a motor. It shows the torque available at each speed, and the load must fit below it. The curves in the figure are schematic, with torque and speed as fractions of their maximum.

• Brushed DC: torque falls in a straight line as speed rises. Stall torque is the highest and no-load speed the highest speed. Maximum power is at half the no-load speed.
• Servo: a flat torque region (limited by current) to near the top speed. The continuous curve (heat limit) is lower than the peak curve.
• Induction on mains: a fixed-speed motor: torque rises to a breakdown peak near synchronous speed. It is not a speed-control machine without a drive.
• Induction on a VFD: constant torque up to base speed, then constant power (field weakening). See AC Motors and Variable-Frequency Drives.
• Stepper: the most torque at rest and a quick fall with speed, with a resonance dip. See Stepper Motors and Drivers.

Brushed DC motors and the motor constants

A brushed DC motor is described by two constants that are the same number in SI units: the torque constant K_t (N·m per ampere) and the back-EMF constant K_e (volts per rad/s). The equations are T = K_t × I and V = I × R + K_e × ω. Every other quantity follows.

Brushed motors are cheap and need only a simple H-bridge (see Driving Motors and Actuators in the Electronics guides), but brushes wear, spark and limit speed and life. Coreless and ironless types are very light and fast, but their small thermal mass makes them easy to overheat.

Brushless and servo motors

A brushless motor has permanent magnets on the rotor and windings on the stator; electronics do the commutation the brushes did. Three Hall sensors give a six-step trapezoidal drive (BLDC, simple and economical). A high-resolution encoder or resolver allows sinusoidal field-oriented control, with smooth torque and precise positioning: that is a servo. Servo motors have low rotor inertia, high peak torque (often three or more times continuous) and a flat torque curve, which makes them the choice for fast, accurate axes. Their cost is the drive and feedback, and the tuning (see Servo Systems and Tuning).

• Speed and torque ratings: a catalog gives continuous torque at stall, peak torque, rated speed, maximum speed and K_t. Torque is proportional to current up to saturation.
• Voltage sets speed: the DC bus must exceed the back-EMF at top speed plus the drops, or the motor cannot reach it. Check the voltage against the speed you need.
• Inertia: low-inertia servo motors accelerate quickly, while higher-inertia types handle larger load ratios more easily (see Mechanical Transmission: Screws, Belts and Gearboxes).
• Frameless and direct-drive motors are built into the machine; torque motors give high torque at low speed without a gearbox.

Induction motors

The three-phase induction motor is the workhorse of industry: rugged, simple and efficient. Its rotor always runs slightly slower than the rotating field: the synchronous speed is n_s = 120 f ÷ poles, and the difference is the slip. On the mains it runs near-constant speed; with a variable-frequency drive it becomes a good variable-speed motor (see AC Motors and Variable-Frequency Drives).

Linear motors and voice coils

A linear motor is a rotary motor unrolled: a flat forcer moves along a magnet track, driving the load directly with no screw, belt or backlash. Speed and acceleration are very high and the stiffness is limited only by the servo, but the force per size is modest, the magnets attract the forcer strongly (it needs good bearings and cooling), and the cost is high. A voice coil is a short-stroke linear actuator like a loudspeaker: force is proportional to current, with no hysteresis, and it is used for fast, small, precise moves (focusing, pick heads, valves). Both need a linear scale and a servo drive.

How to choose

If you need…	Consider	Because
Lowest cost, light loads, positioning at low speed	Stepper (open loop)	No feedback or tuning; holds position
Speed above a few hundred rpm with torque	Servo or closed-loop stepper	Steppers lose torque quickly with speed
Fast, accurate, dynamic motion	Servo	High peak torque, low inertia, flat curve
A pump, fan, conveyor or mixer	Induction motor with a VFD	Rugged, economical, speed control only
A cheap small actuator or toy	Brushed DC	Simple drive, low cost
Smooth, silent motion at low speed	Servo, or a microstepped stepper	Torque ripple matters
Direct drive, no backlash, high precision	Linear or torque motor	No transmission
Explosive or wet environments	Pneumatic or special-rated motors	See Pneumatic and Hydraulic Actuators

Heat, duty and ratings

• Continuous torque is a heat limit. Copper loss rises with the square of current: doubling the current quadruples the heat.
• Ratings assume a stated heat sink and ambient (often 40 °C and an aluminum mounting plate). A motor in a hot enclosure or with poor mounting has less.
• Winding insulation class (B 130 °C, F 155 °C, H 180 °C) sets the maximum winding temperature. Life halves for roughly every 10 °C above the limit.
• Cooling: self-cooled motors lose cooling at low speed, so constant-torque operation at low speed with a self-cooled induction motor needs a separate fan (forced ventilation).
• Duty cycle: a motor can deliver more than continuous torque for short periods; compute the RMS torque over the cycle (see Motion Control Basics).
• Altitude and enclosure: above 1 000 m derate; IP ratings tell you about dust and water, not about cooling.

Related

• Motion Control and Actuators: the overview of this guide.
• Stepper Motors and Drivers, Servo Systems and Tuning and AC Motors and Variable-Frequency Drives: the individual technologies.
• Sizing a Motion Axis: the full sizing method.
• Driving Motors and Actuators: the drive electronics (H-bridge, PWM, flyback).
• Motor Torque and Inertia Matching Calculator.

FAQ

Servo or stepper?

Choose a stepper when the load is light and known, the speed is low (a few hundred rpm), cost matters and open loop is acceptable. Choose a servo when you need speed, torque headroom, accuracy under changing loads, or feedback that tells you when something is wrong.

What is the difference between BLDC and a servo motor?

Mechanically they are similar. The difference is the control: a BLDC drive uses Hall sensors for simple six-step commutation, a servo drive uses an encoder for smooth sinusoidal control and closed position and velocity loops.

Can I run an induction motor above its rated speed?

On a drive above the base frequency the motor enters constant-power operation: torque falls with speed. Check the mechanical limit of the rotor and bearings, and the manufacturer’s maximum speed.

Why does my brushed motor get hot at low speed?

At low speed with a load the current stays high while there is almost no back-EMF to oppose it, so the winding dissipates I²R with little cooling from its own fan.

References

• Hughes, A. and Drury, B., Electric Motors and Drives, 5th ed., Newnes.
• Parker Hannifin, Servo motor sizing and selection guide; Maxon, Selection Guide (motor constants and equations).
• IEC 60034-1, Rotating electrical machines: rating and performance (insulation classes, duty types).

Last reviewed: October 2026. Motor constants and curves are typical examples; use the manufacturer’s datasheet for real values.