Motor Concepts
From spec terms like diameter, power, and torque to how BLDC, brush DC, and stepper motors actually work
How to Read a Motor Spec Table
Spec tables on our product pages are full of terms that can be unfamiliar — diameter, continuous power, torque, and more. The table below covers the core terms that recur throughout the spec tables across this site.
| Term | Meaning |
|---|---|
| Diameter (mm) | The outer diameter of the motor housing. It's the most basic dimension for determining mounting space, and a larger diameter generally allows more torque headroom. |
| Continuous Power (W) | The mechanical power the motor can sustain indefinitely without overheating. It can briefly exceed this figure, but sustained operation above it can overheat and damage the coils. |
| Torque (mNm) | The rotational force at the motor shaft. 1 mNm is the torque produced by 1 mN (about 0.1 gf) of force applied 1 mm from the shaft center. There are several types of torque, covered in detail below. |
| No-load Speed (rpm) | The maximum rotational speed the motor reaches with no load applied. Actual speed drops once a real load is applied. |
| Torque Constant (mNm/A) | The ratio of torque produced per amp of coil current. A key electrical characteristic used to calculate how much current is needed for a given torque. |
| Weight (g) | The mass of the motor alone. For weight-sensitive applications like drones, medical devices, and wearable robotics, power-to-weight ratio (power density) is often the deciding factor. |
Torque Comes in Several Types
Spec tables can list several different items all called "torque." They share the same mNm unit, but each means something different.
| Torque Type | Meaning |
|---|---|
| Continuous Torque | The torque the motor can sustain indefinitely without overheating. This is the reference figure for most normal operating conditions. |
| Stall Torque | The torque the motor can produce while the shaft is held stationary and not turning. It's momentarily high, but sustaining this state generates significant heat in the coils and can cause damage. |
| Peak Torque | The maximum torque allowed for a very brief period, such as during acceleration. |
| Holding Torque (steppers only) | The force that holds a powered, stationary shaft in place. It's why a stepper motor can hold position without a separate brake. |
| Detent Torque (steppers only) | The resistive torque produced purely by the magnetic pull of the internal permanent magnets, even with power off. It's the source of the subtle "clicking" feel when you turn the shaft by hand. |
The Relationship Between Power, Torque, and Speed
Power, torque, and speed are connected by the following relationship.
Power (W) = Torque (N·m) × Angular velocity (rad/s)
In other words, for the same power, a faster-spinning motor produces less torque, and a slower one produces more. When a motor alone can't provide enough torque, a gearhead (speed reducer) is often added to trade off speed for torque.
What Is a Brush DC Motor?
In a brush DC motor, brushes and a commutator — contact components that feed current into the coils wound on the rotor — mechanically reverse current direction as the rotor turns. Because it spins with nothing more than an applied voltage and no separate control electronics, it's the simplest drive method, with inexpensive, simple drive circuitry.
However, because the brushes are in continuous friction contact with the rotor, they wear down over time, limiting service life, and the contacts can generate sparking and electromagnetic interference (EMI). Precision-machined brush DC motors like Portescap's Athlonix® series minimize these drawbacks, making them widely used where cost-performance matters.
What Is a Brushless DC (BLDC) Motor?
As the name implies, a brushless DC (BLDC) motor has no brushes or commutator. Instead, a Hall sensor or encoder senses rotor position, and an external electronic drive switches the current sequence through the coils electronically to match that position. This process is called commutation (electronic commutation) — and it's exactly the role played by PMD's ION®/CME drives and Magellan® ICs, covered on our PMD Motion Control Solutions page.
With no mechanical friction parts, BLDC motors last longer than brush DC motors, can run at higher speed and efficiency, and generate less electromagnetic noise since there's no sparking. The tradeoff is that a dedicated drive electronics package is required for the motor to turn at all, adding system complexity and cost. Portescap manufactures both slotted and slotless (ironless winding) BLDC designs; the slotless design has no cogging (the subtle vibration caused by magnetic attraction between magnets and iron core), making it the go-to choice for applications like aerospace and medical devices that demand smooth, quiet rotation.
What Is a Stepper Motor?
A stepper motor energizes multiple coils arranged around the rotor in a fixed sequence, moving the shaft in discrete, fixed-angle increments rather than spinning it continuously. Each single increment of movement is called a step, and the motor's biggest advantage is that its position can be tracked simply by counting steps — enabling open-loop position control without a position sensor (encoder).
| Type | Characteristics |
|---|---|
| Can-Stack Motor | The simplest, most economical stepper design, built from claw-tooth pole stator cups. Well suited to applications needing reasonable accuracy and moderate torque. |
| Hybrid Stepper Motor | A precision stepper that combines a permanent magnet with many fine gear-like teeth, delivering a smaller step angle and higher torque density than can-stack designs. |
| Disc Magnet Stepper | Uses a thin disc-shaped permanent-magnet rotor for very low inertia and fast response — a design unique to Portescap. |
| Linear Stepper Motor | Moves in discrete steps along a straight line rather than rotating. Enables precise linear positioning without a leadscrew, used in valves, optical focus adjustment, and more. |
Comparing Motor Types at a Glance
| Aspect | Brush DC | Brushless DC (BLDC) | Stepper Motor |
|---|---|---|---|
| Drive method | Mechanical commutation via brushes/commutator | Electronic commutation via external drive | Sequential coil energizing (fixed-angle steps) |
| Service life | Relatively shorter due to brush wear | Long | Long |
| Position control | Requires an external sensor (e.g. encoder) | Requires an external sensor (or sensorless) | Position can be estimated open-loop |
| Efficiency & speed | Moderate | High | Relatively lower (suited to low-speed, high-torque range) |
| System cost & complexity | Low (simple drive circuitry) | High (dedicated electronic drive required) | Moderate |
| Typical applications | Power tools, low-cost actuators | Medical devices, aerospace, robotics, semiconductor equipment | Valve control, printers, robot joints, optical alignment |
Compare Rotation Principles Interactively
Click each tab to compare how the rotor moves and how commutation differs across the three motor types. Watch how the rotor (center, marked N/S) and the surrounding signals (blue coils / brush contacts) move differently for each type.
The rotor spins smoothly and continuously, while fixed brush contacts (the white squares on the left and right) sweep across the commutator, mechanically reversing current direction. An orange spark appears each time a contact sweeps past.
The rotor spins smoothly and continuously, and the outer stator coils are activated electronically in sequence to follow it (electronic commutation). With no mechanical contacts, there's no wear or sparking.
The rotor doesn't spin smoothly — it moves in discrete, fixed-angle "steps." Each time a pair of coils is energized in sequence, the rotor advances by one step.
Next Step
Once you're comfortable with the terminology and principles, answer a few questions in our Motor Selection Guide to find a model matching your requirements right away.
Go to the Motor Selection Guide →