There isn’t much of a debate really between brushed and brushless motors. For new builds and applications, brushless motors account for upward of 90% of the total market. Brushless motors remove the wear and arcing associated with using brushes and are generally more efficient and durable. Legacy systems can often benefit from replacing existing brushed motors with brushless, but in a few cases a brushed motor could be the preferred choice when the control electronics need to remain very simplified. We’ll break down the benefits of brushless motors and then explain why in certain situations brushed motors may be a better choice.
Brushed and Brushless: What’s in a Name?
The main difference between the two motors is in the name: one has brushes and one doesn’t. Brushed DC motors have a slotted iron core with windings that are attached to a commutator. The brushes carry the current to the windings. In brushless motors, on the other hand, electronic commutation determines the sequence for energizing the stator windings rather than using a physical commutator and brushes. They have a simpler mechanical construction but have more complex microprocessors and control architectures, with upward of eight wires needed for the motors and controls.
If brushless motors are simple mechanically and complex logically, then brushed is the opposite. They have more complex mechanical construction due to the commutator bars and brush assemblies, but they only need two to four wires for the motor and control. This makes them easier to control through a linear torque-speed relationship. However, brushes and commutators on the motors wear out, limiting motor life, and because they spark when in use, brushed motors are generally unsuitable for hazardous locations.
Still, brushed motors can have an advantage in applications that require a large through bore, as this can be as large as 70% of the motor’s outer diameter. The through bore can be used, for example, as a route for lead wire, as a mounting area for other hardware, or as an optical path.
Comparing Performance
In most applications, a brushless motor has a general performance advantage over brushed. It has higher power, a smaller form factor, higher torque density, higher speeds, more complex controls for more precise velocity and position, and it easily dissipates heat. Less internal friction due to the lack of brushes also makes for a more efficient and longer-lasting motor.
It might seem then that brushed motors don’t have as much performance to offer their brushless counterparts motors, but they do have other advantages. Because of the high number of slots, brushed motors have lower torque ripple than brushless (for more on torque ripple see our blog Cogging Torque and Torque Ripple: What You Need to Know. They also have high stall torque and smooth, high torque at low speeds.
Brushed or Brushless: Which to Use
When it comes to new builds and applications where high performance and complex control is needed and part of the system design, brushless is the default choice. However, when specific performance requirements are needed for an application, brushed motors can be a better fit. When paired with simple controls, they are useful in certain applications:
- Positioning systems that require a steady hold of the load. The high stall torque (“stand-still” operation) provides this.
- Speed control systems that can use the high torque at low speeds (<100 rpm).
- Large through bore diameter to OD ratio.
- Legacy applications with brushed motors where moving to brushless could be cost prohibitive due to required upgrades in the control architecture.
When to Upgrade to Brushless
Brushed motors in a legacy system can be updated to a brushless motor, but it will require more than just swapping out one motor for another. Contrasted to the simple motor controls of a brushed motor, brushless motors use electronic means to determine the sequence for energizing the stator. This will require upgrading the electronic controls for the system. The motor is just one component of a system, so it’s important to look at the system as a whole to determine if it would benefit from changing to a brushless motor or staying with the current architecture.
While modern, high-performance systems use brushless motors, brushed motors can provide benefits for legacy systems or new systems that require a high stall torque, high torque at low speeds or simple controls. Kollmorgen has the engineering and product expertise, with a broad portfolio of standard and easily modified motors, to service existing analog systems, develop a new brushless system, or upgrade a system from a brushed to brushless motor.
