Brushless Servo Motor vs. Brushed Motor

We live in a motor-driven world. Your Nespresso machine, father’s van, and even animated Santa’s figures in malls are all about motors – tools that convert x type of energy in the mechanical.

Image by Photorama from Pixabay
Image by Photorama from Pixabay

Water, steam, heat, or wind-powered – the motors’ family is big and diverse, but the most used motors nowadays are electric. If you Google „electric motors”, a range of options will appear: from brushed DC to brushless AC servos. Let’s find out more about the “brush” feature. 

A brushless servo motor vs a brushed one

First, let’s discuss the construction differences of brushless vs brushed motors. 

In a brushed motor, a stator with two magnets surrounds a turning rotor (or armature). When connected to a current source, opposite polarities create a magnetic field torque. Because of this, the rotor starts turning around its axis. 

The flow of the electric current between the rotor and stationary part of the machine is switched with a commutator. The commutator consists of a collector (a set of contacts located in the rotor) and brushes (contacts located outside the rotor and pressed to the collector). 

Each time the rotor turns by 180°, the commutator reverses the direction of the electric power to create a one-way torque to keep the motor turning.

Brushless servo motor (for example, BLDC) has permanent magnets (two or more) attached to the rotor which turns around a fixed stator (in an external-rotor configuration). No brushes. A special solid-state circuit is used to control the current supply to the stator. This electronic sensor defines the rotation speed, and the angle of the rotor. It controls transistors, reversing the direction of the electric current, so that the electromagnets can create torque that flows in one direction. Brushless motor servos with permanent magnets are synchronous: rotors and stators turn in this system at the same frequency. 

Pros and cons of brushed motors

Advantages

+       Lower price: brushed motors are cheaper than their brushless brothers. So for your budget, a brushed motor can be a good money-saver.

+       Higher resistance: Brushed DC motors are simple. This simplicity ensures good motor performance even in the most extreme environments. The absence of external components and electronics lets you apply brushed servomotors in dusty, contaminated, and rough environments. 

+       Simple control: Brushed two-wire motors do not require any extraordinary skills for you to run or integrate them. Simple control schemes is what made this type of motors popular among hobbyists and students.  

 

Disadvantages

-       Lifetime: Brushes are the Achilles tendon in these motors. They must be regularly cleaned, checked and replaced if needed. Depending on the material, brushes in BLDC can be more or less robust, fit for lower or higher current and speed, create more or less noise, etc. Precious metals such as gold, silver, and platinum are used in some brushes in order to provide lower contact resistance and prevent graphite dust particles (which is crucial for sensitive optical mechanisms). But as these metals are much softer than graphite, extreme loads are fatal for them. Some brushes need special maintenance with dopants to increase their electrical conductivity. 

-       High risks at high speed and torque: When torque overpasses, brushes can burn out. When speed exceeds, brushes can fly out. When the friction is too high, sparks can occur. 

-       Size: Commutator and brushes make motors bigger and heavier. It makes this motor type mostly inapplicable for robotics, where size matters. 

-       Noise: Commutator produces steadily mechanical noises, which makes such motors unlikely to be used in household appliances were quietness is quite important.

Pros and cons of servo brushless motor 

Disadvantages

+       Higher output: Brushless motors win in speed and torque control. As long as there are no brushes, there is no power loss across them and no other brush-related issues as well. Rejection of commutator in favor of electronics turns a brushless motor into a more efficient smart drive solution. Some studies assume brushless motors to be 5-15% more efficient than their brushed competitors are. When converting electricity into mechanical power, servo brushless motors are more efficient than brushed motors. 

+       Low maintenance: To put it simply - no brushes no crushes. No mechanical or electrical noise, no sparks or overheating. The motor’s lifetime isn’t limited by the brushes’ life anymore.

+       Compact size: As brushless stepper motors do not include commutators, they are more compact and light. Small brushless drives are used in PCs, in most of the drones, home electronics, and, of course, in robotics.

+       Positioning accuracy: Positional sensors create a reliable closed-loop system, ensure high accuracy and rotation repeatability. These sensors require significantly less maintenance than brushes, but at the same time, they are sensitive to vibrations, dust, and other contaminants. So this motor is smart but a bit fussy when it comes to workplace conditions.

Minuses

-       Price: Bad news. Servo brushless motors are more expensive than brushed. Sometimes, an electronic controller can cost almost as much as the motor itself. Good news. The technology was introduced in the 1960s. Since then, servomotors flooded the market in hundreds of sizes and at different prices. So you have a wide choice according to your budget and project needs.

-       Complex control: Brushless servo motors are advanced mechanisms driven by electronics. To control and adjust them, one requires advanced knowledge.

When to apply a small but powerful brushless motor

Brushed, or not brushed, that is the question. To meet the right choice, first, make it clear for yourself: where will you apply the servomotor? What environment will surround the drive? Are the motor size and weight crucial for the finished product? What are the safety standards? What maximum torque should a servo give? What is the budget? 

The above-given comparison shows brushless servomotors in a better light, but the brushed motor can also be your favorite workhorse if chosen, applied, and maintained correctly. It’s silly to buy a forklift if you only want to put a star on the top of your Christmas tree. Same with motors. It makes no sense to use a more efficient, long-living brushless motor for a motorized toy that your child will easily destroy in five minutes (even if it took you a few days to build it). If using for high loads, brushless motors and advanced brushed motors can be equally good. So, brushless motors are 100% right choice when applying to:

Vehicles: Power windows, seats, and air conditioning in cars. Electric bicycles, scooters, and drones. All Tesla hybrids. Powerful servo drives can be found everywhere. The future of motion is electric and brushless.

Image by Thomas Ehrhardt from Pixabay
Image by Thomas Ehrhardt from Pixabay

Industrial engineering: Brushless servo motors are widely used in CNC automation. They are especially useful as they eliminate sparks and fire hazards. As BLDCs are less noisy, they also help to create a more comfortable workplace for CNC human operators.