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Making the choice between Brushed and Brushless DC servo motors

POSTED 12/16/2016  | By: Kristin Lewotsky, Contributing Editor

In engineering, there is no perfect solution, just the best solution for the application at hand. Use cases for motion control vary widely from space exploration applications for which cost is immaterial and reliability requirements absolute to high-speed packaging lines that operate 24/7. Fortunately, design teams have a variety of options from which to choose. One of the key decisions to make is whether to use a brushed DC motor or a brushless DC motor.

Brushed DC motors
Before we can discuss the trade-offs, let’s start with some motor basics. A motor consists of a rotor (also called an armature) and a stator. Although a number of variations exist, including stationary rotors and rotating stators, for purposes of this article, let’s restrict the discussion to a rotary motor with a stationary stator surrounding a central rotor that turns. The stator consists of a pair of permanent magnets with opposing poles, while the armature consists of a crossbar wrapped with wire in opposite directions on each side (see Figure 1). When the two coils are connected to a power source, they act as electromagnets with opposite polarities (see tutorial on brushed DC motors for more details).

Figure 1: A two-pole brushed DC motor consists of a stator with two magnets and a rotor (armature) with the coils wrapped in opposing directions.

Electric motors operate based on the Lorentz force, in which a magnetic field applies a force to a current-carrying wire loop. This causes the rotor to turn about its axis. The torque generated by the Lorentz force is a cross product, which means that essentially once the poles of the electromagnets formed by the rotor windings are aligned with the opposing poles of the stator magnets, the force falls to zero and the rotor stops rotating.

Reversing the direction of the current in the coils will reverse the polarity of the electromagnets, however. The force will reappear and the rotor will resume its motion. If this reversal can take place each time stator moves just past the vertical, the rotor will continue to turn and do useful work.

To change current direction on a frequent and controlled basis, brushed DC motors require a commutator. This is a split ring with one side attached to each coil of the rotor. As the rotor turns, so does the commutator. To apply current, a pair of fixed brushes press against the commutator from opposite sides (see Figure 2). When the commutator/rotor assembly turns each side of the commutator contacts first one brush/current source, then the other, in sequence. As a result, the current in the rotor coils reverses every 180° to keep the motor turning.

Figure 2: In a commutator, brushes (red and blue) contact a conducting split ring (green) to deliver current to a coil. Because the ring is divided, it changes the direction of the curreent every 180 degrees.This is a very simple model for purposes of discussion. As the tutorial explains, for practical reasons, brushed DC motors are typically three-phase or higher.

Brushes can be made in a variety of materials including carbon-based alloys like copper graphite or silver graphite, or precious metals like gold, silver, or platinum. The most appropriate choice depends upon the application.

Graphite brushes are formed in solid pieces. They self-lubricate and tend to be fairly robust. They are a good fit for larger motors running at high speed (above 1000 RPM). The drawback is that they tend to produce debris over time that can contaminate the commutator and cause intermittent failures. It’s essential that they be used at high enough speed to fling off any debris.

Precious metal brushes consist of separate strands, making them more fragile than their carbon-based counterparts. They offer better performance, however, with lower electrical noise and audible noise. They’re more compact and effective in low-duty-cycle applications. They are also a good fit for low-voltage systems because the voltage drop between commutator and brush tends to be low. On the downside, they do not self-lubricate, leading to more wear over time and a requirement for external lubricant.

Pros and cons
Brushed DC motors are workhorses of motion control. They are economical and simple to use. Because they don’t require onboard electronics, they can tolerate extreme environments. Provided the brushes are properly chosen and maintained, brushed DC motors can last a long time. They are a good fit for moderate-to-low speed applications.

Brushed motors require informed use. Past a specified current density, for example, the brushes will burn up. At excess speed, they can fly off the commutator. They may need special accommodation for high-altitude use, such as dopants like molybdenum disulfide or lithium carbonate.

The addition of commutator and brushes increases size. The brushes require regular maintenance, so the motors need to be in an accessible spot. Because the rotor with its windings is on the inside, brushed motors can only dissipate waste heat across the air gap, making thermal management an important issue. Voltage drop across the brushes also acts to decrease efficiency.

Finally, the friction of brush-to-commutator contact further reduces efficiency and generates audible noise. It reduces torque at high speeds. Coupled with flaws on the commutator, friction can also cause arcing and increased electromagnetic interference (EMI); in the worst case, the effects can generate sparks, making these devices unsuitable for explosive environments.

Brushless DC motors
Brushless DC (BLDC) motors, or electronically commutator motors (ECMs), provide an alternative. BLDC motors are permanent-magnet synchronous motors. They can be operated as servo motors, but also as stepper motors. The term also encompasses switched-reluctance motors. For purposes of comparison, let’s consider a common BLDC motor design that is basically a brushed DC motor turned inside out. The permanent magnets are mounted on the rotor, while the stator consists of a laminated cage wound with coils. As a result, the rotor does not need any wiring, nor does the motor need a commutator and brushes.

Although they are classed as DC motors and run off of a DC power source, BLDC motors have much in common with AC motors. In order to keep the rotor turning, the windings of the stator must be energized in sequence???basically, it looks like a switched current source, typically with a sinusoidal waveform when used for servo-motor control. To ensure that the magnetic-field distribution generated by the stator windings tracks with the magnetic field distribution of the rotor, BLDC motors monitor the angular position of the rotor, typically with Hall-effect sensors. This feedback is used to control the switching of current to the coils.

Because BLDC motors do not include brushes and a mechanical commutator, they are more compact than brushed versions. They offer higher output per frame size. The lack of brushes reduces maintenance and enables the rotor to turn at higher speeds without damage. Reduced friction flattens out the speed/torque curve and eliminates arcing, lowering EMI. Moving the heat-generating windings to the outside simplifies thermal management. The approach also reduces rotor inertia, enabling BLDC servo motors to deliver improved dynamic response. Without voltage drop across the brushes, efficiency increases.

On the downside, BLDC motors are more complex than their mechanically commutated counterparts. The onboard electronics add significantly to cost.

As we discussed at the beginning of this article, requirements drive the choice of motor. A budget-constrained project with moderate specifications can do just fine with brushed DC motors. If performance and duty cycle are more important, a BLDC motor might be a better solution. OEMs and end-users should take into account not just motor capabilities but capabilities of their staff to build and maintain the equipment. Only by making an informed choice decision can they arrive at an effective solution.