If your application demands a reliable, time-tested, low cost motor, then brushed DC motor technology may be what you’re looking for. The key here is simplicity. A brushed motor is designed to run off of straight-line DC voltage and can even be connected directly to a properly sized battery. When a DC voltage is applied across the terminals of a brushed motor, a potential difference is achieved and current is induced into the windings on the rotor. The brushes allow this current to flow through a rotating mechanical switch called a commutator. The rotor windings act as electromagnets and while powered form two poles that terminate at the commutator segments. This entire assembly is known as an armature.
While rotating, the commutator allows the direction of the current to reverse two times per cycle. This permits the current to flow through the armature and the poles of the electromagnets attracting and repelling the permanent magnets that encompass the motor’s inner housing. As the energized windings of the armature pass the permanent magnets, the polarity of the energized windings reverses at the commutator. This process is called mechanical commutation and is found only in brushed motors.
During the instant of switching polarity, inertia keeps the rotor going in the proper direction and allows the motor to continue turning. The result is power in its mechanical form measured in watts. Mechanical power is the product of torque multiplied by the rotational distance per unit time (or speed). Torque is the force vector component that rotates a load about an axis and is inversely proportional to speed.
We can see that there is a price to pay for how much power a motor can deliver. The amount of current that flows through the windings directly affects to the torque the motor can produce. Adjusting the supply voltage will force a proportional change in the motor’s speed so the output shaft’s angular velocity (speed) will have to be sacrificed as torque demands increase. There are also other factors that come into play, such as losses. For example, static friction is defined as the friction torque a motor must overcome in order for the shaft to begin turning. Then there are brush contact losses caused by the friction of the brushes upon the commutator. Also, copper losses in the form of heat, sometimes referred to as I2R losses, plays a role.
Although, when torque and speed are measured empirically, the resulting graph may not be perfectly linear in all cases. However, we can see that both torque and speed are inversely proportional and that a linear relationship exists. Because of this, feedback may not even be necessary in all cases. Feedback is usually provided by an encoder, tachometer, or resolver. It tells the servo system where the motor is and what speed the shaft is turning. Taking all this into account, we can establish that a properly designed closed loop servo system will have a predictable response to a controlled input. And thanks to this directly linear relationship, a servo can easily compensate for any unwanted changes introduced into the system.
Coreless Brushed DC Motors
The answer to some of the problems with iron core technology was addressed in the 1940s by Dr. Fritz Faulhaber with the invention of the coreless DC micro-motor. This design opened up a whole new multitude of possibilities for space constrained applications requiring high precision. These motors have a self-supporting, progressive, skew-wound, ironless rotor coil that has demonstrated incredible efficiency when compared to iron core motors. For the first time, DC motors did not require the use of iron laminations in the armature. Thanks to this construction, the rotor is extremely light, yielding a low moment of inertia. In effect, this allowed for faster acceleration resulting in a much smaller mechanical time constant. Another benefit to coreless DC motors is that they can be manufactured in very compact sizes. That is why they excel in space constrained applications. The rotor also rotates smoothly without cogging and the coreless DC motor’s windings have very low inductance. All of these characteristics help reduce brush wear and prevent electro-erosion thus increasing the motor’s lifespan.
Unfortunately, with no iron laminations, coreless motors are somewhat prone to overheating. In some instances, a heat sink can be used to alleviate this problem. Also, cost would have to be factored into most applications as the high precision and repeatability of coreless DC motors comes at a bit of a price. These motors are designed for specific applications and would not be the best choice to use in most consumer products. (See Figure 1)
The most common applications are large OEMs in industries requiring very high precision, primarily medical, aerospace, military, robotics, and automation. Some example products are aesthetic lasers, diabetic insulin pumps, collision avoidance scanners, and unmanned aerial vehicle applications. These applications have demanding micro-positioning needs, dimensional constraints, and, sometimes, vacuum compatibility needs. Coreless DC motors seem to excel in situations where reliability, precision, longevity, and repeatability are of the upmost importance.