Ironcore brushed dc motors

BLDC motor with electronics

BLDC motors can run at much higher speeds than brushed varieties, because there are no mechanical limitations being imposed by the brushes and commutator. Integrated drive electronics sweeten the deal.

Traditionally, the motion control industry has relied heavily on ironcore brushed dc motors for demanding applications. They are capable of very high torque, due in part to their ironcore construction. The rotor is usually a rigid design that provides both a sturdy winding support and excellent heat dissipation. That is the reason more current can be pushed through the windings when torque demands increase — the rotor acts as a heat sink. Low cost is yet another benefit of this motor type when project funding is limited.

However, there are disadvantages to ironcore construction. Its heavy armature requires more energy for overcoming its inertia, which reduces acceleration capabilities. Higher rotor inertia also extends stopping time. Due to the heavier rotor weight, this design is not suitable for some inertia-sensitive applications: Higher rotor inertia limits the dynamic characteristics such as the motor's acceleration and stopping time.

Another problem with the ironcore rotor design is increased inductance. When running at high speeds, the brushes pass over the commutator's segments and imperfections. At each commutation point, when the brush breaks contact with a commutator segment, the energy stored in the motor winding as a magnetic field causes an arc or voltage spike between the brush and commutator segment. This occurs not only during normal commutation, but also in situations where the brushes “bounce” on the rotating commutator. At higher speeds, this results in faster brush wear and electroerosion.

One solution is to employ a precious-metal commutator system: This allows for motor designs smaller than those with carbon-graphite commutation, which take up much more space. Commutation signals are usually cleaner as well.

Because the voltage drop between brushes and commutator is generally small in precious-metal systems, motors can be made to operate at lower voltages. However, precious metal cannot self-lubricate, so these units sometimes exhibit wear on the commutator surface over time.

Coreless brushed dc micromotors

Permanent-magnet steppers for positioning

PMDC stepper motors are useful for positioning and other precision applications; stepping often offers sufficient trackability, and inherent detent toque is useful where power is at a premium.

One answer to ironcore issues is the coreless dc micromotor, invented in the 1940s. This design allows a multitude of possibilities for space-constrained applications requiring high precision. Repeatability is another strength. Each of these motors has a self-supporting skew-wound ironless rotor coil that exhibits efficiency superior to that of ironcore motors. These were the first dc motors that did not require iron laminations in the armature. Thanks to this construction, the rotor is extremely light, with a low moment of inertia — so acceleration is faster and the mechanical time constant is smaller.

Another benefit of coreless dc motors is that they can be very compact. The rotor also spins smoothly without cogging, and the coreless dc motor's windings have very low inductance. These characteristics help reduce brush wear and prevent electroerosion, thereby increasing motor life.

What are their drawbacks? With no iron laminations, coreless motors are somewhat prone to overheating, though heat sinks are sometimes used to alleviate this problem. Coreless dc motors are also more costly; they are designed for specific applications and are not the best choice for most consumer products.

Where are they appropriate? The most common uses are high-precision medical, aerospace, military, robotics, and automation applications of large OEMs. Specifically, coreless dc motors excel in aesthetic lasers, diabetic insulin pumps, collision avoidance scanners, and unmanned aerial vehicle (UAV) applications with demanding micropositioning, dimensional, and even vacuum-compatibility requirements.

Going for longevity: Brushless technology

If an application requires quiet, high-speed operation with low EMI, then brushless dc or BLDC technology is often suitable. Brushless motors offer speed: No mechanical limitations are imposed by the brushes and commutator. Another advantage is the elimination of arcing and electroerosion so common in brushed motors. BLDC motors also possess higher efficiency and generate lower EMI, quite helpful in RF applications.

Friction generated in the bearings is usually the only point of failure in brushless motors; the windings are on the stator, so thermal characteristics are better than those of brushed motors. The stator is connected to the case, so heat dissipation is much more efficient. As a result, maintenance on a brushless motor is unnecessary.