A few characteristics of dc motors maintain linear relationships that allow for very predictable operation. First, if enough voltage is applied across the terminals of a dc motor, the output shaft will spin at a rate proportional to that applied voltage. A designer can determine the ratio of the applied voltage to rated voltage, multiply that value by a dc motor's no-load speed, and derive running speed. Second, measuring and plotting current and torque also gives a straight line — another directly proportional relationship. This means that current demand simply increases with that for torque. Finally, plotting motor torque and speed requires only two points of data: no-load speed and stall torque.
The entire motion control world, including manufacturers and designers, depends greatly on the premise that all these linear relationships hold true. Thankfully, they do because the laws of physics do not change. Even so, despite their simplicity, selecting a dc motor for an application can still be a daunting task. Many variables must be taken into account: dimensions, load, duty cycle, environment, feedback considerations, and so on. Let us decode some motor-operation mysteries to shed some light on the selection process.
Going for simplicity: Brushed motors
If an application demands reliable, time-tested, low-cost motion, then brushed dc motors may be the technology of choice. The key here is simplicity: A brushed motor is designed to run on straight-line dc voltage and can even be connected directly to a properly sized battery.
When dc voltage is applied across a brushed motor's terminals, a potential difference develops, and current is induced in the rotor windings. The brushes allow this current to flow through a rotating mechanical switch called a commutator. Rotor windings act as electromagnets and while powered, they 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 twice per cycle, in turn permitting current to flow through the armature and electromagnet poles, to attract and repel permanent magnets that encompass the motor's inner housing. As the armature's energized windings pass the permanent magnets, the polarity of the windings reverses at the commutator. This process is called mechanical commutation and is only found in brushed motors.
The instant that polarity switches, inertia keeps the rotor spinning in the proper direction and the motor turning. The result is power in its mechanical form, measured in Watts. Mechanical power is the product of torque multiplied by 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:
Prot = M × ω
From this equation, we 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 the torque the motor can produce. Adjusting supply voltage forces a proportional change in motor speed, so the output shaft's angular velocity (or speed) must be sacrificed as torque demands increase.
Other factors also come into play — for example, static friction. This is defined as the friction torque a motor must overcome for the shaft to begin turning. In addition, there are brush-contact losses caused by the friction of the brushes upon the commutator.
Other losses are in the form of heat. This dissipated electrical power is also somtimes called I2R loss:
Pdiss = I2·R
Here, current I is that running through the motor, and R is the teminal resistance.
When torque and speed are measured empirically, the result is not always perfectly linear. Even so, from the equation above, we see that both torque and speed are inversely proportional and that a theoretically linear relationship exists. Because of this, feedback provided by an encoder, tachometer, or resolver is not even necessary in all cases. Where feedback is employed, it reports motor position and angular shaft speed to the servosystem.
In summary, a properly designed closed-loop servosystem gives predictable response to controlled input. What is more, thanks to the directly linear relationships, a servo can easily compensate for any disturbances introduced into the system.
Ironcore brushed dc motors
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
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.
Drawbacks: As with brushed motors, brushless varieties must overcome starting friction as well. Again, this is the sum of torque losses not depending on speed.
Dynamic friction is dependent on speed. In fact, dynamic torque friction is the only quantity defining torque losses that is proportional to speed for BLDC motors. A function of speed (as in metric units, of mNm/rpm) dynamic friction is due to both ball-bearing viscous friction, and Eddy currents in the stator originating from the magnet's rotating magnetic field.
The higher cost of construction makes BLDCs impractical for many applications. A designer can easily spend double for a brushless system, and lose the simplicity of a brushed motor to boot. The motors also require space for housing the control/drive electronics, which must be mounted somewhere if not integrated into the motor.
Also keep in mind that the motor can't be mounted too far away from the drive, because long cable runs tend to introduce noise into systems. (To compensate here, phase leads can be twisted and shielded from sensitive feedback leads to reduce noise.) This said, overall a designer can expect excellent linearity in BLDC motor speed-torque ratios.
Part 2 of this two-part series can be found at motionsystemdesign.com/motors-drives. Call MICROMO at (800) 807-9166.