What is a DC motor?
A DC motor is an electrical machine that converts DC electrical power into mechanical motion. There is a large variety of motors for different applications and power requirements, from tiny devices—just a few millimeters in diameter—for medical equipment, up to custom designs generating thousands of horsepower.
Brushed vs. brushless motors
The two most common types of DC motors are known as brush DC and brushless DC (BLDC). Although the underlying physics is the same, their construction, performance characteristics, and means of control are very different.
Which is best for your application? Well, like many things in life, the answer is “it depends.” There are advantages and disadvantages to a brushed motor vs. a brushless motor. In this article, we’ll take a look at both DC motor technologies and shed a little light on this complex topic.
How brushed motors work
First invented in the 1800s, brushed DC motors are one of the simplest types of motors. They were the first type widely used since they could be powered from early direct-current lighting power distribution systems.
Figure 1: Brush DC motor. (Source: Oriental Motor)
As shown in Figure 1, a typical brush DC motor consists of a rotating armature, and a stationary stator.
The armature (also called the rotor) contains one or more windings of insulated wire wrapped around a soft iron core. The windings form one or more coils and are electrically connected to the commutator, which is a cylinder composed of multiple metal contact segments around the armature shaft. The stator encloses the rotor and contains either permanent magnets or electromagnets to generate a magnetic field. The brushes are electrical contacts, made of a soft material such as carbon, which are spring-loaded to make contact with segments of the commutator as the shaft rotates.
Role of brushes in DC motors
When a DC power source is connected to the brushes, the armature coils are energized, turning it into an electromagnet and causing it to rotate so that its north and south poles align with the stator’s south and north poles respectively. As the commutator rotates, the motion causes the polarity of the current into the armature coil—and the direction of its magnetic field—to reverse. The armature rotates towards its new alignment, the current reverses again, and the armature continues to rotate.
This means of reversing the current is called mechanical commutation—the mechanical rotation of the shaft provides the feedback needed to switch the current polarity.
By varying the arrangement of the windings, several brush DC motor varieties have been developed, with different performance characteristics; there are five basic types. The first four types use coils in both the stator and the rotor (armature), so use electromagnets exclusively.
Types of brush DC motors and their uses
A shunt-wound brush DC motor has the rotor and stator field coils connected in parallel; it runs at constant speed regardless of the load. This self-regulating feature makes it widely used in industrial constant-speed applications.
A series-wound brush DC motor has the two coils wound in series; its speed varies with the load, increasing as the load decreases, but has very high starting torque, so it is widely used for short-duration applications such as automobile starters.
A compound-wound brush DC motor is a combination of the shunt- and series-wound motors, with characteristics of both. Compound-wound motors are usually used when both severe starting conditions are encountered and constant speed is required.
A separately excited brush DC motor has separate supplies for rotor and stator, allowing both high stator field current and sufficient armature voltage to produce the required rotor torque current. This type of motor is used when high-torque capability at low speeds is required.
A permanent magnet brush DC motor contains permanent magnets in the stator, eliminating the need for an external field current. This design is smaller, lighter, and more energy-efficient than other brush DC motor types; it is used extensively in low-power applications up to about 2 HP.
How to control speed of brush DC motors
With commutation performed mechanically, controlling a brush DC motor is conceptually very simple. A fixed-speed motor only needs a DC voltage and on/off switch; varying the voltage changes the speed over a wide range.
For applications requiring more sophisticated control, a common circuit topology such as the H-bridge shown in Figure 2, can be used. By turning on transistors Q1 and Q4 simultaneously, or transistors Q3 and Q2 simultaneously, current through the BDC motor flows in one direction or the other, allowing bidirectional motion.
For speed control, a pulse-width modulated (PWM) signal is used to generate an average voltage. The motor winding acts as a low-pass filter so that a high-frequency PWM waveform will generate a stable current in the motor winding. For more precise speed regulation, a speed sensor such as a Hall-effect sensor or optical encoder can be added to form a closed-loop control system.
Brushed DC motor basics
Brush motors compared to brushless motors are inexpensive and reliable and have a high ratio of torque to inertia. Because they need few or no external components, they are also suitable for operation under rugged conditions.
On the downside, the brushes wear down over time and produce dust; brush motors require periodic maintenance for brush cleaning or replacement. Other disadvantages include poor heat dissipation due to limitations of the rotor, high rotor inertia, low maximum speed, and electromagnetic interference (EMI) generated by brush arcing.
How brushless motors work
The underlying principle of operation for a brushless DC (BLDC) motor is the same as that for a brush DC motor—commutation control using internal shaft position feedback—but its construction is very different.
In contrast to the brush DC motor, the permanent magnet is mounted on the BLDC rotor; the stator is made of slotted, laminated steel and contains the coil windings.
BLDCs also don’t use carbon brushes or a mechanical commutator. Forcing the rotor to rotate is done by successively energizing coils around the stator, and commutation is performed via a complex electronic controller used in conjunction with a rotor position sensor (e.g., photo transistor-LED, electromagnetic or Hall effect sensors).
The BLDC construction method allows it to have less internal resistance and much better heat dissipation in the stator coils. This results in higher operating efficiencies since the heat from the coils can more efficiently dissipate via the much larger stationary motor housing.
The stator windings can be arranged in either a star (or Y) pattern, or a delta pattern. The steel laminations can be slotted or slotless. A slotless motor has lower inductance, so it can run at higher speeds and exhibits less ripple at slower speeds. The main disadvantage of a slotless stator is its higher cost because it requires more windings to compensate for the larger air gap.
The number of poles in the rotor may vary depending on the application. Increasing the number of poles increases torque but reduces maximum speed. The material used to construct the permanent magnets also has an effect on maximum torque, which increases with flux density.
Figure 3: Brushless DC motor (BLDC). (Source: Oriental Motor)
Learn more about what a BLDC is and how it works.
Brushless DC motor control
Since commutation must be performed electronically, BLDC control is considerably more complicated than the simple schemes discussed above, and both analog and digital control methods are used. The basic control block is similar to the brush DC motor approach, but closed-loop control is mandatory.
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There are three primary types of control algorithms used for BLDC motor control: trapezoidal commutation, sinusoidal commutation, and vector (or field-orientated) control. Each control algorithm can be implemented in different ways depending on software coding and hardware design, and each one offers distinct advantages and disadvantages.
Figure 4: Brushless DC motor control methods.
Trapezoidal commutation requires the simplest control circuits and software, making it ideal for low-end applications. It utilizes a six-step process using rotor position feedback. Trapezoidal commutation controls motor speed and power effectively but suffers from torque ripple during commutation, particularly at low speeds.
Sensorless commutation - estimating the rotor position by measuring the back EMF of the motor - provides similar performance to the Hall-effect method at the cost of increased algorithm complexity. By eliminating the Hall-effect sensors and their interface circuitry, sensorless commutation reduces component and installation costs and simplifies system design.
Sinusoidal commutation uses modulation of the carrier frequency to drive the motor, controlling the three winding currents simultaneously, so they vary smoothly and sinusoidally as the motor turns. This technique gives smooth and precise motor control by eliminating the torque ripple and commutation spikes associated with the trapezoidal method. It can be operated as an open loop system or a closed loop system with an added speed sensor and is typically used in mid-range performance applications that need both speed and torque control. The complicated sinusoidal commutation scheme does require additional processing power and control electronics to implement.
Vector control is reserved for high-end applications due to its complex design and the high demands it places on the microcontroller. The algorithm uses phase current feedback to calculate voltage and frequency vectors and commutate the motor. Vector control provides precise dynamic control of speed and torque and is efficient over a wide operating range.
A sensorless technique can also be used; a shunt monitors motor current, and the algorithm compares the results to a stored mathematical model of the motor operating parameters. This method reduces the cost of the feedback devices, but significantly increases the processing requirements of the MCU.
BLDC control strategy comparison
How do the different control strategies stack up? As you might expect, the simple trapezoidal approach has the worst torque control, but doesn’t ask too much of the microcontroller or control device. At the other end of the spectrum, the vector control method (also called field-oriented control, or FOC) provides excellent control of both speed and torque, but the microcontroller requirements are demanding.
Difference between brushed and brushless motors
With no mechanical commutator or brushes to wear out, brushless DC motors are low maintenance and non-sparking. In addition, they have less shaft friction and inertia, less audible noise and much better torque-to-weight ratios (power density), so they’re much smaller in size than a comparable brush DC motor.
Compared to brush DC motors, BLDC motors have several performance advantages. They have high starting torque, and the torque is flat up to rated speed. Due to the real-time electronic control, their speed regulation is precise and insensitive to load variations. Since the heat is generated in the external stator and not the internal rotor, they are easier to keep cool. And the lack of brushes means they produce less electrical noise and can run at higher speeds—up to 100,000 RPM in some cases.
Available brushed and brushless DC motor control solutions
As we’ve seen, although simple brush DC motor control is easy to achieve, more precise BDC control and BLDC control are both anything but straightforward.
The good news is that multiple off-the-shelf solutions are available that match motors with appropriate controllers. At the device level, Arrow Electronics offers numerous motor controllers from leading suppliers, addressing both brushed and brushless motors. In addition, since motor control is a huge market, many suppliers offer development kits, reference designs, and software libraries targeted at DC motor control.
Brushed vs. brushless motors: Which one is best for you?
You have many choices available when it comes to picking the right DC motor technology depending on your application.
How about a space-constrained medical device where maintenance is not an option? Start looking at a brushless solution. Is your primary concern low cost? Perhaps a permanent magnet DC motor is right for you.
Is very precise control needed? Consider a BLDC, perhaps with a digital control strategy. Simple control scheme? Check out a brush DC option.
Either way, now that you understand the relative merits of brushless vs. brushed motor technology, you should be in a better position to make the best choice.
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