Understanding the benefits of miniature brushless dc motor technology could pack some pizzazz into your designs
The demand for permanent magnet brushless dc motors (BLDC) is steadily rising. Their growing use is the result of three important factors: First, a BLDC motor is inherently more reliable than a conventional brush-type dc motor. Second, advances in power semiconductors and microprocessors permit cost-effective control. And third, brushless dc motors offer the response and linearity over a wide speed range that is needed for truly high-performance servo mechanisms.
When coupled with appropriate control electronics, BLDC motors exhibit characteristics much like a conventional dc motor. Therefore, the designer who is familiar with dc motors is already prepared to use BLDC motors. For example, these motors produce torque in direct proportion to input current. And when properly commutated and controlled, a brushless servo will perform in all operating modes that a brush-type servo will, including slow speed scans and rapid point-to-point positioning. In addition, brushless servos are capable of going well beyond the safe operating envelope of the brush-type servo. Together with complementary control electronics, these motors are used in applications from industrial process control to motorized surgical tools and radar pointing systems.
Characteristics of brushless dc motors
• Long operating life — Having neither brushes nor a metallic commutator, the life of a BLDC motor is extended well beyond that of a brushtype dc motor.
• Highly responsive — The high torque to inertia ratios of a BLDC motor yield quick response to commands.
• High speed — Lacking the limitations of conventional communication, BLDC motors can be operated at very high speeds. It has been demonstrated that electronic commutation can support speeds to 150,000 rpm.
• High thermal capacity — The windings of a BLDC motor are located on its stationary member, so heat may be carried away using direct conduction.
• High shaft power — With inherent capacity for high speed and high torque, BLDC motors are able to yield higher levels of shaft power, the product of shaft torque and speed, resulting in extremely high acceleration capability.
• Tolerant of harsh environments — BLDC motors emit no sparks during normal operation because there is no sliding contact between the brushes and metallic commutator. Further, the “hard-wired” connections of a BLDC motor can be easily encapsulated so that the entire assembly may be autoclaved.
• Other significant features — Low thermal time constant; analog or digital commutation schemes; very low audible noise; low radial and axial play; custom designs including integral gearheads, resolvers, encoders, and controls.
Brushless motor types
Three phase brushless motors This type of motor exhibits back-emf waveforms that may be either sinusoidal or trapezoidal, each of the three phases being phased 120 electrical degrees apart. These motors are the most widely used because they offer maximum performance per unit cost. A brushless servo amplifier can be constructed using just six power devices and some appropriate control logic, while rotor position feedback may be as simple as three Hall effect switches. This straightforward control scheme provides a basis for a system that yields reasonably smooth torque over a wide range of operating speeds.
For more precise control at low speeds, the motor can be designed for “sinusoidal” excitation. This approach requires the addition of a feedback device to support motor commutation. Sinusoidal brushless servos are used in the most precise motion control systems.
• Housed brushless dc motors These motors are rugged building blocks suitable for high performance servo applications. They are available in a wide range of configurations and are self-contained assemblies having single or dual output shafts that can be coupled to the driven load.
Applications for housed motors include torque, velocity, or position control, fluid pumps, nut drivers, leadscrews, medical instruments, robotics, factory automation, and XY tables. Other uses include special machine tools, spindles, focusing mechanisms, optical pedestals, and motion simulators.
• Servomotors These motors are designed for use in “closed-loop” systems. They are housed motors with an integral position/ velocity feedback device (Hall sensors, encoders, resolvers, etc.). As the motor shaft turns, the feedback device reports position/velocity to the system controller.
To develop an understanding of the brushless dc motor, it is often helpful to know the construction and function of its parts. Following is a brief overview of motor construction.
Brushless dc motors have three major subassemblies:
• Stator — The current carrying member. Typically a stack of laminations into whose slots coils of wire are inserted.
• Rotor — The magnetic field producing member. Usually a ferromagnetic hub of stainless steel onto which permanent magnets have been bonded.
• Commutator — A combination of electronic components that direct power to the various armature phases at the proper time depending upon rotor position and commanded torque.
The geometry of the BLDC motor is “inverted” with respect to the conventional brush-type motor. That is, for BLDC motors the armature is stationary (the stator) while the field rotates (the rotor). See Figure 1. With the wound armature stationary with respect to the power source, it is practical to “hard-wire” the connections. Therefore, sliding contacts are not required to commutate the BLDC motor.
On the contrary, with the brushtype motor, the armature rotates with respect to the power source. As such, there must be a method to transfer power from the stationary reference into the moving reference. The usual technique to accomplish this is to use a sliding contact such as the graphitemetal brush acting on a metallic commutator.
BLDC electronic commutator (Hall sensor assembly plus drive electronics) — The commutator senses the position of the rotor (field) with respect to the armature and directs power to the stator (armature) windings thereby generating torque through a full rotation. It is comprised of the rotor position sensor(s), power switches, and control logic circuits. All these devices together perform the commutation of a brushless motor. Without the commutator, the armature and field could interact to produce torque, but only through a limited angle of travel.
The overall performance of multiphase brushless dc motors is closely related to the electronics that control them. Assuring a proper match between the motor and its control electronics is more critical to the brushless system than it is for conventional brush-type systems. This is because the essential function of commutation is shared between the motor and the drive electronics. Often it is the effective design and integration of the electronics with the motor that provides the performance advantage that is sought.
BLDC stator — The stator (armature) consists of a bonded stack of laminations and coils of magnet wire. For typical BLDC motors having rotating inner members, the teeth of the stator (armature) point radially inward. Magnet wire is wound to specific coil dimensions and inserted into the slots of the stack. In the case of machine insertion, the wire is wound directly in the slot. For mechanical integrity, coils are either varnished or impregnated with epoxy prior to final machining.
BLDC rotor — The rotor (field) comprises a permeable iron hub (cold rolled or stainless steel) to which permanent magnets are bonded. If the magnets are not shaped prior to bonding, they are ground to shape as an assembly after bonding. Under special circumstances, the rotor may be fitted with a protective sleeve to avoid impact damage or as a redundant measure to retain the magnets at high speed.
The word “commutation” in the context of electrical circuits means simply “switching.” As it applies to dc motors, this refers to switching power from one armature coil to another.
Brushless dc motors lack the brushes and metallic commutator associated with the conventional brushtype motor, but they cannot produce torque, except through a limited angle, without some form of commutation. In the case of a multiphase BLDC motor, the commutator is electronic and noncontacting in nature.
One of the more expedient ways to sense rotor position is to employ Hall sensors, one per motor phase. Hall sensors detect the presence of a magnetic field. Those used in motors for rotor position sensing are typically digital latching devices and are used in conjunction with a trapezoidal or sixstep servo amplifier.
Hall sensors are usually mounted to a printed circuit board and attached to the motor stator such that when the rotor turns, its magnets pass in close proximity to the sensors, thereby triggering them to switch alternately “on” and “off” at the magnet edges. From the output signals of three well-placed Hall sensors, one has sufficient information to support electronic commutation of a BLDC motor.
To understand the timing associated with BLDC motor commutation, see Figure 2.
The relative position of the rotor with respect to the stator must be known with a reasonable degree of accuracy in order to assure proper commutation. Normally, an accuracy of eight electrical degrees will suffice. For a motor having four poles, that equates to two mechanical degrees.