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Motor Types

Overview: Freescale Semiconductor and Motor Control

Electric motors are all around us, from common appliances to our most sophisticated computers. In fact, the technology has been present for over a century, with many of the earliest motor types still in broad use. Motors provide motion. Whether rotating or linear, motors enable us to move people and machines. They impact every aspect of our daily lives. Electric motors are clean and relatively efficient for the tasks they perform when compared to pneumatic or hydraulic alternatives.

Freescale offers comprehensive motor control solutions for virtually all electric motor topologies, including:

Stepper Motor

Stepper motors consist of a doubly salient structure (teeth on both the rotor and stator) and are used primarily in applications requiring precise position control which cannot justify the cost of expensive position feedback sensors. Stepper motors are a fairly new motor type, designed as a replacement for expensive servo motors. As current is switched from one set of stator coils to the next, the magnetic attraction between rotor and stator teeth results in the rotor moving by a small amount to the next stable position, or "step". Since it takes time for the current to be removed from one coil and established in the next (commutation), and since this process results in very little angular displacement of the rotor, stepper motors are typically limited to low speed position control applications.

Brushed DC Motor

DC motors typically consist of a rotating armature coil inside of a stationary magnetic field which is generated by either a permanent magnet or a stationary electromagnet connected in series or parallel with the armature coil (the series connection often being referred to as a Universal Motor). The fact that these motors can be driven by DC voltages and currents makes them very attractive for low cost applications. To convert the armature current from DC into AC (which is required for rotation), a mechanical solution consisting of brushes and a commutator is employed. However, the arcing produced by the armature coils on the brush-commutator surface generates heat, wear, and EMI, and represents the most significant drawback of this motor type.

Brushless DC Motor (BLDC)

Although the name implies a DC motor, it is actually an AC motor. Concentrated coil windings on the stator work in conjunction with surface mounted magnets on the rotor to generate a nearly uniform flux density in the airgap. This permits the stator coils to be driven by a constant DC voltage (hence the name brushless DC), which is simply switched from one stator coil to the next. This process (referred to as COMMUTATION) must be electronically synchronized to the rotor angular position, and results in an AC voltage waveform which resembles a trapezoidal shape. Since there are no brushes or commutator, the BLDC motor does not exhibit the arcing problems associated with a brushed DC motor.

Permanent Magnet Synchronous Motor

Very similar to their BLDC cousins, PMSM motors are driven with sinusoidal voltages and currents which can achieve lower torque ripple than BLDC motors. A sinusoidal flux density exists in the airgap which has been traditionally generated by sinusoidally distributed multi-phase stator windings. However, newer designs achieve this flux density with concentrated stator windings and a modified rotor structure. Rotor magnets may be surface mounted for lowest torque ripple, or buried inside the rotor structure for increased saliency, which increases the reluctance torque of the machine. Field Oriented Control (FOC) is often employed to control these motors, which requires precise knowledge of the rotor angular position.

AC Induction Motor (ACIM)

Invented at the tail end of the nineteenth century, AC Induction Motors were the electric workhorse of the industrial revolution. The rotor consists either of multiphase windings, or the more popular copper or aluminum bars arranged in a structure that resembles a squirrel cage. Essentially a rotating transformer, currents are "induced" in the rotor conductors (secondary) from the stator coils (primary). The absence of permanent magnets makes AC induction motors extremely rugged and robust. Sinusoidal flux density is created in the airgap which is generated by sinusoidally distributed multi-phase stator windings. Field Oriented Control (FOC) is often employed to control these motors, which requires precise knowledge of the rotor angular position. However, due to the damping action provided by the moving rotor conductors, AC induction motors are also capable of simply running open loop from a multi-phase AC supply.

Switched Reluctance (SR) Motor

One of the oldest motor topologies, SR motors utilize concentrated stator windings and contain no permanent magnets. The rotor is a very simple construction of soft iron laminates with no coils. Since the rotor cannot generate its own magnetic field, there is no reactive torque (magnet to magnet) in an SR machine. Instead, both rotor and stator poles demonstrate salient protrusions (doubly salient design) where the flux length is made to vary as a function of angle. This results in the magnetic reluctance also changing as a function of angle, which gives rise to saliency torque. This is the only torque producing mechanism in an SR motor, which tends to result in high torque ripple. However, due to their simple design, SR motors are very economical to build, and are perhaps the most robust motor available. Unfortunately, the high torque ripple also gives rise to audible noise during operation, which has limited the application of SR motors in many applications.

Freescale offers complete solutions for every type of motor control application. Our superior portfolio and breadth of devices includes:

  • 8-bit microcontrollers (MCUs)
  • 16-bit digital signal controllers (DSCs)
  • 32-bit embedded controllers
  • Acceleration and pressure sensors
  • Analog and mixed signal devices

Freescale solutions deliver wide ranging banks of flash and RAM memories, pulse width modulators (PWMs), and configurable timer options. Please refer to our brochure, "Motor Control Technologies" for specific product suggestions to serve each type of motor control application.

Freescale Motor Control Solutions

We are dedicated to providing comprehensive system solutions that not only improve motor efficiency but also minimize system cost and development time.

Freescale's Complete Motor Control Solution