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