Modern Drive Technology
A comparison of the differences between AC induction, synchronous AC, and brushless
DC motors, in terms of construction and drive methods.
What type of motor is considered the
traditional workhorse of the industry? It's a
difficult question to answer because there
are areas where AC motors excel and areas
where DC motors hold a clear advantage.
An AC motor's long life, limited only by the
slow wearing of its bearings, offers a distinct
advantage over the changing of brushes.
However, DC motors are better in areas of
high starting torque, slow speed operation,
rapid controlled acceleration, and precise
positioning. Besides the more traditional
types of AC and DC motors, there are also
brushless AC and brushless DC motors to
consider.
Induction Motors
Let's first review the basic operation
of an induction motor and how it can be
controlled. When you apply voltage on the
stator of an induction motor, the current that
flows induces current in the rotor, i.e., one
magnetic field is set up on the stator, and a
second magnetic field is induced on the
rotor. The interaction of these two magnetic
fields results in motion.
Typical construction of an induction motor
consists a stator, with lamination and turns
of copper wire, and a rotor constructed of
steel laminations, with large slots on its
periphery stacked together to form a
"squirrel cage" rotor (Figure 1). The slots
are filled with conductive material, either
copper or aluminum, and are short-circuited
upon themselves by conductive end rings.
This "one-piece" casting usually has small
integral fan blades for circulating air.
Inverter Control
The induction motor may be
controlled via inverter control techniques.
These use either six-step techniques or
synthesize a sinusoidal waveform to control
motors speed. The frequency of the
waveform controls the motor speed. A
standard induction AC motor would provide
a speed regulation (limited to the slip of the
motor) of approximately 1.5 to 3% of base
speed. With an inverter constant torque can
be provided to base speed and constant
horsepower to 1.5 times base speed. The
low-end controllable speed is about 300rpm.
Some advantages of using this approach
include low initial cost due to motor design
simplicity, reliability, and vibration-free
operation. The technique of using an
induction motor and inverter control works
out best for many adjustable-speed
applications.
Vector Control
An induction motor can also be
operated by vector controls with appropriate
feedback such as an encoder. This control
technology primarily uses a PWM
synthesized sinusoidal waveform to control
speed. Motors designed to be operated with
vector controls typically include a high
efficiency winding, an efficient lamination
design, and high-temperature insulation
materials.
With vector control, tighter speed
regulation approaching 0.01% of set speed
may be attained. A vector control allows
controllable speed ranges from about 5 times
base speed down to zero speed. The
constant horsepower range is about 3.5
times the base speed.
Smooth stopping and highly
efficient operation may be attained by using
a vector control. A standard vector control
with dynamic brake (D.B) resistor may be
used in many applications. Whenever a
motor is stopped faster than if it were
coasting to a stop, it becomes a greater
generator. The energy or power generated
by the motor may be shunted through the
external D.B. resistor and dissipated as heat.
However, if the application has high
inertial loads, consider a line regeneration
("line regen") vector control. The energy or
power generated by the motor is put back
onto the incoming power line (Figure 2).
"Line Regen" controls save energy back to
the power line. Additionally, these designs
operate near unity power factor, so they
provide additional energy savings.
Advantages of this approach include
low initial cost, reliability, the capability of
providing full rated torque from rated speed
down to zero speed, precise speed and
torque control, constant horsepower output
above rated speed, and such programmable
features as controlling
acceleration/deceleration time and tuning.
Induction motors with vector
controls can be applied in high-performance
adjustable-speed applications. For
positioning applications, they can be used
with proper position controllers.
Synchronous Motors
A synchronous motor is like an
induction motor with a slightly different
rotor. The rotor construction enables this
type of motor rotate at the same speed, in
synchronization as the stator field. There
are two kinds of synchronous motors: self
excited (as the induction motor) and directly
exited (as with permanent magnets).
The self-excited -or reluctance
synchronous-motor has a rotor with notches,
flats or teeth on the periphery (Figure 3).
These teeth are also called salient poles.
They create an easy path for the magnetic
field to follow, thus allowing the rotor to
lock "lock in" and run at the same speed as
the rotating field. During operation the rotor
lags a small distance behind the stator field.
This increases with load; and if the load is
increased beyond the motor's capability the
rotor "pulls out" of synchronism.
Brushless Motors
When a synchronous motor is
excited directly, as with permanent magnets,
it is called a brushless motor. The rotor is a
cylinder of permanent magnet alloy; such
materials as ferrite, samarium cobalt or
neodymium can be used. The north/south
poles are, in effect, the design's salient teeth
and prevent the motor from slipping.
Brushless motors are driven by
selectively applying power onto their
windings. In order to know when to apply
power, some type of rotor position feedback,
such as Hall sensors, encoders or resolvers,
must be used. Informing the control when
to switch power from winding to winding is
called electronic communication.
Brushless Controls
When driving a three-phase
brushless motor by energizing two of the
three windings at a time, simple Hall sensor
feedback can be used. Because there are six
different commutation sections in a threephase motor, this control scheme is referred
to as six-step commutation. Some
manufacturers use the term "DC" and, thus,
call their drives "DC brushless."
When sinusoidal waveform is
applied to the motor windings on a
continuous basis, the terms "AC brushless"
is used. The motor has a three-phase
sinusoidal back EMF and is being powered
by a three-phase driving current waveform,
so "extra torque" is available. Or
rephrasing, for the same torque (compared
to a "DC brushless" drive), the "AC
brushless" drive will require less current.
For this reason the AC brushless packages
are popular (Figure 4). In addition, it may
be possible to use a smaller size motor. This
control scheme reduces torque ripple and
provides good low speed operating
conditions. Brushless motors are designed
to provide an optimized-torque-per-package
size and improve the torque-to-inertia ratio.
They also offer rapid incrementing and
accurate positioning.
Inverter controls are presently
available through 500hp. This technology
works adequately in open loop adjustable
speed applications. Table 1 compares the
various closed loop drive technologies.
Vector drives are available up through
500hp. These can be used in adjustable
speed application for improved speed
control, or in a positioning application.
Commercially available brushless
drives typically range through 15hp, custom
designs are range to 60hp, and specialty
designs provide higher horsepower. The
advantage of brushless servos would be the
rapid positioning capability in a smaller
package size.
For proper sizing and selection,
acceleration torque, package size and inertia
matching, all should be considered when
selecting a drive package for the
application.