Specifications

117Motor Control
Introduction
Motor control design for industrial
applications requires attention
to both superior performance
and ruggedness. Maxim’s feature
integration and superior specications
enhance motor controller equipment
precision while improving robustness
in harsh industrial environments.
Motor controllers either control variable
power supplies to the motor or to
electronic switches between the power
supply and the motor. These switches
are precisely timed to open and close to
make the motor rotate most eectively.
The timing is often governed by complex
mathematical equations based on motor
architecture and electromagnetic theory.
Depending on the application, a motor
controller can be as simple as a variable-
voltage generator, a pulsed-DC voltage
source, or a complex signal generator
requiring sophisticated digital signal
processing algorithms to generate the
correct timing. For large motors, those in
the multihorsepower range with multiple
power phases, precise control is essential.
At a minimum, the wrong timing can
result in extreme power use. In the
worst case, wrong timing can destroy
the motor and the installation itself.
Many electric motors have maximum
torque at zero RPM, so these large
motors must be soft-started. To reduce
maintenance to a minimum, the
mechanical mechanisms (clutches)
that traditionally provided this soft-
start capability are rapidly being
replaced by electronic soft-starters or
variable frequency drives (VFDs). In
some applications motors must supply
both forward and reverse tension to
the load; optimally, braking energy
from overhauling loads is fed back
into the AC line using regenerative
VFDs instead of being wasted as heat
in large braking resistors or in high-
maintenance mechanical brakes.
Motor control is a very signicant portion
of the Control and Automation market.
According to U.S. Department of Energy,
motor driven equipment accounts for
64% of the electricity consumed by U.S.
industries. Furthermore, electric motors
consume about 45% of the world’s
electricity according to the International
Energy Agency (IEA) report of May
2011 on global energy consumption
by electric motor driven systems. By
comparison, lighting is a distant second
consuming 19%. With the cost of energy
rising steadily, plant operators look for
ways to reduce energy consumption
while maintaining throughput.
Furthermore, with the availability of
reasonably priced and highly capable
motor controllers for all types of motors,
plant engineers are free to choose motor
types that are less expensive, more
ecient, and require less maintenance.
To put the energy savings opportunity
in perspective, compare motor power
consumption vs. speed when driving
fans and centrifugal pumps. The torque
needed rises with speed, resulting in
power draw that is proportional to
the cube of the speed! In other words,
reducing the speed to one-half of full
speed drops the power to one-eighth
of full power. Even dropping the speed
to 75% of full speed drops the power
consumption to 42% of full power (0.75
cubed = 0.42). It is clear that signicant
savings in energy use can be realized
by even small reductions in speed. This
fact, in turn, justies the use of VFDs in
applications that can tolerate the speed
reduction. Of course, speed reduction
equates to performing the work more
slowly, which, in some cases, directly
impacts throughput. Nonetheless,
there are numerous applications where
motors do not need to run at full speed
to accomplish the work quickly enough.
Pumping out a tank of uid may not
need to be done as fast as possible.
Venting a room may need a full-speed
fan at rst, but once the air is moving a
slower speed may suce. The EIA report
(May 2011) states that it is feasible and
cost eective to save 20% to 30% of total
motor power consumption worldwide.
Certainly adding variable-speed
controllers adds cost to the installation;
however, the forecasted energy
savings will soon oset those initial
expenses. The return-on-investment
calculations are often straightforward.
Interfacing to the Motor
Controller
A very important aspect of every
motor controller in the industrial
control and automation setting is the
communications interface between
the factory control system and the
individual motor controller. All the
block diagrams in the individual motor
controller sections show a control panel
that provides a direct user interface
at the controller and a standard
separately wired communications
interface that connects to the eldbus.
The eldbus ultimately runs back to a
PLC (programmable logic controller)
that sends commands to the motor
controller such as motor start, motor
acceleration, speed adjustment, motor
stop, etc. An additional option exists:
powerline communications (PLC, not to
be confused with programmable logic
controller). This technology gives the
option of sharing command and control
connections with power connections
between the PLC (programmable logic
controller) and the motor controller.
Motor Types
Brushed DC Motors (BDCs)
Brushed DC (BDC) motors are among
the rst motor types put to practical
use and they are still popular where low
initial cost is required. These motors
have a wound rotor armature and either
a permanent magnet stator or eld
wound stator. Brushes slide across the
segments of the commutator on the
rotor to switch the DC power source to
the appropriate windings on the rotor.
BDC motors have their place for two
important reasons: low initial cost and
ruggedness, because no electronics are
needed inside the motor. Because the
motors suer from wear of the brushes,
brush springs, and commutators, they
require high maintenance in intensive-
use applications. Sparking also occurs
between the brushes and the commutator
Overview