Introduction
Basically, four different types of DC motors are used in industrial applications:
series wound, shunt wound, compound wound, and permanent
magnet. Several factors must be considered when selecting a DC motor for
a specific application.
First, decide what the allowable variation in speed and torque can be for a
given change in load. Each type of motor has benefits that are advantageous
for certain applications. The following review will help you decide
which motor may provide better performance in a given application. The
DC motor and drive specifications should always be consulted to determine
the specific speed and torque capabilities of the system. The speed/
torque curves listed below are for illustrative purposes.
Series Wound DC Motors
A series wound DC motor has the armature and field windings connected
in a series circuit. Figure 3-17 shows a series wound DC motor, with an
associated speed/torque curve.
As seen in Figure 3-17, this type of motor configuration features very high
breakaway torque. Typical applications for this motor would be printing
presses, ski lifts, electric locomotives, cranes, and oil drilling.
The starting torque developed can be as high as 500% of the full load rating.
The high starting torque is a result of the fact that the field winding is
operated below the saturation point.
An increase in load will cause a corresponding increase in both the armature
and field winding current, which means that both armature and field
winding flux increase together. As you recall, the torque developed in a
DC motor is the result of the interaction of armature and field winding
fluxes. Torque in a DC motor increases as the square of the current value.
A series wound DC motor will generate a larger torque increase compared
with a shunt wound DC motor for given increase in current.
Conversely, the speed regulation of a series wound DC motor is poorer
than that of a shunt wound motor. As stated above, when the load
increases, so does the armature and field winding current. When the load
is reduced, so is the current, which causes a corresponding decrease in flux
density. As a reminder of DC motor basics, when the field flux is reduced
once the motor is running, a decrease in "hold-back" electromotive force
(EMF) occurs. Therefore, when the load is reduced, speed increases. If the
load were completely removed, the speed of the motor would increase to
infinity-basically until the motor destroys itself. As a safety precaution,
series wound DC motors should always be connected to a load.
Parallel (Shunt) Wound DC Motors
A shunt wound DC motor has the armature and field windings connected
in parallel. Figure 3-18 shows a shunt wound DC motor, with an associated
speed/torque curve.
This type of DC motor is probably the most widely used motor in industrial
applications. As indicated in the figure, this type of motor requires two
power supplies-one for the armature and one for the field winding.
Typical applications for this motor would be printing presses, ski lifts, plastic
extruders, conveyors, and practically any other application where DC
motors are used. Because of the need for two power supplies, this type of
motor is a prime candidate for a DC drive (converter), which usually
includes a low-current field winding exciter (power supply).
With constant armature voltage and field winding excitation, this type of
motor offers relatively flat speed/torque characteristics. The starting torque
developed can be 250-300% of the full load torque rating for a short
period of time. Speed regulation (speed fluctuation due to load) is acceptable
in many cases between 5-10% of maximum speed, when operated
from a DC drive. Regulation of this amount would be typical when operated
from a drive controller, open loop (no electronic feedback device connected
to the motor shaft). As discussed in Chapter 5, speed feedback
devices such as a tachometer generator can dramatically improve the regulation
(down to less than 1%).
Because of the need for two power sources, the shunt wound DC motor
offers the use of simplified control for reversing requirements. Direction of
any shunt wound motor can be changed by simply reversing the direction
of current flow, in either the armature or shunt field winding. The capability
of armature or field reversal is standard on many DC drive modules. (In
many cases, the reversing of flux and direction is accomplished in the field
winding control. The field winding consumes less than one tenth of the
current compared with the armature circuit. Smaller components and less
stress on circuitry is the result when "field reversal" is used for DC motor
control.)
Compound Wound DC Motors
A compound wound DC motor is basically a combination of shunt wound
and series wound configurations. This type of motor offers the high starting
torque of a series wound motor. In addition, it offers constant speed
regulation (speed stability) under a given load. This type of motor is used
whenever speed regulation cannot be obtained from either a series or
shunt wound motor. Figure 3-19 indicates a compound wound DC motor,
with an associated speed/torque curve.
The torque and speed characteristics are the result of placing a portion of
the field winding circuit in series with the armature circuit. This additional
armature winding circuit is not to be confused with the commutating
winding or interpoles. The commutation windings also have a few turns,
but have the duty of neutralizing armature reaction.
When a load is applied, there is a corresponding increase in current
through the series winding, which also increases the field flux. This in turn
increases the torque output of the motor.
Permanent Magnet DC Motors
A permanent magnet motor is built with a standard armature and brushes,
but has permanent magnets in place of the shunt field winding. The speed
characteristic is close to that of a shunt wound DC motor. When adding
the cost of a DC motor and control system, this type of motor is less expensive
to operate, since there is no need for a shunt field winding exciter
supply. Figure 3-20 indicates a permanent magnet DC motor, with an
associated speed/torque curve.
Along with less expensive operation, this type of motor is simpler to
install, with only the two armature connections needed. This motor type is
also simpler to reverse-simply reverse the connections to the armature.
The permanent magnet poles are usually constructed of materials such as
ceramic or alnico (aluminum, nickel, and cobalt). The ceramic magnets are
used for low-horsepower, slow-speed applications because of their low flux level generation. Though this type of motor has good operational
characteristics and lower cost, there are several drawbacks to this type of
motor compared with the others.
Materials such as ceramic have a high resistance to demagnetization. However,
permanent magnets do have a tendency to lose some of their magnetic
strength over use and time. This reduction in magnetic field strength
causes a corresponding reduction in torque output. To counteract this possibility,
some higher-cost permanent magnet motors include windings
built into the field magnets for the purpose of "re-magnetizing" the magnets.
In addition to ceramic or alnico magnets, rare earth magnets are also a
cost-effective means of generating magnetic field flux. This type of magnetic
group includes the "embedded" magnet, which is only one of nine
different magnetic materials available.
Though this type of motor has very good starting torque capability, the
speed regulation is slightly less than that of a compound wound motor.
The overall torque output makes this motor a prime candidate for lowtorque
applications. Peak torque is limited to about 150%. This limitation
is based on the fact that additional "demagnetizing" of the field poles could
occur if more torque was developed.
Specialty DC Motors-PM DC Servomotors
Servomotors are considered "specialty" in that they are used in applications
that require very fast speed response and accuracy. In many cases,
the shaft speed is accelerated from zero to 6000 rpm in hundredths of a
second. The same speed profile could be needed in the deceleration mode,
as well as an immediate reversal of direction.
These types of motors must be designed to handle the stress of acceleration,
plus not fluctuate in speed, once the desired speed is obtained. Special
consideration is given to heat dissipation, since these motors must be
small, yet generate enough torque to operate the machine. The small size
allows this type motor to fit inside small packaging, palletizing, and processing
machines. Typically, these motors are long and narrow, in contrast
to a standard shunt wound DC motor. The long, narrow design results in
low inertia armature assemblies, which can be accelerated quickly. Servomotor
design with permanent magnets affords the smallest space possible.
In comparison, shunt field windings must have laminations wide enough
to generate the necessary field flux, which adds to the total width of the
machine. Figure 3-21 indicates the physical appearance of a typical DC
servomotor.
As seen in Figure 3-21, this type of motor is usually of a totally enclosed
design to seal out most moisture, dust, and moderate contaminates. The
physical frame of the motor acts as a heat sink to dissipate the heat generated.
Many servomotors are used expressly for positioning applications. Therefore,
the motor design allows for a position feedback device such as an
encoder or resolver. Mounting of the servomotor can be easily done by
means of a "C" face (no flange, but tapped holes to receive mounting
bolts) or "D" flange (outside flange with through-holes).
The principle involved in the PM servomotor is exactly the same as the
standard PM DC motor. It has an armature, commutator, and the PM field
for magnetic interaction. The difference comes in the physical size and
shape of the servomotor, as well as the performance and speed characteristics.
Specialty DC Motors-Brushless Servomotors
Another type of DC servomotor uses the high-torque and acceleration
characteristics, but without the use of a commutator or brushes. This type,
called the brushless DC servomotor, takes input three-phase or single-phase
input power and converts it to DC used by the motor windings. The windings
create magnetic flux that interacts with the PM field to generate
motor speed and torque. Figure 3-22 shows the design of the brushless DC
servomotor.
As seen in Figure 3-22, instead of the permanent magnets being mounted
as the field, the magnets are actually part of the rotor. (Note: Since there
are no brushes or commutator, the term "rotor" is used instead of armature,
indicating an AC-type machine design.) A typical brushless DC servomotor
may have multiple poles, such as three N and three S poles. There
would also be corresponding windings in the stator to create the magnetic
interaction. (Note: Because this is an AC-type machine design, the term
"stator" is used instead of "field" or "field windings.")
The rotor of the servomotor is usually laminated iron with magnets
inserted and "press-fit" or epoxied into position. Special high-speed bearings
support the rotor in position. Instead of a standard conduit box, servomotors
usually include a military-style connector. This style features all
connections on one plug or receptacle, with a screw-on ring to ensure positive
contact. This style of connector is resistant to machine vibration and
electrical interference.
The servomotor takes input power and converts it to DC for the main
windings in the stator. Depending on the design of the servomotor, the
control unit may include transistors that are turned on or off to generate
voltage. In the case of a three-phase servomotor, an external servo amplifier
is connected to generate the control voltage for the stator windings.
The main disadvantage of this motor is the inability to develop high starting
torque. In the case of a single-phase servomotor, half of the main
windings are used at any given time. This causes the copper losses to be
somewhat high. However, since transistor switching is used for control of
the brushless DC servomotor, motor life is mainly limited by the bearings,
since there are no commutator segments or brushes to wear out.
Introduction
Basically, four different types of DC motors are used in industrial applications:
series wound, shunt wound, compound wound, and permanent
magnet. Several factors must be considered when selecting a DC motor for
a specific application.
First, decide what the allowable variation in speed and torque can be for a
given change in load. Each type of motor has benefits that are advantageous
for certain applications. The following review will help you decide
which motor may provide better performance in a given application. The
DC motor and drive specifications should always be consulted to determine
the specific speed and torque capabilities of the system. The speed/
torque curves listed below are for illustrative purposes.
Series Wound DC Motors
A series wound DC motor has the armature and field windings connected
in a series circuit. Figure 3-17 shows a series wound DC motor, with an
associated speed/torque curve.
As seen in Figure 3-17, this type of motor configuration features very high
breakaway torque. Typical applications for this motor would be printing
presses, ski lifts, electric locomotives, cranes, and oil drilling.
The starting torque developed can be as high as 500% of the full load rating.
The high starting torque is a result of the fact that the field winding is
operated below the saturation point.
An...
More >>
