Technical Documents - Documentos Técnicos: Servomotors - Permanent-Magnet DC Servomotors - Brush-Type PM DC Servomotors - Disk-Type PM DC Motors
Permanent-Magnet DC Servomotors
Tools & Equipment - DC & AC Electric Motors
Permanent-magnet (PM) field DC rotary motors have proven to be reliable
drives for motion control applications where high efficiency, high
starting torque, and linear speed–torque curves are desirable characteristics.
While they share many of the characteristics of conventional rotary
series, shunt, and compound-wound brush-type DC motors, PM DC servomotors
increased in popularity with the introduction of stronger
ceramic and rare-earth magnets made from such materials as
neodymium–iron–boron and the fact that these motors can be driven easily
by microprocessor-based controllers.
Fig. Cutaway view of a
fractional horsepower permanent-
magnet DC servomotor.
The replacement of a wound field with permanent magnets eliminates
both the need for separate field excitation and the electrical losses that
occur in those field windings. Because there are both brush-type and
brushless DC servomotors, the term DC motor implies that it is brushtype
or requires mechanical commutation unless it is modified by the
term brushless. Permanent-magnet DC brush-type servomotors can also
have armatures formed as laminated coils in disk or cup shapes. They are
lightweight, low-inertia armatures that permit the motors to accelerate
faster than the heavier conventional wound armatures.
The increased field strength of the ceramic and rare-earth magnets
permitted the construction of DC motors that are both smaller and lighter than earlier generation comparably rated DC motors with alnico (aluminum–
nickel–cobalt or AlNiCo) magnets. Moreover, integrated circuitry
and microprocessors have increased the reliability and costeffectiveness
of digital motion controllers and motor drivers or
amplifiers while permitting them to be packaged in smaller and lighter
cases, thus reducing the size and weight of complete, integrated motioncontrol
Brush-Type PM DC Servomotors
The design feature that distinguishes the brush-type PM DC servomotor, as
shown in the figure above, from other brush-type DC motors is the use of a permanent-
magnet field to replace the wound field. As previously stated, this
eliminates both the need for separate field excitation and the electrical
losses that typically occur in field windings.
Permanent-magnet DC motors, like all other mechanically commutated
DC motors, are energized through brushes and a multisegment commutator.
While all DC motors operate on the same principles, only PM DC motors
have the linear speed–torque curves shown in the following figure, making them
ideal for closed-loop and variable-speed servomotor applications. These
linear characteristics conveniently describe the full range of motor performance. It can be seen that both speed and torque increase linearly with
applied voltage, indicated in the diagram as increasing from V1 to V5.
The stators of brush-type PM DC motors are magnetic pole pairs.
Fig. - A typical family of
speed/torque curves for a permanent-
magnet DC servomotor at
different voltage inputs, with
voltage increasing from left to
right (V1 to V5).
When the motor is powered, the opposite polarities of the energized
windings and the stator magnets attract, and the rotor rotates to align
itself with the stator. Just as the rotor reaches alignment, the brushes
move across the commutator segments and energize the next winding.
This sequence continues as long as power is applied, keeping the rotor in
continuous motion. The commutator is staggered from the rotor poles,
and the number of its segments is directly proportional to the number of
windings. If the connections of a PM DC motor are reversed, the motor
will change direction, but it might not operate as efficiently in the
Disk-Type PM DC Motors
The disk-type motor shown exploded view in the following figure has a diskshaped
armature with stamped and laminated windings. This nonferrous
laminated disk is made as a copper stamping bonded between
epoxy–glass insulated layers and fastened to an axial shaft. The stator
field can either be a ring of many individual ceramic magnet cylinders,
as shown, or a ring-type ceramic magnet attached to the dish-shaped end bell, which completes the magnetic circuit. The spring-loaded brushes ride directly on stamped commutator bars.
Fig. - Exploded view of a
permanent-magnet DC servomotor
with a disk-type armature.
These motors are also called pancake motors because they are housed
in cases with thin, flat form factors whose diameters exceed their
lengths, suggesting pancakes. Earlier generations of these motors were
called printed-circuit motors because the armature disks were made by a
printed-circuit fabrication process that has been superseded. The flat
motor case concentrates the motor’s center of mass close to the mounting
plate, permitting it to be easily surface mounted. This eliminates the
awkward motor overhang and the need for supporting braces if a conventional
motor frame is to be surface mounted. Their disk-type motor form
factor has made these motors popular as axis drivers for industrial robots
where space is limited.
The principal disadvantage of the disk-type motor is the relatively
fragile construction of its armature and its inability to dissipate heat as
rapidly as iron-core wound rotors. Consequently, these motors are usually
limited to applications where the motor can be run under controlled
conditions and a shorter duty cycle allows enough time for armature heat
buildup to be dissipated.
Cup- or Shell-Type PM DC Motors
Cup- or shell-type PM DC motors offer low inertia and low inductance
as well as high acceleration characteristics, making them useful in many servo applications. They have hollow cylindrical armatures made as aluminum
or copper coils bonded by polymer resin and fiberglass to form a
rigid “ironless cup,” which is fastened to an axial shaft. A cutaway view
of this class of servomotor is illustrated in the following figure:
Fig. Cutaway view of a permanent-magnet DC servomotor with a cup-type armature.
Because the armature has no iron core, it, like the disk motor, has
extremely low inertia and a very high torque-to-inertia ratio. This permits
the motor to accelerate rapidly for the quick response required in
many motion-control applications. The armature rotates in an air gap
within very high magnetic flux density. The magnetic field from the stationary
magnets is completed through the cup-type armature and a stationary
ferrous cylindrical core connected to the motor frame. The shaft
rotates within the core, which extends into the rotating cup. Spring-brushes
commutate these motors.
Another version of a cup-type PM DC motor is shown in the exploded
view in the figure below. The cup type armature is rigidly fastened to the
shaft by a disk at the right end of the winding, and the magnetic field is
also returned through a ferrous metal housing. The brush assembly of
this motor is built into its end cap or flange, shown at the far right.
The principal disadvantage of this motor is also the inability of its
bonded armature to dissipate internal heat buildup rapidly because of its
low thermal conductivity. Without proper cooling and sensitive control
circuitry, the armature could be heated to destructive temperatures in
Fig. - Exploded view of
a fractional horsepower brushtype
Brushless PM DC Motors
Brushless DC motors exhibit the same linear speed–torque characteristics
as the brush-type PM DC motors, but they are electronically commutated.
The construction of these motors, as shown in the following figure, differs
from that of a typical brush-type DC motor in that they are
“inside-out.” In other words, they have permanent magnet rotors instead
of stators, and the stators rather than the rotors are wound. Although this
geometry is required for brushless DC motors, some manufacturers have
adapted this design for brush-type DC motors.
Fig. - Cutaway view of a brushless DC motor.
The mechanical brush and bar commutator of the brushless DC
motor is replaced by electronic sensors, typically Hall-effect devices
(HEDs). They are located within the stator windings and wired to solidstate
transistor switching circuitry located either on circuit cards
mounted within the motor housings or in external packages. Generally,
only fractional horsepower brushless motors have switching circuitry
within their housings.
The cylindrical magnet rotors of brushless DC motors are magnetized
laterally to form opposing north and south poles across the rotor’s diameter.
These rotors are typically made from neodymium–iron–boron or
samarium–cobalt rare-earth magnetic materials, which offer higher flux
densities than alnico magnets. These materials permit motors offering
higher performance to be packaged in the same frame sizes as earlier
motor designs or those with the same ratings to be packaged in smaller
frames than the earlier designs. Moreover, rare-earth or ceramic magnet rotors can be made with smaller diameters than those earlier models with
alnico magnets, thus reducing their inertia.
A simplified diagram of a DC brushless motor control with one Halleffect
device (HED) for the electronic commutator is shown in
the figure below:
Fig. - Simplified diagram
of Hall-effect device (HED) commutation
of a brushless DC
The HED is a Hall-effect sensor integrated with an ampli-fier in a silicon chip. This IC is capable of sensing the polarity of the
rotor’s magnetic field and then sending appropriate signals to power
transistors T1 and T2 to cause the motor’s rotor to rotate continuously.
This is accomplished as follows:
1. With the rotor motionless, the HED detects the rotor’s north magnetic
pole, causing it to generate a signal that turns on transistor T2.
This causes current to flow, energizing winding W2 to form a southseeking
electromagnetic rotor pole. This pole then attracts the
rotor’s north pole to drive the rotor in a counterclockwise (CCW)
2. The inertia of the rotor causes it to rotate past its neutral position so
that the HED can then sense the rotor’s south magnetic pole. It then
switches on transistor T1, causing current to flow in winding W1,
thus forming a north-seeking stator pole that attracts the rotor’s
south pole, causing it to continue to rotate in the CCW direction.
The transistors conduct in the proper sequence to ensure that the excitation
in the stator windings W2 and W1 always leads the PM rotor field
to produce the torque necessary keep the rotor in constant rotation. The
windings are energized in a pattern that rotates around the stator.
There are usually two or three HEDs in practical brushless motors that
are spaced apart by 90 or 120º around the motor’s rotor. They send the
signals to the motion controller that actually triggers the power transistors,
which drive the armature windings at a specified motor current and
Fig. - Exploded view of a brushless DC motor with Hall-effect device (HED) commutation.
The brushless motor in the exploded view seen in the figure above illustrates a
design for a miniature brushless DC motor that includes Hall-effect com-mutation. The stator is formed as an ironless sleeve of copper coils
bonded together in polymer resin and fiberglass to form a rigid structure
similar to cup-type rotors. However, it is fastened inside the steel laminations
within the motor housing.
This method of construction permits a range of values for starting current
and specific speed (rpm/V) depending on wire gauge and the number
of turns. Various terminal resistances can be obtained, permitting the
user to select the optimum motor for a specific application. The Halleffect
sensors and a small magnet disk that is magnetized widthwise are
mounted on a disk-shaped partition within the motor housing.
Position Sensing in Brushless Motors
Both magnetic sensors and resolvers can sense rotor position in brushless
motors. The diagram in the following figure shows how three magnetic sensors
can sense rotor position in a three-phase electronically commutated
brushless DC motor. In this example the magnetic sensors are located
inside the end-bell of the motor. This inexpensive version is adequate for
Fig. - A magnetic sensor
as a rotor position indicator: stationary
brushless motor winding
(1), permanent-magnet motor
rotor (2), three-phase electronically
commutated field (3), three
magnetic sensors (4), and the
electronic circuit board (5).
In the alternate design shown in the following figure, a resolver on the end cap
of the motor is used to sense rotor position when greater positioning
accuracy is required. The high-resolution signals from the resolver can be used to generate sinusoidal motor currents within the motor controller.
The currents through the three motor windings are position independent
and respectively 120º phase shifted.
Fig. - A resolver as a
rotor position indicator: stationary
motor winding (1), permanent-
magnet motor rotor (2),
three-phase electronically commutated
field (3), three magnetic
sensors (4), and the electronic circuit
Brushless Motor Advantages
Brushless DC motors have at least four distinct advantages over brushtype
DC motors that are attributable to the replacement of mechanical
commutation by electronic commutation.
• There is no need to replace brushes or remove the gritty residue
caused by brush wear from the motor.
• Without brushes to cause electrical arcing, brushless motors do not
present fire or explosion hazards in an environment where flammable
or explosive vapors, dust, or liquids are present.
• Electromagnetic interference (EMI) is minimized by replacing
mechanical commutation, the source of unwanted radio frequencies,
with electronic commutation.
• Brushless motors can run faster and more efficiently with electronic
commutation. Speeds of up to 50,000 rpm can be achieved vs. the
upper limit of about 5000 rpm for brush-type DC motors.
Brushless DC Motor Disadvantages
There are at least four disadvantages of brushless DC servomotors.
• Brushless PM DC servomotors cannot be reversed by simply reversing
the polarity of the power source. The order in which the current
is fed to the field coil must be reversed.
• Brushless DC servomotors cost more than comparably rated brushtype
• Additional system wiring is required to power the electronic commutation
• The motion controller and driver electronics needed to operate a
brushless DC servomotor are more complex and expensive than those
required for a conventional DC servomotor.
Consequently, the selection of a brushless motor is generally justified
on a basis of specific application requirements or its hazardous operating
Characteristics of Brushless Rotary Servomotors
It is difficult to generalize about the characteristics of DC rotary servomotors
because of the wide range of products available commercially.
However, they typically offer continuous torque ratings of 0.62 lb-ft
(0.84 N-m) to 5.0 lb-ft (6.8 N-m), peak torque ratings of 1.9 lb-ft (2.6
N-m) to 14 lb-ft (19 N-m), and continuous power ratings of 0.73 hp
(0.54 kW) to 2.76 hp (2.06 kW). Maximum speeds can vary from 1400
to 7500 rpm, and the weight of these motors can be from 5.0 lb (2.3 kg)
to 23 lb (10 kg). Feedback typically can be either by resolver or
Basic of servomotor control
This information explains the difference between a servomotor and a stepper motor when connected to a servo
driver. It covers the terms used in controlling the pulse train supplied to servomotors by a PCL series
- The difference between a stepper motor and a servomotor configuration is shown below. The design and
construction of the motors are also different.
- Stepper motor operation is synchronized by command pulse signals output from the PCL or a pulse
generator (strictly speaking it follows the pulses). In contrast, servomotor operation lags behind the
I. Connection and operation differences in stepper motors and servomotors
II. Advantages and disadvantages of stepper motors and servomotors
1. Stepper motor
(1) Since stepper motor operation is synchronized with the command pulse signals from a pulse
generator such as the PCL, they are suitable for precise control of their rotation.
(2) Lower cost.
(1) Basically, the current flow from a driver to the motor coil cannot be increased or decreased during
operation. Therefore, if the motor is loaded with a heavier load than the motor's designed torque
characteristic, it will get out of step with the pulses.
(2) Stepper motors produce more noise and vibration than servomotors.
(3) Stepper motors are not suitable for high-speed rotation.
(1) If a heavy load is placed on the motor, the driver will increase the current to the motor coil as it
attempts to rotate the motor. Basically, there is no out-of-step condition. (However, too heavy a load
may cause an error.)
(2) High-speed operation is possible.
(1) Since the servomotor tries to rotate according to the command pulses, but lags behind, it is not suitable for precision control of rotation.
(2) Higher cost.
(3) When stopped, the motor’s rotor continues to move back and forth one pulse, so that it is not suitable
if you need to prevent vibration.
Both motors have advantages and disadvantages. The selection of which type to use requires careful
consideration of the application’s specifications.
Below is a summary of the comparison of stepper motors and servomotors
* ppr = Pulses per revolution
III. What is a deflection counter?
The servomotor rotation lags behind the command pulses from the PCL. This means that when the PCL
completes outputting a number of pulses equivalent to the preset amount, the encoder will take some time
to return all of the pulses. That is why the servo driver includes a "deflection counter."
=> This counter compares the number of command pulses from the PCL and the number of pulses returned
from the encoder.
=> If the number of pulses returned from the encoder is smaller than the number of command pulses output,
the driver will try to rotate the motor some more.
If the number of pulses returned from the encoder is larger than the number of command pulses output,
the driver will attempt to run the motor backward.
When the number of command pulses output from the PCL and the number of pulses returned from the
encoder match, the motor stops.
(In other words, the driver attempts to rotate the motor until the deflection counter is zero.
IV. Output signals from an encoder
An encoder is a kind of pulse generator. It outputs three types of pulse signals: A phase, B phase, and Z
phase (Index signal)
1. A phase and B phase signals
In order to make the pulse per rotation resolution finer and to set the direction of rotation, two pulse
trains with the same cycle length are phase shifted.
This half-pulse deviation is the key. For example, if the A phase pulse rises first, this means the motor
rotation is clockwise (CW). If the B phase rises first, this means the rotation is counter-clockwise (CCW).
That is how to tell the direction of rotation.
2. Z phase (Index signal)
In our example, we will assume that the encoder has 1000 pulse per revolution (1000 ppr) and that it
outputs 1000 A and B phase pulses (1000 rising edges per revolution). But, it outputs a Z phase pulse
only once per one revolution.
If you want to execute a zero return precisely, a stop using only the ORG sensor may cause a deviation
of plus or minus a few pulses each time. Therefore, after the ORG turns ON, count a specified number of
Z phase signals and make this the official zero position.
Supply these A phase, B phase, and Z phase signals from the encoder to the PCL.
(The corresponding terminal names on the PCL50xx series, PCL61xx/PCL60xx series are: EA, EB, and
Since stepper motors have full step and half step operation modes, the number of encoder divisions can
For example, an encoder which puts out a nominal 1000 pulses per revolution (1000 ppr) can be
multiplied as follow:
- 1x => 1000ppr = 0.36°/ pulse
- 2x => 2000ppr = 0.18°/ pulse
- 4x => 4000ppr = 0.09°/ pulse
(The multiplication rate is specified in the servo driver.)
You can specify the multiplication rate to apply (1x, 2x, and 4x) in the PCL. Shown below are the bits used by 3 typical PCL models.
* Multiplication settings of the encoder A/B phase signals
- PCL3013/5014: Set bits 10 and 11 in R16 (environment setting register 1)
- PCL61xx series: Set bits 16 and 17 in RENV2 (environment setting register2) (Bit names: EIM0 and
- PCL60xx series: Set bits 20 and 21 in RENV2 (environment setting register2) (Bit names: EIM0 and