WHAT IS THE MOST EFFECTIVE WAY

TO COMMUTATE A BLDC MOTOR?

Understanding the Methods and Tradeoffs
By: Jason Kelly
www.cui.com

What is the Most Effective Way to

Commutate a BLDC Motor?

BRUSHLESS DC MOTORS
Brushless direct current electric motors, or BLDC motors for short, are electronically
commutated motors powered by a dc electric source via an external motor controller.
Unlike their brushed relatives, BLDC motors rely on external controllers to achieve
commutation. Put simply, commutation is the process of switching the current in the motor
phases to generate motion. Brushed motors have physical brushes to achieve this process
twice per rotation, while BLDC motors do not, hence the name. Due to the nature of their
design, they can have any number of pole pairs for commutation.

BLDC motors provide significant advantages over traditional brushed motors. They
typically offer an efficiency increase of 15-20%, require less maintenance without
brushes to physically wear out, and deliver a flat torque curve at all rated speeds. While
BLDC motors are not a new invention, widespread adoption had been slow due to the
need for complicated control and feedback circuitry. However, recent advancements
in semiconductor technology, better permanent magnets and the growing demand for
greater efficiency has led to BLDC motors replacing brushed motors in many applications.
BLDC motors have found their niche in many industries, including white goods, automotive,
aerospace, consumer, medical, industrial automation equipment and instrumentation.

With the industry moving in a direction that requires BLDC motors in more applications,
many engineers are having to make the switch to this technology. And while the basics of
motor design still apply, the addition of external control circuitry has added another set of
design considerations to the equation. High on the list of design questions is how to obtain
feedback for motor commutation.

MOTOR COMMUTATION
Before delving too far into feedback options for BLDC motors, it is important to understand
why they are necessary. BLDC motors come in single phase, 2-phase, and 3-phase
configurations; the most common configuration being 3-phase. The number of phases
match the number of windings on the stator while the rotor poles can be any number of
pairs depending on the application. And because the rotor of a BLDC motor is influenced
by the revolving stator poles, the stator pole position must be tracked in order to
effectively drive the 3 motor phases. Hence, a motor controller is used to generate a 6-step
commutation pattern on the 3 motor phases. These 6-steps, or commutation phases,
move an electromagnetic field which causes the permanent magnets of the rotor to move
the motor shaft.

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What is the Most Effective Way to

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Figure 1:
Six-step pattern
for BLDC motor
commutation

Using this standard motor commutation sequence the motor controller can then use a high
frequency pulse-width modulated (PWM) signal to effectively reduce the average voltage
observed by the motor, thus varying motor speed. This setup also allows for a large amount
of flexibility in design by allowing for one voltage source to be used for a wide range of
motors even if the dc voltage source is much larger than the rated voltage of the motor. In
order for this system to maintain its efficiency advantages over brushed technology, a very
tight control loop is required between the motor and controller. This is where feedback
technology becomes important; for the controller to maintain accurate control of the
motor, it must always know the exact position of the stator in relation to the rotor. Any
misalignment or phase shift in the expected and actual position may result in undesirable
behavior and a decline in performance. There are many ways to achieve this feedback for
the commutation of BLDC motors, but the most common are Hall effect sensors, encoders,
or resolvers. Additionally, some applications rely on sensorless commutation techniques.

POSITION FEEDBACK
Since the inception of the brushless motor, Hall effect sensors have been the workhorse for
commutation feedback. For 3-phase control, only 3 sensors are required, and with a very
low per-unit cost, they are easily the most economical option to achieve commutation from
a pure BOM cost perspective. Hall sensors are embedded into the stator of the motor to
detect rotor positon, which is used to switch the transistors in the 3-phase bridge to drive
the motor. The three Hall effect sensor outputs are commonly noted as the U, V, and W
channels. While Hall sensors are an effective solution for commutating BLDC motors, they
only address half the needs of a BLDC system.

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What is the Most Effective Way to

Commutate a BLDC Motor?

Hall effect sensors will allow a controller to drive a BLDC motor, but unfortunately its
controls are limited to speed and direction. With a 3-phase motor, Hall effect sensors can
only provide angular position within each electrical cycle. As pole pair counts increase,
the number of electrical cycles per mechanical revolution increase and as BLDC use
becomes more commonplace, so increases the need for accurate position sensing. To
ensure a robust and complete solution, the BLDC system should provide real-time position
information, so that the controller can track not only speed and direction, but distance
travelled and angular position as well.

Figure 2:
Three-phase bridge
driver circuit

The most common solution to address the need for tighter position information has been
to add an incremental rotary encoder to the BLDC motor. Incremental encoders are often
added in addition to Hall effect sensors within the same control feedback loop system. The
Hall sensors are used for motor commutation, and the encoder is used to track position,
rotation, speed, and direction with far greater accuracy. Since hall sensors only provide
new position information at each Hall state change, their precision is limited to six states
per electrical revolution; for a two-pole motor, this results in only six states per mechanical
revolution. Compared with incremental encoders that offer resolutions in the thousands
of PPR (pulses per revolution), which can then be decoded into four times as many state
changes, it becomes obvious why there is a need for both.

However, because motor manufacturers are having to put both Hall effect sensors and
incremental encoders onto their motors, many encoder manufacturers have moved
to offering incremental encoders with commutation outputs, often simply called
commutation encoders. These encoders are designed to provide their traditional
quadrature A and B channels (and sometimes a once-per-turn index pulse channel Z)
along with the standard U, V, and W commutation signals required by most BLDC motor
drivers. This saves motor designers the unnecessary step of installing both Hall effect
sensors and an incremental encoder.

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What is the Most Effective Way to

Commutate a BLDC Motor?

Although the advantages of this approach are compelling, there is a significant tradeoff
to this method. As noted previously, for a BLDC motor to be efficiently commutated,
the rotor and stator positions must be known. This means great care must be taken to
ensure that the commutation encoder’s U/V/W channels are properly aligned to the BLDC
motor’s phases.

For optical encoders that have fixed patterns on their optical disks and Hall effect sensors
that have to be manually placed, the process to achieve correct alignment of a BLDC motor
is both iterative and time consuming. The method involves additional equipment including
a second motor and an oscilloscope. To align an optical encoder or a set of Hall effect
sensors, the BLDC motor must be backdriven with a second motor; then, as the motor is
being rotated at constant velocity by the second motor, an oscilloscope is used to monitor
the counter-electromotive force (also known as back electromotive force or back-EMF) of

Figure 3:
Six-step Hall effect
outputs and
trapezoidal motor
phases

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What is the Most Effective Way to

Commutate a BLDC Motor?

the three motor phases. The resulting U/V/W signals from the encoder or Hall sensors must
be checked against the back-EMF waveform presented on the oscilloscope. If there are
any variances between the U/V/W channels and the back-EMF waveform, an adjustment
must be made. This process can take upwards of 20 minutes per motor, requires extensive
laboratory facilities, and is a major source of frustration when using BLDC motors. Although
optical commutation encoders consolidate the burden to installing only one technology,
the downside of implementing an optical commutation encoder is its lack of versatility.
Because optical encoders use fixed patterns on their optical disks, the motor pole count,
quadrature resolution, and motor shaft size must be known before ordering.

Figure 4:
Desired alignment
of commutation
channels and motor
phases

CAPACITIVE COMMUTATION ENCODERS

CUI Inc has addressed both problems by offering an enhanced commutation encoder
based on the patented capacitive technology used in their AMT family of products.
Optical encoders use very small LEDs to transmit light through a disk with notches at
specific intervals to generate output patterns. AMT encoders can be described in a similar
manner, but instead of transmitting light via LEDs, an electrical field is transmitted. In the
place of an optical disk is a PCB rotor containing a sinusoidal-patterned metal trace that
modulates the electrical field. The receiving end of the modulated signal is then passed
back to the transmitter where it is compared against the original via a proprietary ASIC.
This technology uses the same principle as the Vernier digital caliper, which is known for
its reliability and accuracy.

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What is the Most Effective Way to

Commutate a BLDC Motor?

Figure 5:
Capacitive encoder
operation

The AMT31 Series Commutation Encoder provides incremental outputs A/B/Z, as well as
commutation outputs U/V/W. With a design including a capacitive ASIC and onboard MCU
the encoder generates its outputs digitally. This is significant because it allows the user to
digitally set the zero position of the encoder with a single press of a button. Simply lock the
BLDC motor into the desired phase state and zero the AMT31 encoder with the AMT One
Touch Zero™ module or the AMT Viewpoint™ programming GUI. This removes the need to
backdrive the motor, or to view any of the output signals with the oscilloscope, effectively
removing 20 minutes from the assembly process.

And because of the capacitive technology, the quadrature resolution and commutation
outputs can be adjusted dynamically. Users simply connect the AMT31 encoder to the
AMT Viewpoint GUI, select from a list of 20 quadrature resolutions (up to 4096 PPR) and
7 standard pole pair options (up to 20 poles), and hit ‘Program’. This has advantages in
development, allowing engineers to make any changes to prototypes quickly and easily,
and also aids production supply-chain management by allowing a single stock-keeping unit
(SKU) to be used for multiple motor controls of different resolutions and BLDC pole counts.
In addition to each unit supporting multiple resolutions and pole pair counts, the encoder
housing is designed for easy assembly while offering several mounting options and multiple
sleeve sizes to suit commonly used motor shaft diameters.

The AMT Viewpoint GUI also brings an unprecedented level of design support to the AMT31
series encoder. When connected to AMT Viewpoint, diagnostic data from the AMT31
encoder can be downloaded and used to avoid potential failures and down time in the field.

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What is the Most Effective Way to

Commutate a BLDC Motor?

CONCLUSION
A tight control loop with high accuracy allows BLDCs to excel in many areas. Increased
accuracy means less power wasted, higher precision, and more end user control over
the BLDC operation. BLDC motors are currently being implemented in a diverse range of
applications, including surgical robotics, driverless automobiles, assembly line automation,
and will soon find their place in many other fields not yet conceived. And as the market
for BLDC motors grow, the requirements remain the same: robust, efficient motors that
offer low cost, high accuracy position sensing feedback. When paired with a BLDC motor,
the AMT31 series encoder is able to deliver valuable savings in time during installation,
while also streamlining development and manufacturing. With its high versatility, the
ability to be programmed and zero set in seconds, and with its compatibility with the
AMT Viewpoint GUI, the AMT31 encoder is well-aligned to meet the needs of the quickly-
growing BLDC market.

View CUI’s AMT31 Commutation Encoders

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