Page 28 - Industrial Technology July 2021

MACHINE BUILDING

EFFECTS OF PWM

DRIVES, MOTORS & CONTROLS

BRUSHLESS DC MOTORS ARE INCREASINGLY POPULAR OPTIONS FOR A HOST OF CRITICAL APPLICATIONS
WHERE SIZE, PERFORMANCE AND RELIABILITY ARE IMPORTANT CONSIDERATIONS. BUT DESIGNERS OF
MOTION SYSTEMS CAN FACE CHALLENGES WHEN SELECTING OR DEVELOPING PWM ELECTRONICS TO DRIVE
THESE MOTORS. MATTHIEU BOUAT, APPLICATION ENGINEER AT PORTESCAP, DISCUSSES THE PARAMETERS
THAT DESIGNERS SHOULD CONSIDER WHEN DEVELOPING THE ELECTRONIC CONTROL ASPECT OF THE
MOTION SYSTEM, PARTICULARLY WHERE FACTORS SUCH AS BATTERY LIFE AND THE OVERALL SIZE AND
WEIGHT OF THE PACKAGE ARE KEY CONCERNS.

n any brushless DC motor driven system, the role of but if these extremes are not needed, then a lower
the control electronics, the amplifier, is to vary the supply voltage will be beneficial to reduce the current
supply voltage or the current, or both, to achieve the ripple. Further, operating under the same load point
Idesired motion output of the motor. There are a with a lower power supply voltage will also increase the
number of different options for the amplifier. A linear duty cycle, which will reduce the current ripple even
amplifier adapts the power delivered to the motor by more. Generally, it is important to keep
linearly changing the voltage or current. It dissipates the duty cycle of the PWM as far as
the power which is not delivered to the motor, possible from 50%, which is the
resulting in the need for a large heat sink to worst case.
dissipate the power, increasing the amplifier A second option is to increase the PWM
size and making it more difficult to integrate frequency. With a shorter cycle time, the
into the application. current will have less time to rise, meaning
In contrast, a chopper amplifier less ripple. Portescap recommends using
modulates the voltage (and current) by PWM frequencies not less than 50 kHz for
switching on and off the power brushless DC motors. PWM frequencies of
transistors. The primary 80 kHz or more would be even more
advantage is that it saves appropriate for motors having very small
power when the transistor is electrical time constant.
off. This helps save on the battery life of Finally, we can consider increasing the
the application, causes less heating and inductance, adding external inductance
allows a smaller size of the electronics. components to slow down the rise and fall of
Most commonly, chopper amplifiers use a the current and hence reduce current ripple.
PWM (pulse width modulation) method, However, while adding inductances of, for
varying the duty cycle at a fixed example, several tens of µH can work on
frequency to adjust the voltage or paper, in practice there may not be the
current within the desired target room to integrate these components,
value. particularly in applications where space is
The switching frequency is a fixed limited. Therefore, it is usually wiser to explore the two
parameter, making it easy for electronic designers to filter other options first.
acoustic and electromagnetic noise generated. It is useful, PWM has many advantages over other forms of
however, to understand some other basic physical control, and it is easy to see why it is the most widely
phenomena to avoid unexpected performance issues. Schematic cross-section of a slotless brushless DC motor used solution for driving brushless DC drivers. The
Most notably, the switching on and off of the transistors starting step 3 affordability and availability of electronic components
leads to current rise and fall at each cycle, which can be make it simple to set an adequate PWM voltage and to
problematic. proportional to the square of motor speed and to the use a high PWM frequency, reducing ripple and avoiding
While the average current – established by the duty square of motor current. Iron losses have a direct impact the need to use additional inductances. This minimises
cycle of the switching electronics – defines the motor on motor power, and it is easy to see, being a squared additional energy dissipated by Joules losses, helping to
torque, the ripple current generates extra Joules losses relationship, how a high ripple current can quickly maximise battery life, while enabling designers to
(heat) and can have huge impact on the RMS (root mean generate significant iron losses. minimise the size and weight and the electronics, which
square) current, without any corresponding increase in is key for example in portable devices with embedded
torque. Since there are no brushes the ripple current Reducing current ripple electronics.
doesn’t compromise the lifetime of the brushless DC motor It is important, therefore, to keep current ripple as low as As ever, it can pay dividends to engage with a
as it would in a brushed DC motor, but it wastes energy possible, and there are a number of ways in which we can knowledgeable supplier at the earliest stages of a design.
(potentially compromising battery life in portable do this. The first is to consider reducing or adapt the Portescap engineers are on hand to help you define
applications) and may require a heat sink, adding size and power supply voltage, given that current ripple is directly suitable electronics to work with brushless DC motors
weight to the application. The ripple current also causes proportional to this voltage. A high voltage can be useful in even the most demanding applications.
iron losses, forming circulation eddy currents that are where the application requires high speed or high power, MORE INFORMATION: www.portescap.com

28 INDUSTRIAL TECHNOLOGY • July 2021