High-Performance Drivers for Motor Control
By Maurizio Di Paolo Emilio for Mouser Electronics
Electric motors represent an essential component in the industrial sector, where they are used for fans, pumps,
and other types of machinery in a wide variety of applications. The traditional AC motors that have been around
for over a century are the simplest types of induction motors, but they can waste a significant amount of
energy. DC motors are a class of rotary electrical machines widely used in various applications. A DC motor's
speed can be controlled by modulating the voltage according to the needs of the application. This can save
significant energy since the motor only works as much as the situation requires.
Features of electric drives
An electric motor is a "reversible" machine capable of transforming electrical energy into mechanical energy. The
term reversible indicates that it can do the inverse operation, changing its name to Generator. Conceptually,
and often also practically, the two are the same thing. A fixed (stator) and mobile (rotor) part always
constitute a motor. The various types of motor are distinguished from each other by how the magnetic fields are
generated:
- Continuous Motors (DC): static field, generated by magnets or by windings in the stator; they are available
in a wide range of voltages; the most popular being 12V and 24V.
- Alternate Motors (AC): dynamic field, generated by the interaction between fields generated by the currents
and rotor. The rotation of the rotor is synchronised with the frequency of the supply current (Synchronous
AC motor).
- Brushless motors: static field, generated by rotating magnets which are fixed on the rotor.
In DC motors, the generation of the magnetic field is carried out by the stator. The magnets can be permanent
(ferrites, for example) in low-power motors, while they are generated by dedicated windings in the medium and
large power motors, also called the wound field. The power is brought to the rotor by rotating collectors and
brushes that are subject to wear. They have excellent characteristics of robustness and reliability. The motor
speed is controlled by adjusting the DC voltage applied to the armature winding. Depending on the application, a
full-bridge converter, half-bridge or just a pulse width modulation (PWM) converter is used.
DC motors are also widely used in servo applications where speed and accuracy are essential. To meet design
requirements regarding speed and accuracy, microprocessor-based closed-loop control and information on the rotor
position are crucial. Maxim's Hall effect MAX9921 sensor provides information
on
the rotor position. A Hall-effect sensor is a transducer that varies its output voltage in response to a
magnetic field. The Hall effect sensor is composed of a sensitive element coupled to a magnet contained in an
airtight container that detects the variation of the magnetic field flux when a ferromagnetic material body
(metal projection) is approached and moved away.
The device can work ideally from zero frequency up to a few kHz. Hall effect devices are used as proximity
sensors, positioning, speed and current sensing. Unlike a mechanical switch, it is a long-lasting solution
because there are no mechanical wear problems.
The DC brushless motor (BLDC) is a direct current electric motor with a permanent magnet rotor and a rotating
magnetic field stator. Unlike a brushed motor, therefore, it does not require sliding electrical contacts
(brushes) on the motor shaft to operate. This means less mechanical resistance and less opportunity for sparks
forming, and a considerable reduction of maintenance.
A very similar motor is the stepper motor, which differs from the brushless motor in that the stator is not
powered all the time, but cyclically supplies the various electromagnets to generate a rotation or obtain a
precise position. In a brushless motor, the rotor is non-winding and instead has permanent magnets, while the
magnetic field generated by the windings on the stator is variable.
Since the motor operates in direct current, to realise the rotation of the magnetic field generated in the
stator, an electronic circuit composed of a bank of power transistors controlled by a microcontroller that
controls the switching of the current, commands the current inversion and therefore the rotation of the magnetic
field.
Since the controller must know the position of the rotor concerning the stator to determine the orientation of
the magnetic field, it is usually connected to a Hall effect sensor. The efficiency of these machines is on
average higher than the asynchronous direct current motors (Figure 1).
Figure 1: Block diagram for brushless motor control. (Source: Maxim
Integrated)
BLDC motors are a particular type of synchronous motor. This means that the magnetic field generated by the rotor
and the stator have the same frequency. The BLDCs are subdivided into three categories: single-phase, 2-phase
and 3-phase. The number of phases corresponds to the number of windings on the stator.
Motor Drives
The motor driver covers a wide range of applications in the most different industrial and civil sectors,
particularly in electronic instrumentation and various computer peripherals. Each application has a different
requirement of power and characteristics to be satisfied, for example, speed, piloting, torque control. Position
or speed adjustment are some requirements that a control system must manage; to cope with these characteristics
on the market, we find a series of control modules for DC motors and steppers. The DC motor finds space in
automation and robotics; the operating principle is based on the interaction of two magnetic fields that create
attraction and repulsion (stator and rotor). The stepper motor finds space in precision applications and
requires pulses to be driven. The servomotor, on the other hand, is an electromechanical system equipped with a
mechanical part and a feedback electronics for driving; it requires an appropriate control system to carry out
particular operations.
Driver ICs
Designers are faced with increasing pressure to improve design efficiency further and stand out in the
competitive market. This can be achieved, for example, by reducing overall energy consumption and optimising
thermal management. The primary function within the motor is switching, applying a current through the motor
windings at precisely the right time. Switching is controlled by algorithms that reside on a microcontroller or
digital signal processor (DSP). The motor control algorithms are generally very sophisticated as they have to
make the right switching decisions under different engine load conditions.
Features that simplify projects include integrated power MOSFETs and an ultra-low power supply architecture that
provides integrated current limitation and flexible current regulation mode. Monitoring and safety functions
such as overvoltage, short circuit and over-temperature protection, together with fault diagnostics, ensure high
performance.
The MAX14871 full-bridge
driver offers a
low-power solution for voltages between 4.5 V and 36 V. This driver reduces power dissipation and provides a
charge-free design for reduced external components and low current supply. Integrated current control requires
minimal external components and includes three adjustment modes (Figure 2).
Figure 2: Block diagram and circuit application for the MAX14871. (Source:
Maxim Integrated)
Given the increasing need for energy saving and silent running of the motor in various applications, brushless DC
motors (BLDC) are suitable for a wide range of situations. To improve efficiency, Toshiba's controllers take
advantage of InPAC (Intelligent Phase Control) technology. Toshiba's InPAC
technology compares the relationship between the current phase (current information) and the voltage phase (Hall
effect signal) and provides feedback to the motor current control signal to automatically adjust the phase.
Toshiba TC78B0 integrated circuits are designed to control the rotation speed of the motor by modifying the PWM
work cycle. These devices are equipped with three-phase full-wave drive, sinusoidal PWM drive, overcurrent
detection circuit and thermal shutdown circuit.
Figure 3: Block diagram for the TC78B015FTG. (Source: Toshiba)
TC78B015FTG
operates with a power supply between 6V and 22V while the model TC78B015AFTG requires a power supply from 6V to
30V. Both devices support output currents up to 3A and support Hall-effect devices, as well as offering a range
of protection features including thermal shutdown, overcurrent detection and motor block detection
(Figure 3).
ON Semiconductor offers its driver STK984-090A for three-phase
brushless DC motor made up of a power MOSFET. STK984-090A-E has an integrated shunt resistor and thermistor.
Various protective functions are incorporated against over-temperature, over-current, over-voltage and low
voltage. The BLDC motor drive circuit can be efficiently designed with a reduced PCB area (Figure
4).
Figure 4: Application schematic for the STK984-090A. (Source: ON
Semiconductor)
In order to reduce size and weight in motor drive applications, Texas Instruments has brought DRV832x gate drivers
to the market.
These drivers are based on an intelligent gate drive architecture that eliminates lots of traditional
components. The drivers allow the control current to be set to optimise power loss and electromagnetic
compatibility. The drivers are offered with or without a buck regulator or with three integrated current shunt
amplifiers. Each option is available in versions with a serial diagnostic interface (Figures 5 and
6).
Figure 5: Block diagram for the DRV8320H. (Source Texas Instruments)
Figure 6: Charge pump architecture for the DRV832X. (Source Texas
Instruments)
Conclusion
BLDC motors offer many advantages over traditional ones. The development of powerful magnets has enabled the
production of BLDC motors capable of producing the same power as brushed motors but with a smaller footprint.
Motor control offers the possibility of improving efficiency during the design phase. Understanding the control
needs of each type of engine and the most appropriate style for a given application can help ensure greater
efficiency in any context. Modern microcontrollers with gate drivers are perfect for providing the level of
performance and computational functionality needed to develop high-efficiency control loops. Motor control in
industrial projects is essential, mainly when used in robotic systems and numerical control machines, as well as
in a whole series of precision drives.
Maurizio Di Paolo
Emilio holds a Ph.D. in Physics and is a telecommunication engineer and journalist. He has worked on various
international projects in the field of gravitational wave research. Working as a software/hardware developer in
the data acquisition system, he participated as the designer of the thermal compensation system (TCS) for the
optical system used in the Virgo/Ligo Experiment (an experiment for detection of the gravitational wave that
achieved the 2017 Nobel Prize in Physics). Actually, he collaborates with University of L'Aquila and INFN to
design devices for radiobiological and microscopy applications and new data acquisition and control systems for
space applications. Moreover he works in the software/hardware engineering field as editor and technical writer.
He is the author of several books published by Springer, as well as numerous scientific and technical
publications on electronics design.
Singapore
