Motor Control and Power Supply Design
   

Motor Control and Power Supply Design


By Daniel Torres


Introduction


Nowadays, most of the devices that help make life easier and comfortable—things like washing machines, refrigerators, fans, air conditioners, power tools, blenders—require motion control.

All of these devices consume energy to produce motion. The way this energy is efficiently used depends on the control systems, electric machine design, control algorithms, etc. One of the biggest challenges that we face as a human race is the efficient use of energy and most of the efforts to improve this challenge are targeted at motion control systems. Hence, many energy-saving advances are coming from improved motor control techniques, frame design, materials and manufacturing precision.

More efficient control techniques were developed years ago, but the required CPUs to perform such complex algorithms and computations were too expensive for cost-sensitive markets like the appliance market. This situation has changed in recent years; high-performance digital signal controllers have been developed at lower costs with all the required features to perform these complex control algorithms.

Another area where energy-saving advances are being developed is power conversion. Power conversion systems are used to convert electric power from one form to another. During this process a certain amount of energy is lost due to the inherent power consumption of the system, efficiency of the topologies, the control techniques and the electronic devices used. Most of the power conversion controls are performed by analog circuitry, but the requirements, of the new energy-saving regulations have increased and frequently have become harder to meet with analog control systems.

The use of microcontrollers (MCU) and DSCs has opened new frontiers in this regard. Today, it is feasible to achieve 98-percent efficiency in power conversion systems through the use of digital control techniques and complex mathematical computations executed by high-performance low-cost digital signal controllers.


Motion Control Design Challenges


Several kinds of motors are used in motion control, including brushless DC motors (BLDC), brush-commutator permanent magnet DC motors, linear motors and stepper motors.

System engineers must not only choose the right kind of motor for the mechanical task, but they must also choose the appropriate control loop scheme for encompassing both the mechanical and electrical time-variant responses of the system. Tuning this control loop is often done in the design stage of the drive electronics.

Developers face a number of design variables because each type of motor has a unique set of requirements for the drive electronics. Designing drive electronics is further complicated by the electric motors themselves, which, by their inductive nature, are prone to producing electromagnetic interference (EMI), radio frequency interference (RFI) and destructive high-energy transients. Drive electronics design must prevent EMI and RFI, while still withstanding transient over-voltage and over-current conditions.

BLDC motors are very popular for many applications. A BLDC motor does not have a commutator and is more reliable than a DC motor. A BLDC motor has other advantages over an AC induction motor. BLDC motors achieve higher efficiency by generating the rotor magnetic flux with rotor magnets. They are used in high-end home appliances (such as refrigerators, washing machines and dishwashers), high-end pumps, fans and other devices that require high reliability and efficiency.

BLDC motors are widely used in pump, fan and compressor applications because of their robust structure. A common feature of these applications is that they do not require position information; only speed information is required, and only to perform control. BLDC motors can be used without complex control algorithms.

In the BLDC motor, the rotor position must be known to energize the phase pair and control the phase voltage. If sensors are used to detect rotor position, then sensed information must be transferred to a control unit.

This requires additional connections to the motor, which may not be acceptable in some applications. It may be physically impossible to make the required connections to the position sensors. Also, the additional cost of the position sensors and the wiring may be unacceptable. The physical connection problem could be solved by incorporating the driver in the motor body; however, a significant number of applications do require a sensorless solution due to their low-cost nature.

A permanent magnet synchronous motor (PMSM) provides rotation at a fixed speed in synchronization with the frequency of the power source, regardless of the fluctuation of the load or line voltage. The motor runs at a fixed speed synchronous with mains frequency, at any torque up to the motor's operating limit. PMSMs are therefore ideal for high-accuracy fixed-speed drives.

A three-phase PMSM is a permanently excited motor. Boasting very high-power density, very high efficiency and high response, the motor is suitable for most sophisticated applications in mechanical engineering. It also has a high overload capability. A PMSM is largely maintenance-free, which ensures the most efficient operation.

Precise speed regulation makes a PMSM an ideal choice for certain industrial processes. PMSMs have speed/ torque characteristics ideally suited for direct drive of large-horsepower, low-RPM loads.

Synchronous motors operate at an improved power factor, thereby improving the overall system power factor and eliminating or reducing utility power factor penalties. An improved power factor also reduces the system's voltage drop and the voltage drop at the motor terminals.

A PMSM abandons the excitation winding and the rotor turns at the same speed as the stator field. The PMSM's design eliminates the rotor copper losses, giving very high peak efficiency compared with a traditional induction motor. The power-to-weight ratio of a PMSM is also higher than induction machines.

Progress in the field of power electronics and microelectronics enables the application of PMSMs for high-performance drives, where, traditionally, only DC motors were applied.


Power Conversion Design Challenges


The main purpose of a power supply is to provide regulated, stable power to a load, regardless of power grid conditions. The switched-mode power supply (SMPS) is one type of power supply that has been widely used in office equipment, computers, communication systems and other applications because of its high efficiency and high energy density.

An SMPS fully digitally controlled by software running on a digital signal controller (DSC) has many advantages over mixed-analog and processor-controlled implementations. These include programmability, adaptability, reduced component count, design reusability, process independence, advanced calibration ability and better performance.

By using full digital control, an SMPS system becomes flexible and can also realize complex control arithmetic that improves efficiency and lowers cost. A controller-based SMPS system integrates high-performance digital signal processing with power electronics, providing a new method for design of power electronics and the typical high-level control and communication capability an SMPS system requires.


Brushless DC Motor Control Theory


The BLDC motor is a rotating electric machine with a classic three-phase stator like that of an induction motor; the rotor has surface-mounted permanent magnets. It is also referred to as an electronically commuted motor. There are no brushes on the rotor, and the commutation is performed electronically at certain rotor positions. The stator is usually made from magnetic steel sheets. The stator phase windings are inserted in the slots (distributed winding) or can be wound as one coil on the magnetic pole. The air gap magnetic field is produced by permanent magnets, the rotor magnetic field is constant. The magnetization of the permanent magnets and their displacement on the rotor is chosen so that the shape of the back-EMF (the voltage induced in the stator winding due to rotor movement) is trapezoidal. This allows a DC voltage, with a rectangular shape, to be used to create a rotational field with low torque ripples.


Figure 1: Three-Phase Voltage System for a BLDC Motor

The motor can have more then one pole-pair per phase. The number of pole-pairs per phase defines the ratio between the electrical revolution and the mechanical revolution. For example, the BLDC motor shown has three pole-pairs per phase, which represents three electrical revolutions per mechanical revolution.

The rectangular, easy to create, shape of the applied voltage makes controlling and driving the motor simple. However, the rotor position must be known at certain angles to be able to align the applied voltage with the back-EMF. Alignment of the back-EMF with commutation events is very important; when this is achieved, the motor behaves as a DC motor and runs at maximum efficiency. Thus, simplicity of control and performance makes the BLDC motor the best choice for low-cost and high-efficiency applications. Figure 1 shows the waveforms applied to a three-phase BLDC motor.


Permanent Magnet Synchronous Motor Control Theory


Thanks to sophisticated control methods such as vector control, a PMSM offers the same control capabilities as high-performance four-quadrant DC drives.

Vector control is an elegant control method of a PMsynchronous motor, where field-oriented theory is used to control space vectors of magnetic flux, current and voltage. It is possible to set up the coordinate system to decompose the vectors into a magnetic-field-generating part and a torquegenerating part. Then the structure of the motor controller (Vector Control controller) is almost the same as for a separately excited DC motor, which simplifies the control of permanent magnet synchronous motor. This vector control technique was developed in the past specifically to achieve a similarly good dynamic performance in PM-synchronous motors.

In this method, it is necessary to break down the fieldgenerating part and the torque-generating part of the stator current to separately control the magnetic flux and the torque. To do so, you must set up the rotary coordinate system connected to the rotor magnetic field. This coordinate system is generally called "d,q-coordinate system." Very high CPU performance is needed to perform the transformation from rotary to stationary coordinate systems.

Figure 2 shows the block diagram of the required task a CPU must perform in a vector control technique

To perform vector control:

  • Measure the motor quantities (phase voltages and currents)
  • Transform them into the two-phase system (α , β ) using Clarke transformation
  • Calculate the rotor flux space-vector magnitude and position angle
  • Transform stator currents into the d,q-coordinate system using Park transformation
  • The stator current torque (isq) and flux (isd) producing components are controlled separately by the controllers
  • The output stator voltage space-vector is calculated using the decoupling block
  • The stator voltage space-vector is transformed back from the d,q-coordinate system into the two-phase system fixed with the stator by inverse Park transformation
  • Using the sinewave modulation, the output three phase voltage is generated


Figure 2: Permanent Magnet Synchronous Motor Vector Control Block Diagram


Benefits and Features of the 56F8013 DSC


The Freescale MC56F801x DSC family is well-suited to digital motor control, combining a DSP's calculation capability with an MCU's controller features on a single chip. These hybrid controllers offer many dedicated peripherals, such as pulse width modulation (PWM) module(s), an analog-to-digital converter (ADC), timers, communication peripherals (SCI, SPI, I2C), on-board flash and RAM.

  • Up to 32 MIPS at 32 MHz execution frequency
  • On-chip memory includes high-speed volatile and non-volatile components
  • 16 KB of program flash
  • 4 KB of unified data/program RAM
  • Flash security feature prevents unauthorized accesses to its content
  • Flash protection prevents accidental modifications
  • JTAG/Enhanced On-Chip Emulation (EOnCE) for unobtrusive, real-time debugging
  • Four 36-bit accumulators
  • 16- and 32-bit bidirectional barrel shifter
  • Parallel instruction set with unique addressing modes
  • Hardware DO and REP loops available
  • Three internal address buses
  • Four internal data buses
  • MCU-style software stack support
  • Controller-style addressing modes and instructions
  • Single-cycle 16 x 16-bit parallel multiplier-accumulator (MAC)
  • Proven to deliver more control functionality with a smaller
  • memory footprint than competing architectures


Motion Control Proposed Solution


BLDC systems combine the positive attributes of AC and DC systems. In contrast to a brush DC motor, BLDC systems use a type of permanent magnet AC synchronous motors with a trapezoidal back-EMF waveform shape, and electronic commutation replaces the mechanical brushes in the DC motor. Although this control method will generate torque glitches during phase commutation, it satisfies most applications in which rotor speed is the control target.

PMSM motors with a sinusoidal back-EMF waveform shape can also be used in a BLDC system. But the phasor angle between stator flux and rotor flux is maintained between 60° electrical and 120° electrical. Torque ripples will occur during operation, but average torque will remain constant, meeting the requirements of most low-end applications. Figure 3 shows a block diagram that can be used to implement either PMSM vector control or a BLDC motor control.


Figure 3 : Generic PMSM/BLDC Motor Control Solution Block Diagram

The information from back-EMF zero crossing can be used to determine the rotor position for proper commutation and to determine which power transistors to turn on to obtain maximum motor torque. The cheapest and the most reliable method to sample back-EMF zero crossing information is to feed the resistor network samples' back-EMF signal into ADC inputs or GPIOs. In sensored control structure, phases are commutated once every 60° electrical rotation of the rotor. This implies that only six commutation signals are sufficient to drive a BLDC motor. Furthermore, efficient control implies synchronization between the phase Bemf and the phase supply so that the Bemf crosses zero once during the non-fed 60° sector.

As only two currents flow in the stator windings at any one time, two phase currents are opposite and the third phase is equal to zero. Knowing that the sum of the three stator currents is equal to zero (star-wound stator), the anticipated instantaneous Bemf waveforms can be calculated. The sum of the three stator terminal voltages is equal to three times the neutral point voltage (Vn). Each of the Bemfs crosses zero twice per mechanical revolution, and as the Bemfs are numerically easy to compute, thanks to the signal processing capability of the 56F8013, it is possible to get the six required items of information regarding the commutation.


Switched-Mode Power Supply Proposed Solution


A generic SMPS AC/DC system is comprised of two parts: the primary side is the AC/DC converter with power factor correction (PFC); the secondary side is a full-bridge DC/DC converter. The AC/DC system uses an interleaved PFC boost control structure, which includes a full-bridge rectifier, two interleaved parallel BOOST PFC circuits, and two assistant switches to realize the zero voltage switch (ZVS) of the main switches. Implementing a ZVS algorithm reduces the components' stress and improves efficiency, which allows the design to eliminate the reverse recovery output diodes. The DC/DC converter uses a ZVS phase-shifted full-bridge control structure implemented in software with a current doubler rectifier. This reduces the size of the filter inductor and improves efficiency.

A circuit diagram for a 56800/E-based switched-mode power supply (SMPS) is shown in Figure 4. The entire system is controlled by two 56800/E devices. The primary-side device accomplishes all control of the PFC system, which includes the two main switches and two ZVS switches. The secondary-side device accomplishes all control of the DC/ DC phase-shifted full-bridge converter, which includes four main switches and two synchronous rectifiers. The functions performed in software for the PFC and DC/DC converter include: two digital PI regulators in the power system, control of all switches, soft start, digital generation of sine reference for the primary PFC, communication, power supply protection and supervisor functions.


Figure 4: AC/DC Block Diagram


References


www.freescale.com

Application Note AN1931 "Three-Phase PM Synchronous Motor Vector Control Using DSP56F80x" Freescale Semiconductor

Application Note AN3115 "Implementing a Digital AC/DC Switched-Mode Power Supply using a 56F8300 Digital Signal Controller" Freescale Semiconductor

Design Reference Manual DRM070 "Three-Phase BLDC Motor Sensorless Control Using MC56F8013" Freescale Semiconductor

Design Reference Manual DRM077 "PMSM and BLDC Sensorless Motor Control Using the 56F8013 Device" Freescale Semiconductor

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Daniel Torres is an applications engineer at Freescale Semiconductor, experienced in digital signal controllers, ColdFire controllers and 8-bit MCUs. He is focused on motor control and power management.


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