30-07-2012, 03:55 PM
3-Phase BLDC Drive Using Variable DC Link Six-Step Inverter
Variable DC Link.pdf (Size: 1.02 MB / Downloads: 69)
Introduction
This paper describes the design of a 3-phase BLDC drive using a variable DC link six-step inverter, based
on Freescale’s MC56F8013 dedicated motor control device.
Recently, small high-speed BLDC motors have become very popular in a wide application area. The
BLDC motor does not have a mechanical commutator and is, consequently, more reliable than the DC
motor. Small high-speed BLDC motors have very low inductance compared to conventional BLDC
motors. When PWM control is applied to the phases of a BLDC motor, the current follows the rectangular
PWM voltage shape. This rapidly changing current magnetizes and demagnetizes the motor iron at a
frequency equal to the PWM frequency. Due to magnetic hysteresis losses, the motor can become hot
enough to be damaged and the high current ripple will cause other losses. Because of the special control
required by the motor, the method adopted in this reference design uses a variable DC link six-step
inverter r to generate the desired voltage for the motor. The motor then requires only a conventional
three-phase inverter for commutation.
The concept of the application is a high-speed BLDC motor with closed-loop speed-control. It serves as
a design example of a 3-phase BLDC drive with variable DC link six-step inverter, using a Freescale
digital signal controller.
This reference design includes basic motor theory, system design concept, hardware implementation,
and the software design, including the FreeMASTER software visualization tool.
Freescale Controller Advantages and Features
The Freescale MC56F801x family is well suited to digital motor control, combining the DSP’s calculation
capability with the MCU’s controller features on a single chip. These digital signal controllers offer many
dedicated peripherals such as pulse width modulation (PWM) modules, analog-to-digital converters
(ADC), timers, communication peripherals (SCI, SPI, I2C), and on-board Flash and RAM.
BLDC Motor
A brushless DC (BLDC) motor is a rotating electric machine where the stator is a classic 3-phase stator,
like that of an induction motor, and the rotor has surface-mounted permanent magnets (see Figure 2-1).
In this respect, the BLDC motor is equivalent to a reversed DC commutator motor, in which the magnet
rotates while the conductors remain stationary. In the DC commutator motor, the current polarity is altered
by the commutator and brushes. On the contrary, in the brushless DC motor, the polarity reversal is
performed by power transistors switching in synchronization with the rotor position. Therefore, BLDC
motors often incorporate either internal or external position sensors to discern the actual rotor position;
alternatively, the position can be detected without sensors.
BLDC Motor Control Using a Variable DC Link Six-Step Inverter
The BLDC motor is driven by rectangular voltage waveforms coupled with the given rotor position (see
Figure 2-2). The generated stator flux interacts with the rotor flux generated by a rotor magnet, defining
the torque, and thus speed, of the motor. The voltage waveforms must be properly applied to the two
phases of the 3-phase winding system, to keep the angle between the stator flux and the rotor flux close
to 90° to generate maximum torque. To achieve this, the motor requires electronic control for proper
operation.
Commutation
Commutation provides the creation of a rotation field. As explained previously, for proper operation of a
BLDC motor it is necessary to keep the angle between the stator and rotor flux close to 90°. With six-step
control we get a total of six possible stator flux vectors. The stator flux vector must be changed at a certain
rotor position. The rotor position is usually sensed by Hall sensors. The Hall sensors generate three
signals also comprising six states. Each of the Hall sensor states corresponds to a certain stator flux
vector. All Hall sensor states with corresponding stator flux vectors are illustrated in Figure 2-4. The same
figure is illustrated in tables Table 2-1 and Table 2-2.
The next two figures depict the commutation process. The actual rotor position in Figure 2-5 corresponds
to the Hall sensors’ state ABC[110] (see Figure 2-4). The actual voltage pattern can be derived from the
Table 2-1. Phase A is connected to the positive DC bus voltage by the transistor PWM_AT, phase C is
connected to ground by transistor PWM_CB, and phase B is not powered.
As soon as the rotor reaches a certain position (see Figure 2-5), the Hall sensor state changes its value
from ABC[110] to ABC[100]. From Table 2-1 a new voltage pattern is selected and applied to the BLDC
motor.
Speed and Voltage Control
Commutation ensures proper rotor rotation of the BLDC motor, while the motor speed depends only on
the amplitude of the applied voltage. The amplitude of the applied voltage is adjusted by the variable DC
link six-step inverter using pulse width modulation. The required speed is controlled by a speed controller.
The speed and voltage controllers are implemented as conventional PI controllers. The difference
between the actual and required speed (voltage) is the input to the PI controller. Using this difference, the
PI controller controls the duty cycle of PWM pulses fed to the variable DC link six-step inverter,
corresponding to the voltage amplitude required to keep the desired speed. See Figure 2-7.