Sensorless drive for high speed brushless dc motor based on the virtual neutral voltage

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Sensorless Drive for High-Speed Brushless
DC Motor Based on the Virtual Neutral
Voltage
Abstract
Introduction:
In recent years, high-speed brushless dc (BLDC) motor,
which due to its high efficiency, compactness, low cost, and
maintenance compared with a brush dc motor, is receiving more
and more interest in industrial automation, especially on blowers
and compressors. Since the rotation speed of a high-speed motor
can reach up to tens of thousands revolutions per minute(r/min)
due to the development of bearing technology, a proper drive
method to ensure low loss and high efficiency in a wide speed
range becomes a critical issue. Meanwhile, it is well known that
the BLDC motor requires six discrete rotor positions for the
inverter operation. These are typically generated by Hall sensors
mounted on a motor. However, it is a well-known fact that these
sensors increase the cost of the motor and need special
mechanical arrangements to be mounted. To break through the
aforementioned restriction, many position sensorless methods
have been considered as potential solutions, thus prompting the
sensorless operation of the BLDC motor to be a hot issue in

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Page number 2
recent years. Therefore, to cope with these two issues effectively
will promote the development of the high-speed BLDC motor.
Existing system:
Generally, the BLDC motor can be driven by either pulse-
width modulation (PWM) or pulse-amplitude modulation
(PAM) techniques. It is well known that the inverter, which
applied to most of the BLDC motor drive systems, is controlled
by the PWM scheme for varying the voltage. When the PWM
control is valid, the dc-link voltage is fixed, and the duty of the
inverter is controlled by the speed and load conditions. Also, it
is popular in the low-speed BLDC motor control. Unfortunately,
for a high speed BLDC motor, the high-frequency and large-
range current ripple will inevitably increase the copper and rotor
iron losses. The PAM scheme is another popular control mode
for the BLDC motor, especially at high speed. For PAM control,
120-degree commutation control, i.e., the so called six-step
mode is generally used with lower switching frequency than the
PWM control. Meanwhile, the dc-link voltage can be adjusted
according to the error between the speed and its reference.
Moreover, the conclusion that the PAM control for the BLDC
motor can provide lower harmonic content and higher efficiency
than the PWM control shows the superiority of PAM control at
high speed

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Proposed system:
The front-end phase-controlled rectifier can provide an
adjustable dc-link voltage, while the back-end three-phase
inverter can achieve the commutation control. In particular, due
to the very small phase inductance and resistance, a low initial
dc voltage from the phase-controlled rectifier can be beneficial
to limit the startup current. Three Y-connected resistors of same
value are connected across the motor terminals to obtain the
virtual neutral point “S.”
In order to satisfy the commutation control of high-speed
BLDC motor in a wide speed range and improve the motor
efficiency, a hybrid drive method combining PWM and PAM
techniques is proposed. When the motor runs at low speed, the
PWM control is adopted with a fixed dc-link voltage that can be
easily produced by the rectifier. When the motor speed reaches a
threshold value, i.e., the enough back EMF can counteract the
dc-link voltage, the PAM control works with an adjustable dc-
link voltage. However, it should be noted that for a high-speed
motor with low phase inductance and resistance, the reason why
the PAM control is not employed at low speed is the difficult
generation of a low dc-link voltage from the rectifier to drive the
motor.

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Page number 4
Advantages:
 Simplicity of implementation, robust to electronic noise,
and eliminating the need for the motor neutral wire.
Applications:
 Motor drives
Block diagram
INPUT AC
SUPPLY
CONTROLLED
RECTIFIER
FILTER THREE PHASE
INVERTER
THREE PHASE DRIVER
CIRCUIT
PIC CONTROLLER WITH
BUFFER
5V DC
12V DC
THREE PHASE
LOAD

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Page number 5
Tools and software used:
 MPLAB – microcontroller programming.
 ORCAD – circuit layout.
 MATLAB/Simulink – Simulation.

Sensorless drive for high speed brushless dc motor based on the virtual neutral voltage

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Sensorless drive for high speed brushless dc motor based on the virtual neutral voltage