Transcript Dual Winding Method of a BLDC Motor for Large Starting Torque

Dual Winding Method of a BLDC Motor for
Large Starting Torque and High Speed
IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER
2005
G. H. Jang and C. I. Lee
Department of Precision Mechanical Engineering, Hanyang University,
Seoul 133-791, Korea
Student : Chien-Hung,Chen
Professor : Ming-Shyan,Wang
Date : 24th-DEC-2010
Outline
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Abstract
Introduction
DUAL WINDING METHOD AND ITS INVERTER CIRCUIT
A. Dual Windings in Parallel Connection
B. Inverter Topology for Dual Windings
PROTOTYPING AND EXPERIMENTAL VERIFICATION
CONCLUSION
REFERENCES
Abstract
•
This paper presents a novel winding method and its inverter circuit
to drive a brushless dc (BLDC) motor to high speed with large
starting torque.
•
The proposed winding coils are composed of dual windings, i.e.,
main and auxiliary windings, and the proposed invertercircuit has
nine switches.
•
In the starting period, it energizes both main and auxiliary windings
in parallel connection to generate large starting torque.
•
After the motor is accelerated, the proposed inverter circuit is
automatically switched to energize main winding, which generates
small back-electromotive force, to reach high speed.
•
The proposed method is applied to the spindle motor of a hard disk
driveand its effectiveness is verified through experiment.
Introduction 〔1/2〕
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A BRUSHLESS dc (BLDC) motor has been widely used in the driving source of
many precision electromechanical devices not only because it has high
efficiency but also because it has good controllability.
[1]. The linear torque–speed characteristics of a BLDC motor make it easy to
control its speed over the wide range. However, a BLDC motor cannot be run
at high speed with large starting torque at the same time because its inherent
back-electromotive force (EMF) is proportional to the speed. In order to run a
BLDC motor at high speed, back-EMF constant is designed to be small in
order to reduce the voltage drop due to back-EMF. However, it generates small
starting torque because back-EMF constant is identical to torque constant, and
it consequently results in a long transient period. It is one of the drawbacks of a
BLDC motor in high-speed applications.
Several researchers have addressed the driving methods to run the motor at
high speed with large starting torque. Jang and Kim proposed a method to
drive a BLDC motor at high speed with large starting torque by utilizing a
bipolar-starting and unipolar-running algorithm.
Introduction 〔2/2〕
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[2]. It uses the existing windings of a BLDC motor, but it cannot improve the
torque–speed characteristics better than those of the unipolar or bipolar
winding configuration of the BLDC motor. Tsai et al. presented a winding
method and a driving method to run the motor at high speed with large starting
torque.
[3]. They proposed a variable winding BLDC motor, i.e., two sets of windings
for a BLDC motor. It has the serial and parallel windings together. The former is
used for low-speed operation with large torque, which results from series
connection, and the latter is used for high-speed operation with small backEMF constant, which results from parallel connection. However, their inverter
circuits require additional switching devices and more complex control logic,
and two sets of windings of their motor are required to have the same backEMF constant.
This paper presents a novel winding method and its inverter circuit to drive a
BLDC motor at high speed with large starting. torque. The proposed BLDC
motor and inverter circuits are prototyped, and their effectiveness is verified
through the experiment.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔1/11〕
A. Dual Windings in Parallel Connection
Fig. 1 shows the proposed winding method and inverter circuit of
the BLDC motor. The winding coils of each phase are composed of
dual windings, i.e., main and auxiliary windings in parallel
connection. The proposed inverter circuit can be controlled in such
a way that the current flows in either main and auxiliary windings or
main winding only.
Fig. 1. Dual winding configuration and its inverter topology.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔2/11〕
Torque–speed characteristics of a BLDC motor can be derived as follows [4]:
where and are the supplied voltage and the rotating speed,
and
, and
are torque constant, back-EMF
constant, and phase resistance of main and auxiliary windings, respectively. In
the case where only the main winding is energized, the starting torque and the
maximum speed at no load can be written as follows:
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔3/11〕
Fig. 2. Current flow through the auxiliary winding of phase AB when the
conventional inverter energizes the main winding of phase AB.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔4/11〕
In the case where both main and auxiliary windings are energized, the starting
torque can be written as follows:
It can be concluded from (2) and (4) that energizing both main and auxiliary
windings in parallel connection always generates larger starting torque than
energizing only the main winding.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔5/11〕
In the case where both main and auxiliary windings are energized, the maximum speed
at no load can be written as follows:
In the proposed dual windings, the number of coil turns of the main winding is
designed to be smaller than that of the auxiliary winding. It makes the torque constant
and the back-EMF constant of the main winding smaller than that of the auxiliary
winding. It can be concluded from (3) and (5) that energizing the main winding always
generates higher maximum speed than energizing both main and auxiliary windings in
parallel connection because the denominator is bigger than the numerator in (5).
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔6/11〕
B. Inverter Topology for Dual Windings
• The proposed dual windings cannot be properly operated by the conventional inverter
circuit. In the high-speed range when only the main winding is energized, back-EMF
voltage is generated in the auxiliary winding. It allows the current to flow along any
closed path including the auxiliary winding and it may generate negative torque.
• Fig. 2 shows the undesirable current flow through the auxiliary winding of phase AB
when the main winding of phase AB is energized by using the conventional inverter
circuit. Back-EMF voltage of the auxiliary winding makes the negative current flow
through the auxiliary winding which results in negative torque. Eventually, it prevents
the BLDC motor from being accelerated to high speed.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔7/11〕
• Fig. 3 shows the measured phase current at main and auxiliary windings when only
the main winding is energized by using the conventional inverter circuit. It shows that
negative current flows in the auxiliary winding even though the auxiliary winding is not
energized by the conventional inverter circuit. This research develops an inverter circuit
as shown in Fig. 1 to prevent the negative current from flowing in the auxiliary winding
due to back-EMF at high speed.
Fig. 3. Measured phase current at main and auxiliary windings when only the main
winding is energized.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔8/11〕
• A developed inverter circuit energizes the main and auxiliary windings in parallel
connection at starting period, and it energizes only the mainwinding at high speed.
• As shown in Fig. 1, the proposed inverter circuit has nine switches, i.e., the upper
three, middle three, and lower three switches. Each of the three middle switches is
composed of one n-channel MOSFET and four diodes. In these middle switches,
drain and source voltage of the MOSFET is determined by the voltage of the main
and auxiliary windings, and the gate voltage is controlled by supplied voltage.
• Table I shows the firing sequences of the proposed inverter circuit.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔9/11〕
• Fig. 4. shows the operation of both main and auxiliary windings at the starting period by
the proposed inverter circuit. When inverter switches of A , A, B, C and B are turned on,
the current flows through the solid lines of main and auxiliary windings of phase A and B,
respectively, as shown in Fig. 4.
Fig. 4. Current flow by the operation of both main and auxiliary windings.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔10/11〕
• Once the motor is accelerated, the back-EMF voltage of the auxiliary winding is
increased. When it reaches higher than the supplied voltage, the source voltage of the
MOSFET connected to the auxiliary winding is bigger than the gate voltage and the
corresponding middle inverter switch is automatically turned off. Then, it does not allow
the current to flow through the auxiliary winding in the speed range higher than this
threshold speed. The firing sequence at high speed is exactly the same one as thatof
the starting period as shown in Table I.
DUAL WINDING METHOD AND ITS
INVERTER CIRCUIT 〔11/11〕
• Fig. 5 shows the operation of the main winding at high speed by the proposed
invertercircuit. Even though the inverter switches A , A, B, C and B are turned on as
shown in Table I, the source voltage of the inverter switch A connected to the auxiliary
winding is bigger than the gate voltage and the inverter switch A is automatically turned
off. The current flows through the solid lines of the main winding of phases A and B as
shown in Fig. 5.
Fig. 5. Current flow by the operation of main winding at high speed.
PROTOTYPING AND EXPERIMENTAL
VERIFICATION 〔1/3〕
• The proposed BLDC motor and its inverter circuit are prototyped for a spindle motor of
a hard disk drive (HDD). The operating speed is 10 000 r/min and the number of coil
turns of the main and auxiliary windings are 25 and 50, respectively. The proposed
inverter circuit is also developed as shown in Fig. 1, and its switching action is
controlled by a TMS320F240 digital signal processor (DSP) from Texas Instruments.
• Fig. 6 shows the measured torque–speed curves of the prototyped motor with the
operation of dual windings and main winding. The proposed motor with dual windings
has a starting torque of 15.9 mN m which is 60% larger than the motor operated by
only the main winding. In the speed range lower than 6000 r/min, the main and
auxiliary windings are energized to maximize the torque.
Fig. 6. Measured torque–speed curves.
PROTOTYPING AND EXPERIMENTAL
VERIFICATION 〔2/3〕
• From 6000 r/min, the back-EMF voltage of the auxiliary winding starts to be higher than
the supplied voltage so that the corresponding middle inverter switch connected to the
auxiliary winding is automatically turned off.
• In the speed range higher than 6000 r/min, the developed inverter energizes only
the main winding so that it reaches the same maximum speed of 14 000 r/min which
can be obtained by the operation of the main winding.
• The proposed dual winding operation reduces the starting time of the BLDC motor
with a 3.5-in disk to reach 10 000 r/min from that of a conventional motor by 16.7%.
PROTOTYPING AND EXPERIMENTAL
VERIFICATION 〔3/3〕
• Fig. 7 shows the measured efficiency curves of the prototyped motor with the
operation of dual windings and main winding. Efficiency of the prototyped motor with
dual windings is bigger than that with the main winding in the speed range lower than
6000 r/min because the torque output of the former is bigger than the latter.
Fig. 7. Measured efficiency curves.
CONCLUSION
• This paper has developed a novel winding method and its inverter circuit to drive a
BLDC motor to high speed with large starting torque. The proposed BLDC motor and
inverter circuits were prototyped, and their effectiveness was verified through
experiment. The proposed inverter circuit can automatically switch from dual windings
to main winding to maximize the torque output and high-speed operation. It may be
effectively applied to reduce the starting time of high-speed electromechanical devices
with heavy load. It may also reduce the power consumption of the BLDC motor with
frequent on–off operation because the proposed dual winding has high efficiency
during the startup period.
REFERENCES
[1] E. Grochowski and R. F. Hoyt, “Future trends in hard disk drives,” IEEE Trans.
Magn., vol. 32, no. 3, pp. 1850–1854, May 1996.
[2] G. H. Jang and M. G. Kim, “A bipolar-starting and unipolar-running method to drive a
HDD spindle motor at high speed with large starting torque,” IEEE Trans. Magn., vol. 41,
no. 2, pp. 750–755, Feb. 2005.
[3] M. C. Tsai, M. C. Chou, and C. L. Chu, “Control of a variable-winding brushless
motor with the application in electric scooters,” in Proc. IEEE Int. Conf. Electric
Machines and Drives, 2001, pp. 922–925.
[4] J. R. Hendershot and T. J. E. Miller, Design of Brushless Permanent- Magnet Motors.
Oxford, UK: Oxford Univ. Press, 1994.
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