Control of the Brushless DC Motor in Combine Mode
A brushless DC (BLDC) motor is considered to be a high performance gear motor due to its low maintenance cost, versatility,

adequate torque and speed, and high reliability. Normally, a simple BLDC motor driver is composed of a permanent magnetic

rotor and three stator coils. In each controlling step, two out of three coils are used to

generate rotating magnetic field, while the floating coil induces a back electromotive

force (emf) and feeds the generated current back to the controller as noise. In this work,

combine mode scheme is proposed to control a BLDC motor in such a way that the excess

current is accumulated via additional switching circuit for further utilization. Therefore,

a BLDC motor can drive the load in concurrent with generating power. The proposed switching

circuit has been designed and simulated on PSpice. The results show that up to 75% of input

voltage can be charged to the storage capacitor.
This paper provides a technical review of position and speed sensorless methods for

controlling Brushless Direct Current (BLDC) motor drives, including the background analysis

using sensors, limitations and advances. The performance and reliability of BLDC motor

drivers have been improved because the conventional control and sensing techniques have

been improved through sensorless technology. Then, in this paper sensorless advances are

reviewed and recent developments in this area are introduced with their inherent advantages

and drawbacks, including the analysis of practical implementation issues and applications.

The study includes a deep overview of state-of-the-art back-EMF sensing methods, which

includes Terminal Voltage Sensing, Third Harmonic Voltage Integration, Terminal Current

Sensing, Back-EMF Integration and PWM strategies. Also, the most relevant techniques based

on estimation and models are briefly analysed, such as Sliding-mode Observer, Extended

Kalman Filter, Model Reference Adaptive System, Adaptive observers (Full-order and

Pseudoreduced-order) and Artificial Neural Networks.
For the past two decades several Asian countries such as Japan, which have been under

pressure from high energy prices, have implemented variable speed PM motor drives for

energy saving applications such as air conditioners and refrigerators [1]. On the other

hand, the U.S.A. has kept on using cheap induction motor drives, which have around 10%

lower efficiency than adjustable PM motor drives for energy saving applications. Therefore

recently, the increase in energy prices spurs higher demands of variable speed PM motor

drives. Also, recent rapid proliferation of motor drives into the automobile industry,

based on hybrid drives, generates a serious demand for high efficient PM motor drives, and

this was the beginning of interest in BLDC motors.
BLDC motors, also called Permanent Magnet DC Synchronous motors, are one of the motor

types that have more rapidly gained popularity, mainly because of their better

characteristics and performance [2]. These motors are used in a great amount of industrial

sectors because their architecture is suitable for any safety critical applications.
The brushless DC motor is a

synchronous electric motor that, from a modelling perspective, looks exactly like a DC

motor, having a linear relationship between current and torque, voltage and rpm. It is an

electronically controlled commutation system, instead of having a mechanical commutation,

which is typical of brushed motors. Additionally, the electromagnets do not move, the

permanent magnets rotate and the armature remains static. This gets around the problem of

how to transfer current to a moving armature. In order to do this, the brush-

system/commutator assembly is replaced by an intelligent electronic controller, which

performs the same power distribution as a

brushed DC motor [3]. BLDC motors have many advantages over brushed DC motors and

induction motors, such as a better speed versus torque characteristics, high dynamic

response, high efficiency and reliability, long operating life (no brush erosion),

noiseless operation, higher speed ranges, and reduction of electromagnetic interference

(EMI). In addition, the ratio of delivered torque to the size of the motor is higher,

making it useful in applications where space and weight are critical factors, especially in

aerospace applications.
The control of BLDC motors can be done in sensor or sensorless mode, but to reduce

overall cost of actuating devices, sensorless control techniques are normally used. The

advantage of sensorless BLDC motor control is that the sensing part can be omitted, and

thus overall costs can be considerably reduced. The disadvantages of sensorless control are

higher requirements for control algorithms and more complicated electronics [3]. All of the

electrical motors that do not require an electrical connection (made with brushes) between

stationary and rotating parts can be considered as brushless permanent magnet (PM) machines

[4], which can be categorised based on the PMs mounting and the back-EMF shape. The PMs can

be surface mounted on the rotor (SMPM) or installed inside of the rotor (IPM) [5], and the

back-EMF shape can either be sinusoidal or trapezoidal. According to the back-EMF shape, PM

AC synchronous motors (PMAC or PMSM) have sinusoidal back-EMF and high performance

Brushless DC Motor have trapezoidal back-EMF. A PMAC motor is typically excited

by a three-phase sinusoidal current, and a BLDC motor is usually powered by a set of

currents having a quasi-square waveform [6,7].
Because of their high power density, reliability, efficiency, maintenance free nature

and silent operation, permanent magnet (PM) motors have been widely used in a variety of

applications in industrial automation, computers, aerospace, military (gun turrets drives

for combat vehicles) [3], automotive (hybrid vehicles) [8] and household products. However,

the PM BLDC motors are inherently electronically controlled and require rotor position

information for proper commutation of currents in its stator windings. It is not desirable

to use the position sensors for applications where reliability is of utmost importance

because a sensor failure may cause instability in the control system. These limitations of

using position sensors combined with the availability of powerful and economical

microprocessors have spurred the development of sensorless control technology. Solving this

problem effectively will open the way for full penetration of this motor drive into all low

cost, high reliability, and large volume applications.
The remainder of the paper is arranged as follows. Section 2 describes the position and

speed control fundamentals of BLDC motors using sensors. Next, Section 3 explains the

control improvements applying sensorless techniques, describing the motor controller model

and the most important techniques based on back-EMF sensing. Section 4 also briefly

analyses the sensorless techniques using estimators and model-based schemes. In addition,

Section 5 compares the feasibility of the control methods, and describes some relevant

implementation issues, such as open-loop starting. Finally, Section 6 provides an overview

for the applications of BLDC motor controllers, as well as conclusions are drawn in Section

7.
PM motor drives require a rotor position sensor to properly perform phase commutation

and/or current control. For PMAC motors, a constant supply of position information is

necessary; thus a position sensor with high resolution, such as a shaft encoder or a

resolver, is typically used. For BLDC motors, only the knowledge of six phase-commutation

instants per electrical cycle is needed; therefore, low-cost Hall-effect sensors are

usually used. Also, electromagnetic variable reluctance (VR) sensors or accelerometers have

been extensively applied to measure motor position and speed. The reality is that angular

motion sensors based on magnetic field sensing principles stand out because of their many

inherent advantages and sensing benefits.
2.1. Position and Speed Sensors
As explained before, some of the most frequently used devices in position and speed

applications are Hall-effect sensors, variable reluctance sensors and accelerometers. Each

of these types of devices is discussed further below.
2.1.1. Hall-effect sensors
These kinds of devices are based on Hall-effect theory, which states that if an

electric current- carrying conductor is kept in a magnetic field, the magnetic field exerts

a transverse force on the moving charge carriers that tends to push them to one side of the

conductor. A build-up of charge at the sides of the conductors will balance this magnetic

influence producing a measurable voltage between the two sides of the conductor. The

presence of this measurable transverse voltage is called the Hall-effect because it was

discovered by Edwin Hall in 1879
Unlike a brushed DC motor, the commutation of a high performance BLDC Motor

is controlled electronically. To rotate the BLDC motor the stator windings should be

energized in a sequence. It is important to know the rotor position in order to understand

which winding will be energized following the energizing sequence. Rotor position is sensed

using Hall-effect sensors embedded into the stator [9].
Most BLDC motors have three Hall sensors inside the stator on the non-driving end of

the motor. Whenever the rotor magnetic poles pass near the Hall sensors they give a high or

low signal indicating the N or S pole is passing near the sensors. Based on the combination

of these three Hall sensor signals, the exact sequence of commutation can be determined.

Figure 1 shows a transverse section of a BLDC motor with a rotor that has alternate N and S

permanent magnets. Hall sensors are embedded into the stationary part of the motor.

Embedding the Hall sensors into the stator is a complex process because any misalignment in

these Hall sensors with respect to the rotor magnets will generate an error in

determination of the rotor position. To simplify the process of mounting the Hall sensors

onto the stator some motors may have the Hall sensor magnets on the rotor, in addition to

the main rotor magnets. Therefore, whenever the rotor rotates the Hall sensor magnets give

the same effect as the main magnets. The Hall sensors are normally mounted on a printed

circuit board and fixed to the enclosure cap on the non-driving end. This enables users to

adjust the complete assembly of Hall sensors to align with the rotor magnets in order to

achieve the best performance [10].