Question

A report that show

The Brushless DC (BLDC) Motor. needs to understand the construction of BLDC Motor, the operation of BLDC Motor, the terminal characteristics of BLDC Motor, and the speed control of the BLDC Motor.

The report should include the following: ➢ Title ➢ Report outline ( ➢ Introduction about BLDC Motor. ➢ Description of the construction, operation, terminal characteristics, speed control of BLDC Motor. ➢ Conclusion includes the advantages and disadvantages of BLDC Motor.

Please if you cant send PDF file here i can give you my email to send it to, if you want that let me know in comments.

Answer 1

BRUSHLESS DC (BLDC) MOTOR

Brushless DC (BLDC) Motor:

A brushless DC electric motor (BLDC motor or BL motor), also known as electronically commutated motor (ECM or EC motor) and synchronous DC motors, are synchronous motors powered by direct current (DC) electricity via an inverter or switching power supply which produces electricity in the form of alternating current.

Introduction:

A motion system based on the DC motor provides a good, simple and efficient solution to satisfy the requirements of a variable speed drive. Although dc motors possess good control characteristics and ruggedness, their performance and applications in wider areas is inhibited due to sparking and commutation problems. Induction motor do not possess the above mentioned problems, they have their own limitations such as low power factor and nonlinear speed torque characteristics (Ramu Krishnan 2009). With the advancement of technology and development of modern control techniques, the Permanent Magnet Brushless DC (PMBLDC) motor is able to overcome the limitations mentioned above and satisfy the requirements of a variable speed drive. The permanent magnet machines have the feature of high torque to size ratio. They possess very good dynamic characteristics due to low inertia in the permanent magnet rotor. Permanent magnet machines can be classified into dc commutator motor, Permanent Magnet Synchronous Motor (PMSM) and Permanent Magnet Brushless DC (PMBLDC) motor. The permanent magnet dc commutator motor is similar in construction to the conventional dc motor except that the field winding is replaced by permanent magnets. PMBLDC motors are generated by virtually inverting the stator and rotor of PM DC motors. The ‘DC’ term does not refer to a DC motor. These motors are actually fed by rectangular AC waveform. 26 The PMSM and PMBLDC motors have similar construction with poly-phase stator windings and permanent magnet rotors, the difference being the method of control and the distribution of windings. The PMSM motor has sinusoidally distributed stator windings and the controller tracks sinusoidal reference current. The PMBLDC motor is fed with rectangular voltages and the windings are distributed so as to produce trapezoidal back emf (Kenjo & Nagamori 1985).

The advantages of using brushless DC motor are as follows,

High Speed Operation - BLDC motors can operate at speed above 10,000 rpm under loaded and unloaded conditions .

Responsiveness and Quick Acceleration - Inner rotor BLDC motors have low rotor inertia, allowing them to accelerate, decelerate, and reverse direction quickly.

High Reliability - BLDC motors do not have brushes, have life expectancies over 10,000 hours .

High Power Density - A good weight/size to power ratio.

Construction of BLDC Motor:

A commutator-brushes arrangement helps in achieving unidirectional torque in a typical dc motor. Obviously, commutator and brush arrangement is eliminated in a brushless dc motor. Here, an integrated inverter/switching circuit is used to achieve unidirectional torque. That is why these motors are, sometimes, also referred as 'electronically commutated motors'.

Just like any other electric motor, a BLDC motor also consists of two main parts a stator and a rotor. Permanent magnets are mounted on the rotor of a BLDC motor, and the stator is wound for a specific number of poles. Also, a control circuit is connected to the stator winding. Most of the times, the inverter/control circuit or controller is integrated into the stator assembly. This is the basic constructional difference between a brushless motor and a typical dc motor.

A typical controller provides a three-phase frequency-controlled supply to the stator winding. The supply is controlled by logical control circuits and energizes specific stator poles at a specific point of time. This can be understood from the below animations about working of BLDC motors.

Types of BLDC motors:

There are two types of BLDC motors based on their construction/design: (i) inner rotor design & (ii) outer rotor design. Regardless of these types, note that the permanent magnets are always mounted on the rotor and winding on the stator.

1. Inner rotor design (inrunner): this is a conventional design, where the rotor is located at the core (center) and stator winding surrounds it.
   

   A commutator-brushes arrangement helps in achieving unidirectional torque in a typical dc motor. Obviously, commutator and brush arrangement is eliminated in a brushless dc motor. Here, an integrated inverter/switching circuit is used to achieve unidirectional torque. That is why these motors are, sometimes, also referred as 'electronically commutated motors'. Just like any other electric motor, a BLDC motor also consists of two main parts a stator and a rotor. Permanent magnets are mounted on the rotor of a BLDC motor, and the stator is wound for a specific number of poles. Also, a control circuit is connected to the stator winding. Most of the times, the inverter/control circuit or controller is integrated into the stator assembly. This is the basic constructional difference between a brushless motor and a typical dc motor. A typical controller provides a three-phase frequency-controlled supply to the stator winding. The supply is controlled by logical control circuits and energizes specific stator poles at a specific point of time. This can be understood from the below animations about working of BLDC motors. Types of BLDC motors There are two types of BLDC motors based on their construction/design: (i) inner rotor design & (ii) outer rotor design. Regardless of these types, note that the permanent magnets are always mounted on the rotor and winding on the stator. Inner rotor design (inrunner): this is a conventional design, where the rotor is located at the core (center) and stator winding surrounds it. Inner-rotor BLDC motor (Credit: Kaspars Dambis - flickr) Outer rotor design (outrunner): In this configuration, the rotor is external. i.e. stator windings are located at the core while the rotor, carrying permanent magnets, surrounds the stator.

2. Outer rotor design (outrunner): In this configuration, the rotor is external. i.e. stator windings are located at the core while the rotor, carrying permanent magnets, surrounds the stator.
   

Torque-speed Characterstics:

There are two torque parameters used to define a BLDC motor, peak torque and rated torque. During continuous operations, the motor can be loaded up to rated torque. This requirement comes for brief period, especially when the motor starts from stand still and during acceleration. During this period, extra torque is required to overcome the inertia of load and the rotor itself. The motor can deliver a higher torque up to maximum peak torque, as long as it follows the speed torque curve. Figure 2.12 shows the torquespeed characteristics of a BLDC motor. As the speed increases to a maximum value of torque of the motor, continuous torque zone is maintained up to the rated speed after exceeding the rated speed the torque of the motor decreases. The stall torque represents the point on the graph at which the torque is maximum, but the shaft is not rotating. The no load speed, ωn, is the 40 maximum output speed of the motor (when no torque is applied to the output shaft). If the phase resistance is small, as it should be in an efficient design, then the characteristic is similar to that of a shunt DC motor. Figure 2.12 Torque vs Speed Characteristics of BLDC Motor The speed is essentially controlled by the voltage, and may be varied by varying the supply voltage. The motor then draws just enough current to drive the torque at this speed. As the load torque is increased, the speed drops, and the drop is directly proportional to the phase resistance and the torque. The voltage is usually controlled by chopping or PWM. This gives rise to a family of torque/speed characteristics in the boundaries of continuous and intermittent operation. The continuous limit is usually determined by heat transfer and temperature rise. The intermittent limit may be determined by the maximum ratings of semiconductor devices in the controller, or by temperature rise. In practice the torque/speed characteristic deviates from the ideal form because of the effects of inductance and other parasitic influences. The linear model of a DC motor torque/speed curve is a very good approximation.

Speed Control of BLDC Motor:

Speed control of BLDC motor is essential for making the motor work at desired rate. Speed of a brushless dc motor can be controlled by controlling the input dc voltage / current. The higher the voltage more is the speed.

Many different control algorithms have been used to provide control of BLDC motors. The motor voltage is controlled using a power transistor operating as a linear voltage regulator. This is not practical when driving higher-power motors. High-power motors must use PWM control and require a microcontroller to provide starting and control functions.

The control algorithm must provide three things:

• PWM voltage to control the motor speed
• Mechanism to commutate the motor
• Method to estimate the rotor position using the back-EMF or Hall Sensors

Pulse-width modulation is used to apply a variable voltage to the motor windings. The effective voltage is proportional to the PWM duty cycle. When properly commutated, the torque-speed characteristics of the BLDC motor are identical to a dc motor. The variable voltage can be used to control the speed of the motor and the available torque.

The commutation of the power transistors energizes the appropriate windings in the stator to provide optimum torque generation depending on the rotor position. In a BLDC motor, the MCU must know the position of the rotor and commutate at the appropriate time.

The speed control can be closed loop or open loop speed control.

Open Loop Speed Control – It involves simply controlling the dc voltage applied to motor terminals by chopping the dc voltage. However this results in some form of current limiting.

Closed Loop Speed control – It involves controlling the input supply voltage through the speed feedback from the motor. Thus the supply voltage is controlled depending on the error signal. The closed loop speed control consists of three basic components.

A PWM circuit to generate the required pwm pulses. It can be either a microcontroller or a timer IC.

A sensing device to sense the actual motor speed. It can be a hall effect sensor, a infrared sensor or a optical encoder.

Trapezoidal Commutation of BLDC Motor:

One of the simplest methods of control for dc brushless motors uses what is termed Trapezoidal commutation. In this scheme, current is controlled through motor terminals one pair at a time, with the third motor terminal always electrically disconnected from the source of power.

Three Hall devices embedded in the motor are usually used to provide digital signals which measure rotor position within 60 degree sectors and provide this information to the motor controller. Because at any time, the currents in two of the windings are equal in magnitude and the third is zero, this method can only produce current space vectors having one of six different directions. As the motor turns, the current to the motor terminals is electrically switched (commutated) every 60 degrees of rotation so that the current space vector is always within the nearest 30 degrees of the quadrature direction. The current waveform for each winding is therefore a staircase from zero, to positive current, to zero, and then to negative current. This produces a current space vector that approximates smooth rotation as it steps among six distinct directions as the rotor turns.

In motor applications such as air conditioners and refrigerators use of Hall-Effect sensors is not a viable option. Back-EMF sensors that sense the back EMF in the unconnected winding can be used to achieve the same results.

The trapezoidal-current drive systems are popular because of the simplicity of their control circuits but suffer from a torque ripple problem during commutation.

Sinusoidal Commutation for BDLC Motors:

Trapezoidal commutation is inadequate to provide smooth and precise motor control of brushless dc motors, particularly at low speeds. Sinusoidal commutation solves this problem.

This is because the torque produced in a three phase brushless motor (with a sine wave back-EMF) is defined by the following equation:

Shaft Torque = Kt [I_RSin(δ) + I_Y Sin(δ +120) + I_B Sin(δ o+240)]

where:

δ is the electrical angle of the shaft,

Kt is the torque constant of the motor and

I_R, I_Yand I_B are the phase currents.

Assuming phase currents sinusoidal: I_R = I₀Sinδ; I_Y = I₀Sin (δ +120); I_B = I0Sin (δ +240)

Therefore,

Shaft Torque = 1.5I₀xKt

Sinusoidal commutated brushless motor controllers attempt to drive the three motor windings with three currents that vary smoothly and sinusoidally as the motor turns. The relative phases of these currents are chosen so that they should result in a smoothly rotating current space vector that is always in the quadrature direction with respect to the rotor and has constant magnitude. This eliminates the torque ripple and commutation spikes associated with trapezoidal commutation.

Sinusoidal commutation results in smoothness of control that is generally unachievable with trapezoidal commutation. However, while it is very effective at low motor speeds, it tends to fall apart at high motor speeds. This is because as speed goes up the current loop controllers must track a sinusoidal signal of increasing frequency. At the same time they must overcome the motor back-EMF that also increases in amplitude and frequency as speed goes up.

This degradation continues as speed increases. At some point motor current phase shift crosses through 90 degrees. When this happens torque is reduced to zero. With sinusoidal commutation, speeds above this point result in negative torque and are therefore not achievable.

Advantages of Brushless DC motor:

Brushless DC motors have many advantages our the traditional brushed DC motors. Few of the advantages are discussed below in detail:

• Brushless DC motor does not have any carbon brushes, which reduces frequent replacement requirements of brushes and maintenance costs.
• Brushless DC motors have better performance and efficiency as compared to the brushed DC motors due to the involvement of electronic control enabling high-level control over the speed and position of the motor. Brushless DC motor lifespan is approximately 6 times higher than the counter brushed DC motor.
• Brushes can cause high sparks which may result in short life or complete burnout of brushed DC motor. However, in the case of brushless DC motor, due to no spark issue, there are fewer chances of burnout due to sparking issues.
• Brushless DC motors are available in small compact sizes and also provide high torque to weight ratio making it suitable for many robotics and medical applications involving robotic arms and robotic legs.
• Brushless DC motor produces comparatively low operating noise as compared to other motors of the same ratings. As in other motors, there is continuous contact of brushes resulting in noise and sparking during contact. Therefore, brushless DC motors are given preference where electrical noise needs to be avoided.
• As traditional commutation based on mechanical setup is replaced by the modern electronic commutation system resulting in more control and fewer chances of failure due to previously discussed reasons of wear and tear.
• Unlike other motors, brushless DC motor has low no-load current making it suitable to run it at low or no load.
• Brushless DC motors can provide maximum torque continuously during rotation, while brushed DC motor can provide maximum torque at an only specific point of the rotation. For the same torque rating, the brushed motor will require a much bigger magnet as compared to the brushless DC motor. This results in a very compact and small-sized brushless DC motor proving very high torque rating.
• Brushless DC motors can have a feedback control to monitor and control the speed and torque, resulting in accurate torque and speed control providing higher efficiency, low power consumption, and long battery life in the case where the motor is operating using some batteries.
• Brushed DC motors have the heating issue and do not cool quickly due to the presence of electromagnet in the center of the motor. On the other hand, the brushless DC motor does not have an electromagnet in the center reducing the heating issue.

Disadvantages of Brushless DC motor:

Just like in all the other devices, brushless DC motors also have few shortcomings as compared to other motors. As brushless DC motor outperforms brushed DC motor in many cases, however, brushless DC motor also has few shortcomings which are discussed below:

• The cost of a brushless DC motor is comparatively higher as compared to brushed DC motor and the electronic controller also increases the cost of overall setup, as in a traditional motor, low-cost mechanical commutation setup involving brushes is used.
• When brushless DC motor is operated at low speed, slight vibrations occur during low-speed rotation. However, vibrations reduce at high speed.
• Due to the inherent natural vibration frequency of brushless DC motor, sometimes this natural frequency can match or can come closer to the vibration frequency of the body or plastic parts resulting in the occurrence of resonance phenomenon. However, this resonance can be minimized by adjustment, and it is common to observe resonance phenomenon in many brushless DC motor based devices.
• Brushed DC motors are easy to operate having simple wiring as the positive terminal is connected to positive and negative terminal is connected to negative wire and motor starts to rotate. However, in the case of brushless DC motor, wiring and operation of the motor are not that simple due to the involvement of electronic control and its link to all the electromagnets.

In short, brushless DC motor has many advantages our the traditional brushed DC motors such as low maintenance cost and less frequent maintenance requirement. They are also compact in size and provides high torque with better speed control and efficiency. On the other hand, it also has a few disadvantages like higher cost, resonance issue, and complex wiring setup due to the involvement of electronic control.