BLDC ( BRUSHLESS DC ) MOTOR EVERYTHING YOU NEED TO KNOW !!!

BLDC 

 BRUSHLESS DC  MOTOR 

EVERYTHING YOU NEED TO KNOW !!!

• INTRODUCTION
• PRINCIPLE
• CONSTRUCTION 
• WORKING & OPERATION
• APPLICATION
• ADVANTAGE / DISADVANTAGE
• ARCHITECTURE 
• HALL EFEECT SENSOR
• DRIVE STRATEGIES

SIMPLE BLDC MOTOR

INTRODUCTION

Electrical equipment often has at least one motor used to rotate or displace an object from its initial position. There are a variety of motor types available in the market, including induction motors, servomotors, DC motors (brushed and brushless), etc. Depending upon the application requirements, a particular motor can be selected. However, a current trend is that most new designs are moving towards Brushless DC motors, popularly known as BLDC motors.

A brushless DC motor (known as BLDC) is a permanent magnet synchronous electric motor which is driven by direct current (DC) electricity and it accomplishes electronically controlled commutation system (commutation is the process of producing rotational torque in the motor by changing phase currents through it at appropriate times) instead of a mechanically commutation system. BLDC motors are also referred as trapezoidal permanent magnet motors.

Unlike conventional brushed type DC motor, wherein the brushes make the mechanical contact with commutator on the rotor so as to form an electric path between a DC electric source and rotor armature windings , BLDC motor employs electrical commutation with permanent magnet rotor and a stator with a sequence of coils. In this motor, permanent magnet (or field poles) rotates and current carrying conductors are fixed.

The armature coils are switched electronically by transistors or silicon controlled rectifiers at the correct rotor position in such a way that armature field is in space quadrature with the rotor field poles. Hence the force acting on the rotor causes it to rotate. Hall sensors or rotary encoders are most commonly used to sense the position of the rotor and are positioned around the stator. The rotor position feedback from the sensor helps to determine when to switch the armature current.

This electronic commutation arrangement eliminates the commutator arrangement and brushes in a DC motor and hence more reliable and less noisy operation is achieved. Due to the absence of brushes BLDC motors are capable to run at high speeds. The efficiency of BLDC motors is typically 85 to 90 percent, whereas as brushed type DC motors are 75 to 80 percent efficient. There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges.

CONSTRUCTION

BLDC motors can be constructed in different physical configurations. Depending on the stator windings, these can be configured as single-phase, two-phase, or three-phase motors. However, three-phase BLDC motors with permanent magnet rotor are most commonly used.

Stator of a BLDC motor made up of stacked steel lamination to carry the winding. These windings are placed in slots which are axially cut along the inner periphery of the stator. These windings can be arranged in either star or delta. However, most BLDC motors have three phase star connected stator.

Each winding is constructed with numerous interconnected coils, where one or more coils are placed in each slot. In order to form an even number of poles, each of these winding is distributed over the stator periphery.

The stator must be chosen with the correct rating of the voltage depending on the power supply capability. For robotics, automotive and small actuating applications, 48 V or less voltage BLDC motors are preferred. For industrial applications and automation systems, 100 V or higher rating motors are used.

Rotor

BLDC motor incorporates a permanent magnet in the rotor. The number of poles in the rotor can vary from 2 to 8 pole pairs with alternate south and north poles depending on the application requirement. In order to achieve maximum torque in the motor, the flux density of the material should be high. A proper magnetic material for the rotor is needed to produce required magnetic field density.

Ferrite magnets are inexpensive, however they have a low flux density for a given volume. Rare earth alloy magnets are commonly used for new designs. Some of these alloys are Samarium Cobalt (SmCo), Neodymium (Nd), and Ferrite and Boron (NdFeB). The rotor can be constructed with different core configurations such as the circular core with permanent magnet on the periphery, circular core with rectangular magnets, etc.

                 

Hall sensor provides the information to synchronize stator armature excitation with rotor position. Since the commutation of BLDC motor is controlled electronically, the stator winding should be energized in sequence in order to rotate the motor. Before energizing a particular stator winding, acknowledgment of rotor position is necessary. So the Hall Effect sensor embedded in stator senses the rotor position.

Most BLDC motors incorporate three Hall sensors which are embedded into the stator. Each sensor generates Low and High signals whenever the rotor poles pass near to it. The exact commutation sequence to the stator winding can be determined based on the combination of these three sensor’s response.

HALL SENSOR AT RED SPOT 

NOTES:

•    Three phases of coil windings  wrapped around stator teeth  (slotted motor)

•     Permanent magnets mounted in  rotor

•     Energize phases to create  electromagnet and pull rotor  magnets to align with phases

•     Pole pairs: (number of magnets in  rotor) / 2

•     Back EMF: voltage generated in  windings as motor rotates

•    Voltage constant of motor (Kv):  speed generated per volt applied

•    Torque constant (Kt): torque per  amp applied

§Relates electrical and mechanical  quantities

PRINCIPLE

                   

BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves.

When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate.

motor stator is excited based on different switching states. With the switching of winding as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate. 

Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction

Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit.

Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft.

Working & Operation

The underlying principles for the working of a BLDC motor are the same as for a brushed DC motor;
i.e., internal shaft position feedback. 
In case of a brushed DC motor, feedback is implemented using
a mechanical commutator and brushes. With a in BLDC motor, it is achieved using multiple feedback sensors.  The most commonly used sensors are hall sensors and optical encoders.

Note: Hall sensors work on the hall-effect principle that when a current-carrying conductor is exposed to the magnetic field, charge carriers experience a force based on the voltage developed
across the two sides of the conductor. If the direction of the magnetic field is reversed, the voltage developed will reverse as well. For Hall effect sensors used in BLDC motors, whenever rotor magnetic poles (N or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can be used to determine the position of the shaft.

In a commutation system – one that is based on the position of the motor identified using feedback sensors – two of the three electrical windings are energized at a time as shown in figure 4. In figure 4 (A), the GREEN winding labeled “001” is energized as the NORTH pole and the BLUE winding labeled as “010” is energized as the SOUTH pole. Because of this excitation, the SOUTH
pole of the rotor aligns with the GREEN winding and the NORTH pole aligns with the RED winding labeled “100”. In order to move the rotor, the “RED” and “BLUE” windings are energized in the
direction shown in figure 4(B). This causes the RED winding to become the NORTH pole and the BLUE winding to become the SOUTH pole. This shifting of the magnetic field in the stator produces
           

APPLICATION

• Cars

• Consumer products

• Industrial machinery

• Robotics

• Camera equipment

ADVANTAGE

• It has no mechanical commutator and associated problems

• High efficiency due to the use of permanent magnet rotor

• High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed

• Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors

• Long life as no inspection and maintenance is required for commutator system

• Higher dynamic response due to low inertia and carrying windings in the stator

• Less electromagnetic interference

• Quite operation (or low noise) due to absence of brushes

DISADVANTAGE

• These motors are costly

• Electronic controller required control this motor is expensive

• Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors

• Requires complex drive circuitry

• Need of additional sensors

ARCHITECTURE 

•     Rotor position feedback through  sensors or sensor-less techniques
•     Control unit generates next  commutation signals using control  algorithm
•     Gate drivers convert signal to  MOSFET lines
•     Power Inverter switches the supply  voltage (VDC) on each of the three  phase windings
•     Additional current sensors aid in  control algorithm (not required for all  strategies)

• To drive 3 phases, use a modified 3 leg H-Bridge design

• Use N-Channel MOSFETs with low Ron

• Simplest control is enable gates with logic high or disable  with logic low

• If MOSFETs 1 and 4 on at same time: current shoot-through!

• Need “dead time” to avoid current shoot-througH Some half bridge driver chips use “adaptive gate driving” and  measure top/bottom drive voltages from incoming signal

• Software can also add extra time between enabling top and bottom MOSFETs

• If want to extend control use PWM to control the gate  voltage between logic high and logic low

HALL EFFECT SENSOR

•     Analog or digital outputs

•    Sense magnetic field strength

•    Cheap sensor (<$2)

•     Hard to get precise numbers for magnetic  field strength 
•     Typically digital output

•    BLDC location:

1)Embedded in stator

2)External to rotor

    

DRIVE / CONTROL STRATEGIES

     Three main drive strategies:

1)Trapezoidal Commutation
2)Sinusoidal Commutation
3)Field Oriented Control

TRAPEZOIDAL COMMUTATION

•    Good for high speeds
•    Can use crude position sensing (Hall effects)
•    Simplest control architecture (back EMF or Hall effect)

•    Block, six step, sensored six step

•    Three Hall effect sensors are mounted 120o apart in  stator

•    Change state every 60o

•    Energize two phases with a high or low and cycle

•    Derive commutation table from Hall Effect sensor  states

•     Shows 1 electrical cycle (with 6 electrical states)

•      Total electrical cycles per mechanical revolution is equal  to pole pairs

•     Same for speed

•    Use PWM strategies to vary speed

•     Also measure time between Hall Effect transitions for  speed

•     Acoustic noise produced from blocky steps in  current

•     Not good for low speeds

SINUSOIDAL COMMUTATION

•     Good for low speed precise motion

•     Requires precise position sensing (encoders, resolvers, AMR sensor) 

•     Medium complexity controllers (PI controller)

FIELD ORIENTED CONTROL

1.       Hybrid performance at both speeds
2.       Requires precise position sensing(encoders , amrs, resolvers)
3.       High complexity cotrollers (PI controllers )                                                                                                                                                                                                     

•    Control stator currents via  torque and flux components

•        d is direct flux
•        q is quadrature torque

•    Use Clarke transformation to  go from 3-phase to 2-phase  time variant
•    Use Park transformation to go  from 2-phase time variant to  time invariant (d, q)
•      Need high resolution position  feedback
•      Speed and position loops  wrapped current loops