BRUSHLESS DC MOTOR 1 • 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 instead of a mechanical commutation system. • BLDC motors are also referred as trapezoidal permanent magnet motors. 2 Construction • Consists of stator and rotor. • Stator is made up of silicon steel stampings with slots. • The slots accommodate armature windings. • This winding is wound with specified number of poles (even). • This winding is connected to a DC supply through an inverter. 3 Simplified BLDC Motor Rotor and Stator 4 • Rotor is a permanent magnet. • Number of rotor poles is same as that of stator. • Rotor shaft carries rotor position sensor. • It provides information about the position of shaft at any instant to the controller which sends signal to the electronic commutator. • The function of electronic commutator is similar to that of the mechanical commutator. 5 • Out of the three coils on stator only one can be energised. • When it is energised it becomes an electromagnet. • The operation of a BLDC motor is based on the interaction between the PM and the electromagnet. • When the stator coil is energised, opposite poles of rotor and stator get attracted. • As a result, the rotor poles get attracted to the energised stator coil. 6 • As the rotor nears coil A, coil B is energised. • This process continues. • The DC current required in each coil is as follows. 7 • Unlike brushed DC motor, the commutation of BLDC motor is electronically controlled. • Rotor position should be known in order to know the sequence of energizing stator coils. • Hall sensors and optical encoders are used for this purpose. 8 • Hall Effect Sensors are devices which are activated by an external magnetic field. • A magnetic field has two important characteristics flux density, (B) and polarity (North and South Poles). • The output signal from a Hall effect sensor is the function of magnetic field density around the device. • When the magnetic flux density around the sensor exceeds a certain pre-set threshold, the sensor detects it and generates an output voltage called the Hall Voltage, VH. 9 10 • 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. 11 • 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 %, whereas as brushed type DC motors are 75 to 80 % efficient. • There are wide varieties of BLDC motors available ranging from small power range to fractional horsepower, integral horsepower and large power ranges. 12 CIRCUIT DIAGRAM-Open Loop 13 Block diagram 14 Circuit Diagram-Closed Loop 15 • A BLDC motor accomplishes commutation electronically using rotor position feedback to determine when to switch the current. • Feedback usually entails an attached Hall sensor or a rotary encoder. • The stator windings work in conjunction with permanent magnets on the rotor to generate a nearly uniform flux density in the air gap. • This permits the stator coils to be driven by a constant DC voltage (hence the name brushless DC), which simply switches from one stator coil to the next to generate an AC voltage waveform with a trapezoidal shape. 16 Difference Standard DC motor • Magnetic field is stationary in stator, rotor poles switch polarity due to commutation to provide constant rotation. Brushless DC motor • Magnetic field of rotor is fixed. Magnetic field in stator poles is electronically commutated, provides rotating magnetic field. • Motor contains internal position encoder to provide position feedback to the control system. 17 • Current in the DC motor is automatically switched to different windings by means of a commutator and brushes to create continuous motion. • In a brushless motor, the rotor incorporates the magnets, and the stator contains the windings. • Brushes are absent and hence in this case, commutation is implemented electronically with a drive amplifier that uses semiconductor switches to change current in the windings based on rotor position feedback. 18 Advantages • • • • • • • • • Higher efficiency and reliability Smaller and lighter Better speed versus torque characteristics Faster dynamic response Long operating life Noiseless operation Higher speed ranges Elimination of sparks as there are no brush. Reduced friction. 19 Disadvantages • • • • Requires complex drive circuitry Requires additional sensors Higher cost Some designs require labor. (Hand wound stator coils) 20 Applications • Consumer: Hard Drives, CD/DVD Drives, PC cooling fans, toys, RC airplanes, air conditioners. • Medical : Artificial heart, microscopes, centrifuges, arthroscopic surgical tools, dental surgical tools and organ transport pump system. • Vehicles: electronic power steering, electric vehicles. • Airplanes : an electric self-launching sailplane 21 Link • https://www.youtube.com/watch?v=43JMIuw VrY4 • https://www.youtube.com/watch?v=VaWGJV HiJC8 22
