24-11-2012, 02:44 PM
BRUSHLESS DC MOTOR AND ITS CONTROL
BRUSHLESS DC.pptx (Size: 2.03 MB / Downloads: 57)
BRUSHLESS DC MOTOR
It has no brushes and slip rings.
Electronically commutated motors,powered by a DC electric source via an integrated inverter, which produces an AC electric signal to drive the motor; additional sensors and electronics control the inverter output.
It uses position detector and inverter to control the armature current.
DRIVE CIRCUIT
Optical sensor used as position decoder.
Phototransistor PT1.PT2,PT3 are placed 120’ apart, exposed to light in sequence by the revolving shutter.
By switching action rotor rotates continously
COMPONENTS OF A BLDC MOTOR
STATOR :
Steel laminations are stamped out with high quality stamping die.
Most BLDC motors have 3 stator winding connected in star fashion.
One or more coils are placed in stator slots
Each winding forms even number of poles.
ROTOR
Made of permanent magnet.
Can vary from 2 to 8 pole pairs with alternate north and south pole.
Number of magnetic poles also effects the step size and torque ripples.
More poles give less step and less torque ripples.
The higher the number of poles, the higher the torque but lower the speed.
HALL EFFECT SENSORS
It is a transducer that varies its output voltage in response to magnetic field.
Used for proximity switching , position , speed detection and current sensing application.
Most BLDC motors has 3 hall sensor embedded into the stator on non driving end of rotor.
DRIVE ELECTRONICS
Inverter (drive transistor are most expensive parts , generally 6 transistor for the three phases of three phase motor).
Logic circuits, generate the required PWM(Pulse Width Modulation ) signals.
Heat sink and thermal management hardware to manage the heat generated in the switching process.
AC front end including filter capacitor for the conversion of the input voltage to a suitable DC voltage.
BLDC MOTOR DRIVE CIRCUIT
Braking chopper is an electrical switch,limits DC bus voltage by switching the braking energy to a resistor where the braking energy is converted to heat.
Automatically activated when the actual DC bus voltage exceeds a specified level depending on the nominal voltage of the variable-frequency drive.
CURRENT CONTROLLER
T-I block performs the conversion from refrence torque to the peak refrence current.
Conversion of torque to current assumes pure rectangular current waveform due to motor inductance.
Hall decoder block to extract the bemf information from hall effect signals.
ADVANTAGES
High dynamic response
High efficiency
Very high torque to inertia ratio
Very high peak torque
No arcing(used in explosive env.)
Higher speed range
Noiseless operation