BLDC Motors

BRUSHLESS D.C MOTORS

Prepared by Krishan Arora Assistant ProfessorLovely Professional University

IntroductionClassical DC motors are no doubt good and simple but inefficient in some ways.Although dc motors possess good control characteristics and ruggedness, their performance and applications are inhibited due to sparking and commutation problems. The Permanent Magnet Brushless DC (PMBLDC) motor is able to overcome the limitations mentioned above and satisfy the requirements of a variable speed drive.

ContentsAbstract.Introduction.Basics of a motor.ClassificationHall EffectBLDCM.Working procedureConstructionComparison TablesCharacteristicsDrive CircuitsAdvantages.Disadvantages.Applications.

AbstractPermanent magnet (PM) brushless DC motors (BLDCM) are generated by virtually inverting the stator and rotor of PM DC motors.These motors are actually fed by rectangular AC waveform. The advantage is the removal of brushes, leading to eliminate many problems associated with brushes. Another advantage is the ability to produce a larger torque because of the rectangular interaction between current and flux.

MOTOR MOTOR >> Converts electrical energy to mechanical energy.

ORIGIN... >> Nicola Tesla introduces electric motor in December 1889. >> He identifies the principle of rotating magnetic field.

Main parts of brushed motor...Commutator :>> Facilitate collection of current from the armature conductors.

Brush :>> Collects current from the commutator.

Classification

HALL EFFECT: THE DISCOVERYDiscovered by Edwin Hall in 1879.

Quantum Hall Effect discovered in 1975

Hall-EffectIf a current-carrying conductor is kept in a magnetic field, the magnetic field exerts a force on the moving charge carriers, tending to push them to one side of the conductor, producing a measurable voltage difference between the two sides of the conductor.

Lorentz Force:F = q[E + (v x B)]Lorentz Force:F = q[E + (v x B)]

Hall voltage is produced by charge accumulation on sidewallsCharge accumulation balances Lorentz ForceCharge accumulation increases resistance

BLDCMHas no brushes and commutators.Rotation of the rotor depends on the accurate position with stator.Detected by Hall Sensor, mounted on rotor, shifted at 60 or 120 phase shift.Electronic commutation used to vary the PWM duty-cycle for speed control, using software.

Working of BLDCMAs there is no commutator ,the current direction of the conductor on the stator controlled electronically.Rotor consists the permanent magnet where as stator consist a no. of windings. Current through these winding produces magnetic field and force.Hall sensor used to determine the position during commutation.

Commutation of BLDCM Brushless DC motor requires external commutation circuit to rotate the rotor.

Rotor position is very important.

HALL SENSOR senses the position of the coil accurately.

Working Procedure

When electric current passes through a coil in a magnetic field, the magnetic force produces a torque which turns the motor.

Force in Motor:F=ILBF = Force B = Magnetic FieldL = Length of ConductorI = Current in Conductor

Torque in Motor:T = IBA sin A = LWL = Length of WindingW = Width of Winding

Working Procedure Halls Sensors sense the position of the coils.

The Decoder Circuit turns appropriate switches on and off.

The voltage through the specific coils turns the motor.

ConstructionBLDC motors have many similarities to AC induction motors and brushed DC motors in terms of construction and working principles respectively. Like all other motors, BLDC motors also have a rotor and a stator.

StatorSimilar to an Induction AC motor, the BLDC motor stator is made out of laminated steel stacked up to carry the windings. Windings in a stator can be arranged in two patterns; i.e. a star pattern (Y) or delta pattern (). The major difference between the two patterns is that the Y pattern gives high torque at low RPM and the pattern gives low torque at low RPM. This is because in the configuration, half of the voltage is applied across the winding that is not driven, thus increasing losses and, in turn, efficiency and torque.

Steel laminations in the stator can be slotted or slotless as shown in Figure 2. A slotless core has lower inductance, thus it can run at very high speeds. Because of the absence of teeth in the lamination stack, requirements for the cogging torque also go down, thus making them an ideal fit for low speeds too (when permanent magnets on rotor and tooth on the stator align with each other then, because of the interaction between the two, an undesirable cogging torque develops and causes ripples in speed). The main disadvantage of a slotless core is higher cost because it requires more winding to compensate for the larger air gap.

BLDC Motor Stator

RotorThe rotor of a typical BLDC motor is made out of permanent magnets. Depending upon the application requirements, the number of poles in the rotor may vary. Increasing the number of poles does give better torque but at the cost of reducing the maximum possible speed. Another rotor parameter that impacts the maximum torque is the material used for the construction of permanent magnet; the higher the flux density of the material, the higher the torque.

Disassembled view of a brushless dc motor

Brushless dc motor = Permanent magnet ac motor + Electronic commutator

Speed Torque Characteristics

Drive CircuitsThere are two Drive circuits used in Permanent magnet Brushless DC Motor :

Unipolar Drive

Bipolar Drive

Unipolar Drive

Bipolar Drive

AdvantagesIn BLDC motor PM are on the rotor & electromagnets are on the stator controlled by software. Thus the advantages are:

Because of computer control it is more precise & more efficient.There is no sparking & less electrical noise.Voltage and current rating is high.High speed can be achieved.As there is no brushes it requires no servicing.It has longer life.Low Radio Frequency Interference and electromagnetic Interference.

DisadvantagesRequires Complex Drive Circuitry.

Requires additional Sensors.

Expensive.

Some designs require manual labor. (Hand wound Stator Coils)

ApplicationsPMBLDC motors are increasingly being used in a wide spectrum of applications: domestic equipments, automobiles information technology equipment industries public life appliances transportation aerospace, defence equipments, power tools, toys, vision and sound equipments medical and health care equipment ranging from microwatts to megawatts.

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