BrushLess DC (BLDC) Motor
The brushless DC motor(BLDC Motor) is actually a Permanent-Magnent Synchronous Machine ( It is not a DC motor).
- It is named like brushless DC motor, because its characteristics resemble to that of DC shunt motor with constant field current.
- Though it is powered from variable DC source, it is internally an AC motor.
- The commutation is not done by brushes but by static switches.
- The construction of the stator is similar to that of traditional Induction motors.
- The appearance of the brushless DC motor is shown here.
- It consists of a stator with multiphase winding(usually 3 phase), a permanent-magnet rotor, and a rotor position sensor.
- Brushes are not used for the commutation. It is performed electronically by switching the current to the motor.
- BLDC motors also require measurement of the position of the rotor to find its speed of rotation and to generate a supply current at the same frequency.It is done by means of a position-sensing arrangement that usually consists of either a hall-effect sensor or optical encoder.
- The permanent magnet rotor is typically made either of rare earth magnets or of ceramic magnets(ferrites). The rare earth magnets are having good magnetic properties but they are expensive. So mostly ceramic magnets are used.
- The BLDC motors can be rated up to 250 kW at 50000 rev/min.
Features of BLDC Motor:
- In conventional DC motors, the supply voltage is limited by brush wear and sparking that can occur at the commutator. So transformers are required to step down the supply voltage. Such problems will not arise in BLDC motors, because the commutation is performed electronically without the need of brushes.
- In general the armature winding is on the stator and thus most of the losses are in stator. So mostly cooling is not required.
- In brushless DC motor, by sealing the stator, submersible units can be built.
- The construction is easy and similar to Induction motors and thus suitable for automated production.
The operation of Brushless DC motor is similar to that of Permanent-Magnet DC motor.
- A fixed magnetic field is generated by the permanent magnets interacts with the perpendicular field induced by the currents in the stator windings, thus creating a mechanical torque.
- As the rotor turns in response to this torque, the angle between the stator and rotor fields is reduced and thus the torque as well.
- To maintain the torque acting on the rotor, the power electronic switching circuit is incorporated in the BLDC motor.
- The switching circuit connected with the stator windings switches the supply current to the stator so as to maintain a constant angle δ = 90°, between interacting fields.
- Because the current is continually switched between windings as the rotor turns, the current in each stator windings is actually alternating, at the frequency proportional to the number of motor magnetic poles and the speed.
- The electronic switching device consists of a rotor position sensor, fixed on the shaft, and an electronic switching module that can supply each stator winding.
- The total back emf of the inverter is obtained by piecewise addition of the motor phase voltages and is a constant voltage, proportional to motor speed.
- Diagrams of the phase-to-phase back emf's and the switching sequence of the inverter are shown below. From the figures it is verified that the addition of the three-phase voltages leads to a constant voltage.
- In practice, we cannot obtain the exact trapezoidal back emf waveforms as shown below as a consequence of voltage ripple. Additional phase windings on the stator could solve the problem.
The simplified voltage equation is given by:
V = kaωm + RwI
T = kT I
ka = kT
V = motor voltage
ka = armature constant
ωm = mechanical speed
Rw = winding resistance
T = motor torque
kT = torque constant
I = motor (armature) current
Phase voltages and SCR switching sequence for the brushless DC motor drive
Brushless DC motors are used in the design of servo loops in control systems( computer disk drives, propulsion systems for electric vehicles)
- Controllability over a wide range of speeds.
- Capability of rapid acceleration and deceleration.
- Convenient control of shaft speed and position.
- No mechanical wear or sparking problem due to commutation.
- Better heat dissipation capabilities
- Need for more complex power electronics than the brush type dc motor for equivalent power rating and control range
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