CONTROLLERS FOR PERMANENT MAGNET BRUSHLESS DC MOTORS

CONTROLLERS FOR PERMANENT MAGNET BRUSHLESS DC MOTORS

The control of PERMANENT MAGNET BRUSH LESS DC motors can be accomplished by various control techniques using conventional six pulse inverters which can be classified in two broad categories as voltage source inverter (VSI) and current source inverter (CSI) based topologies. The controllers can further be divided on the basis of solid state switches and control strategies. The PERMANENT MAGNET BRUSH LESS DC Motor needs rotor-position sensing only at the commutation points, e.g., every 60°electrical in the three-phases; therefore, a comparatively simple controller is required for commutation and current control.

The commutation sequence is generated by the controller according to the rotor position which is sensed using Hall sensors, resolvers or optical encoders. These sensors increase the cost and the size of the motor and a
special mechanical arrangement is required for mounting the sensors. The system reliability also reduces due to the additional components and wiring. Therefore, the control complexity and high cost of the drive hold back the
widespread use of PERMANENT MAGNET BRUSH LESS DC motors. Reduced cost controllers for PERMANENT MAGNET BRUSH LESS DC motors are more in demand and many schemes and algorithms for reduced cost controllers are shown.
 

The cost reduction of controllers for PMBLDC Motor drives can be accomplished by two approaches, namely topological approach and control approach. In the
topological approach, the number of switches, sensors and associated circuitry used to compose the power converter is minimized, whereas, new algorithms are designed and implemented in conjunction with the converter to produce
the desired characteristics, in the control approach. To begin with the topological approach, topologies with more than one switch per phase, but less than conventional two switches per phase can be considered for low cost
applications. However, there are some conventional topologies (i.e. six switch topology) for low cost applications. As the majority of applications of these motors are at low power levels, therefore, single phase AC mains fed PERMANENT MAGNET BRUSH LESS DC Motor drives are considered here.

1. Load commutated converter topology
2. Buck converter-CSI based topology
3. Ćuk converter-CSI based topology
4. Conventional VSI based topology
5. Three phase four switch topology
6. 3-ph. 4 switch topology without input rectifier
7. C-dump topology
8. Split supply converter topology
9. Buck converter based topology for bifilar wound two phase PMBLDC Motor
10. Variable DC link converter topology
11. SEPIC converter based topology
    
    Unipolar excitation for PMBLDC motors, need less electronic components and use a simple circuit a compared to conventional bipolar excitation of PMBLDC motors. This leads to converter cost minimization and opens up scope for substantial applications where cost matters more than the accuracy of control.
    For proper operation of a PMBLDC motor, the flow of current in the stator windings must be synchronized to the instantaneous position of the rotor and therefore, the current controller must receive information about the position of the rotor. However, the presence of the position sensor is undesirable in many applications; therefore, position sensorless schemes may be employed in which rotor position information is deduced from the voltages and currents in the motor windings.

LOAD COMMUTATED CONVERTER TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTORS

A single phase AC mains input based thyristorised load commutated converter topology as shown in the figure, has been used on a current source inverter.
Four-quadrant operation, current sensorless control and wide operating speed range are good features of the proposed topology. However, the requirement of a big inductor for high capacity applications has been a major disadvantage of this topology.

BUCK CONVERTER-CSI BASED AND ĆUK CONVERTER-CSI BASED TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTORS

Some modifications in this thyristorised drive based on the buck and Ćuk topology (schematic shown in Figure) have been shown for reduction of harmonics and cost as well.

CONVENTIONAL VSI BASED CONVERTER TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTOR

The topologies with switches less than one per phase modified from the basic VSI topology (conventional six switch configuration as shown in Figure).

One such reported topology is a three phase four switch topology shown in
Figure below,

which has been tested with different schemes like PWM [Pulse Width Modulation] and hysteresis current control methods. It has been modified for power factor correction, resulting in a topology with a total of six switches as shown below.

 This topology has single phase to three phase conversion with sinusoidal input current close to unity power factor. This topology enables regenerative braking due to bidirectional power flow between ac input and PMBLDC motor via the DC link. This topology requires a symmetric PWM scheme for switching control, which can be generated using a digital signal processor (DSP) or a
field programmable gate array (FPGA).

C-DUMP CONVERTER TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTORS

Another topology in this category is the C-dump converter topology shown in Figure which has (n+1) switches for an n-phase machine. For a three phasePERMANENT MAGNET BRUSH LESS DC Motor it has four sets of power switches and power diodes (one switch and one diode per set), of which three are connected with phase windings and one remaining set is connected with the capacitor for energy recovery. Since each phase has only one switch, the current in it could only be unidirectional; hence, it is very much similar to the half wave converter driven PERMANENT MAGNET BRUSH LESS DC Motor in operation.

SPLIT SUPPLY CONVERTER TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTORS

A half bridge power converter topology known as a split supply converter topology shown in Figure has also been used for PERMANENT MAGNET BRUSH LESS DC MOTOR having one switch per phase and only two diodes for rectification. This topology can be used with bifilar winding after incorporating some modifications; however it reduces motor utilization.

BUCK CONVERTER BASED TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTORS

A buck converter based two phase PERMANENT MAGNET BRUSH LESS DC Motor drive with bifilar winding as shown in Figure can be used for low voltage applications only. Another topology, which combines the advantages of C-dump converter and split supply converter topologies, has been reported in the literature for the control of PERMANENT MAGNET BRUSH LESS DC Motor.

VARIABLE DC LINK CONVERTER TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTOR

This topology named the variable DC link converter topology shown in
Figure has variable DC link voltage, four quadrant operation and low voltage rating power switches as major strengths. Some topologies have been shown which provide power factor correction (PFC) as well while controlling the operation of PERMANENT MAGNET BRUSH LESS DC Motors.

SEPIC CONVERTER TOPOLOGY FOR PERMANENT MAGNET BRUSH-LESS DC MOTORS

A SEPIC converter based unipolar control has been reported as one such topology shown in Figure. These topologies have also been claimed as low cost controllers for PERMANENT MAGNET BRUSH LESS DC motors.