Construction and Working:
The armature receives a supply from a dc source through brushes and commutators. The brushes and commutator constitutes an inverter sensitive to the shaft position.
In a similar way, a synchronous motor may be considered to operate as a de motor. In a synchronous machine the field rotating whereas the armature is stationary but it should be supplied by an inverter controlled by shaft-position sensing signals.
The static inverter with the shaft-position sensitive controller can very well be regarded as an electronic commutator serving the same function as does the mechanical commutator.
This facilitates the operation of synchronous motor as a versatile variable-speed drive like a de motor but having a mechanical commutator and brushes.
Thes motors produce no brush or commutator particles or gases as by-products of operation. These motors are capable of operation submerged in fluids, combustible gases and may even be hermetically sealed.
They provide a fairly linear output torque vs input current characteristic, which lends itself to servo-applications.
The stator winding of a brushless dc motor may be conventional three-phase winding of a synchronous motor or the conventional armature winding of a dc motor. However, in both cases, the stator winding has to be supplied from a static inverter triggered by shaft-position sensitive signals so that the supply frequency is proportional to the shaft speed.
The dc field winding is placed on the rotor and supplied from a static dc source through slip-rings mounted on the motor shaft.
The equivalent block diagram of a brushless dc motor is shown in fig.
A 3-phase inverter is equivalent to a three-segment commutator. If the stator winding is similar to the armature winding of an ordinary dc machine, six symmetrical tappings from the winding can be taken out and the stator winding may be supplied from a six-phase SCR bridge inverter.
This is equivalent to six segment commutator. There may be many other variations of the electronic commutation arrangement. For instance, as individual stator coils may be controlled by series SCR switches.
However, three segment commutation with a three-phase static inverter is widely used for drives.
It is, therefore, obvious that a 3-phase synchronous motor when supplied by a 3-phase inverter behaves like a simple dc motor but the SCRs of the inverter should be triggered in proper sequence and instant proportional to the position of the rotor shaft.
The SCRs may be turned off naturally owing to the nature of the load (synchronous motor). The synchronous motor may be operated at any power factor (leading, lagging behind or unity) by variation of the dc field excitation.
When the motor is operated at leading power factor, the inverter can be operated with load commutation i.e. the SCRs are turned off naturally. Otherwise, a force commutated inverter should be employed.
Since the system behaves like a conventional separately excited dc motor, the speed can be controlled by the variation of the dc supply to the inverter or to the field.
The speed is inversely proportional to the field strength. The torque-speed characteristics are similar to those of a separately excited dc motor but are slightly more drooping in this case.
The shaft-position sensing unit normally consists of a toothed disc on a free motor shaft end and two sets of three position detecting sensors.
The detecting sensor may be an oscillator triggered into action for a definite period by the toothed disc or may be an optical unit using phototransistor, light sources producing pulses at definite intervals or may be a Hall device for sensing a magnetic pulse for the required period of time.
The brushless dc motor may be controlled either by a voltage fed inverter, a current fed inverter or a cyclo-converter. Open-loop control is simple. To achieve higher accuracy closed-loop control should be employed.
Applications:
The brushless dc motors are generally more expensive for the same kw rating that conventional (commutator and brush) motors but possess certain advantage too such as little or no maintenance, much longer operating life, no risk of explosion or possibility of RF radiation due to arcing, higher efficiency, rapid response etc.