Adjustable Speed Drive with a Brushless DC Motor

The brushless dcmotordescribed consists ofasyn- chronous motorwitha rotorposition sensor anda three-phase bridge inverter. Thecommutation intheinverter circuit isper- formed bytheinduced voltage ofthearmature windings. Inthis situation, since auxiliary commutation devices suchascapacitors, inductors, andauxiliary thyristors arenotrequired, thesimplest inverter circuit isobtained. However, thecommutation ability decreases atlowspeedandstarting difficulty results. A novel starting procedure isdescribed whereby thechopping oftherectifier output voltage isaccomplished bycontrol ofthedelay angle. In order toobtain arapid interruption ofthemaincircuit current, an auxiliary thyristor antiparalleled tothesmoothing reactor isused. Finally, anewmethodofspeedsensing thateliminates theneedofa tachogenerator isproposed. Speedstabilization withanaccuracy about 5percent inaspeed rangeof10:1isperforned withthis new control system.