The Research Of6Phases Permanent Magnet Brushless Direct Current Motor Control Technology

The PM BLDCM (short for Permanent Magnet Brushless Direct Current Motor,the same to the following) is one kind of self-controlled PMSM, and its back EMFwave is trapezoidal, phase current wave is rectangular. It has the advantages of separateexcitation motor, which has dropping machinery characteristic, while avoiding theproblems caused by machinery commutator. For the most commonly used3phases PMBLDCM, if its trapezoidal back EMF wave top flat width reach to120°E, and the phasecurrent top flat is in the same phase position with the trapezoidal back EMF wave topflat, at this time, the electromagnetism torque is no ripple, and its high power densitycan be shown clearly.In fact, for the3phases PM BLDCM, the top flat width of phase BEMF is hard toreach to120°E, and at single side of PWM, the OFF phase has current under someconditions. And the non-squareness phase current will cause torque ripple during noncommutation area. At commutation area, because of the changing rate of current isdifferent between the turn on phase and turn off phase, the non commutation phasecurrent is unstable, which causes much more torque ripple. The torque ripple causesnoise and shake, impose restrictions on the range of speed for motor, and reduce thepower density of motor independently. This become the limitation of PM BLDCM forwider speed range and more powerful.This dissertation compares and analyses the phase BEMF top flat width of3phasesand6phases(dual3phases) PM BLDCM, and makes the following conclusions: Forthe3phases60°E phase bandwidth motor, the top flat width of BEMF can reach to110°E at the best condition, but for6phases PM BLDCM, the top flat width of BEMFcan reach over120°E easily. The6phases PM BLDCM could solve the problem ofprinciple torque ripple, which is hard to overcome by control method for3phases PMBLDCM.Next, the dissertation analyzes and recommends the method of PWM-ON-PWM,by30°E rotor position signal, which easily eliminates torque ripple by the OFF phase continue current.Third, analyzes the reason for non squareness phase current, come up with the ideaof peak current control method. Gives the way of squareness phase current formationunder the condition of the PWM frequency is stable. And gives an example forcalculate phase current rise and fall rate with the parameter and frequency is known.Fourth, by the theory of commutation of PM BLDCM considering inductance andresistance, put forward the control method of shorten commutation time while keepingthe non commutation phase current reach to expectation, in order to provide foundationfor non commutation area current to form like squareness wave. Analyzes therelationship of magnetism coupling between dual3phases winding for6phases PMBLDCM, which proves that the current changing rate of turn ON phase and turn OFFphase is nearly the same. This will provide advantages of reducing torque ripple for6phases PM BLDCM.In order to checking the theory above, we designed a312V11kw controller of6phases PM BLDCM for electronic vehicle. Finally we got the turn ON phase currentand non commutation phase current scope by initial experiment. From current scope, itclearly shows that the turn ON phase current is close to squareness wave, while the nonon commutation phase current is nearly stable. The changing rate of phase current isclose to theory calculation, which proves the effectiveness and exactness of the controltheory shown in this dissertation.From the conclusion above we can expect that the6phases PM BLDCM can beused in the field of wide speed range, low noise and high power.