Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
The rectangular field synchronous motor drives are usually applied in low power, wheel hub motor driven vehicles. The common name of this drives is commutatorless or brushless DC drive (BLDC). The name “refers to” a mature DC machine construction, the permanent magnetic excitation is on the rotor, the winding is on the stator, the mechanical commutator is substituted by electrical commutation. At the rotor, the flux density distribution of the permanent magnetic excitation has a rectangular shape. Generally the stator has a three phase winding, but five phase vehicle motor also exists. Fig.6.7. represents a construction of a three phase drive.
The main element of the circuit is a voltage source inverter that is similar to Fig.5.6, but the control method is different. To reach smooth torque proportional to the i e DC current, such current should be switched to the phases, which shape and phase position should be synchronized to the rotor position. The best current shape selection depends on the spatial distribution of the rotor flux density and the flux linkage considering the three phase coils that is represented by K a, K b, K c torque factors. The origin of the name is from the calculation of the machine torque: m=K a i a +K b i b +K c i c. Fig.6.7b shows the K a, K b, K c torque factors of the most common three phase machine construction and the maching current shape . The K a, K b, K c torque factor amplitude is K m =kNDℓB max (N: number of turns, D, ℓ: machine dimensions, B max : maximum flux density of the rectangular field, k: machine constant), the amplitude of i a, i b, and i c currents are equal to the i e DC current. The hatched areas in the torque time function represent the participation of the a-phase in the torque development. For reversing the torque direction the control of the phase current should be shifted by α=180°. The control mainly consists of two phase conduction states succession, the three phase conduction is only during changes. The conduction states are cyclically altering with 60° appointed by the inverter controller with va, vb, vc electronic switches. The rectangular field synchronous machine drive is simple and it has a great dynamical behavior, but it has a disadvantage: the field weakening (shifted synchronized) mode does not provide smooth torque and the speed range can be only slightly expanded.