Brushless DC Motor Drives (BLDC)

Basics: This section deals with properties and control of brushless DC motors drives with permanent magnet excitation (BLDC, PMDC). In the first part characteristics of contemporary permanent magnets (PM) that are used in electric motors are presented along with the simplified ways of their modeling. As an example a pendulum is given, which consists of swinging coil over a stationary PM, and the influence of PM modeling simplifications upon the dynamic trajectories of movement is discussed. Further on, a model of PMDC drive is derived on a transformed d-q and also a contra model in which no transformation of variables is used, with the commutation taking place according to the state of physical (natural) variables.

However, the problem of nonholonomic constraints is not undertaken while dealing with PMDC modeling. In a classical DC motor with mechanical commutator, the existence of such constraints is evident, because the connection of each armature's coil to the external circuit depends on the rotor position. In case of electronic commutation, a switching of windings' supply is controlled also by the rotor position angle, but topographic structure of circuits remains fixed and the switching is carried out by abrupt changes of impedance values of power electronic switches. In this section many characteristics and transient curves for BLDC drives are presented and a comparison is made between results obtained from both types of models: d-q transformed and untransformed ones. This may justify the choice regarding the kind of the model to be used in particular applications, depending on the dimension of a whole system and required rigorousness of results.

The results presented cover DC drives operation with and without control system intervention. PID control is discussed in its application to a given profile of speed and rotor position movement; also, inverse dynamics method is introduced. Numerous examples of DC drive problems are included, using two typical BLDC motors with given data.

Introduction

For 160 years DC direct current machines have played an important role in electric drives. The basic advantages associated with their application in drives include: easy adjustment of rotational speed, uncomplicated start-up and reversal, stable operation for small speeds as well as good dynamic properties ensuring fast reaction to changing parameters of power supply. DC machines with classical design consist of a stator with electromagnets which are powered by the excitation current and whose role is to generate a magnetic field of excitation. The windings of the rotating armature are connected to the mechanical commutator with the graphite-metal brushes slipping over it. They constitute the electrical connection between the movable armature windings and external circuits. The mechanical commutator, which is a device mostly of historical significance, plays the role of a mechanical rectifier, which converts AC current with the frequency corresponding to the rotor's rotational speed into the DC current outside the armature. At its time, the mechanical commutator was an outstanding device, which was however troublesome during exploitation and costly in terms of cost of investment. It was also the weakest link in the system in the sense of reliability of operation as well as required frequent service and regular overhaul. A modern brushless DC machine (BLDC) displays two fundamental differences in contrast to the mechanically commutated DC machines. First of all, it does not have a mechanical commutator over which the brushes forming the electrical node used to slip. A static electronic commutator is used in its place, whose role is the commutation of the current in armature windings in the function of the angle of rotor position. Hence, the principle governing DC machines is preserved, i.e. the machine is self-commutating. In the characteristics of the machine the basic effect involves the fact that along with the increase of the load the machine tends to slow down unless it’s supplied with external speed control for the stabilization of the speed. As a result of this slowing of rotational speed the armature current tends to increase and this leads to a new equilibrium point of the operation. The second relevant difference between a classical mechanically commutated machine and up-to-date motor involves the replacement of electromagnets exciting the main magnetic field with adequately selected permanent magnet assembly. This solution is rendered possible as a result of magnetic parameters and other utility parameters and ultimately the commercial value of permanent magnets. They contain rare earth elements, such as neodymium (Nd), samarium (Sr) among others.

The application of permanent magnets improves the efficiency of a machine since there are no power losses in the excitation windings and leads to the decrease of machine mass. However, in terms of the construction and thermal requirements of machine operation there is no advancement since the permanent magnets installed in the machine and providing the excitation flux require adequate operating conditions which don’t permit the deterioration or a decay of the magnetic field from the magnets. These requirements basically involve the limitation of the temperature inside the permanent magnet motor, limitation of the influence of armature reaction in a way that ensures that irreversible demagnetization of magnets does not occur and not extending air gaps in order to prevent overloading of permanent magnets. One has to bear in mind that the permanent magnet DC machines (PMDC) have to be designed in manner that ensures their operation over a number of years without deterioration of the exploitation parameters. Another important difference between the classical mechanically commutated DC machine and a brushless one concerns the number of windings of the armature and, subsequently, the current waveform on the DC side. In a commutator machine the usual number of windings varies around a couple dozen, as a consequence of which there is an adequately large number of commutator segments. In connection with this, DC current contains very small pulsations since the commutation occurs every couple of degrees of rotor's angle of rotation. Consequently, the electromagnetic torque generated by this machine tends to demonstrate small pulsations. In BLDC machines of the most common engineering design there are three phase windings of the armature, which is reflected by three branches of an electronic commutator (rectifier controlled by the angle of rotor position). This results in considerable current and torque pulsations generated by the machine since the commutation occurs every 60º of the angle of rotor rotation, alternatively in the anode and cathode group of the electronic commutator. It’s obviously possible to increase the number of the armature windings and number of commutator branches thus leading to the reduction of current pulsation; however, two negative effects follow. One of them is associated with the need to use a more extensive and expensive electronic commutator, while the other one involves an increase of commutation losses and decrease of the efficiency of the drive. A final remark that can be made at the beginning of this introduction is that brushless DC machines with permanent mag nets can vary considerably in terms of their engineering structure. First of all, there can be minute machine serving as servo-drives in technology, household appliances and vehicles. Besides, there are larger machines, which are applied in electric drives of automatically controlled devices, including drives in manipulator joints. Finally, there are high power machines with the parameters of the drives used in industrial machinery, for example in steel mills or vessels. BLDC machines may have a various number of phases, have cylindrical construction and in some applications they can have a form of a disk with immobile armature and rotating magnets. The final solution can serve for use in low revolution gearless drives. BLDC machines need not have low revolution ranges, as ones discussed before, but also can operate under rotational speeds exceeding 10,000 [rev/min]. The number of the available versions is large and still growing.