Position Sensing

34.9.8 Position Sensing

(a)

(b)

FIGURE 34.108 Hard-chopping mode conduction paths: (a) both It can be seen from the preceding discussion that to control

devices on: positive voltage applied to motor phase and (b) T1 and T2 the SR motor satisfactorily, the motor phases are excited at the

turned off: negative voltage applied to motor phase. rotor angles determined by the control method. It is therefore essential to have knowledge of the rotor position. Furthermore, the rotor-angle information must be accurate and have high

the rising-inductance region. This means that even if a phase resolution to allow implementation of the more sophisticated nonlinear control schemes that can minimize torque ripple is excited, the current in the motor phase will not increase and optimize the motor performance. in the rising-inductance region. Therefore, at higher speeds, This means that the performance of an SR drive depends on the turn-on angle must be placed before the beginning of the the accurate position sensing. The efficiency of the drive and increasing inductance region, so that the phase current will its torque output can be greatly decreased by the inaccurate have an adequate time to increase before the back-emf becomes position sensing, and the corresponding inaccurate excitation high. angles. It has been demonstrated that at high motor speeds, an In addition, the time available for the current to rise after error of only 1 ◦ may decrease the torque production by almost turn-on becomes less and less as the speed of the motor

8% of the maximum torque output.

increases. This is due to the fact that the available conduc- Traditionally, the rotor-position information has been mea- tion time is lower for constant switching angles as the speed of sured using some form of mechanical angle transducer or rotation increases. This can be seen by considering that speed encoder. The position-sensing requirements are in fact similar is the time rate of change of angle. Thus, as the speed increases, to those for brushless PM motors. However, although position there will be a point when the current level never rises to the sensing is required for the motor operation, the position- chopping level. At this point, the single-pulse mode of oper- measurement sensors are often undesirable. The disadvantages ation will come into effect and the current will decrease or of the electromechanical sensors include the following: remain constant throughout the increasing-inductance zone.

An example of a single-pulse mode current waveform was seen (a) The position sensors have a tendency to be unreliable in Fig. 34.105b.

because of environmental factors such as dust, high As the current is not commutated in the single-pulse mode,

temperature, humidity, and vibration. the control in this mode consists only of controlling the on and

(b) The cost of the sensors rises with the position resolu- off angles. The turn-on angle θ on can be placed at some point

tion. Hence, if high-performance control is required, in advance of the rising-inductance region where the phase

an expensive high-resolution encoder needs to be inductance is low, so that the current can increase at a faster

employed.

rate before the increasing-inductance region. The angle can be (c) There is an additional manufacturing expense and advanced up until maximum allowable current occurs at the

inconvenience due to the sensor installation on the peak of the waveform (this may even mean switching on in

motor shaft. In addition, consideration must be given the previous decreasing-inductance zone). The actual control

to maintenance of the motor because of the mechan- turn-on and turn-off angles for the single-pulse mode, for a

ical mounting of the sensors, which also adds to the given load torque and speed, can be determined by simulating

design time and cost.

the motor equations. (d) Mechanical position sensors entail extra electrical The speed at which a changeover between single-pulse and

connections to the motor. This increases the quan- chopping-mode occurs is called the base speed. Base speed is

tity of electrical wiring between the motor and the defined as the highest speed at which the chopping mode can

motor drive. This wire normally needs to be shielded

984 M. F. Rahman et al. from electromagnetic noise and thus further adds to conventional synchronous machine. The major difference

the expense of the drive system. between the Syncrel and conventional synchronous and induc- (e) The allocation of space for the mounting of the posi- tion machines is in the rotor structure. In both the induction tion sensor may be a problem for small applications machine and the synchronous machine, there is a source of (such as for motors used in consumer products).

flux in the rotor itself. In the case of the induction machine, this flux is produced by currents resulting from an induction

Hence, to overcome the problems induced by rotor-position mechanism, and for the synchronous machine, there is a field transducers, researchers have developed a number of meth- winding wound on the rotor that is fed with the dc current to ods to eliminate the electromechanical sensor for deriving produce flux. The permanent-magnet synchronous machine position information. This is achieved by indirectly determin- replaces the wound field on the rotor with a permanent mag- ing the rotor position. Such methods are commonly termed net. The Syncrel, on the other hand, does not have any source sensorless rotor-position estimation methods. The term sensor- of flux on the rotor, but instead the rotor is designed to dis- less seems to imply that there are no sensors at all. However, tort the flux density distribution produced by the sinusoidally there must be some form of sensor used to measure the rotor

distributed mmf.

position. In fact, the term sensorless position estimation in Sinusoidally wound reluctance machines were traditionally reality implies that there are no additional sensors required to used in the fiber-spinning industry because of their syn- determine position apart from those that measure the motor chronous nature. This made it simple to keep a large number electrical parameters to control the motor. These are normally of machines running at the same speed using just the frequency current- or voltage-measuring circuits. of the supply to the machines. These machines were direct-on- Hence, all sensorless position estimation methods for the line-start machines. This was facilitated by the presence of an SR motor use some form of processing on electrical wave- induction machine starting cage on the rotor. This cage was

forms of the motor windings. In essence, the major difference also essential to damp out oscillations in the rotor speed when between sensorless position detection, and the electromechan- running at synchronous speed. It should be pointed out that ical sensors mentioned above, is that there is no mechanical these machines are not considered to be Syncrels – a Syncrel connection of the sensor to the motor shaft. Therefore, sen- does not have an induction machine cage on the rotor. A Syncrel sorless position detection involves electrical measurements is absolutely dependent on an intelligent inverter drive in order only. to start the machine and to stabilize it when running. The lack

of a requirement for a starting cage means that the rotor design can be optimized for best torque and power performance.

34.10 Synchronous Reluctance

The revival of interest in the Syncrel in the early 1980s was

Motor Drives

motivated by the development of low-cost microprocessors and reliable power electronics, coupled with the perception

34.10.1 Introduction

that the Syncrel may be more efficient and simpler to con- trol in variable-speed applications compared to the induction

In recent years, there has been a revival of interest in reluc- machine. The control simplicity is achieved in practice, mainly tance machines. Two main machines have been the focus because one does not have to locate the flux vector in order of this interest: the switched-reluctance machine (SRM) and to implement vector control. The potential for improved effi- the synchronous reluctance machine (Syncrel). The SRM is a ciency and torque density compared to the induction machine machine that does not have sine-wave spatial distributed wind- is very much dependent on the rotor design. The Syncrel has ings, but instead has concentrated coils and a doubly salient the advantage over the switched-reluctance machine; in that it rotor and stator structure. The operation of this machine is produces relatively smooth torque naturally, and it uses a con- highly nonlinear in character, and normal ac machine model- ventional three-phase inverter. Therefore, inverter technology ing techniques cannot be applied in a straightforward manner developed for the induction machine can be applied directly. to describe its operation. The SRM drive has been considered in detail in an earlier section.