Renesas H8S/2635 Handbuch PDF herunterladen (Seite 15)

Of course presented in above figure situation due to some non-linearity effects is very hard to

realize. The effect of non-linearity can be observable when the motor is running very slowly as a

slight kick at the commutation points. There is also a second drawback. Namely, the non-

linearity and ripple in the torque tend to produce a velocity modulation in the load. Fortunately,

in a system with a velocity feedback and high gain even small changes in the velocity will

produce big error signals, what gives us demands to change the torque in such a way to keep

the velocity as constant.

In the sine wave motor in difference to the trapezoidal motor we don't change the basic

sinusoidal torque characteristic. We can think about this motor that it is possible to run this

motor by applying sinusoidal currents to the motor windings. Of course, each of these currents

must have the phase shifted by 120

. Now if we want to have a smooth rotation at low speeds

and without torque ripples we need a high resolution device to control the commutation. This

device in general is more complicated than resolver for the trapezoidal motor, because we need

some reference table from which it can generate the sinusoidal currents, which additionally are

multiplied by the torque demand signal. This last operation allows determining the absolute

amplitude of the currents. It is good to mention that in 3-phase motor, it is sufficient to

determine the currents in two of the windings and this will give us automatically the information

about the third one. In this kind of the motor we also need some feedback loop, but we are not

limited only to the velocity. In other words, in the feedback loop we can put velocity or position

as well and at the end we will get the same effect.

3.3. Hall sensor.

According to the basic definition a hall sensor is a special kind of sensor or device which works

on base of the Hall Effect to make measurements of the magnetic field and current. Hall

sensors can be use for switching, positioning, speed detection and current sensing. All of these

features are very useful and were adopted into the brushless motors. Namely, it goes out, that

hall sensors are very useful in detecting the actual position of the rotor. In the already presented

example with 3-phases motor, there are 3 hall sensors. We can read the binary output states on

them, and then we can calculate the actual position of the rotor. Unfortunately, the resolution

in this approach is not too high and is only 60

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Renesas H8S/2635 Handbuch Seite 15