The characteristics of AC induction motor studied earlier, which concluded that due to the construction of the motor, it is impossible to independently control the torque producing current and magnetic flux (see equation 1-7). Therefore, the control performance AC induction motor were very poor compared to that achieved by the DC motor.
With the development of new types of power control devices, such as Insulated Gate Bipolar Transistor (IGBT) and the increasingly powerful computational tools used with microcontrollers, we now have variable speed control to achieve benefits equal to the engine. The following are some of the methods used initially tried to get better engine performance AC. Observe that the results are very poor reason for not have much application.
Then we have a brief explanation of the modern concepts that apply to get some benefits in accordance with the type of process control.
VOLTAGE CONTROL BY CHANGE
Figure 3-1 shows the resulting torque vs. speed characteristic of an induction motor when the voltage applied to the armor varies, maintaining constant frequency.
From this figure it is clear that when the supply voltage decreases, the torque also decreases, which is not acceptable if you want to control motor speed. From equation (1-7) we see that both IM and I2, are directly proportional to the supply voltage. As in this type of torque control varies roughly with the square of the armature voltage to V <Vnom. Then for V> Vnom can occur magnetic core saturation.
In general, the control supply voltage is not recommended for practical applications.
CONTROL BY CHANGE OF FREQUENCY.
Figure 3-2 shows the characteristic torque vs.. speed of an induction motor power for various frequencies and constant tension.
The increase in feeding frequency, assuming constant stress, causes the magnetizing current decreases in inverse proportion IM, reducing the torque generated.
On the other hand, a decrease in the frequency does not greatly increase the torque, after IM rises too high and enters the saturation region.
The feed rate control is only practical when you want to operate with the attenuated field and above the base rate (nominal).
CHANGE CONTROL ROTOR RESISTANCE.
Figure 3-3 shows the torque vs. characteristic. speed of an induction motor for various rotor resistance.
Notice in this figure that the maximum torque is maintained in the range of R2N <r2 <R2C (R2N: R2C nominal resistance: Critical resistance). R2 actually may be less than R2N, however in this case can occur on excessive current for starting the machine.
For the features shown in Figure 3-3 can be noted that this type of control can have practical applications. The problem is that to access the rotor to vary the resistance r2, it is necessary that it be the type winding and slip rings exist making it larger, expensive and excessive maintenance.
SPEED CONTROL TYPE V / F constant.
Of the three methods of control seen, the latter is feasible only when you want maximum torque throughout the speed range of variation. However, for rotor motors with squirrel-cage type this control can not be applied.
Comparing the torque expression given in equation (1-7) with the DC motor (equation 1-3) note that corresponds to the current IM Ip (field), while I2 corresponds to the armature current Ia. On the other hand, we have:
or in the case of nominal values
where we have:
is a relationship, whose value must be equal to the maximum magnetic flux in the machine, multiplied by a constant.
For constant torque control voltage and frequency should be variable, just enough so that magnetic flux is controlled and maintained at its maximum. This can be achieved if the voltage and frequency range so that:
Making this kind of control, torque characteristics vs. speed of an induction motor is as shown in Figure 3-4. With this, at least for steady state, the induction motor starts to have operating characteristics similar to a DC motor.
