Presents the block diagram of variable speed DC motor in which we present the main parts that compose it. See Figure 8.

Figure 8

The numbered parts are as follows:

# 1. Speed ​​reference value by entering the comparator block. It is usually a voltage value provided by a potentiometer or 0-10V standard signals / 4-20mA from a controller.

# 2. Speed ​​error. It is the result of the comparison of signals known reference value of speed and speed feedback.

# 3. Block speed controller. Reacts with proportional-integral features (PI) to the speed error which receives its input. 4 The signal is output.

# 4. Torque reference value. Named because compared with current feedback, which is proportional to the torque developed by the motor.

# 5. Torque error. It is the result of the comparison of signals known reference value of torque and current feedback.

# 6. Torque controller block. Reacts with proportional-integral features (PI) to the torque error which receives its input. The signal 7 is output.

# 7. Thyristor firing command. DC signal is converted into pulses that trigger after entering the trigger circuit. The firing angle is such that the DC power received by the motor armature is needed to keep the engine speed desired reference value.

# 8. Current feedback. Is the signal representing the armature current, measured directly by a DC current transformer or indirectly by a small value resistor (shunt resistor).

# 9. Current feedback. This is the current measurement signal of armor but indirectly through AC current transformers located at the entrance of the power stage. This current value must be rectified by diode bridge and become VDC through a resistor.

# 10. Speed ​​feedback. It is the speed measurement signal in an indirect way by taking a sample of the armature voltage (see equation 2-2). This value has an inherent error rate due to the drop as Ia ra, which is why the drive loses precision when trying to control the speed. To minimize this problem applies IxR compensation technique.

# 11. Speed ​​feedback. It is the speed measurement signal directly. The velocity meter is attached directly to the motor shaft. There are two types of such meters which are called: Tachogenerator (Dinamo-tach) and Encoder. The tachometer provides a DC analog voltage type signal proportional to the speed of the motor shaft and polarity depending on the direction of rotation. Using the tachometer speed allows accuracy within ± 0.1%. The encoder provides four digital signals out of phase for two, which provide information on speed, rotation and position. Using the encoder allows a speed accuracy within ± 0.01%.

It has the following examples:

a) The instructions from an operator control station or other input, are sent to the regulator, see Figure 9. The controller compares the instructions with the feedback voltage and current and sends the appropriate signal to the trigger circuit. This stage conditions the trigger pulses that activate the transistors or thyristors, causing your driving. In some designs, the regulator and trigger circuit are united in a single digital circuit.

Figure 9

Voltage feedback provides an indication of engine speed and indicates the current engine torque as seen in equation (1-1) and (1-2) in the first half.

From these equations and neglecting the drop in inductance La, we have:

From the above equation we see that the armature voltage feedback V is an indirect way of measuring the engine speed n.

Ideally, measuring the value of counter electromotive voltage Ea but this is impossible, therefore it is best to measure the armature voltage applied to and subtract from it a voltage drop equal to the ra Ia, this is possible thanks to the drop voltage across any resistance is proportional to the drop in the resistance of the armature.

Typical settings on the controller include: minimum speed, maximum speed, current limit (torque), IR compensation (load) and adjust the rate of acceleration.

b) For a more precise control of speed, an encoder or tachometer may be mounted on the motor to give a feedback signal, see Figure 10, which is proportional to the actual motor speed. The quality of these and the regulator determines the overall accuracy of the drive.

Figure 10

Figure 9 shows that the armature and field coils are fed by separate circuits. Usually the food you receive the field is constant, thereby facilitating the torque control feature, which depend only on the current applied to the armature.

Figure 11 shows the aforementioned control mode, which is of the "Vector Control" because it has the ability to independently control current flow and producing an engine torque in order to accurately control the torque and power.

The angle "d" is 90 ° due to the mechanical position between the switch and the brushes, so the torque is directly proportional to the armature current (Ia) as If is constant. Note that with increasing engine load, speed tends to decrease and so the drive must be able to provide the at higher engine to compensate for this increased load and thus maintain constant speed.

Figure 11