Block Diagram

The block diagram of a dc drive is shown above. It does not show all details. The DC motor has not been represented in the form of a block diagram and the details of the load the motor drives have also not been shown. The block diagram functions as follows.

For the system described here, the output of the system is the speed of the motor. Hence when this system is to be controlled in closed-loop, the parameter that is to be set is what that speed should be. It is denoted to be W_ref. In order to control the speed in closed-loop, we need a feedback signal too. It can be obtained in several ways. A digital tacho or an analogue tachogenerator can be used. It is assumed that an analogue tachogenerator is used here. It is coupled to the motor shaft and its output voltage varies linearly with its speed. Let the speed feedback signal be W_f This signal can be compared with the speed reference signal and the error can be processed by a controller. The controller can be of one of several types. It can be an integral controller, or a PI controller and PDF (pseudo-derivative feedback) controller or a PID controller or a rule-based fuzzy logic controller. Here both the controllers used are PI (proportional plus integral) controllers. A PI controller can lead to fast response and zero-error for a step input.

The PI controller for speed has as its input the error between the two signals, W_{ref and} W_f. If the speed feedback signal W_f is lower than the reference signal W_ref , it means that the DC motor speed is running below the set speed and it needs to be accelerated. In order to accelerate the motor, it should develop greater torque. To develop greater torque, its armature current has to increase. Hence the output of speed controller is set to function as the reference signal for armature current. It will be a voltage corresponding to armature current with an appropriate coefficient linking the two quantities. When W_f < W_ref , the difference causes the output of speed controller to increase. Since the output of speed-controller is set to function as the armature current reference signal, an increase in the value of speed-controller output would in turn lead to an increase in the armature current.

The rectifier circuit is made up of SCRs and the SCRs have a current rating. Hence it is necessary to ensure that the current through the SCRs remains within a safe level. Hence the output of speed controller is limited at both ends. Its maximum value corresponds to the safe level for SCRs. It is not normally the rated current of the motor and it is usually set at a value ranging from 1.5 times to 2 times the rated armature current. The reason is that the motor may have to develop more than the rated torque under transient conditions to achieve fast response. In order to ensure that the motor armature current remains within its rated value, another supervisory loop may be used. Another option is to use a circuit-breaker. The instantaneous trip action in the circuit breaker can be due to magnetic effect and the overload trip can be due to thermal action. A bi-metallic strip within the circuit-breaker expands due to temperature and would trip circuit-breaker. The lower limit on the output of speed-controller would correspond to zero current in the armature, since the motor current in this scheme cannot be in the reverse direction.

The current controller has two inputs, the reference current signal which is the output of the speed controller and a feedback signal proportional to the armature current. The feedback signal can be obtained in several ways. A current transformer can be introduced in the path of ac current from the ac supply. Another option would be to use a DC current transducer that makes use of a Hall-effect sensor or isolated opamp. The transducer used produces a voltage proportional to the current in the armature. The difference between these two signals is processed by another PI controller and its output is also limited to correspond to 0^o and 180^o firing angle. The output of current controller may vary between 0 V and 10 V, with 0 V corresponding to 180^o firing angle and 10 V corresponding 0^o firing angle. If the firing angle be a and the output of current controller V_C, then

As the output voltage of current controller increases due to the difference between the reference signal and the feedback signal corresponding to armature current, the firing angle is advanced towards 0^o and the average output voltage of the bridge rectifier increases. This in turn leads to increased torque generation and the motor accelerates.

If the speed reference is brought down suddenly, the current in the motor cannot be reversed and hence the motor slows down due to friction and the load. This process can be slow.

The question that can be raised is whether we need the current loop. The answer is that it improves the performance. If there is a change in the supply voltage even by a small amount, the output of the bridge circuit tends to a fall a bit for the same firing angle. The reduction in output voltage causes a large change in the armature current, with the speed remaining more or less. The current loop comes into action, correcting the firing angle to the required value. The time constant of the armature, due to its inductance and resistance, tends to be of the order of a few tens of ms and the mechanical time constant, due to the moment of inertia of motor and load and the friction, is of the order of a few tenths of a second. If a current controller is not used, the speed would have to change before the speed controller can come into action. Since the mechanical time constant is about at least 10 times greater, there would be a significant change in speed if there be no current controller.

Normally a filter may be necessary in the feedback circuit for speed. The tacho signal usually contains a small ripple superimposed on its dc content. The frequency of the ripple is usually dependent on the speed and the lower the speed is the lower is the frequency of this ripple. Hence the time constant of the filter may have to be set to correspond to the lowest speed at which the motor would be required to run. Usually the motor speed does not have to vary over a range larger than 0.1 p.u to 1 p.u. Since the power output varies proportionately with the speed, there is usually no justification to run the motor at an extremely low speed. The next section describes how the simulation is carried out. The routines are explained with the help of pseudo-code that can be understood by a reader with some knowledge of one of the programming languages such as C, PASCAL, BASIC, Fortran or Matlab.

 
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