Current limiting featuer in a Thyristor DC drive, working of current amplifier. Uses of Current Limiting Feature.

Current Limiting feature in a DC drive....

Output of the Speed Amplifier is connected to the next stage of Current Amplifier through a preset potentiometer called "current limit pot". This potentiometer restricts the voltage level going to the Current Amplifier. If dc drive is functioning correctly, restricting this voltage level can restrict the current taken by the motor to any value from zero to full load current. To understand different uses of this feature, a brief description of the working of the two closed loops ( speed and current ) will be in order.

When a dc drive is started and certain speed reference voltage is given, the output voltage of the ramp circuit starts increasing slowly. This voltage is connected to the input of the speed amplifier.The output of this amplifier starts going towards the saturation level ( either +supply voltage or -ve supply voltage depending upon the configuration of the opamp and reference polarity.) This voltage, in turn fed to the current amplifier, pushes the current amplifier voltage also towards its saturation level slowly. The rising voltage starts firing the thyristors thereby generating gradually increasing armature voltage. The motor starts accelerating. In the process the tachogenerator mounted on the dc motor starts generating more and more voltage. At one instance, the voltage generated by tachogenerator, which reaches the input of the speed amplifier, matches the reference voltage at the input of the speed amplifier. Any tendency of tachogenerator voltage increasing beyond the reference voltage, drops the output of the speed amplifier. The net result is that the motor starts running at a constant speed. Here, let us assume that the mechanical load is such that the current feedback generated is 1 volt. You will notice that the voltage output from the speed amplifier is also 1 volt. Now, in this condition, if the motor is mechanically loaded more, it demands more current from the thyristors. This extra current can not come from the supply without an increase in armature voltage. Increased mechanical load first drops the speed of the motor slightly. Decreased back emf of the motor allows the motor current to increase.

( motor current = [applied voltage - back emf ] / armature resistance ). Almost in no time, the drop in speed is compensated by the speed amplifier taking a corrective action because of the closed loop feedback. If the output of the speed amplifier is monitored, it will be seen that the output has gone up and matched the current feedback level. This is tricky. It appears as if the feedback is deciding how much should be the reference ! More and more load on the motor will go on increasing the feedback signal level and a point will be reached when the output of the speed amplifier can match the current feedback level. From this point onwards, the motor current can not increase to meet the demand from the mechanical load, and as a result the speed will start falling down. This point is the point where the current has started limiting itself. Any amount of load put on the motor will only result in drop in speed. This is Current Limiting. So long as the motor is demanding current less than the set value, the Speed closed loop will work perfectly holding the speed at the set value. Further rise in load will result in reduction in speed.

Now that the current limiting is clear, let us see the uses of this feature in a dc drive.

- To limit the current to a safe level.

- To test the tachogenerator signal by running the motor under current limit. Without drive running in current limit, it is impossible to conclude if the tachogenerator is at fault. The closed loop action makes it impossible. But by putting the drive in current limit, we are actually breaking the feedback loop thereby making it possible to diagnose.

- It helps in initial trials of the drive or motor, where excessive load can cause damage.

- It can also help in deciding the correct polarity of the tachogenerator required.

- It can be used to control tension in winder application where speed is not required to be maintained but it is the tension which is to be maintained constant.