The function of the Current Loop Amplifier (CLA) is to control/regulate the current of the motor which results in the control of the motor torque. The current of the motor is sensed either by a resistive shunt or with a Hall effect sensor. The control of over current should be fast in order to protect the power semiconductors during starting/stopping of the motor or in the event of fault. Also the steady state error of the current should be zero (as any error in torque affects the speed). These requirements can be met by using a ``PI" (Proportional Integral) compensator.
The current signal is filtered, scaled and then applied to the CLA. The output of the RLA which acts as a current command, is applied to the other input. The CLA subtracts both the input signals and generates the error signal. The error signal is applied to the proportional-integral (PI) compensator. In a 3-phase SCR amplifier like one used at the GMRT, the motor current has a 150 Hz component along with the DC component. As the current is sampled and fed back to the loop amplifier, the 150 Hz component of the current gets injected into the loop. This is like injecting a noise into a system. In order to avoid oscillations in the loop, the current loop compensator is designed to heavily attenuate the 150 Hz signal component. The filtered output of the error amplifier is applied to the 4-quadrant power amplifier.