Drives are at the heart of an automation system. They are the controlling element for DC and AC motors, which develop the torque to turn the wheels of industry.

In DC drive systems, an armature and field exciter are used to control the two separate elements in a shunt wound DC motor. SCRs are used to vary the DC voltage output, which has a direct bearing on the speed of the motor. The DC drive is the simplest of drive systems. The disadvantage of using SCRs for the drive power structure is the inherent nature of line notching. This notching is caused by the phasing on and off of the six SCRs located in the drive input section.

Digital DC drives are the latest entry into DC drive technology. With digital technology, precise control of speed and torque can be realized. Speed and current controller circuits use feedback to make small changes in armature supply and field exciter operation. Measuring and scaling circuits sample the actual speed and current output. Summing circuits take the error signal and translate those signals into corrective actions. Higher speed accuracy (<1% regulation) is obtained by the use of a tachometer generator or tach. When operated in the speed regulation mode, the drive closely monitors the speed-feedback signal. Armature voltage (or EMF) control would give a 1 2% regulation characteristic. IR compensation will improve the drooping speed due to load. When operated in the current regulation mode, the drive closely monitors the value of...