DC drives control the speed of DC motor system by supplying voltage, and thus assist it to run at the right speed. The motor speed is directly and inversely proportional to armature voltage and field current, respectively. So, the function of a DC drive system is to influence both or either of the variables to keep the speed at the required level.
Nowadays, a range of thyristor devices is used to control the speed of DC motors.
DC Drive Operation
The DC drive system is reliable and cost effective. They show more efficiency and precise control than AC drives in many systems, especially their use in high power and regenerative systems.
DC drives and motors have been replaced by VFDs and AC motors in many modern systems, but they are still widely used in various industrial and commercial applications such as elevators, winders, spindle drivers, crushers, machines for producing paper, and more.
The Basic Elements of a DC Drive System
The essential elements of a DC drive system are:
DC drive input. For operating average or small motors, DC drives run on a single-phase and three-phase power supply for operating small and large motors, respectively. For full wave rectification in both case, they require the use of four and six thyristors, respectively.
Rectifier bridge. It is an arrangement of four or more diodes in a bridge circuit configuration. Its function is to convert an AC input into a DC output. It requires single- or three-phase power supply for its operation.
Field supply unit. It creates a steady motor flux by providing a steady voltage to the field winding. The DC drives in permanent magnet motors don’t have this unit.
Speed regulation unit. The unit’s main task is to send correct signals to the firing circuit by comparing the desired speed with feedback signals.
Firing circuit: The firing circuit supplies thyristors the gate pulses so that the former ones stay ON for a temporary period to generate shifting armature voltage.
DC Drives Working Principle
A DC drive system controls the speed of a DC motor. It has been done by calculating the amount of armature voltage and field current, which are directly and inversely comparative to the speed. The DC drives control the motor speed by applying two core principles:
- If the motor speed is higher than the required speed, the DC drive reduces the amount of field current to reduce the motor flux, which in result decreases the armature counter-electromotive force.
- On the other hand, the flow of armature current increases in case if the armature electromotive force is low, which in result increases the torque and speed of the motor.