There remains a strong market for dc motor drive systems, despite the inexorable growth of the ac motor drives market. Why should this be, when modern ac drives can often be configured to ape the performance of their dc counterparts? Well, energy efficiency is one reason, particularly in situations where the motor is subject to frequent braking
The dc drive comes in two basic configurations: non-regenerative and regenerative. The former controls motor speed and torque in one direction only (quadrant I), and is capable of reversing - again, with motor speed and torque in one direction only (quadrant III). Regenerative or four-quadrant dc drives are capable of controlling not only the speed and direction of motor rotation, but also the direction of motor torque. When the drive is operating in quadrants I and III, both motor rotation and torque are in the same direction and it functions as a non-regenerative unit. In quadrants II and IV, however, the motor torque opposes the direction of motor rotation, which provides a very useful, controlled braking force. Under braking conditions, the drive is able to convert both the motor and connected load's mechanical energy into electrical
energy that can be returned to the mains.
So, thanks to this ability, four-quadrant regenerative dc drives offer an extremely energy efficient system, capable of reducing demand from the incoming ac supply. If these drives are operated on equal driving/braking cycles, the running cost is only the electrical losses in the motor and drive. By comparison, an ac drive generally uses a braking resistor to control ramp down, dissipating the energy as heat, which is wasted to the atmosphere. Continued use in this mode is extremely wasteful. The ability to control the rate of braking is also lost when using braking resistors, whereas the four-quadrant regenerative dc drive is fully controllable in both motoring and braking modes, and conserves the maximum amount of energy.
The dc drive package offers energy efficient improvements in other areas, too. Lower speed motor losses are not as significant as those of ac motors, and the dc drive package has better power conversion efficiency across a wider speed range. At lower motor speeds, the dc drive provides better power/torque conversion, typically from 10:1 to more than 100:1 speed ranges, so there is no need to 'oversize' the motor to achieve useable low-end torque.
A four-quadrant dc drive is more energy efficient when dealing with torque at start and near zero speeds. It develops full rated torque at, or near zero speed in exactly the same way as throughout the entire speed range. This is because the torque is generated by the linear interaction of the two magnetic fields of the armature winding and the field winding. The commutator ensures that the axes of these magnetic fields are constantly kept perpendicular to each other, thereby in the optimum torque producing position. The resultant torque is practically a linear function of the armature and field currents.
The four-quadrant dc drive can also set the required current levels in each winding independently to meet a certain load requirement without the need for complicated algorithms. This is because the interaction between the two is practically zero. In contrast, an ac motor needs special cooling arrangements to allow full torque to be developed near zero speed. And as there is no need for intermediate energy storage (a dc link-fed inverter requires an electrolytic capacitor bank and choke), the four-quadrant dc drive is also far more compact than an equivalent ac inverter.
Sprint Electric's 340XRi, 680XRi and 1220XRi DC drives are designed to regenerate energy back into the mains supply under braking without the need for complex intermediate storage, resistive dumping or additional power bridges. Using little panel space, and mounting on a standard D