Regenerative DC drives boost energy efficiency

Edited by the Engineeringtalk editorial team Apr 28, 2006

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.

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.

This is an important feature in these days of greater attention to energy conservation.

Four quadrant regenerative DC drives offer an extremely energy efficient system using their ability to return the braking energy to the mains thereby lowering demand from the incoming AC supply.

If these drives are operated on equal driving/braking cycles, the cost of running the drive is only the electrical losses in the motor and drive.

By comparison, an AC drive generally uses a braking resistor to control down ramps dissipating the energy as heat 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.

A 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 other energy efficient improvements.

In comparison lower speed motoring losses are not as significant as those in AC motors.

The DC drive package has better power conversion efficiency across a wider speed range.

At the lower motor speeds, the DC drive package provides better power/torque conversion, typically from 10:1 to in excess of 100:1 speed ranges, eliminating the 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.

The DC drive 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 two DC armature and field currents.

The heat dissipation in the windings at a given torque will be constant at any (including zero) speed, and therefore the cooling arrangements can be dealt with in a standard fashion.

The DC motor is usually of an open-frame through-vent construction.

This suits the purpose of its applications, and allows advantages to be made in terms of ease of cooling, and therefore compactness, ie a smaller frame size for a given power rating.

Conversely, the AC motor is normally of totally enclosed fan-cooled construction, which places a further burden on the cooling arrangements.

The four quadrant DC drive can also independently set the required current levels in each winding to meet a certain load requirement without the need for complicated algorithms since 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.

Due to the fact that there is no need for intermediate energy storage (as per DC link-fed inverter), ie no electrolytic capacitor bank and choke, the four quadrant DC drive is also far more compact than the equivalent AC inverter.

Sprint Electric's 340XRi, 680XRi and 1220XRi DC drives offer a solution to four quadrant regenerative drive applications.

Using little panel space and mounting on standard DIN rail, these drives are designed for use with motors up to 1.8kW.

With four quadrant operation, the torque can be reversed very gently or almost instantly, as required.

Although compact in size, the 340XRi, 680XRi and 1220 XRi offer all the system functionality required for four quadrant regenerative drive applications.