DC Drive

A DC drive is compact and cost effective, has extremely high starting torque and a speed range of 1:100 or more. It can run smoothly down to the lowest speeds and has a high overload capacity.
Varying the armature voltage is all that it needed to control the speed of a DC motor. The torque is proportional to the current and can be kept constant over the whole speed range. As it is simply a case of converting three-phase current to DC, the units are compact and easy to handle. The simple relationship between voltage and speed also makes the system easy to understand and operate.
Thanks to their high power density, DC motors are small in size with short acceleration and braking times. With four-quadrant operation, the torque can be reversed very gently or almost instantaneously, as required.
In these days of global warming and greater attention to energy conservation, perhaps it is the DC motors ability to run in reverse as a generator for rapid braking applications that is its best feature. It is completely natural for a DC drive and an example of how important it is to consider different engineering solutions.
The simple control performance is easily adaptable, resulting in the high level of flexibility needed for solving complex drive problems. The wide control range and low speed capability make the drive suited to a wide range of requirements. High dynamic response ensures rapid reaction and smooth running ensures low mechanical stress and long life.
The converters have low losses and high efficiency and the regenerative capabilities allow power to be returned to the supply during braking. DC drives, in most cases, do not require additional measures to reduce interference emissions or to increase resistance to interference.
DC drives do not require electrolytic capacitors, so there are no onboard components with a limited life expectancy. DC drives and motors are also established mature products, so they are slow to become obsolescent. Heat dissipation is low and there is no degradation of motor bearings due to high frequency switching. There are no IGBTs to fail due to thermal cycling and no problems with reflected waves.
Digital converters are user friendly and easy to operate. A keypad and display allow simple adjustment of parameters during operation and remote monitoring and diagnosis is possible via standard interfaces and a bus system. Developments of DC motors have also concentrated on improving the use of the available space and reducing maintenance.
The result is a co-ordinated system that uses the latest materials and allows a brush service life of more than 15,000 hours, similar to that of the bearings. High quality, modern insulation materials ensure a high level of availability for modern DC motors.
DC drives do not rely on mathematical models to enhance their performance and are inherently reliable due to a low number of power components. They do not require output chokes to protect the motor windings. Line commutated thyristor converters make up the core of the power section. They operate with a high level of efficiency and minimal switching losses. It is simple to feed power back into the supply when reversing the motor and the blocking voltage of the thyristor enables the converters to be used with high supply voltages.
A DC converter needs only 25% to 75% of the space of an equivalent AC drive, depending on the power. The DC motor has a high power density and small dimensions and these are often deciding factors when the drive itself has to move or space is cramped. For example, the travel mechanism of cranes, on oil rigs, ski lifts, printing machines, electric vehicles, extruders, marine equipment and many industrial shop floor applications.

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