The direct current drive system offers various advantages, which produce significant installation and operational cost savings:
In addition to all above mentioned advantages the lower price of the system and of its installation and commissioning must be considered.
In the last 30 years, direct-current motors have been increasingly employed in almost all applications, and now practically any kind of operating machinery makes extensive use of this type of motor. Over this period, direct-current motors have spread their usage worldwide, mainly for their unique features, which make them the simplest, most reliable and economical system available on the market.
A choice based on a thorough application evaluation and not following a fashion often leads to the selection of the direct-current solution, particularly because of its lower cost and higher reliability. It is recognized that any economic evaluation between alternate solutions must consider both the Initial Investment Cost and the Life Cycle Cost, the last one including both maintenance and losses costs. The evaluation made on the investment payback time is significantly in favour of the direct-current solution for the majority of applications.
The direct-current motors are extremely easy to use and to control, due to the very simple relationship between voltage and speed which characterizes their operation. The technical progress and the user-friendly simplicity of the actual AC-DC static converters, very easy to program, contribute to strongly reduce commissioning time and to easily optimize the overall system performance on the basis of the actual operating conditions. The direct-current motors are so widely used and well-known that at present most maintenance technicians worldwide have the capability to install and commission them very easily and to provide prompt maintenance.
The electrical drive system consisting of a DC motor and an AC-DC static converter is very reliable in all operating conditions. Also, a carefully implemented maintenance programme allows regular motor operation with very high availability over the installation lifetime. It must also be noted that the AC-DC converters do not overstress the motor insulation with high voltage peaks and therefore significantly increase the motor lifetime potential. New technologie and the recent engineering progress in designing new generation motors allow for the development and construction of smaller machines but with improved characteristics and increased reliability in all their components, particularly the commutator-brush holding system.
One of the outstanding features of the direct current drive systems is the very simple speed regulation over a wide speed range, at both constant torque and constant power, so allowing significant savings in converter sizing. The wide regulation adaptability makes the directcurrent system highly preferable, when compared to alternate solutions, especially in those applications requiring accurate speed precision and high torques at low speeds (paper machines, printing machines, etc.), large overloads (extruders, mixers, etc.), fast accelerations (flying shears, cranes, etc.) or operation at constant power (coilers,etc.).
The laminated-frame direct-current motors feature a very high power-to-weight ratio and therefore, represent the best solution to fit in the operating machines, since they allow a significant space saving and the use of lighter structures. This characteristic is particularly useful in the applications involving moving structures. Moreover, the modular construction allows the motor to be installed in very limited spaces. Also, the converter system has smaller dimensions than an inverter of the same rating.
The direct-current motor is generally very quiet, throughout the whole regulation range, basically due to the voltage waveform coming from the DC converter, which does not make use of high commutation frequencies. For the same reasons, the direct-current solution has also a lower environmental impact due to the electromagnetic compatibility (European Directive EEC 89/336 and following Amendments) which greatly simplifies the procedures for CE marking even in case of very complex systems.