ACor DC motor comparison
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Direct current motors are still alive and kicking. People have been predicting the demise of the direct-current motor for over 20 years. However directcurrent motors are far from admitting defeat and their advantages, although rarely highlighted, are undeniable. Designed to transform electrical energy into mechanical energy, electric motors are divided into two categories: direct-current motors on the one hand and alternating-current motors on the other. In addition, there are two different type of applications, constant speed or variable speed. The direct-current motor is of simple design, and consists of a brush-commutator assembly allowing electric current to be transferred. When it comes to variable speed applications, the direct-current motor is associated with a variable speed drive controlling the speed at which the motor runs. It is powered by direct current. The alternating-current motor, powered by sinusoidal current, consists of a motor and a sophisticated electronic drive. Since the middle of the 1980s, the fields of automation and power electronics have made great progress resulting in the enhancement of sophisticated technologies and the devaluation of simpler technologies. Presented as being outdated, direct-current machines have little by little been replaced by alternating current motors (also known as three-phase asynchronous motors). Today, alternating current motors represent over 96% of the world’s electric motors, most of which operate at fixed speed with no variable speed drive. However, given the new tendencies aiming at improving energy efficiency, it seems necessary to reexamine the usual practices and reasoning concerning electric motors as it is well known that variable speed can trigger real energy savings.

In an industrial context where the alternating-current motor is often preferred, it seems important to have another look at the criticisms commonly levelled at direct-current motors. Are they true or false? Defects attributed to direct-current machines… What should be pointed out… False: Although it is true that the technology used in direct-current machines is simple, it is also recognised that it has proved itself. The operation of the brush-commutator system is simple to grasp and operates easily with limited maintenance. Their cost of purchase is much higher than that of alternating...

Characteristics of direct-current motors Main applications of direct-current technology Direct-current technology is particularly well-suited to high-power applications (above 250KW) using variable speed as it is able to adapt to different operating speeds. “The wide regulation adaptability of Direct-current motor makes it a perfect solution for applications requiring accurate speed precision and high torques at low speeds, large overloads, fast accelerations or operation at constant power” specifies Livio Ghersini, direct-current sales manager at Anslado Sistemi Industriali S.p.A. “This...

operations can be scheduled in advance according to the behaviour of the motor and are therefore less costly than breakdowns and unscheduled production hold-ups. • If a problem arises, the repairers and maintenance technicians can intervene quickly on directcurrent machines. Their close proximity to the brush manufacturers and their stocks of spare parts and components means less down-time for the machine. For this reason, obtaining help for a directcurrent machine is easy. Operation Direct-current machines are easy to control. Control and accuracy of torque Flexible speed Direct...

pour une application spécifique, qu’il s’agisse d’un train, d’une éolienne, ou encore d’un turbogénérateur » confirme Laurent Cebuslki, Directeur R&D chez Mersen, pour l’activité Applications Electriques. « Le balai est aujourd’hui encore l’objet de recherches faisant appel à des compétences pointues en chimie, matériaux, génie des procédés, électrotechnique, tribologie, afin de trouver la meilleure alchimie parmi une multitude de paramètres. Il peut s’inscrire dans un cycle de fabrication pouvant durer jusqu'à trois mois! Le balai a incontestablement de l’avenir, et les innovations sur...

What parameters should be considered when choosing a new motor? Available budget (motor and drive) Application in question Available surface Operating environment of the machine Required torque Required nominal power Required nominal service Required voltage Degree of protection of the enclosures Required type of speed (variable or constant) Required range of speed Need to adjust the speed accurately Starting current Need for overload capacity We have been too hasty in casting aside the direct-current motor in favour of the alternating-current motor for all applications. In fact the...

Sources: Des moteurs électriques plus efficaces – More efficient electric motors (Mesures 822, February 2010) The strengths of the direct current motor: flexibility, reliability, energy saving (AsDE – Power Distribution Association, Medium Small Power Motors Group) Les variateurs et moteurs continu n’ont pas dit leur dernier mot – Direct-current motors and drives have not said their last word (Mesure 802, February 2008) Official Journal of the European Union, regulation No. 640/2009 • Machines à courant continu - Constitution et fonctionnement / Techniques de l’ingénieur Direct-Current...