Direct current motors are still alive and kicking.

People have been predicting the demise of the direct-current motor for over 20 years. However direct-current 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 Presented as being outdated, direct-current machines have little by little been replaced by alternating current motors (also known as three-phase asynchronous motors).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 re-examine the usual practices and reasoning concerning electric motors as it is well known that variable speed can trigger real energy savings.

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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…

They are outdated. 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 current machines.

False: If you compare the [direct-current motor and drive] solution with the [alternating-current motor and drive] solution, the alternating-current machines are in reality more expensive to purchase.

Their technology is not very well-known today.

True: The maintenance of direct-current machines is often outsourced which does not make it easy for the teams using them to learn about their technology. Moreover, technical and scientific education today puts more emphasis on the operation of alternating-current motors. However, direct-current machines have no secrets for manufacturers and maintenance companies. The latter can train and support the users of motors who so wish.

They lead to high maintenance costs. False: Direct-current motors do have so-called “wear parts” (brush-commutator system) which require regular maintenance, but the total maintenance costs over the entire lifetime of a direct-current machine (motor and drive) are equivalent to the cost of replacing one single alternating-current machine drive (on average, every 7 years), since the drives used in alternating-current machines are expensive electronic parts.Moreover, while replacing a brush is a simple operation for the maintenance operators, which can be done quickly and easily, an electronic fault means a specialist must be systematically called out, with no control over the time this will take or the cost of the repair.

It would seem that the disadvantages attributed to direct-current machines are not all justified or true. Let us take a closer look at their characteristics to avoid allowing electrical machines which are still running perfectly to die out.

The advantages attributed to direct-current machines are rarely mentioned whereas the disadvantages which are always heaped on them are neither justified nor true if we are careful to consider a period of use corresponding to their average lifetime of approximately 15 years.

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Characteristics of direct-current motors

Main applications of direct-current technologyDirect-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 technology also permit to obtain high reliability for high-power applications”, adds Jean-Marc Gobert, Ansaldo direct-current motors sales manager for France at EMG-EMCO.

Robustness• Direct-current motors are built from robust materials which guarantee a long life. Direct-current

machines which are over 50 years old can be found on certain sites. This means that the initial cost of purchase of the motor is quickly covered.

• Direct-current motors are extremely resistant and can give optimum operation whatever the production environment may be (heat, humidity, dust, etc.). They are historically common in the most demanding industries (metallurgy, etc.).

Design• The design of direct-current motors allows them to operate in degraded mode in case of

commutation or electronic defect. This type of operation is a real guarantee of safety to ensure production continuity, as the machine will continue to run, even if a problem occurs, up to 50% of its yield in some cases.

• Direct-current motors, with their simple design, are extremely reliable. They are used in particular for demanding applications which cannot tolerate the slightest reliability failure, such as lifting motors.

Maintenance• The behaviour of the direct-current motor is easy to observe. Failures in these motors are easily

identified due to the repercussions on the brush-commutator system, which are visible to the naked eye. No particular knowledge of electronics is required for this. For this reason, maintenance

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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 direct-current 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 direct-current machine is easy.

Operation Direct-current machines are easy to control.Control and accuracy of torqueFlexible speedDirect relationship between voltage and speed Direct relationship between current and torque Large overload/overtorque capacity whilst maintaining the same level of accuracy Recovery of electrical energy during braking (inherent to direct-current motor technology)

Size• When confronted with a problem of space rationalisation, direct-current motors have many

advantages. For equal power, the size of a direct-current motor (i.e. the size of the motor and drive) is less than the size of an alternating current motor (motor and drive).

• The reduced size of direct-current motors also has a positive impact on the overall cost.

Costs• Simulations of the overall cost of usage of a direct-current machine over 15 years (taking into

account all of the parameters, such as the cost of purchase, maintenance costs, costs linked to operating losses if the machines are immobilised, the life-expectancy of the machine, etc.) demonstrate that direct-current machines (motor and drive) cost less than alternating-current machines (motor and drive). “The direct-current drive is only made of the “redresseur” element of the alternating-current drive which is in addition composed of an “onduleur”, much more complex and expensive” confirms Mr Jacques Urban, drive experts at Gefran.

- What is the brush in the direct-current motor used for? The brush is a component made of graphite which transmits the electrical current between a fixed part and a moving part in a motor or generator. The brushes are held in a brush-holder to ensure they are permanently in contact with the commutator. Thanks to the properties of carbon, the brush lays a thin carbon layer on the commutator bars during transmission of the electric current, a process which makes it possible to assess how the motor works. Although it is one hundred years old, the brush is a “living” solution in more than one way. New brush grades are still being developed today in order to optimise the performance of direct-current motors and adapt to new applications. « Il existe aujourd’hui plus d’une centaine de nuances de balais différentes, chacune est élaborée et optimisée

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kW Hp Min Max Min Max Min Max Motor Drive Total Min Max

Alternating current

8 000 12 000 14 400 21 000 22 400 33 000 16 000 20 000 36 000 58 400 69 000

Direct current

10 000 15 000 7 050 14 100 17 050 29 100 6 600 0 6 600 23 650 35 700

Global price over 15 years

(€)

NB: for powers of 150-375kW, we have reused the study assumptions which appeared in Mesures 802: 250 kW variable speed drive, 4 quadrant variable speed drive, 5,000 hours of use per year. Alternating current: 350 kW motor, declassification being obligatory due to heat.

150-375 200-500

WINDING

Examples of applications Paper mills

Power

Technology

PRICEMotor

purchase price (€)

Drive purchase price (€)

Total purchase price (€)

Cost of maintenance over 15 years (5000

h/years) (€)

pour une application spécifique, qu’il s’agisse d’un train, d’une éolienne, ou encore d’un turbo-générateur » 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 lesquelles nos équipes travaillent actuellement lui laissent augurer d’excellentes perspectives! » ajoute-t-il avec enthousiasme.

– Where can direct-current motors be found?

Industry- Extruders

- Rubber mixers - Winders (paper mills) - Rolling mills - Printing machines - Slitting machines - Winder machines (Steel plants, paper plants) - Wire drawing machines - Cement works machines Transport - Railway traction motors - Ski-liftsHandling - Various (test benches) - Lifting (gantry cranes, cranes) - Lifts (Eiffel Tower)

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– What parameters should be considered when choosing a new motor?

Available budget (motor and drive)Application in questionAvailable surfaceOperating 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 accuratelyStarting current Need for overload capacity

Conclusion:

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 direct-current motor is ideal for many industrial, transport and handling applications. It is particularly suited to high-power applications using variable speed and operating in extreme environments due to its robustness, its degraded operating mode and its reliability. The direct-current motor is easy to handle and simple to use. Its behaviour is easy to observe, allowing preventive maintenance to be scheduled. It is well-known to motor repairers who can intervene quickly, thus allowing limited down-time of the machine. It is also a good solution to modern problems of space, thanks to its reduced size. Finally, its cost of purchase is quickly offset by its long life-expectancy and it proves to be cheaper than alternating-current machines if we consider the overall cost of use over several years. The direct-current motor has definite advantages. It is advisable to think about the qualities and inconveniences of both types of technology before opting for one or the other.

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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 machines – Constitution and Operation / Engineer's techniques (François Bernot, Doctor in engineering sciences, Engineer of the École Supérieure d’Électricité, University lecturer at the UTBM (Belfort).• Machines à courant continu - Constitution et fonctionnement / Techniques de l’ingénieur - Direct-Current machines – Construction / Techniques for engineers (François Bernot, Doctor in engineering sciences, Engineer of the École Supérieure d’Électricité, University lecturer at the UTBM (Belfort).• Moteurs asynchrones - Choix et problèmes connexes / Techniques de l’ingénieur -

Asynchronous motors – Choices and related problems / Techniques for engineers, 10 June 1996 (Maxime Dessoude)• Standard Handbook for Electrical Engineers – 15th edition - Section 20: Motors and Drives (Donald Fink & H. Beaty, MacGraw-Hill)• Cahier Technique Schneider Electric n°207 – Les moteurs électriques…pour mieux les piloter et les protéger - Schneider Electric Technical Book No.207 – Electric motors… to better control and protect them (E. Gaucheron)

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