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Motors and Drives


Heat dissipation remains a major consideration for designers of motor-driven systems. Bill Bertram looks at the state-of-the-art and casts one eye into his crystal ball.


La dissipation de la chaleur reste une préoccupation majeure pour les


concepteurs des systèmes motorisés. Bill Bertram examine les technologies de pointe et consulte sa boule de cristal.


Die Wärmeableitung ist für Entwickler von Systemen mit Motorantrieb immer noch ein wichtiger Faktor. Bill Bertram informiert sich über den Stand der Technik und wirft einen Blick in die Kristallkugel.


Cool life lessons for electric motors


F


irst, let us look at the basics of motors and their thermal management. Heat is a byproduct of the conversion of


electrical input to mechanical output, as performed by a motor or other rotating electrical machine. In fact, it is part of the inefficiency, as is noise. Motor designs are steadily becoming more efficient, which means they are producing less heat – but still enough to require consideration by designers of motor-driven systems. The simplest form of heat dispersal


is through conduction into the surrounding air or structure of the driven machinery. To assist with this, a heat sink – usually a finned block of aluminium with a large surface area – can be affixed to the motor to absorb the heat and dissipate it quickly to atmosphere. A variation of this is found on many industrial motors, which have a finned cover over much of their body length.


Another common method for


keeping electric motors cool is forced draught air cooling. Industrial motors


often have an integral fan, mounted on the rear of the output shaft, so that it spins at the same rate as the driven load. This is protected by a perforated cowl, which also protects inquisitive fingers from the fan blades. This type of motor is often referred to as ‘totally enclosed fan cooled’ (TEFC). A variation on this, often used with servo motors, is an independently excited electric fan. A cooled motor can operate at higher load and is likely to have a longer working life than an uncooled motor. Forced ventilation is also used when the speed of the motor is controlled by a frequency inverter. At low speeds the fan borne by


motor’s shaft becomes inefficient so a force ventilation is needed to deliver the volume of the cooling air required to cool the motor down.


Liquid cooling


A liquid coolant has a greater capacity than ambient air to absorb heat from a motor. Water, glycol or other liquids can be used as coolant. With water there is the option to use an open loop cooling system, in which water, typically


Improving the efficiency of electrical conversion and new methods of thermal management may mean that in future motors run far cooler than they do today.


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