Power Transmission


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 from the mains supply, is circulated around the motor to absorb heat, then discharged into a drain.


However, closed loop systems are also used and, indeed, must be used with non-water cooling. In these, the coolant is constantly recirculated through a heat exhanger to cool it before it is reused. The heat exchanger can be simply air cooled, with the heat dissipated to atmosphere, or the heat energy can be stored and/or transferred for use elsewhere.


There are other, less common, cooling systems, too. For instance, laboratory researchers developing super high- performance motors may find it necessary to use ultra-cold liquid nitrogen as the cooling agent. Elsewhere, subsea ROVs (remote operated vehicles) typically use very hardworking servomotors as propulsion drives, yet do not have an engineered cooling system; instead, exploiting the ocean depths as an ‘infinite heat sink’.


Cool and exotic


More exotic methods of motor cooling also exist and may become more common as motor uses and performance develop. For instance, in some high- performance applications heat generation


may be extreme and therefore require rapid removal. In such cases it may be possible to cool the windings directly by having enclosed coolant channels running along the stator slots and between the windings.


It is even possible to consider directly cooling the conductors by immersion in an electrically non-conductive coolant.


Suitable fluids for this include deionised water (poor electrical conductor) and transformer oils (which are specially formulated to be non-conductive). Other methods of cooling include spraying oil directly onto the end-turns of the conductors.


For industrial engineers, heat dissipation is often not a problem; motors either have enough free air flow or their standard cooling fan is more than adequate. However, consideration must be given to thermal management if a motor is to be used in:


l A hot climate; l In a confined space where natural air flow is likely to be restricted;


l If it is possible for the motor to come into contact with flammable materials or with users’ hands;


l If thermal expansion could become an issue.


Instead, machinery engineers should bear in mind that the heat generated by electrical motors is the result of electrical and mechanical losses, ie, inefficiencies. Therefore, if a motor is found to be running hot, it may be worth looking at the efficiency of the drive system.


Replacing an older motor with a modern high-efficiency unit may solve thermal issues as well as reducing power consumption. Alternatively freeing a sticking bearing may pay dividends. l


Bill Bertram is with motor maker Marathon Electric, Eibergen, The Netherlands. www.regalbeloit.eu


www.engineerlive.com 25


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29