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. But until then, heat dissipation remains a major consideration for designers of motor-driven systems. Bill Bertram, of motor maker Marathon Electric, looks at the state of the art and casts one eye into his crystal ball.
First, 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.
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 exchanger 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:
• a hot climate,
• in a confined space where natural air flow is likely to be restricted,
• if it is possible for the motor to come into contact with flammable materials or with users’ hands,
• 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, i.e. 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. Another possibility is that it may be practical to recover the heat energy through a water jacket and use it elsewhere.
In fact, the cooling of rotating machines is codified in the standards IEC 34.6 and AS 1359.21, which provide guidelines on which cooling arrangements are likely to be appropriate in particular situations.
In an odd turn of events, automotive engineers may make the next big leap in electric motor development. Electric and hybrid vehicles are developing apace, and for them, dissipating heat from the electric motor is a major issue. The obvious solution is a radiator system, similar to those already found on cars, but the potential returns are such that it is worth exploring other options – and any new technology may transfer into industrial drives and other fields.