The featured image for this article is a line drawing of a drive system from a Hamburg Metro Car, an SKF DT4. [5] … AC traction motors (commonly induction motors) are the standard for modern trams, replacing older DC motors to provide higher efficiency, better reliability, and reduced maintenance. These motors, often running at 60–200 kW, power the bogies and enable regenerative braking to feed energy back into the overhead line. They are controlled by variable-frequency inverters for smooth acceleration.

The Modern Tramway of February 1951 carried an article by ‘Eltee’ entitled ‘Traction Motor Trends’ about the recent changes in electric motors in trams. [1: p33-34]

“The present trend towards the use of lightweight high-speed electric motors for traction purposes, a trend exemplified at its best by the motors used in in the P.C.C. cars in America, and those of similar design now being introduced at Blackpool and and Glasgow and on the Continent, justifiably prompts the query as to why motors were not, in the past, built as they are today. There are actually several reasons for this, some highly technical, but one of the more important is that the need for efficient ventilation of motors was not sufficiently appreciated in the early days.

“When a motor is running and current is passing through its conductors these conductors are heated by the passage of current, just as are the conductors of an electric stove though much less so. The power represented by this heat is lost to the motor, and called the ‘copper loss’. Another source of loss is the rotation of the armature in the motor magnetic field; the alternating magnetism through the armature caused by its rotation brings about power losses in armature iron, which also appear as heat. In running, then, the motor gets heated, and if there were no means of dissipating this heat the motor would get hotter and hotter until something melted.

“In practice this does not happen, as the motor casing is in contact with the air around it, and when the casing is hot it loses heat to this air, doing so all the more readily when the car it is driving is moving and there is a certain amount of draught. Many years ago this was the only way of cooling the tramway motor, hence a large motor had to be used simply to ensure that there was enough casing area to dissipate the heat generated. Some additional armature cooling was given by the provision of axial ducts in the armature, aided by a few radial ducts. In this way some slight fanning action was given by the moving armature, swirling the air in the motor casing and conveying the heat from the armature more readily to the outside casing for dissipation into the atmosphere.

“A later development introduced what is now known as ‘series ventilation’, the self-ventilated motor being introduced about 1910. In such a motor a fan is mounted on the non-commutator end of the armature, and two sets of openings are made in the same end of the motor casing. The fan draws air through the armature axial ducts when the motor is running and expels it through one of the casing openings; this assists to keep the armature cool. This air, in the first place, is drawn in through the other set of openings and over the field coils before turning round and entering the armature ducts; in this way the field also is kept cool, but the ventilation of the armature suffers because the air is already somewhat warmed by its passage over the field coils.

“A further development, common from about 1920 onwards, is known as ‘parallel ventilation’, in which there are two parallel air streams through the motor. A twin fan is fitted to the non-commutator end of the armature, and openings made in both ends of the motor casing. The fan draws a stream of cool air over the commutator, round the armature surface and over the field coils before expelling it. The other half of the fan draws an air stream under the commutator and through the armature axial ducts, thus keeping the interior of the armature cool and dissipating most of the iron losses effectively.

“With a motor as efficiently ventilated as this it is possible to ‘force’ the motor more without its getting too hot; that is, in more technical language, a motor can have a higher rated power. Consideration of the above method of ventilation readily shows that if the motor armature rotates more quickly the attached fan will draw more air through it, ventilate it even more thoroughly, and permit even more ‘forcing’ by the passage of greater currents. This, in essence, explains the present trend towards motors of high rotational speed; the efficient ventilation possible on such motors permits more power to be passed through them than through motors of similar size with less effective ventilation.

“The above being understood, two further points are worthy of emphasis. One is that the greatest losses occur in a motor when it is starting and running slowly; the best ventilation occurs when it is running quickly. Cars on a town route will thus need bigger motors than similar cars on an interurban route on which there is a lot of free running, providing their maximum speeds are equal. The second point is that, if motors have been used on a service on which their capabilities are being fully employed the gear ratio must not be altered, because, although the speed of the cars can thereby be improved, such a measure will not only increase the currents passed through the motor (for more power will be required from the motor) but will also decrease the average speed of rotation of the motor. resulting increased “losses” and impaired ventilation will both tend to raise the operating temperature of the motor. and so reduce its life, unless it was known that hitherto it had been used well below its capacity and was operating at relatively low temperatures.” [1: p33-34]

Since ‘Eltee’ was writing at the beginning of the 1950s, much has changed!

Improvements in the ventilation of tramcar electric motors since 1950 have centred on a move away from traditional forced-air cooling in direct current (DC) motors to advanced, sealed, and integrated systems used with modern AC traction, enhancing reliability and reducing maintenance. [2][3]

Modern three-phase AC motors allow for lighter, more compact, and more powerful motors. These motors are often less sensitive to heat and easier to cool than older designs. [2][3]

Modern tram design integrates motors directly into the bogies, with ventilation systems designed as part of the overall low-floor, compact carriage architecture, ensuring better cooling airflow in restricted spaces. [3]

Many modern motors are now completely enclosed, utilizing improved heat sinking and specialized cooling fan designs rather than drawing in outside air, reducing the impact of dust and water on electrical components. [2][3]

The use of GTO-inverters and modern power electronics reduces motor heat generation compared to older resistor-controlled DC motors, reducing the load on ventilation systems. [2][3]

Improved insulation materials allow motors to operate safely at higher temperatures, reducing the strain on the cooling systems and improving longevity. [2][3]

Modern electric trams utilize motors to generate electricity during braking, returning power to the grid or charging on-board batteries/supercapacitors. The use of battery-power and on-board storage can allow trams to pass through city centres or other sensitive areas without overhead wires. [3][4]

Electric motors are ideal for rapid urban transport because their higher torque at low speeds allows speedy departures from stops on a network. It also allows tramcars to handle hilly terrain better than internal combustion engines.

Electric motors are roughly 90% efficient at converting energy into motion. In contrast, diesel engines lose about 60-70% of fuel energy as heat. [4]

Additionally, unlike internal combustion engined vehicles that consume fuel while stopped, electric trams use virtually no power when stationary. [4]

AC motors have been shown to improve reliability and decrease downtime compared to traditional DC motors. But they have significantly lower maintenance needs than internal combustion engines, having far fewer moving parts and not needing oil changes, spark plugs, filters, and complex exhaust systems. The high torque of electric motors at low speeds eliminates the need for heavy, expensive multi-stage gearboxes common in internal combustion engined vehicles. [4]

Trams typically have a service life of about 30 years, roughly double that of diesel-powered buses. They typically produce no local pollutants like nitrogen oxides or particulates, which is critical for city air quality and meeting climate targets. Electric propulsion is significantly quieter than internal combustion engines, reducing noise pollution in densely populated areas. In addition, electricity can be generated from various sources, including renewable energy (wind, solar, hydro), making the system future-proof as the power grid decarbonizes. [4]

Increasingly in an urban environment public transport is heading underground. Because they emit no exhaust fumes, electric trams can safely operate in tunnels and underground stations where diesel engines cannot.

Internal flexibility is increased as the need for bulky and heavy engines and fuel tanks is eliminated. The net gain is a more friendly user experience, faster loading and unloading at stops and increased passenger capacity. [4]

It is not surprising that many cities around the UK, and across the world, are seeking to reintroduce trams and to increase the size of their networks.

References

1. ‘Eltee’; Traction Motor Trends; in The Modern Tramway Volume 14 No. 158; The Tramway and Light Railway League, February 1951, p33-34.
2. https://en.wikipedia.org/wiki/Electric_locomotive, accessed on 12th May 2026.
3. https://en.wikipedia.org/wiki/History_of_trams, accessed on 12th May 2026.
4. https://medium.com/@blaisekelly/why-trams-are-cheaper-than-buses-6d929192624a, accessed on 12th May 2026.
5. https://evolution.skf.com/new-drive-systems-for-mass-transit, accessed on 12th May 2026.