5 advantages of a direct-drive motor

October 14 2016

TM4 offers its SUMO direct-drive powertrains for the bus and commercial vehicle markets. They represent the best compromise between efficiency and simplicity for our customers looking to electrify their commercial vehicle platforms. But why did TM4 come up with this design choice, you  may ask?

When TM4 first started to develop motor and inverter systems for the commercial vehicle market, our original idea, as a company mainly involved in automotive projects, was to design a high RPM and high power density electric motor that would be used with a gearbox, ideally a multispeed one.

We planned to use this motor for both the commercial vehicle and the high performance passenger car markets. The problem was there was no suitable gearbox commercially available on the market for us to use with that high torque and high RPM motor.  Sure, we found some gearboxes, but they were and still are at the prototype stage and very expensive.

After making short and long term analysis of industry and technology trends, we determined that a direct-drive motor was the best type of motor for medium and heavy duty buses and trucks.

Here are a 5 key advantages that this design offers:

1. Efficiency

The transmission can be responsible for approximately 5 to 10 % energy losses inside the powertrain due to mechanical friction. Considering that the battery packs remains the most expensive and important component inside an electric vehicle, making the most of the energy carried by the vehicle is critical.

Single and multi-speed gearboxes can, in some situation, be part of the solution when it comes to increasing efficiency of a powertrain. This is especially right for traditional gasoline cars, but it is also true for electric powertrains using AC induction motors. While the peak efficiency of these motors can be as high as permanent magnet motors, the high efficiency can only be attained at a certain operating sweet spot. Outside of the latter, efficiency falls fast.

TM4’s permanent magnet and reluctance assisted AC motor not only have high peak efficiency, but maintain a >94% efficiency over the majority of the operating range, which surpasses any potential efficiency gain that could be achieved with a gearbox.

2. Installation

The bus and trucks OEMs we visited and interviewed during our design phase did not want to make major changes to their vehicles or, for that matter, their production lines, simply for the purpose of incorporating a new powertrain. Fitting as much battery as possible is already quite a challenge.

For this reason they wanted the motor and power electronic components to fit between the H beam of the truck or in the engine bay of the city bus, ideally, keeping the same type of driveshaft arrangements. Knowing all the space taken by the diesel engine and the automatic transmission typically found in a bus, this can easily be achieved with a direct-drive motor.

Direct-drive in-wheel or hub motors, on the other end, often require a special axle or the customization of other vehicle parts in order to be properly integrated, which means that the added price often offsets any potential benefits. Direct-Drive single motor is simply easier to install and integrate with the existing bus and truck axles available today.

 Typical truck or high floor bus chassis.



One way to maximise reliability is to minimize the amount of parts within your drive system. This is what we aimed to do with our direct-drive motors. Removing the gearbox means that you can remove out of the equation the most mechanically complex and maintenance intensive part of an electric vehicle.

With a direct-drive motor connected to the axle, the only mechanical wear you have in your powertrain is the driveshaft bearing and the axle itself. Both are designed to last the lifespan of the vehicle. Our direct-drive motors, if used in the conditions prescribed by TM4, will last 1 million kilometers without any maintenance being required.

4. Maintenance

Speaking about maintenance, there are of course obvious costs associated with it. For a vehicle operator, it means expenses related to the training of support personnel, spare parts storage and vehicle downtime. While this will remain a reality even with electric vehicles, we need to keep in mind that one of the selling arguments in favor of EV’s is that they’re expected to reduce all of the expenses incurred for vehicle maintenance.

Maintenance is also about being able to access, diagnose, and replace a product as easily as possible if there is a problem. A motor system integrated with multiple other components such as a gearbox assembly or inside a wheel might be hard to access on a standalone basis and ultimately, failure of one of the component might prevent you to use the motor altogether. Such scenarios can make maintenance more complex or costly. TM4 direct-drive motors require no maintenance.

 5. ROI

Considering all of the items mentioned above, everything comes down to maximising the ROI of the vehicle owner or fleet operator. We want to provide the best torque density a customer can get for the money, both in term of the initial investment and long term expenses, and also maximise the system efficiency, which can also be translated in cost savings. We’ve found that keeping a simple powertrain architecture based on direct-drive motors offers the best compromise in that regard.

TM4 has not only validated this hypothesis theoretically during the SUMO product line design, but the millions of on-road kilometers achieved by buses and trucks using these products since their launch in 2012 also tend to confirm this proposition. Of course, to benefit from all of this, the products should be industrialised in order to be readily available and affordable. This is why TM4 has heavily invested into its Canadian and Chinese development and production facilities in order to bring down the price of its electric powertrain technologies and ensure that it can cover the growing worldwide demand for these products.


In conclusion, while TM4 is not the only company providing direct-drive motor to the bus and truck market, the torque density achieved by our systems is unequaled on the market. This is thanks to the many innovative technologies found inside our motors and power electronics.  To learn more about our products or if you have any questions, please visit our products and technology sections or contact us.

Olivier Bernatchez
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    Leave a Reply

    1. I have found the post on direct drive very informative! I only recently came upon it, and wish I’d seen it sooner. It does a great job of pulling different topics/disciplines/perspectives — something unique and valuable here. Thanks — hope you keep it up!

    2. Olivier Bernatchez
      good evening Tecnicar SRL is an Italian company that builds electric vehicles, which would like information for the conversion of a thermal bus fleet to be converted into electric and it is vanholl A309 and mercedes-benz citaro 520, I would like to receive information and data sheets of your engines and electronics for buses of about 12m, email address tecnicar@hotmail.it

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