Motoring Along: Understanding the role of motors in electric trucks
Posted: September 18, 2018 by Jim Park
A row of Sumo motors from TM4 await shipping. They may be similar in appearance, but could have different ratings that increase the motor’s length but not the diameter.
TORONTO, Ont. — Do you know who makes the motor in your washing machine? Do you care? Do you know who will make the motor on your first electric truck? Will you need to care? That’s just one of the many paradigm shifts we’ll be dealing with as electric drivetrains emerge in the trucking industry.
North American fleets have always had vehicle spec’ing choices. From engines and transmissions to wheel seals and cab lighting options, OEM data books list dozens of options for even a single component. Will we have the same options when spec’ing electric powertrains? Probably not, and we probably won’t need them. Motors will be sized for the application, with certain engineering spec’s such as rpm, input voltage, and output torque and horsepower already considered. The OEM will just install the motor from a supplier that meets the need.
“You’re talking about an industry where for 50 years truck buyers could pick any component they wanted,” says Rick Mihelic, director of future technologies studies at the North American Council for Freight Efficiency, and a former vehicle development engineer at Peterbilt. “To not get their way with the truck builder will be a major change in behavior for North American fleets. In Europe or China, you get the truck that the maker gives you. It will take some getting used to here.”
It’s likely OEMs will have a portfolio of options based on the application — stop and go, straight on-highway, length of haul, terrain and its inherent regenerative charging abilities – and the output to get the job done. Fleets will care what goes onto their trucks once there’s some field history on components. Right now, it’s crystal ball territory.
Differentiators will include batteries and power management systems, probably not the motors, Mihelic believes. “It will be different for everybody, because just like the ECM on a Cummins engine, probably there’s something completely different than a Paccar MX engine, and a Detroit Diesel engine,” he says. “They’ve all tuned and optimized the performance toward what they think is important to their customers. And it’s not the same thing on every engine. So, I bet the battery trucks will be the same way.”
A technician prepares to insert a winding into a motor stator. Induction motors have only one moving part and no surface-to-surface contact to cause wear.
Right-sizing the motor
At a basic level, an induction motor (or brushless motor) contains two main parts: a stator, which is stationary and contains the wire windings that carry alternating current through the motor; and a rotor, which is basically the driveshaft. The rotor also has a series of wire windings and magnets that respond to the magnetic field produced in the stator when a current is applied to those windings. In the simplest of terms, this causes the rotor to turn. Other types of electric motor have contact points called brushes that induce the current. The induction motor has no friction points except the rotor bearings.
Electric motor manufacturers have proprietary rotor and stator designs that each company believes handles a specific task most efficiently. This would be like two diesel engine manufacturers each using different-shaped piston bowls and different injection rate-shaping strategies to achieve their goals. The principles are the same, the execution is a little different. That’s one ingredient in the secret sauce.
The physical shape and size of the windings (the all-important secret sauce) also affects efficiency and power output. The output of the motor can be tailored to an application by increasing the number of windings, and thus the “length” of the motor. Motor design diameters tend not to change within a series because the shape and length of each leg of the stator winding play a role in the design’s efficiency. This would be somewhat like comparing one manufacturer’s 13-liter engine to its 15-liter engine. The basic design doesn’t change much, but it’s sized for a particular application.
“To make an electric motor more powerful or less powerful you have to change the thickness, [some might be inclined to refer to this dimension as the length] of the motor,” says Patrice Dupont, business development manager at TM4, Dana’s now in-house motor provider and business partner. “To get more torque out of an electric motor you need to be able to put more current through it. More current means more windings. For example, a 90-kilowatt motor that might generates 1,000 Newton meters of torque might be 100 millimeters thick [long]. A 400-kilowatt motor that can generate — I’m just throwing out numbers here — 2,500 Newton meters [Nm], might be 355 millimeters thick [long]. So, we will modify the internal dimensions of the motor, but the diameter doesn’t change, only the length in this case.”
To get the right motor for the application, an OEM will define certain parameters like gross vehicle weight, desired gradeability, and top road speed, while the motor manufacturer will offer a motor or line of motors that meet the criteria.
In TM4’s case, its Sumo MD product line for medium-duty vehicles includes nine models with output ranging from 162 kw and 1,590 Nm at 3,250 rpm, to 265 kW and 2,760 Nm at 3,500 rpm.
“The OEM defines what they need, and we make sure that the right motor is selected for the end user or the platform,” Dupont says.
You might see parallels with diesel engines here. In the case of a diesel, a 13-liter engine would come in a variety of ratings the customer can select, but with electric motors it’s more of an engineering decision. You can’t vary the output of an individual motor, but you might be able to spec’ a different motor if the OEM will allow it.
Size and speed
Electric motors, like diesel engine, have sweet spots of efficiency. Because diesel engines have very narrow sweet spots for torque and fuel efficiency, transmissions are used to match engine speed with road speed. Electric motors have much broader sweet spots, but there are still practical limits.
Dupont says electric motor makers, knowing certain things about the intended application, can configure the motor so that the efficiency zone matches the duty cycle.
“Using a city transit bus as an example, if we know the average speed of that bus is 16 kilometers per hour and the motor is turning at 1,800 rpm at that speed, we can move the power map of the motor towards 1,800 for best efficiency without using any gear reduction. But it’s a design change at the engineering level. You can’t do on the fly,” Dupont says.
Similarly, in a medium-duty package delivery van that runs at typical road speeds in an urban environment, the motor can be engineered to efficiently operate within that range and without any gear reduction – as long as the vehicle doesn’t wildly exceed that speed and doesn’t stay at elevated speeds for prolonged periods.
Applications such as the transit bus and the package van can get away without the need for a transmission because the vehicle speed range and the engine sweet spot are fairly well aligned if the motor has been properly designed for the application.
With a vehicle that operates at higher speeds most of the time, like highway trucks, gear reduction will be needed to maintain the motor’s efficiency and torque output. And there’s another advantage to using gear reduction in high-speed applications. It allows for smaller, lighter and more electrically efficient motors.
“The amount of current going through the smaller motors required to generate the torque is much higher,” says Dupont. “Therefore, the power electronic package will grow in size and cost. The trend today, whether it’s commercial or passenger, is to go towards higher-rpm motors.”
When we speak of gear reduction, we are talking about two or three gears — not the 10 or 13 like a diesel needs. In many cases those gear sets can be integrated into the axle or the shaft side of the motor, so there’s a big gain in packaging efficiency as well.
And when it comes to higher-rpm motors, the range is probably 5,000-6,000 rpm with final drive reductions of somewhere between 5:1 and 10:1. That’s actually fairly slow on the electric motor speed spectrum, but there are certain mechanical limits to transmission input speeds that will likely force designers of electric powertrains to constrain output speeds. ” That will be one of the limiting factors in diminishing the size of the motor because, as I said earlier, the faster you spin, the smaller the motor can be,” Dupont adds.
The other wild card in an electric powertrain is the battery. Engineers will be working very hard to optimize the charge and discharge cycles of the battery, and the motors obviously have a large role to play there.
“How you pull energy turns into a big impact [on] the life of the battery,” Mihelic says. “Everybody may be doing that a little bit differently. You know, one vehicle may get super acceleration and another guy will say, ‘Trucks don’t need motorcycle-like acceleration.’ They may favor their batteries more and tune down that acceleration so it’s a more reasonable energy draw. Those are the sort of choices that I think motor and truck makers be making to differentiate their products, because it will all come down to battery range.”
Today, the diesel engine could be seen as the heart of the powertrain, with other components like transmissions and rear axles complementing what the engine does. In an electric system, the motor is part of a bigger puzzle with several other components factoring into the motor choice, like the power electronics, batteries, and cooling needs.
My washing machine motor has been purring away down in the basement now for almost 20 years. I have no idea who made it, what color it is, or how many kilowatts it needs. And that suits me just fine. Maybe electric truck motors will be the same.
Motor weight could be a concern with heavy-duty installations due to the un-sprung mass at the front of the axle. The design objective is to make the motors as small and light as possible.
Motor output is usually referred to in kilowatts (kW) rather than horsepower. Torque output is often referred to in Newton-meters (Nm) rather than foot-pounds.
One horsepower is equal to 0.745699872 kilowatt.
For example, 350 kw = 470 hp
One Newton-meter is equal to 0.7375 foot-pounds.
For example 1,000 Nm = 737 lb-ft