Right Sizing Electric Drive Systems

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Performance Comparison

How “right-sized” mid-voltage can outperform high voltage traction systems for EV bus & truck

Date: June 20, 2018
Motor Weight (kg) Transmission System Weight (kg) Continuous Torque (Nm) Continuous Power (kW) Motor Voltage
340 Direct Drive 340 1852 281 750Vdc
90 2-Speed 205 3036 150 360Vdc

Summary

This study evaluates the impact that a multiple speed transmission can have on greatly improved driving performance of a commercial EV. In order to highlight this significant impact on vehicle performance, the two motors that are selected for this study intentionally have very different rated power outputs. The 340kg direct drive motor has almost twice the rated power of the smaller traction motor used in this analysis. The much smaller 90kg motor is combined with an IEdrives 2-speed in this analysis. Both motors have been in production for a number of years, and can be found in commercial trucks and buses around the world.

Another intentional aspect of this comparison is the selection of motors with a dramatic difference in their operating voltage. The large direct drive traction motor must be operated on at least 600Vdc to get any useful performance. It should be noted that the direct drive motor loses 55kW of power, dropping to 225kW continuous rating if the system voltage is only 600V. Peak performance for the direct drive motor requires 750Vdc. In order to maximize the performance of the direct drive motor, this study uses the 750V performance mapping for the direct drive motor.

Discussion on System Voltage

Historically, the dominant electric drive system used over the past 20 years for non-grid connected commercial electric trucks and buses has been the very heavy, very large, direct drive traction motor. These motors range from 300kg to 550kg in mass, and produce peak torque from 2000 Nm to mid-4000 Nm. In order to achieve these large torque outputs, the motor must consume very large quantities of electrons, converting the additional electrons into a stronger magnetic field. Unfortunately, high current flow in turn generates high internal heating due to the resistance in the wire windings. Elevated temperature within the winding begins the deterioration and eventual failure of the winding insulation. To reduce the current induced heating required at say 350Vdc, the amount of current needs to be reduced. The easiest way of reducing current is to increase voltage. Using this relationship, vehicle voltage has been driven ever higher by the needs of survivability of direct drive motors and is now commonly in the 700+Vdc range.

While the reduced current flow resulting from a higher system voltage is helpful in reducing the breakdown of the insulation for all motors, it does add considerable cost to the rest of the vehicle’s balance of plant components. There are more than twice as many battery cells compared to a 350V system. As well, each one of those cells needs a BMS connection and cooling. The high voltage battery pack itself will be larger, and therefore heavier when using generally available cells. Battery cells can be designed specifically for high voltage in order to mitigate some of the growth in the battery pack.

Another issue with high voltage packs is charging. An electrical grid connected charging system now has to invert 220 3-phase, or maybe 480Vac, and step that up to 750+Vdc instead of a nearly similar voltage of 350+Vdc. There are also all of the 3-phase VFD drives for the power steering pump motor, the air compressor motor, the AC compressor motor that cannot take 750Vdc directly, therefore requiring a significant kW capacity in a dc to dc step down buck. Another point of concern should be about the durability for all of the switching relays, circuitry and IGBTs that now have to handle a 750V in-rush versus a significantly milder 350V.

Study Parameters

This study compares the performance of a hypothetical 18 ton city bus using a direct drive, electric motor against a much smaller electric traction motor combined with an IEdrives 2-speed transmission. The direct drive traction motor has a rated power of 281 kW and rated torque of 1852 Nm. This motor has been exclusively used as a direct drive motor. The smaller motor is a 150 kW continuous rated motor combined with an IEdrives 2-speed transmission.

When combined with the IEdrives 2-speed, the small motor system has over 3000 Nm of continuous torque output rating, more than 50% greater than the direct drive motor.

All of the vehicle parameters are held constant and identical for both drive systems. These assumptions, and the grade changes, are listed in the top right corner of each graph below.

It is the practice of IEdrives to studiously avoid the use of motor torque that is greater than the Continuous rating for any motor. The continuous torque line is labeled Motor Cont Nm in the graphs below. The Peak torque line, labeled Motor Peak Nm, and the region between it and the continuous rating, should never be considered for any vehicle “must accomplish” performance point calculation. IEdrives considers vehicle operation above the continuous motor rating to be at best, unreliable.

Top Speed Performance - 0% Grade

Top vehicle speed is purely a function of available motive power. The direct drive motor has 130kW more of rated power at 281kW than the small motor at 150kW. As such, it is not surprising that the direct drive motor can drive this 18 ton vehicle to a higher top speed of 120 km/h compared to the 90km/h capability of the small motor. What should be considered, at least for city buses, is that very few bus operators outside of the USA want their drivers pushing their bus to 120km/h. China has mandated a national maximum speed of 70 km/h for city buses. India is either 70km/h or 80km/h maximum speed depending upon the city. If the top maximum vehicle speed were only 80km/h, the direct drive would never spin faster than 2700 rpm in this example. Even the high torque direct drive would benefit greatly by adding a reduction gearbox that could convert the unused rpm above 2700 into useful rpm. A reducer around 1.3:1 would have the bus reach 80 km/h around the 3500 motor rpm mark. This additional reduction would also prove to be highly valuable for increasing the gradeability of this drive.

2% Gradeability

At a 2% grade, the extra power of the direct drive motor continues to be able to out speed the small motor/2-speed drive system. However, in the bus markets of China or India, the small motor drive has been able to maintain the bus’s maximum speed of 70 km/h up to a 2% grade. The additional speed capability of the direct drive heavy motor would only generate a speeding fine for those above speeds above 70km/h in China. While this additional speed of the heavy motor is not useful in China or India, all of the cost of the higher voltage system components, warrantee costs, and greater weight of battery and motor would always be present on this vehicle.

10% Gradeability

As the graphs on this page show, there is a dramatic difference in these two drives as the grade is increased from 2% to 10%. The direct drive powered bus experiences a 95km/h speed loss as it approaches a 10% grade. The sensation to a passenger on that vehicle would be one of very rapid speed loss, and most likely, a great concern as to whether the vehicle will be able to climb the entire hill. In fact, a 10% grade is the maximum slope that the direct drive powered bus would be able to reliably climb.

In comparison, the small motor/2-speed drive system loses only 43km/h, dropping down to approximately 25km/h. The small motor/2-speed drive system would be able to easily start from rest on a 10% grade and quickly accelerate to 20 km/h. It is important to note that all of these speed points are well below the rated torque for the small motor. As such, the small motor powered vehicle would be highly reliable on this grade, even when subjected to very hot ambient temperatures.

15% Maximum Reliable Gradeability

The positive impact of an electric vehicle having more than 1 gear becomes very evident when we look at the maximum reliable grade that a small motor with almost 1/2 less power but has the help of a 2-speed. This small motor/IEdrives 2-speed combination allows this vehicle to start from rest and then to modestly accelerate to 5-10 km/h on a 15% grade. If it were necessary, IEdrives can increase the reduction of 1st gear and provide this vehicle with the ability to start on 20% grades or steeper. What is important to note as well, is that the 2-speed bus would be able to start and maintain this speed on a continuous basis. In comparison, the direct drive would not be capable of climbing a 15% grade, certainly if it has just climbed a 5% and then a 10% getting to the 15% portion. The vehicle would need to be parked before the 15% grade and then allowed to fully cool down the motor and controller. This would make peak torque was once again available for its rated 30 seconds……subject to a 25C ambient temperature. It would also be very important that the 15% hill was short in length or have a driver who is good at backing down hills.

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