Recommended Motor Assessment Techniques

White Paper

For Commercial EVs

June 10, 2018

 

Client: White Paper

 

Vehicle: 16 ton City Bus

 

Motor: UQM PP250+

 

Transmission: IEdrives EVT2-R 2-speed transmission (7.56, 2.16)

 

Summary: This document provides a description of the assessment techniques used by IEdrives in evaluating the potential performance of electric trucks and buses.

 

Traction motors are characterized and differentiated from one another through the use of many different single value parameters. They are described by their power, their torque, or their current consumption to name a few. What is critical to realize, is that for all of these values, they are only accurate at a few static points or at best, over small regions in which the motor operates. A vehicle in use is anything but static. It stops and then starts, it climbs hills, it enters onto highways, all the time changing its speed and acceleration. In order to quickly visualize all of these different vehicle performance points, IEdrives has found that plotting the motor rpm and torque at different speeds and under different road conditions onto the motor’s torque map to be very helpful. This approach is illustrated later in this document.

 

Before starting the commentary, a discussion of a motor’s peak torque versus its continuous torque, and just how useful either are to vehicle’s performance needs to occur first. By definition, ‘Peak’ torque is only available under certain circumstances. What is not generally understood is just how limited those circumstances actually are. ‘Continuous’ torque, by definition should always be available from the motor as long as the inlet coolant temperature requirements are met. IEdrives uses the definition of ‘Continuous’ to mean that the motor can maintain that torque and rpm output (power) indefinitely or until bearing maintenance is required.

The primary driver as to what peak torque might be, or how long it is available, or what continuous torque might be is the measured temperature of the motor and the motor controller at any given moment. In order to offer a useful length of life, the motor OEM must prevent the windings of their motor and the components of their controller from overheating. While it is possible for a 50kg motor to produce 1200Nm of torque, it will only do so for a very short time. Total life of such a motor would be measured in tens of hours versus some minimum number of years. To a much smaller extreme, this is the challenge of all traction motors.

 

Above a maximum temperature, the motor windings and/or the controller components start to be irreversible damaged due to the heat created by current flowing through them at high rates. As determined by each motor OEM, which is primarily driven by material properties that are common to all motors and controllers, they set an absolute maximum temperature for both components. When that temperature is exceeded, the control logic immediately de-rates the drive and reduces the current. As the temperature continues to rise or refuses to reduce, the control logic will further reduce the maximum allowed current. The ‘Continuous’ torque output curve is that line in which the ability to pull heat out of the motor and the controller matches the generation of thermal energy due to current flowing through the system, generating resistive heating. This means that if a motor or controller is already at the continuous rated internal temperature due to use, then there is zero additional torque available (more current flow allowed) above that continuous output. It has been IEdrives experience that the full Peak Torque output is generally available only 1 time each day, unless the vehicle experiences a sustained period of inactivity. Also, that moment of the full time allotment of peak torque from the motor is only the first few minutes when a “cold” motor/controller is turned on and used for the first time that day. For all other future instances after that, the amount of torque greater than the continuous rating is at best a small time percentage of the “full” time provided by the Motor OEM…..or not available at all due to the elevated motor and controller temperatures.

 

Small mass motors (<100kg) typically have 10-30 seconds of peak power availability with a cold motor. Larger mass motors (>200kg) are typically described by the OEM as having a significantly longer peak torque availability. This time span, for a stone cold motor, is anywhere from 60 to 120 seconds for that first high torque demand. This is the case because these large motors have a lot more “cold” thermal mass in which to bleed off thermal energy from the windings for later removal by the motor coolant. It is important to note that these larger motors also will take much longer to rid itself of this greater thermal energy, which means the next future torque output greater than the continuous rating is further away in time. There is no free lunch.

 

The danger of using Peak Torque in analysis for real roads:

An area in which this loss of peak torque has often presented itself is in climbing real hills, particularly steep hills or long shallower hills. On steep hills, say 15% grade as an example, the peak torque of a motor map provided by the OEM, may indicated that the motor could produce sufficient torque for that vehicle to climb a 15% grade by exceeding the motor’s continuous rating. However, for this to be even possible true, the motor needs to be “cold”, and the hill needs to be short enough! Here is why it is unlikely that the motor is “cold” and can produce torque in excess of its continuous rating.

Let’s use climbing that same 15% grade as an example. An aspect of climbing hills that is often overlooked in an engineering analysis is that most 15% grades have a section that is only 5%, and then 8%, and then 12% that precede the 15% portion. It is not possible for the motor to be fully “cold” when it gets to the 15% portion of the climb after it has pushed a vehicle up those preceding hills. This means that it is therefore not possible to have all of the peak torque potential to be actually available from the motor. The question for the driver is “How much is available?”, “Is it enough to get over this hill?” At best, the availability of torque in excess of the continuous rating is unreliable in these conditions. The vehicle may make it up the hill, and then again it may not. It is for this reason that in our drive assessments, IEdrives insists that for the “must meet” performance points of the vehicle, we only will state that a particular electric drive can accomplish this task if those required torque and speed points from the motor are below the motor’s continuous torque line.

Comment on Motor Continuous Ratings:

A majority of motor manufacturers around the world have come to use the strong definition of ‘Continuous’ described above. IEdrives has found that there are some motor OEM’s who put a time limit on their “continuous” power capability. That time limit is typically not noted on their motor performance graphs. An interested party should always ask the motor OEM what their definition of continuous is, it may be much less than the strong definition of “sometime next week is not a problem”. In reality, a “continuous” torque (power) output that has a time limit is at best a misrepresentation and overstatement of the motor’s actual capability. If that time limit is 5 minutes, it is a gross misrepresentation. A 30 minute time limit is a smaller degree of misrepresentation.

Peak Torque versus Continuous (Rated) Torque

The peak and continuous torque lines versus motor rpm are shown above. The red line is the Motor Continuous Torque often referred to as the “Rated Torque”, the blue line is the Motor Peak Torque.

While it is obvious from the graph above, it is important to remember that even the ‘rated’ or continuous torque value that is listed in a motor spec, is only true for a small region of the overall motor map. In this case, the rated torque is described by the OEM as 520 Nm, which is true for the first 1000 rpm. After the 1000 rpm mark, the torque drops off and reflects the constant power region for this motor of a nominal 180kW.

For the vehicle described above, all of its maximum speed points for level ground are below the continuous torque output line, even at the 90 km/h point. If 90km/h were a mandatory maximum speed on flat level ground, then the 2.16:1 2nd gear does a great job of optimizing the entire rpm range of the motor in order to deliver superior performance over the rest of the rpm range. Another way of stating this is that the 2nd gear selection optimizes the maximum gearing assist available to the entire motor’s rpm range. The selection of the 1st gear ratio will become more obvious in the hill climb section below.

Minimum speed on grade

One of the particular requirements for this bus was the ability to maintain 60 km/h on a 3 % grade. At 3.5% grade, the 60 km/h speed point lies right at the continuous torque line at approximately 3700 rpm. Depending upon the OEM, and how conservative they are with defining their motor’s continuous torque points, this vehicle may be able to reach 65 km/h on this grade. The last two speed points of 90 and 80 km/h are not plausible in the least. 80 km/h lies just below the Peak Torque line, but to get to that speed, the motor has to push the bus through 40, 60 and 70 km/h. Even if the bus was not accelerating at some maximum allowed current and was just holding a steady speed of 60 or 65 km/h, that motor is already providing its maximum rated output. That output is defined by a maximum allowed internal motor and controller temperature, which it would already be at that temperature.

Performance on mandatory maximum grades

Here is another illustration of a “must meet” performance point in the requirements for this bus. The requirement was to be able to climb a 20% grade, fully loaded to GVWR and do so at a minimum speed of 10 km/h. There was not a requirement to be able to start from rest on this grade. However, with this particular electric motor and 2-speed transmission, this bus would be able to start from rest on a 20% grade fully loaded….even on a hot day. As the text in the graphs indicates, this bus would then be able to quickly accelerate to around 15 km/h before the combination of thermal heating and the reducing vertical gap (difference) between potential peak torque and continuous torque narrows significantly.

It is only because this particular motor OEM has proven over the years to put forward solid, conservative continuous torque data, IEdrives is willing to suggest that the bus could reach 20 km/h. However, the grade would need to be a rather long hill to allow sufficient time for the bus to reach 20 km/h.

The acceleration rate from 0 to 10 km/h would be stunning compared to an identical diesel powered bus…..absolutely stunning. However, as the continuous torque available approaches the motor torque required line (green), the unused “extra” torque that would be available to power the acceleration energy is diminishing.

It is at these types of vehicle performance points that IEdrives does find value in the shape and location of the Peak Torque curve. From 0 km/h, the torque required to start the bus moving would be nearly constant as the rest the green, 1st gear line indicates, somewhere around 410 Nm. At 0.1 km/h, there is a minimum of 110 additional Nm of highly reliable continuous motor torque available to accelerate the bus to a higher speed. At 500 rpm (approximately 2.5 km/h), the potential peak torque of this motor grows dramatically to 900 Nm. Since the bus had to climb some level of grade to get to this 20% portion, we know that the full potential peak torque from 500 to 2000 rpm will not be there. However, because the motor only has had to operate below the continuous torque value, its internal temperature is also below the maximum allowed. As such, it is entirely reasonable, and correct, to assume that there will be at least some of that potential peak torque available for 2 to 4 seconds. At that time, the internal temperature will quickly climb to the maximum allowed and there will only be continuous torque available once again. However, in that time period, the bus is now going 15 km/h.

Maximum reliable gradeability

A 25% gradeability was not required by this particular bus OEM. However, this graph may also help illustrate that even with taking this analysis approach, there is still significant subjectivity to a vehicle performance study. Normally, IEdrives would only characterize that this motor would only be able to maintain existing speed, up to a maximum of 8-10 km/h upon entering a 25% grade. However, based upon multiple experiences with this motor OEM, IEdrives has found their continuous torque data to be repeatedly conservative. As such, IEdrives would be confident in stating that this bus could also start from rest on a 25% grade. Acceleration would be slow but steady to at least 5 to 8 km/h. On a very cold day, which would offer fantastic thermal rejection through a superior cooling system, this bus might even approach towards 15 km/h using this particular motor.

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