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.