As part of the development process for Ramjet, we have been testing a pair of brand new motors, as well as an old (and heavily abused) motor that was used in C-XeV – Driven’s 2012 competitor.
The aim of the testing was to calculate the comparative efficiency of the two motors, and develop an insight into how the motors perform at different speeds and powers (and see what a really bad motor looks like as well!).
During a Greenpower F24+ race, the motor is run using two 12V Lead Acid batteries wired in series to generate the required 24V. Bench testing has shown that an average current draw of ~22Amps or less is required to reach the end of a race without over discharging the batteries, or ‘falling off the cliff’. If the battery is discharged to the extent that the voltage begins to collapse, lap times at the end of a race can be severely compromised. This can be seen in C-XeVolution’s performance in the Greater London F24+ Heat, when compared to Project E. (504 compared to 711: http://bbk-online.net/gpt/lap213.htm)
Over the course of the race, Project E lost 6 seconds compared to its fastest lap, whilst C-XeVolution dropped nearly 40 seconds. In the race, C-XeVolution was consistently consuming an average of 24 Amps, whilst Project E was consuming 21 Amps.
On a race by race basis, current consumption is controlled by gear ratio. In order to most effectively select a gear ratio, a good understanding of the motor must be gained. Adding further complexity to the mix is the fact that the profile of each circuit varies so wildly. At Dunsfold Park, Project E’s current consumption stayed consistently between 18 Amps and 23 Amps, whilst at the National Final in gusty conditions, the current dropped as low as 15 amps on the main straight, and topped 35 Amps uphill. As such, simply investigating the motor performance at the intended average current would not be enough.
Test Procedure and Equipment
The electrical lab-car, built at the start of the year to develop and test Driven’s 2014 electrical system, was used to test the motors. The test motors were attached to another, spare motor, which acted as a generator powering a number of bulbs. The number of bulbs in the circuit, and thus the torque applied to the test motor, was controlled using a separate arduino board.
Using a current clamp and a voltmeter, we recorded the power into the drive motor, and out of the generator across a range of different output currents. We also used a tachometer to measure the motor’s shaft speed. It’s been assumed that at a given rpm, the generator’s efficiency will be same, independent of the motor being used to drive it. Thus by comparing the two new motors on a plot of efficiency against rpm, we can eliminate the efficiency of the generator from the equation.
Following the first test session, the motors were run in, unloaded and from a power supply, for 3 hours and 12V and 3 hours at 24V. Once the motors had been allowed to cool down after this, they were tested again. We decided to add some more load to the test rig for the second set of tests, to understand the efficiency at lower rpms/higher currents. These loads were not as stable in their power consumption as the bulbs however, and so there is less confidence in the results at a higher current.
Below are three plots. The first shows the motor/generator system efficiency against motor RPM, and the second shows efficiency plotted against current output from the generator, which is broadly analogous to load on the motor when racing. The final plot is a repeat of the first, but with C-XeV’s motor included.
Compared to the two new motors, C-XeV’s old motor ran at a significantly higher speed when unloaded. To achieve the same current output from the generator, it would require more current from the power supply than the new motors. Due to the higher speed of the generator, its voltage output was higher, masking somewhat the motor’s inefficiency. It is inarguable however, that it is less efficient at the speeds experienced during a Greenpower event that the two new motors.
When comparing the two new motors, there is a noticeable difference, with Motor 1 having an efficiency 5% or more greater than Motor 2. The margin increases towards the top end of the powers seen during a Greenpower F24+ event.
After the run in period, both motors showed a lower peak efficiency. The gap between the two motors at lower speeds decreased, and the peak efficiency for both motors appeared to be at a higher current. The second set of tests also showed the folly of allowing current to reach 30 amps or above. Efficiency quickly dropped off and the motor began to noticeably heat up.
Based on this testing, it appears that the motor’s peak efficiency lies between 13 and 16 Amps. It is also known, based on Peukert’s Law that a batteries effective capacity decreases the more quickly it is discharged (Have a look at wikipedia: http://en.wikipedia.org/wiki/Peukert’s_law, or Chipping Sodbury’s page: Greenpower Science).
To me, this suggests there may be an alternative race strategy, that involves running less aggressively, at peak motor efficiency and a low battery discharge rate, for a portion of the race, before moving to a low efficiency, high power mode for a blast to the line. Whether this strategy is quicker over the duration of a race would depend on the battery’s Peukert Constant, the change in motor efficiency at different currents, and the overall aerodynamic efficiency of the car.
Further work will involve further running in of the motors, before another testing session, and performing the same series of tests on the motors fitted to Project E and C-XeVolution (Driven’s last two cars) for the 2014 season for comparison. Both motors have been well run in, and neither has been significantly overheated, which will make for an interesting comparison both to the brand new motors, and the very much overheated motor from C-XeV.
A new work stream opened up by this testing is to investigate the possible effectiveness of a split strategy, running slightly slower at a super high efficiency, to allow a final blast towards the end of a race. In order to determine the worth of this strategy, a better understanding of the battery and motor performance will be needed.