Bench Testing Conclusions

In order to draw any real conclusions on the relative performance between the motor controller under Six-Step control and FOC, it is useful to plot the models as formulated in the previous sections. Figure 5.5 shows the three line of best fit models on one plot.

90

Figure 5.5 - Graphical representation of the linear line of best fit models for power input under Six-Step Control and FOC, and power generated as a function of motor speed.

We can see that there is a slight difference in the amount of power input required to drive the load between Six-Step Control and FOC. This is more predominant at low speeds.

Next we compare the overall system efficiency of the motor-generator setup. This includes all the power losses in the driving motor and motor controller as well as the generating motor and motor controller. Total system efficiency for the driven motor under Six-Step Control is calculated using equations (5.10) and (5.11) below.

ߟ_͸ܵݐ݁݌ൌ ܲܩ݁݊ ܲ_͸ܵݐ݁݌ (5.10) ߟ_{଺ௌ௧௘௣}ൌ െͺͲǤͷͻʹ ൅ ͳͶǤͲͻͶͲݒ௪ ͻͷǤͻͻ ൅ ͳ͸ǤͲʹͻ͸ݒ_௪ (5.11)

Total system efficiency for the driven motor under FOC is calculated using equations (5.12) and (5.13) below. ߟ_ܨܱܥ ൌ ܲܩ݁݊ ܲ_ܨܱܥ (5.12) ߟ_ிை஼ൌ െͺͲǤͷͻʹ ൅ ͳͶǤͲͻͶͲݒ௪ ͹ͻǤʹͻ ൅ ͳ͸Ǥ͵͸Ͳͳݒ_௪ (5.13)

Plotting these two equations yields Figure 5.6.

70 60 50 40 30 20 10 1200 1000 800 600 400 200 0 Speed [km/h] Po w er [W ] Power in [W] 6Step Power in [W] FOC Power out [W] Variable

91

Figure 5.6 - Graphical representation of the bench test setup total system efficiency for the driven motor under Six-Step Control and FOC.

From this we can see that the motor driven under FOC has a greater efficiency than that of Six-Step Control at low speeds. However at higher speeds, the motor driven under Six-Step Control appears to have an advantage in efficiency for these implementations of Six-Step Control and FOC.

The maximum efficiency achieved during this testing of 0.73. If we make the assumption that the generator motor and motor controller have approximately the same efficiency as the driving motor and motor controller (all components of the drive side and generator side are identical), then we can say that the maximum efficiency of the drive motor and motor controller seen in this testing is around 0.865.

Also worth noting is that the overall system efficiency for both Six-Step Control and FOC has a positive slope at the point where the tests ended. This indicates that the maximum efficiency point was not reached. To find this maximum efficiency point, the tests need to be run to a higher speed which would be possible using a higher voltage. However given the maximum rating of the motor controllers is 40V, it was decided not to pursue higher voltage testing for the risk of blowing the motor controllers due to voltage spikes caused by the power supply when its load is decreased suddenly.

The final figure in this analysis shows the difference in efficiency between the two control techniques, Δηtotal given by equations (5.14) and (5.15) below.

߂ߟ ݐ݋ݐ݈ܽ ൌ ߟ͸ܵݐ݁݌െ ߟܨܱܥ (5.14) ߂ߟ_ݐ݋ݐ݈ܽൌ െͺͲǤͷͻʹ ൅ ͳͶǤͲͻͶͲݒݓ ͻͷǤͻͻ ൅ ͳ͸ǤͲʹͻ͸ݒ_ݓ െ െͺͲǤͷͻʹ ൅ ͳͶǤͲͻͶͲݒ_ݓ ͹ͻǤʹͻ ൅ ͳ͸Ǥ͵͸Ͳͳݒ_ݓ (5.15)

Plotting this equation yields the difference in total system efficiency for Six-Step Control and FOC against speed as shown in Figure 5.7.

92

Figure 5.7 - Plot showing the difference in total system efficiency for Six-Step Control and FOC as a function of motor speed.

Interpretation of this plot gives a maximum difference in efficiency of -0.0143 at 13km/h meaning that the Six-Step Control implementation achieved an efficiency that is 1.43% lower than that of the FOC implementation at a speed of 13km/h. However as speed increases, the difference in efficiency becomes less until it reaches the point where the efficiency of Six-Step Control is the same as the FOC implementation at a speed of 50.6km/h. Beyond that speed, the efficiency of the motor driven under Six-Step Control appears to be greater than that of FOC.

It is suspected that the PMSM driven under FOC is able to offer greater efficiency than when driven under Six-Step Control for the full motor speed range. However the FOC firmware and motor controller hardware would need further development to realise this. Reasons for the bench testing showing the efficiency for the FOC implementation only being greater than that of Six-Step Control for low motor speeds include:

x Electrical noise on current measurement signals causing inaccurate current readings. This noise becomes greater with increasing current.

x Delays in current readings due to analogue signal filtering both in hardware and software causing the vector control algorithm to drift from the optimal output vector. The drift becomes greater with increased motor speed.

From this bench testing, we can conclude that the motor driven under this implementation of FOC showed increased efficiency over that of Six-Step Control for motor speeds below 50km/h in the application of the electric skateboard hub motor.

Additionally, it was observed that the drive motor produced significantly less audible noise when run under FOC than that of Six-Step Control.