Direct-Drive Scooter Motors

The controller was originally designed for the B.W.D Scooter, a prototype electric kick scooter with custom hub motors in both wheels. The scooter, shown in Figure 71, was built as part of the Edgerton Center Summer Engineering Workshop in 2009. The two 500W motors, introduced as a case study in Section 3.2.1, have trapezoidal back EMF and adjustable-timing Hall effect sensors, so they are designed for BLDC commutation. This controller attempts to execute sinusoidal commutation with field-oriented control on these motor simultaneously.

Figure 71: The B.W.D. Scooter, test vehicle for this controller, as two integrated 500W hub motors in its wheels.

The two motors tested differ only in the number of turns per phase of their windings. The front motor has 60 turns per phase, while the rear motor has 90 turns per phase. This gives the rear motor a higher torque, but lower top speed. In all other ways they are identical. The test power supply is a 33V, 4.4Ah lithium iron phosphate battery pack.

Before testing the field-oriented control scheme, a baseline operating point was established for comparison. The baseline used is sinusoidal commutation with no opportunity for phase advance (fixed timing). In this case, q-axis current is measured and used to maintain torque control. This baseline control scheme is depicted in Figure 72, a slight modification of Figure 68.

Figure 72: A baseline controller with q-axis control only. The phase advance angle is always zero, i.e. the coil timing is fixed to the Hall effect sensors.

In this baseline test the d-axis controller is eliminated and the phase advance angle is fixed at zero. This means the commutation, though still sinusoidal, is fixed to the Hall effect sensors. The default timing is set by rotating the Hall effect sensors at no load until the motor is spinning at it slowest stable operating speed. As in Figure 16, current lag is expected when the motor is loaded at speed. Figure 73 shows the results of this baseline test.

Figure 73: Results of the baseline test with q-axis control only. As expected, some current lags behind onto the d-axis.

As predicted, current begins to lag behind voltage and onto the d-axis. While the q-axis controller still does its job, maintaining the proper q-axis current for the requested torque, the magnitude of the total current vector is increased due to the d-axis component. The total current magnitude is what determines dissipation in the winding resistance, so the presence of d-axis current yields more dissipation for the same amount of torque, or conversely less torque for the same amount of dissipation. In other words, the motor efficiency is lower.

In this baseline test, there is a region of constant q-axis current (17A) from 49 to 51 seconds.

During this time, the speed increases from 220 to 500rpm (280rpm increase). The d-axis current increases from 4A to 7A and the total current magnitude increases from 17.5A to 18.4A. As speed increases further, the ratio of d-axis to q-axis current increases. This ratio is the tangent of the angle by which current lags the q-axis.

Now for comparison, the same motor and load are controlled using the modified synchronous current regulator. The result is shown in Figure 74.

Figure 74: The same motor and load as the baseline, now controlled with the modified synchronous current regulator.

Immediately, the difference is clear. With the d-axis controller running, d-axis current is held near zero during the entire course of acceleration. This is accomplished by advancing the phase of the voltage, as in Figure 17, to accommodate for current lag. The exact amount of phase advance is controlled in real time to keep the d-axis current at zero. In this case, it varies from 0º to 13º electrical. This slight difference, equivalent to moving the sensors by less than 2º

mechanical, has a large impact on motor efficiency.

Though the torque command is different, there is a region of fairly constant q-axis current in this test as well between 22 and 24 seconds. The average current is about 15A in this window. The speed increases from 300rpm to 540rpm (an increase of 240rpm) during this period of time. The acceleration difference is proportional to the difference in q-axis current (15A vs. 17A).

However, in this case there is no d-axis current. As a result, the magnitude of the total current vector is the same as the q-axis current. Only toque-producing current contributes to dissipation.

This is clearly a more efficient operating point.

The controller can execute field-oriented control on both motors simultaneously. It does not contribute much new information to the theoretical discussion, since the motors are independent, but it demonstrates the computational efficiency of the control algorithm. Figure 75 is offered as a simple confirmation that the control works the same way with the two scooter motors running simultaneously.

Figure 75: Field-oriented control of two motors simultaneously. d-axis current stays near zero at all speeds.

As expected, d-axis current is held near zero over the entire range of tested speeds and current loads. Voltage phase is not plotted in this case, but it also varies dynamically to compensate for current lag. At the same speed, the rear scooter motor requires more phase advance, since it has a higher inductance.