Rugged Line Current Capability Tests

In line with the launch of our Rugged line, we want to publish test results showing their current handling capability.
These tests are intended to give an impression of how much current the different controllers can handle (in typical applications like vehicle traction drives) before thermal throttling occurs.

Test Setup

The tests were conducted using the VESC foc_openloop function. This function induces a fixed current into the motor at a fixed ERPM, regardless of rotor position. As a result, the current mostly does not generate torque, but instead produces a significant amount of heat in the motor.

For this reason, we chose one of the largest motors in our shop for the tests: the QS-Motors QS 273-V4. Whether the current generates torque or heat is not relevant for the controller itself, since the electrical stress caused by the current is the same.

The ambient conditions were a nice German summer day at roughly 20°C ambient temperature.

All tests were conducted under natural convection, with no forced airflow over the heatsinks. This means the tests serve as a benchmark for how long the controllers can sustain a given current as a peak load. Continuous current capability is strongly dependent on airflow. Those results will be published in a separate post.

Results
M-Rugged 88V
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Open PDF: M-Rugged-88_450A_500ERPM.pdf

L-Rugged 88V
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Open PDF: L-Rugged-88_700A_500ERPM_run1.pdf

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Open PDF: L-Rugged-88_700A_500ERPM_run2.pdf

L-Rugged 117V
Tests were done with 500ERPM and 1000ERPM to showcase there is no significant difference.
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Open PDF: L-Rugged-117_700A_1000ERPM.pdf

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Open PDF: L-Rugged-117_700A_500ERPM_run2.pdf

XL-Rugged 88V
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Open PDF: XL-Rugged-88_1000A_500ERPM_run1.pdf

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Open PDF: XL-Rugged-88_1000A_500ERPM_run2.pdf

XL-Rugged 117V
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Open PDF: XL-Rugged-117_1000A_500ERPM_run1.pdf

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Open PDF: XL-Rugged-117_1000A_500ERPM_run2.pdf

Remarks
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Remarks on Loss Composition

Scientifically speaking, the total losses of the converter can be categorised into conduction losses and switching losses.

Conduction losses depend on the RMS current seen by the switches (and all conductors in the converter). They can be described using Ohm’s law:

P_cond = I² × R

These losses depend on the output phase current, assuming that the contribution from current ripple is negligible.

Switching losses occur every time one of the power electronic switches turns on or off. This happens at roughly three times the zero-vector frequency:

f_sw = f_zv / 2
f_sw_total = 6 × f_sw

The amount of energy dissipated during each switching event is related to the output voltage, and therefore to the motor RPM.

As a first-order approximation, this means that switching losses depend on motor RPM, while conduction losses do not. Since conduction losses usually dominate in inverter applications, it is reasonable to assume that the total losses primarily depend on output current. This is why we did not test at multiple RPMs.

Remark on the XL

As shown in the test data, both the XL-Rugged 88V and 117V tests were cut short by motor temperature. In a real application, this would usually not occur, since the current in the motor would be converted into torque rather than heat. This means that the controller could provide full current significantly longer than shown in othe tests.
Even so, the data speaks for itself: the controllers can sustain over 1000A for a significant amount of time.

Conclusion

These current tests are not intended to define exact current ratings for our controllers. Instead, they are meant to give an impression of how capable the controllers are under high-current conditions.

The results are especially representative of vehicle applications, where vehicles usually reach top speed within a few seconds, after which the current draw is reduced significantly. The controllers therefore offer very strong thermal performance for vehicle applications, as they can typically sustain peak current throughout the entire acceleration phase.