Appendix
Motor Parameter Measurement - Calibration Load Specifications
The board calibration process analyses the hardware to measure any deviation from its design values of board parameters. This will help improve the accuracy of motor parameter measurements. The board calibration step of motor parameter measurement is optional. However, we recommend that, at the least, it is run once every time there is a change in the development board.
The board calibration process requires three equal-value resistors to be connected to the three-phase inverter output as shown in Figure 1.
Figure 1
The equation in Figure 2 provides the recommended range of calibration resistance values based on the parameter of the board that is being calibrated:
Figure 2
In this context, Ifullscale and VBUS are defined as follows:
- Ifullscale — full-scale current for the board
- VBUS — voltage applied to the inverter's three-phase bridge
During the calibration process, the specialized firmware applies a series of voltage pulses on the calibration resistors and measures the resulting current. The pulse train has the following properties:
- Amplitude of VBUS
- Pulse train duration of 2 ms with each pulse lasting 50 μs
- Duty cycle of 46%
Since the pulses applied have an amplitude of VBUS, the calibration resistors must be voltage-rated to handle at least half of VBUS. The pulse train also imposes resistor size requirements due to the pulses' energy that needs to be dissipated with a nominal temperature rise to prevent resistance changes. Since the voltages for low and high-voltage applications vary vastly, the resistor values vary as seen below. Factoring all the above requirements we have the following values of resistors for the boards supported:
- dsPICDEM™ MCLV-2 Development Board - 10 Ω and 21 millijoules
- dsPICDEM™ MCHV-2 Development Board and dsPICDEM™ MCHV-3 Development Board - 50 Ω and 450 millijoules
To ensure accurate calibration, use resistors with:
- 1% tolerance specification or better
- Low-temperature coefficient