A geotechnical project needed a precision soil core sampler: regulated speed under changing load, and a higher maximum torque than the stock tool delivered. Rather than design a drill from scratch, we selected the best off-the-shelf model, documented its teardown step by step, and rebuilt its drive. The result made 50% more torque than stock, and we delivered ten prototype units with an assembly manual.
Getting there took three levers, and the order matters: heat, copper, then control.
Torque is current, and current makes heat
In a permanent-magnet motor under field-oriented control (FOC), the controller measures the phase currents and uses the Clarke and Park transforms to express them in a frame that rotates with the rotor. In that frame, the d-axis current (Id) controls the rotor’s magnetizing flux and the q-axis current (Iq) controls torque output. In steady state both are constant, so ordinary PI loops can regulate them (Microchip AN1078). The same application note contrasts this with traditional six-step commutation, which produces oscillations in the torque.
Two consequences follow. More torque means more Iq, so somewhere there is a ceiling on current. And the way you split current between Id and Iq decides how much torque each amp buys. On the drill, that ceiling was thermal.
Levers 1 and 2: heat, then copper
Heat. Relocating the speed controller to an external aluminum heatsink enclosure raised its current limit, and with it the motor’s torque limit. It is an easy lever to overlook, because it looks like packaging rather than engineering. No firmware change makes up for a controller that is running hot, and no gain schedule can command current the hardware cannot survive.
Copper. Higher current needs conductors that can carry it. We moved the harnesses to heavier-gauge wire, with modular connectors for higher current and easy reassembly, and modified the casing to fit the new parts.
Lever 3: control, with FOC and MTPA tuning
With the thermal ceiling raised, we tuned the controller’s FOC and maximum-torque-per-amp (MTPA) parameters, which raised torque 50% over stock, and reprogrammed the motor velocity curve for regulated speed.
MTPA is the rule that picks Id and Iq so a requested torque is produced with the least current. According to MathWorks’ documentation of its MTPA block, a surface-mount permanent-magnet motor reaches maximum torque with zero d-axis current below base speed. An interior-magnet motor also produces reluctance torque, so both current components are computed to keep current to the minimum the torque needs. Above base speed, field weakening adjusts the d-axis current to keep the required voltage within limits. Getting the motor type and its parameters right is what makes MTPA worth having.
Measure on a testbed, not by feel
We built a custom testbed to benchmark speed and torque, so every change was compared against the same baseline. The same habit shaped an electric pusher-trailer project: we fitted a 1:10 scale model SUV and a chassis dyno with a sensor suite to build an empirical model, ran FOC on a customizable electronic speed controller, and used a touchscreen workstation for parameter tuning, data logging and web dashboards. Tuning a motor without a way to load it and measure it is guessing with extra steps.
A short tuning checklist
| Step | Why it comes first |
|---|---|
| Identify the motor type: surface or interior magnets | It decides the MTPA strategy |
| Measure the motor’s parameters | Generic profiles give generic results |
| Find the thermal current limit on the bench, at your worst ambient | It sets the real torque ceiling |
| Tune the current loops before the speed loop | The speed loop is only as good as the current loop under it |
| Log speed, current and temperature for every change | So you can prove the gain against the stock baseline |
What to do this week
- Measure your baseline: peak torque and speed under load for the current design, on a rig you can repeat exactly.
- Find out what limits current today (controller temperature, battery, wiring or a firmware setting) before you touch a single gain.
- Confirm your motor type and parameters so your FOC and MTPA settings are built on the real motor, not the catalog page.
If you want a second pair of eyes on a motor-control problem, send us a brief.