Choosing a Controller for Hub and Mid-Drive Motors such as QS138

Mid-drive motors like the QS138 are a popular choice for 125cc-class conversions. Matching a controller to them follows the same logic as any PMSM.

Start from the motor data. Get the motor's rated and peak phase current, pole pairs, back-EMF/KV and feedback type from the motor supplier. Do not rely on marketing "kW" figures.

Match phase current to your torque target. A typical 125cc-class conversion runs at about 8–9 kW, with roughly 120–150 A of battery (bus) current and no more than about 350–450 A of phase current. Compare that with the T-Series ratings:

• T-400 (≤150 A peak DC, ≤400 A peak phase, 4.5 kW rated / 12 kW peak): matches the current envelope, suitable when 8–9 kW is needed in bursts rather than continuously.
• T-500 (≤250 A peak DC, ≤500 A peak phase, 8.5 kW rated / 18 kW peak): the better choice when 8–9 kW must be sustained (long climbs, payload, hot climates), with phase and bus current limits set in the tuning tool to the motor's and battery's own limits.

Always confirm against the motor's peak current limit and the battery's discharge rating.

Match the voltage. Most such builds use 72 V packs; T-150 to T-1000 cover 48–72 V platforms.

Check feedback. Most QS motors come with Hall sensors; some versions offer encoders. T-Series accepts Hall ABC or GMR ABZ on every model.

Let the controller learn the motor. T-Series runs motor parameter identification (Ld, Lq, flux linkage, resistance) in about a minute, so you don't need to type in parameters from scratch.

Cooling. If the controller sits in a closed frame with little airflow, choose the liquid-cooled T-500 (T-400 is air-cooled only).

Motor data in hand? Send it to us and we will confirm the right model.

Note: QS is a trademark of its owner; Vector is not affiliated with it.