These are my options:

If used with a BLDC motor of lower power with lower maximum allowed current, what are the chances that such a controller can be easily tweaked to lower the max current limit? (e.g. replacing a resistor with a different value)

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    \$\begingroup\$ Look at the second one ... it's a pretty fair guess R20 is the current measurement shunt. If you can measure it (ha ha, it's just a wire) you can replace it with one scaled to meet your needs. Don't scale it down for increased current, of course... \$\endgroup\$ – Brian Drummond Dec 2 '17 at 23:53
  • \$\begingroup\$ First time I see a question seeking a chance, my guesstimate is 20%. \$\endgroup\$ – Harry Svensson Dec 2 '17 at 23:57
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    \$\begingroup\$ Limiting the current will only add heat to the controller with rising switched voltage drop. Chances are you will never be able to keep it cool enough and never get full acceleration. Did you know DC motors get up 8~10x rated current on full startup acceleration. Never underestimate your load effects, Look for a 30A controller. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 3 '17 at 1:42

There's a good chance - if you can find the current shunt and replace it with one of higher value. If the value is not known then you can measure it by passing a known current through the shunt (eg. 1A) and measuring the voltage drop across it.

Current shunts are usually made from an alloy with low temperature coefficient. Do not be tempted to just use a length of copper wire, as to get sufficient resistance it will either be too thin (resulting in high operating temperature and increased resistance) or have higher inductance which might upset the current measuring circuit.

Reducing the current limit (within reason) to protect a motor with lower current rating than the controller should have no serious side effects. If you go ridiculously low then FET switching currents may start to affect the measurement.

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