back-emf is proportional(ish) to motor speed; current is proportional(ish) to motor torque; What I didn’t know was that the difference between applied voltage and back-emf is largely (entirely?) due ...

The current limit circuit (see diagram) compares this voltage to an adjustable negative voltage using an op-amp wired as a comparator – unlike many op-amps, LM324 op-amps can be used as comparators ...