Systems that have controlled parameters and closed-loop feedback mechanisms are generally more robust and less susceptible to failure. For example, vehicle engines regularly use temperature sensors and tachometers to ensure that the operating conditions stay within the designed scope. If temperature and engine speed weren’t controlled or even monitored, the design would need to be significantly more robust and costly to be able to withstand the worst-imaginable scenarios. Constraints allow designs to be more efficient.
With brushed DC motors, a prime example of a constraint that often is not capitalized is the maximum allowed current. In many systems today, maximum current is unbounded, limited only by the small DC resistance of the motor plus the RDS(on) of MOSFETs. Then fault-protection schemes are the only line of defense for preventing component damage. As a result, power-delivery stages are often overdesigned, temperatures can reach high levels, and predicting corner-case behavior can be challenging.
When motors are spinning, a back electromotive force (back EMF) develops on the winding. Directly proportional to the RPM, back EMF counteracts the externally applied voltage across the motor terminals. Steady-state current through a brushed DC motor equals the applied voltage minus the back EMF, divided by the resistance of the winding (Equation 1):
When a motor is prevented from turning (stalls) while being electrically driven, there is no back EMF, and the current will reach the full applied voltage divided by the resistance. This happens if the load torque is greater than the motor’s stall torque, or if there’s simply a jam that stops movement.
The other situation that involves much higher current than normal operating levels is when a motor begins to spin up. Initially the back EMF is zero, and the current rises as quickly as the motor inductance allows. When the current peaks, the motor will be moving and some back EMF will be present, so the peak will