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Grasping brushed DC motor current

23 Nov 2015  | Ross Eisenbeis

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Systems that have controlled parameters and closed-loop feedback mechanisms are typically 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 defence for preventing component damage. As a result, power-delivery stages are often overdesigned, temperatures can reach high levels, and predicting corner-case behaviour can be challenging.

Unbounded current
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 be lower than the stall current.

The waveforms in figures 1 and 2 show measured current during spin-up, runtime and stall using a Maxon Motor RE 30 310007 brushed DC motor.

Figure 1: Motor spin-up current.

Figure 2: Motor stall current.

With 24 V applied, the motor consumed 29 A during spin-up (700 W) for about 1 ms, while the operational current was 2 A (48 W continuous). When the rotor was held and prevented from moving, stall current was 34 A. This motor requires that continuous current be kept under 3.5 A to prevent overheating and damage.

Whenever high amounts of current are involved, two primary consequences come to mind: supply-voltage drop and heating.

Supply-voltage drop
Quick demands of current require capacitance to source the energy and maintain a stable voltage. A power supply by itself has limited capacitance on the output, and interconnects to the motor system will have inductance that limits response time. For these reasons, local bypass capacitance is needed.

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