What determines the level of current required by a series-wound DC motor at various RPMs?

The level of current required by a series-wound DC motor at various RPMs is primarily influenced by the load connected to the motor. In a series-wound motor, the field windings are connected in series with the armature, which means that the current flowing through the armature also flows through the field windings. As the load increases, the motor has to work harder to maintain or increase RPM, which leads to an increase in armature current to produce the necessary torque. This relationship creates a situation where the current draw is directly proportional to the load: heavier loads require more current to maintain speed, while lighter loads allow for less current.

In contrast, the voltage supplied to the motor can impact the overall speed and performance, but it does not primarily dictate the current drawn in relation to RPM. The motor's design and construction also play a role in defining its characteristics and performance, but they do not dynamically determine current based on changing load conditions like the load itself does. The frequency of the AC supply is irrelevant to a DC motor, as it operates on direct current and is not influenced by frequency factors that apply to AC motors. Therefore, the load connected to the motor is indeed the key factor affecting the current draw at various operational speeds.