Which characteristic is typical for a series direct current motor under no-load conditions?

A series direct current motor under no-load conditions typically draws a small current due to the low torque requirement when there is little to no load attached to the motor. This is a fundamental characteristic of series motors, where the field windings are connected in series with the armature. Because the magnetizing current in the field coils is dependent on the armature current, under no-load conditions, the armature draws only sufficient current to overcome friction and windage losses.

The small current draw leads to lower field excitation, which in turn reduces the magnetic field strength. While this might seem disadvantageous, it aligns with the operational behavior of series motors, which are efficient for applications requiring variable speeds and high starting torque with loads. In no-load or light load scenarios, the series motor operates effectively with minimal current consumption, making it advantageous for certain applications.

The nature of the series motor means that the other options—high current draw, constant excitation current, and unstable voltage output—do not apply in the same way. These traits are typically associated with different motor configurations or operational conditions, not with a series DC motor running without a load.