Method for determining a gain error of a current measuring device

An apparatus (17) for ascertaining a gain error of a current measuring device (8, 9, 10) of a sensor unit (7) of an electrical machine (1) having a stator winding (2) with three phases (U, V, W), a current and/or voltage source (4) connected to each of the three phases (U, V, W) and a rotor (5) that is mounted rotatably about an axis of rotation (6), wherein the sensor unit (7) includes three current measuring devices (8, 9, 10), wherein each of the current measuring devices (8, 9, 10) is respectively assigned to a different one of the three phases (U, V, W), and wherein the apparatus (17) is configured to

• a) apply, with a voltage source (4), a test voltage (11) to the three phases (U, V, W), wherein the test voltage (11) comprises at least one first voltage pulse (12) with a first voltage vector and one a second voltage pulse (13) with a second voltage vector, wherein the first voltage vector and the second voltage vector are each oriented in different directions perpendicular to the axis of rotation (6),
• b) acquire, with the sensor unit (7), during the application of the test voltage (11), a first current value of a first phase (U) of the three phases (U, V, W), a second current value of a second phase (V) of the three phases (U, V, W) and a third current value of a third phase (W) of the three phases (U, V, W),
• c) ascertain a sum current vector based on the acquired current values of all of the three phases (U, V, W), and
• d) compare the sum current vector with an ascertained or stored reference current vector, and ascertain a one a gain error based on the comparison.