The correct answer is: A. brush axis is at 45° electrical to the field axis.
A repulsion motor is a type of electric motor that uses the repulsion principle to produce torque. The repulsion principle states that when two magnetic fields of the same polarity are brought close together, they will repel each other. In a repulsion motor, the stator has two windings, a main winding and a control winding. The main winding is connected to the power source, and the control winding is connected to a switch. When the switch is closed, current flows through both windings, creating a magnetic field. The magnetic field from the main winding is concentrated at the poles of the stator, while the magnetic field from the control winding is concentrated at the brushes. The two magnetic fields interact, creating a torque that turns the rotor.
The maximum torque is developed when the brush axis is at 45° electrical to the field axis. This is because at this angle, the magnetic fields from the main winding and the control winding are at right angles to each other, creating the strongest possible repulsion force.
Option B is incorrect because when the brush axis coincides with the field axis, there is no repulsion between the two magnetic fields, and therefore no torque is produced.
Option C is incorrect because when the brush axis is at 90° electrical to the field axis, the magnetic fields from the main winding and the control winding are in the same direction, and therefore there is no repulsion between them. As a result, no torque is produced.
Option D is incorrect because it is not one of the possible answers.