The phase of armature current varies with field excitation. The current has larger values for lower and higher values of excitation. In between, the current has minimum value corresponding to a particular excitation (see graph on right). The variations of formula_3 with excitation are known as formula_4 curves because of their shape. For the same mechanical load, the armature current varies with field excitation over a wide range and so causes the power factor also to vary accordingly. When over-excited, the motor runs with leading power factor (and supplies vars to the grid) and when under-excited with lagging power factor (and absorbs vars from the grid). In between, the power factor is unity. The minimum armature current corresponds to the point of unity power factor (voltage and current in phase). As in a synchronous motor, the stator of the machine is connected to a three-phase supply of voltage formula_5 (assumed to be constant), and this creates a rotating magnetic field within the machine. Likewise, the rotor is excited with a DC current formula_6 to act as an electromagnet. In normal operation the rotor magnet follows the stator field at synchronous speed. The rotating electromagnet induces a three-phase voltage formula_7 in the stator windings as if the machine were a synchronous generator. If the machine is considered to be ideal, with no mechanical, magnetic, or electrical losses, its equivalent circuit will be an AC generator in series with the winding inductance formula_8 of the stator. The magnitude of formula_7 depends on the excitation current formula_6 and the speed of rotation, and as the latter is fixed, formula_7 depends only on formula_6. If formula_6 is critically adjusted to a value formula_14, formula_7 will be equal and opposite to formula_5, and the current in the stator formula_17 will be zero. This corresponds to the minimum in the curve shown above. If, however, formula_6 is increased above formula_14, formula_7 will exceed formula_5, and the difference is accounted for by a voltage formula_22 appearing across the stator inductance formula_8: formula_24 where formula_25 is the stator reactance. Now the stator current formula_17 is no longer zero. Since the machine is ideal, formula_7, formula_28 and formula_5 will all be in phase, and formula_17 will be entirely reactive (i.e. in phase quadrature).