The minimum armature current of the synchronous motor corresponds to operation at unity power factor.
A synchronous motor is an AC electric motor that rotates at a constant speed regardless of the load on the motor. This is because the speed of a synchronous motor is synchronized with the frequency of the AC power supply.
The armature current of a synchronous motor is the current that flows through the armature windings of the motor. The armature current is proportional to the torque that the motor produces.
The power factor of a synchronous motor is the ratio of the real power (the power that is used to do work) to the apparent power (the product of the voltage and current). A power factor of unity means that all of the power that is supplied to the motor is used to do work.
When a synchronous motor is operating at unity power factor, the armature current is at its minimum. This is because the motor is not producing any reactive power. Reactive power is the power that is used to create a magnetic field in the motor.
When a synchronous motor is operating at a power factor that is less than unity, the armature current is greater than the minimum. This is because the motor is producing reactive power. Reactive power does not do any work, but it does cause the current to flow through the motor.
Therefore, the minimum armature current of the synchronous motor corresponds to operation at unity power factor.