The correct answer is: A. high inductance of field and armature circuits
When a DC series motor is connected to an AC supply, the inductance of the field and armature circuits causes the current to lag the voltage. This lagging current results in a low power factor.
The inductance of a coil is a measure of its opposition to changes in current. When an AC voltage is applied to a coil, the current will lag the voltage because the coil takes time to build up a magnetic field. The inductance of the field and armature circuits in a DC series motor is high, which causes the current to lag the voltage.
The power factor is a measure of how efficiently a circuit converts electrical energy into mechanical energy. A power factor of 1 indicates that all of the electrical energy is converted into mechanical energy. A power factor of less than 1 indicates that some of the electrical energy is lost as heat.
The low power factor of a DC series motor connected to an AC supply is caused by the inductance of the field and armature circuits. This lagging current results in a loss of power, which can be reduced by using a power factor correction device.