Alternating Current Series Motor Operation

An alternating current series motor, often simply called an AC series motor or sometimes a universal motor, is a type of electric motor.

Let's understand how a series motor works to determine what type of current it can run on.

Understanding Series Motor Principle

A series motor has its armature winding and field winding connected in series. The direction of rotation in a DC motor depends on the direction of the current in the armature and the field. Specifically, it depends on the relative direction between the armature magnetic field and the main field magnetic field.

In a DC series motor, the current flowing through the armature is the same as the current flowing through the field winding. If you reverse the direction of the supply voltage, the current direction reverses in both the armature and the field windings simultaneously.

• Original condition: Armature current (I_a) creates armature field (→), Field current (I_f) creates main field (→). Torque is produced based on this relative direction.
• Reversed current: Armature current (-I_a) creates armature field (←), Field current (-I_f) creates main field (←). Both fields reverse direction.

Since both magnetic fields reverse at the same time, the relative direction between them remains the same. Therefore, the direction of the torque produced does not change. This principle is key.

AC Operation

When an alternating current is applied to a series motor, the current direction reverses periodically. Because the armature and field windings are in series, the current reverses in both windings at the same instant. Just like in the DC case with reversed polarity, both magnetic fields reverse direction together. Consequently, the direction of the torque produced remains constant (in one direction), and the motor continues to rotate.

However, standard DC series motors face challenges when run on AC, such as:

• High reactance of the field winding causes significant voltage drop and reduces power factor.
• Eddy currents and hysteresis losses in the field core increase heating and reduce efficiency.
• Severe sparking at the commutator due to the alternating flux inducing voltage in the armature coils undergoing commutation.

An AC series motor is specially designed to minimize these issues for efficient AC operation. It typically has:

• A laminated field core to reduce eddy currents.
• Fewer turns in the field winding to reduce reactance.
• Compensating windings and inter-poles to reduce sparking and improve commutation on AC.

DC Operation

Since the fundamental principle of torque production in a series motor works with current flowing in the same direction through both armature and field, an AC series motor (designed for AC) can also run on DC supply. When connected to DC, it operates like a regular DC series motor, albeit with some performance differences compared to AC operation (e.g., speed characteristics, torque).

Thus, a motor specifically designed as an AC series motor can operate using both AC and DC power supplies. This is why they are sometimes called universal motors, commonly found in appliances like vacuum cleaners, blenders, and power tools.

Based on this principle, an alternating current series motor can run on both AC and DC.