- AC Series Motor Defined: An AC series motor is a modified version of the DC series motor adapted to run on alternating current (AC).
- Modification Needs: Necessary modifications include reducing eddy currents and improving the power factor to enhance performance with AC supply.
- Compensating Windings: These are crucial for maintaining torque and motor speed, available in conductively and inductively compensated forms.
- Circuit Diagrams: Detailed circuit diagrams illustrate the different types of compensating mechanisms in series motors, explaining their operational advantages.
- Practical Applications: AC series motors are widely used in household appliances, showcasing their versatility and the practical benefits of their design enhancements.
AC series motors are modified versions of the DC series motor. They retain the series connection between the field and armature but are designed to operate satisfactorily from an AC supply. A motor designed to run on either AC or DC is commonly called a universal motor. Connecting an unmodified DC series motor to AC causes the following effects:
- Torque remains unidirectional because the armature and field currents reverse at the same time.
- Alternating magnetic flux induces eddy currents in the yoke and field cores, increasing iron loss and heating.
- The field and armature inductance adds reactance, so the current lags the supply voltage and the power factor falls.
- Transformer emf in armature coils short-circuited by the brushes during commutation causes excessive brush sparking.
A standard DC series motor therefore performs poorly on AC. Laminating the yoke and field core reduces the eddy currents. This is one required modification to the DC series motor.
The power factor can be improved by reducing the field and armature reactance. Designers use fewer turns in the series field together with a low-reluctance magnetic circuit and larger pole area.
Reducing the number of field turns alone would lower the field mmf and air-gap flux, increasing speed and reducing torque. The magnetic-circuit changes help retain useful flux, while a compensating winding opposes armature reaction and improves commutation. AC series motors use two compensating-winding arrangements:
- Conductively compensated motors.
- Inductively compensated motors.
Conductively Compensated Type of Motors
In a conductively compensated motor, the compensating winding is connected in series with the armature and placed in stator slots. Its axis is 90 electrical degrees from the main-field axis, so its magnetomotive force opposes the armature reaction.
Inductively Compensated Type of Motors
In an inductively compensated motor, the compensating winding is short-circuited and has no direct electrical connection to the armature circuit. The transformer action makes the armature winding act as the primary and the compensating winding act as the secondary. The induced compensating current opposes the armature current’s magnetic effect.
The following schematic shows a single-phase AC series motor with a compensating winding and interpoles.
Solid-state phase-angle control is widely used to vary the speed of universal motors. Common applications include portable drills, hair dryers, fans and kitchen appliances. The compensating winding counters armature reaction, while the interpoles produce a commutating emf that reduces brush sparking as current reverses in the short-circuited armature coils.
Even after measures to reduce series-field inductance, its reactive voltage drop remains higher than that of the armature or compensating field because the main-field flux is needed to produce torque. The interpole winding connects in parallel with a non-inductive shunt, as illustrated above.
