Alternators and AC Motors: Principles and Operation
Alternators
Start Operation
Alternators operate on the principle of electromagnetic induction. Rotating conductors within a magnetic field induce a sinusoidal electromotive force (EMF). This EMF connects to an external circuit via slip rings and brushes. In practice, rotating the magnetic field poles (inductor) while keeping the conductors (induced) stationary is more efficient. E = ΔΦ/Δt.
Alternator Constitution
Alternators consist of two circuits: the inductor and the induced.
Inductor Circuit
This circuit comprises electromagnets with windings arranged to create alternating north and south poles. Direct current (DC) feeds the inductor windings through slip rings and brushes. Since alternators don’t generate DC, two methods provide the DC excitation:
- Exciter Generator: A DC generator coupled to the alternator shaft. A shunt resistor connected to the exciter regulates the field current.
- Auxiliary Alternator: A separate alternator coupled to the main alternator shaft. A three-phase rectifier bridge connected to the auxiliary alternator eliminates the need for brushes in the inductor circuit.
Induced Circuit
This circuit typically consists of three windings placed 120° apart within a laminated magnetic core. The windings are usually connected in a star configuration with the neutral grounded. Each phase winding comprises multiple coils connected in series to increase the generated EMF.
Alternator Frequency
The frequency of the generated AC is directly proportional to the alternator’s speed and the number of pole pairs in the inductor circuit.
Alternator Coupling
Multiple alternators can be connected in parallel to increase power output or to connect to the power grid. The following conditions must be met for successful coupling:
- Equal Voltage: The alternators must have the same voltage.
- Equal Frequency: The alternators must operate at the same frequency.
- Same Phase Sequence: The phase sequence of the alternators must match.
- In-Phase Voltages: The voltages of the alternators must be in phase. This is achieved by adjusting the speed and excitation current of the alternators.
AC Motors
Asynchronous Motor
Asynchronous motors operate based on magnetic induction. A rotating magnetic field induces currents in the rotor, generating torque. E = ΔΦ/Δt. The rotor speed never reaches the speed of the rotating magnetic field; otherwise, no voltage would be induced. Torque is generated by the interaction of the magnetic field (B), conductor length (L), and current (I): F = B x L x I.
Rotating Magnetic Field
Three coils, placed 120° apart and connected to different phases, create a rotating magnetic field. The synchronous speed (Ns) is given by: Ns = 60f / p, where f is the frequency and p is the number of pole pairs.
Slip
Slip (S) is the difference between the synchronous speed (Ns) and the actual rotor speed (n), expressed as a percentage: S = (Ns – n) / Ns * 100. Slip varies with the mechanical load; increased load results in higher slip, torque, and current draw.
Starting Methods
Starting an AC motor requires a high starting torque and current. Various starting methods are used to limit the inrush current, including resistance starting, autotransformer starting, star-delta starting, electronic starters, and variable frequency drives (VFDs).
Acceleration and Load
As the mechanical load increases, the motor torque and current draw also increase. If the load exceeds the motor’s maximum torque, the motor will stall.
Motor Characteristics
The motor characteristic curve shows the relationship between motor torque and speed.
Slip Ring Motors
Slip ring motors offer better speed control but are more expensive and require more maintenance.
Single-Phase Motors
Single-phase motors are suitable for low-power applications.
ESCOM Three-Phase Motor
Similar to a standard three-phase induction motor, but with a modified rotor design.