AC current is controlled primarily by regulating voltage and impedance (resistance in AC circuits).
Methods of AC Current Control
Several methods are used to control AC current, each suited for different applications:
1. Voltage Regulation at the Source (Generator)
- Rotating Field Current Control: The AC voltage produced by a generator is regulated by controlling the current in the rotating field (rotor). By adjusting the field current, the induced EMF (electromotive force), and consequently the output voltage, is controlled. Changes in voltage directly affect the current flow.
- Automatic Voltage Regulators (AVRs): AVRs are used in generators to maintain a stable output voltage despite variations in load. They achieve this by automatically adjusting the field current.
2. Transformers with Tap Changers
- Tap-Changing Transformers: These transformers have multiple taps on their windings, allowing for small changes in the turns ratio. By switching between these taps, the output voltage, and therefore the current supplied to the load, can be adjusted.
- On-Load Tap Changers (OLTCs): OLTCs can change taps while the transformer is under load, providing continuous voltage regulation.
- Off-Load Tap Changers: These require the transformer to be de-energized before changing taps.
3. Impedance Control
- Resistors: Adding resistance in series with the circuit limits the current flow. This is a simple but inefficient method, as energy is dissipated as heat in the resistor.
- Reactors (Inductors and Capacitors): Inductors and capacitors introduce reactance, which opposes the flow of AC current. By varying the inductance or capacitance, the current can be controlled. Variable inductors or switched capacitor banks can be used.
- Variable Transformers (Variacs): A variac is an autotransformer with a continuously adjustable output voltage. It can be used to control the current by varying the voltage supplied to the load.
- Solid-State Devices (Thyristors, Triacs, and IGBTs): These devices can rapidly switch AC current on and off, or control the conduction angle, allowing for precise current control. They are used in applications such as light dimmers, motor speed controllers, and power supplies.
4. Phase Angle Control
- Thyristors/Silicon Controlled Rectifiers (SCRs): Used in AC voltage controllers, SCRs can be triggered at different points in the AC cycle. By controlling the firing angle, the amount of voltage (and hence current) delivered to the load is regulated.
Example Applications
- Dimming Lights: Triacs are commonly used in dimmer switches to control the current flowing through a light bulb, thus adjusting its brightness.
- Motor Speed Control: AC motor speed can be controlled by varying the frequency of the AC power supply using variable frequency drives (VFDs). VFDs utilize solid-state devices to control both voltage and frequency.
- Power Transmission: Tap-changing transformers are essential for maintaining stable voltage levels in power transmission networks despite fluctuating loads and line conditions.
In summary, AC current is controlled through various techniques, including voltage regulation at the source, tap-changing transformers, impedance control with resistors, reactors, and solid-state devices, and phase angle control using thyristors. The choice of method depends on the specific application and desired level of control.
