The primary difference lies in how each method controls power delivery to a load connected to an AC source. On-off control regulates power by switching the load on for several complete AC cycles and then off for a number of cycles. In contrast, phase control adjusts the power by connecting the load to the AC supply for only a portion of each cycle.
Here's a breakdown:
On-Off Control
- Mechanism: Uses thyristor switches (or other switching devices) to connect the load to the AC source for a specific number of full AC voltage cycles. Then, the load is disconnected for a similar duration, repeating this cycle.
- Operation: Entire AC cycles are either passed to the load or blocked.
- Application: Suitable for applications where precise, instantaneous power control isn't crucial, such as resistive heating elements.
- Advantage: Simple control logic.
- Disadvantage: Slower response time due to switching entire cycles.
Phase Control
- Mechanism: Employs thyristor switches to connect the load to the AC supply during a portion of each AC cycle. The point in the AC cycle where the thyristor is triggered to conduct determines how much power is delivered.
- Operation: The firing angle of the thyristor is adjusted to control the conduction period within each cycle.
- Application: Used in applications requiring finer control over power delivery, like light dimmers and motor speed control.
- Advantage: Finer control over power delivery within each cycle, allowing for a more responsive system.
- Disadvantage: Can introduce harmonics into the AC line due to the non-sinusoidal waveform.
Summary Table
| Feature | On-Off Control | Phase Control |
|---|---|---|
| Power Control | By switching entire AC cycles on/off | By controlling the conduction angle within each cycle |
| Switching | Entire AC cycles | Part of each AC cycle |
| Response | Slower | Faster |
| Harmonics | Lower | Higher |
In short, on-off control provides power by allowing or blocking entire AC cycles, whereas phase control regulates power by controlling the fraction of each AC cycle for which the load is connected to the source.
