Thyristor controller
Thyristor controllers or thyristor power controllers provide the power or current or voltage . Typical applications can generally be found where electrical power has to be varied, but this is ultimately converted into thermal energy . The actuators are used, for example, in industrial furnace construction or in plastics processing.
functionality
A thyristor controller is used to control the power consumption of electrical consumers. This is done by reducing the rms value of the AC mains voltage . For this purpose, the sinusoidal course of the mains voltage is cut with the help of thyristors at the beginning of each half- wave, in that a thyristor always ignites a certain time after the zero crossing of the alternating voltage. Thus, the consumer is only switched on during part of the sinusoidal oscillation ( phase angle control ), which reduces its current and power consumption.
Thyristor power controllers are operated single-phase or three-phase with an alternating voltage . They are controlled by a controller and vary the switch-on time of the mains voltage for the load .
Where the operating conditions do not require otherwise, pulse burst mode is recommended . Here whole mains voltage full waves are switched to the load or blocked. For example, a controller requests 60% output level (for example via a 4–20 mA signal, which corresponds to 0–100%). The thyristor power controller switches 60% of the full waves to the load, while it blocks 40%. The operating mode is to be regarded as unproblematic. Unwanted luminance fluctuations (flicker effect) can only occur if the network is too weakly designed.
Some operating conditions require switching in every half-wave and thus very fast operation. Examples are controlled systems with very fast behavior or a required current limitation with low element resistance in the cold state. In this operating mode, the controller changes the phase angle α which marks the thyristor ignition time. A half-wave corresponds to 180 ° el. (Degree electrical). The controller can adjust the phase angle from 0 ° el. (max. power) up to 180 ° el. (no service) vary. In a 50 Hz network, the controller switches every 20 ms and behaves very quickly. The voltage edges when switching on result in disadvantages such as EMC interference potential or control reactive power, even with ohmic consumers. This situation can be counteracted with line filters or, if the system is of a corresponding size, with compensation systems.
Subordinate regulation
As mentioned, the controllers change their on / off ratio based on the controller output level. With a required output level of 50%, the controller would switch a full wave on and a full wave off in pulse group operation. Considering this fact only, changes in output power result from fluctuations in the mains voltage. In fact, the regulators have a subordinate regulation. In the most complex case, they vary the output power proportionally to the controller output level. The controllers react to changes in the mains voltage by varying the switch-on and switch-off ratio.
Monitoring the heating elements
In addition to current limitation, thyristor power controllers offer additional functions for monitoring and protecting the elements:
Heating elements are often operated in parallel. The partial load failure monitoring signals the failure of a heating element. The heating element can be replaced when the system is next shut down.
R-Control limits the temperature of temperature-sensitive heating elements. Most heating elements are PTC thermistors, they increase the resistance with the temperature. At a maximum permissible temperature, the heating element has a defined resistance; this is defined on the actuator. The power output is limited by the controller and the maximum permissible temperature is not exceeded.
Control of DC voltage
Thyristor controllers can also be used to control DC voltage, but they differ fundamentally from those described above. When operating with DC voltage, an additional quenching thyristor and a capacitor must be used to switch off the main thyristor. Alternatively, GTO thyristors can be used. Regardless of the component used, such an arrangement is called a DC chopper.
In contrast to the thyristor rectifier, there is no rectification with the thyristor controller. With suitable control, transformers can also be operated on a thyristor controller.
Areas of application
Thyristor actuators are u. a. used to control electric heating in hardening ovens. In addition to the phase control, the pulse packet control is also used. Here, several full waves are controlled without a gate. Switching processes only take place in the zero crossing . This has the advantage that the power grid is not "contaminated" by the high-frequency components that occur during phase control.
Thyristor controllers are used in soft starters to be able to start squirrel cage asynchronous motors with controlled current and torque. Triac actuators in particular are widely used in the area of low power. Examples are dimmers , power controls for vacuum cleaners , washing machines and drills with stepless speed control.
Individual evidence
- ↑ a b c Manfred Schleicher, Winfried Schneider: Thyristor power controller - Basics and tips for the practitioner . Jumo, Fulda 2011, ISBN 978-3-935742-04-7 ( free full text [PDF; 1.7 MB ]).