What is a thyristor regulator?
A thyristor regulator is an electrical switch that turns on and off AC power at one of the operation parts. The temperature of the electric furnace is control by receiving a signal from the "control part" such as a controller and control the electric power. Compared with mechanical switches, it has the features of high-speed switching, which enables precise temperature control, small noise during operation, and long service life due to the absence of contact, and is widely used in the industrial temperature control field.
Thyristor regulator configuration
A thyristor configuration of three parts: an anode, a cathode, and a gate. It turns on when current flows through the gate, and turns off when current becomes zero.
This property can be used to control AC power by varying the timing of current through the gate.
In an AC circuit, current flows in both positive and negative directions, but a thyristor can only allow current to flow in one direction, so in the case of a single-phase circuit, two thyristors are configuration in parallel in the opposite directions.
In the case of three-phase, the basic configuration is a six-arm (six gates = six arms) single-phase thyristor regulator x three, but there is also a three-arm (three gates = three arms) configuration. Compared to the three-arm configuration, the six-arm configuration has the advantages of control, matching with the transformer load, and reduced outflow current, so at Chino, the six-arm specifications is the standard.
Types of Thyristor Regulators
• Phase control method
The phase control method control the power supplied to load by changing the time (phase) when current flows through the gate terminal of a thyristor (silicon control rectifier). In other words, it is a method of changing the time for which power is supplied to the load.
• Frequency division control method
The division control method is a method of control the ratio of energization time (ON-OFF time) within a certain period. Unlike the phase control method, the division control method switches (ON-OFF) the thyristor near the zero-cross point of the power supply voltage, so it is called zero-cross switching, and has features that the noise generated during switching is extremely small compared to the phase control method.
Since the power supply voltage is always applied to the heater when it is turned on, it is not suitable for use with heaters whose resistance changes significantly with temperature, as a large current flows when cold. It is suitable for temperature control of heaters with a small resistance temperature coefficient, such as nichrome and iron-chrome heaters.
Feedback Method
The "phase control method" is further divided into three types: voltage feedback type, power feedback type, and current feedback type, depending on the change in electrical resistance due to the temperature of the heating element material.
• Voltage feedback type
For heating elements such as iron-chrome or nickel-chrome, which have small changes in electrical resistance, the voltage feedback type is used.
Regular temperature: approx. 600℃~1000℃
• current feedback type
For heating elements such as platinum or molybdenum, whose electrical resistance is extremely small at low temperatures and whose electrical resistance changes by 6 to 12 times when heated, current feedback type is used.
Regular temperature: Approximately 1200℃~1600℃
• Power feedback type
For heating elements such as silicon carbide (SiC), whose resistance changes with the heating temperature and whose electrical resistance deteriorates over time to nearly four times its initial value, a power feedback type is used.
Regular temperature: Approximately 1500℃~1800℃
Model selection
- The relationship between the heater temperature and resistance value is examined, and the phase/frequency division and feedback method are determined. In the case of a transformer load, the phase control method is used, and feedback method is determined from the heater resistance temperature coefficient.
- Calculate the heater current.
Single phase: Heater current = heater rated power / power supply voltage
Three-phase: Heater current = heater rated power / power supply voltage / √3 - Taking into account heater manufacturing tolerances and power supply voltage fluctuations, current capacity of the thyristor is given some margin depending on the heater type and whether or not feedback is used.
| Nichrome type: |
With feedback: 110% or more of the calculated value
No feedback: 120% or more of the calculated value |
| Pure metals: |
More than 120% of the calculated value |
| Silicon carbide type: |
More than 120% of the calculated value |
| Transformer load: |
More than 130% of the calculated value |
- From the above results, select a larger thyristor that is close to current capacity of the thyristor.
