[Application of Solid State Relay in Reformation of Furnace Temperature Control System]: Abstract: The original control system of drying equipment and heat treatment equipment commonly used in factories is analyzed. The principle and performance of solid state relay are emphatically expounded and compared with AC contactor. The method of using solid state relay instead of AC contactor to control furnace temperature is put forward. The schematic diagram is designed and applied to field practice. ...
ABSTRACT: The original control system of drying equipment and heat treatment equipment commonly used in factories is analyzed. The principle and performance of solid state relay are emphatically expounded and compared with AC contactor. The method of using solid state relay instead of AC contactor to control furnace temperature is put forward. The schematic diagram is designed and applied to field practice. It effectively improves the accuracy of temperature control and the working environment.
There are many equipments for drying and heat treatment of workpieces in factory production. Such as box electric furnace, infrared heating furnace, melting furnace, drying box, well tempering furnace, quenching furnace, high temperature salt furnace, medium temperature salt furnace and oil furnace, their working temperature ranges from 0 to 1 200 C. Some of these equipment are purchased, some are designed and manufactured by the company itself. Most of the equipment control cabinets are aging seriously and have been used for nearly 20 years. This kind of equipment uses recorder to directly control the contactor to heat continuously. The control accuracy is low, generally in the range of (+5%), and the noise is high and the energy consumption is high. With the development of the company's manufacturing capability and technology, the requirement for surface treatment and temperature control of workpiece is becoming higher and higher. During the Eleventh Five-Year Plan period, the company invested part of its funds to carry out systematic transformation. In the process of transformation, we use foreign technology for reference. The new system uses three-meter separation. The temperature measuring points are sampled more, distributed uniformly, and the temperature control accuracy is high, generally within (+2%). Especially after using solid-state relay, it reduces both noise and no-load loss.
1. Basic situation and working process of original furnace temperature control system
The principle of the original furnace temperature control system is shown in Fig. 1. The system uses a recorder to directly control the AC contactor for continuous heating. The AC contactor is frequently operated, noisy and severely ablated. It needs to be replaced every other time. Temperature sampling points are few, the real temperature in the furnace is not accurately reflected; temperature control accuracy is not high, temperature fluctuation range is large, recording effect is not good.
2. Furnace Temperature System Controlled by Solid State Relay
Classification of 2.1 Solid State Relays
Solid state relay (SSR) is a kind of contactless electronic switch with relay characteristics developed by combining separate electronic components, integrated circuits (or chips) and hybrid microcircuit technology. It has the characteristics of long life, high reliability, fast switching speed, low electromagnetic interference, no noise, no spark and so on.
According to the output load power supply, it can be divided into AC solid state relay, DC solid state relay and AC solid state relay.
AC solid state relay can be divided into voltage zero-crossing type and random type according to switching mode, bi-directional thyristor output type and unidirectional thyristor reverse parallel type according to output switching element, and pin-in type for printed circuit board according to installation mode.（ Natural cooling, without radiator) and fixed on the metal bottom of the device (cooling by radiator); in addition, the input end has a wide range of input (DC 3 ~ 32 V) constant current source and series resistance current limiting mode.
DC solid state relay can be divided into optical isolation type, high frequency magnetic isolation type and transformer coupling type according to the input end, and high power triode type and power field effect tube type according to the output end.
AC/DC solid-state relay is divided into photovoltaic coupler type and magnetic isolation type.
Principle Analysis of 2.2 Solid State Relay
As shown in Figure 2, solid-state relay is a non-contact switching device with relay characteristics, which uses semiconductor devices instead of traditional electrical contacts as switching devices. It is a four-terminal active device. There are two input control terminals, two output terminals, and optical isolation between input and output. The input terminal is added with DC or pulse signals to a certain current. After value, output 1, 2 can be changed from break state to on state, 1 can be connected to AC or DC circuit [1].
Solid state relay is equivalent to a contactless switch. As shown in Figure 3, the power supply (AC or DC) is controlled by giving the input control signal.
In the case of inductive load, when SSR is turned off by on-state, due to the inconsistent phase of current and voltage, a large voltage rise rate d_/ DT (commutated d_/ dt) is added to both ends of the bidirectional thyristor. If the value exceeds the commutated d_/ DT index of the bidirectional thyristor (typical value is 10 V/mus), the delay shutdown will occur. To failure. The unidirectional thyristor operates in a unipolar state and is only affected by the static voltage rise rate d_/dt (typical value is 100 V/us). The enhanced SSR consisting of two unidirectional thyristors in reverse parallel is much higher than the commutated d_/dt consisting of one bidirectional thyristor. Therefore, it is suitable for inductive or capacitive loads. Enhanced SSR was used.
Advantages and Disadvantages of [$page] 2.3 Solid State Relay
Compared with conventional AC contactors, solid-state relays have advantages, but they also have disadvantages such as large on-off voltage drop and break-state leakage current, as shown in Table 1.
Temperature Control System Based on 2.4 Solid State Relay
According to the characteristics of zero-crossing AC solid-state relay, Yokogawa series of Japan or Huachino series of temperature control meters are selected.