Improvement of Temperature Control System of Far Infrared Drying Furnace
Improvement of Temperature Control System of Far Infrared Drying Furnace
Yang Wenliang
At present, the temperature control system of domestic steel drum far-infrared drying furnace adopts automatic control of thyristor. Since most of the thyristor components sold on the market have short life and high price, there are still many circuit controls using AC contactors. Some of our control cabinets also use AC contactors for circuit control. The moving coil type temperature indicating regulator we use is a single-limit product with only adjustable high temperature limit (upper limit) and no adjustable low temperature start (lower limit). When the furnace temperature is below the upper limit, the control loop is started immediately, see Figure 1. The contact of the temperature relay of the regulator is equivalent to the outside of the preset temperature as a normally open contact. The working principle is as follows: the switch K is closed, the line is connected, the contactor coil J is energized, the armature is pulled, the main contact is closed, the furnace is powered, and heating is started. When the temperature rises to the pre-determined temperature (upper limit), the temperature relay contact T in the regulator opens, the contactor coil J loses power, the armature is released, the contactor main contact is broken, and the furnace stops heating. When the temperature in the furnace drops below the preset temperature (upper limit), the temperature relay contact T of the regulator is closed, the contactor coil J is energized, the armature is pulled, the main contact is closed, and the heating starts again, so that the work is repeated.
Figure 1 Direct control of the front temperature relay
ABC-three-phase power supply; HK-automatic air switch; J-contactor coil; DR-load resistor; RDB, RDc-fuse; K-switch; T-temperature relay contact
However, in actual production, since the steel drum is heated by the assembly line in the furnace, the unheated steel drum is continuously input, and the heated steel drum is continuously outputted, and is greatly affected by external factors, so the temperature variation range is large. When the temperature variation range is small and the ±°C alternating oscillation occurs near the pre-set upper limit value, the contactor main contacts are frequently broken, that is, in an oscillating state in which the desired contact is constant. At this time, the arc burning time of the contact is too long, the moving and static contacts often stick, and the contactor coil is often burnt due to the large current when the clutch is engaged, so that the service life of the contactor is greatly shortened. Sometimes the burnt coils are replaced at both ends for three days, which seriously affects the normal production and the maintenance cost is also large. In response to this problem, we have conducted careful research. It is believed that if the contactor is always sucked or released during this time, the contactor can be prevented from being frequently disconnected.
To this end, we connect a delay-closed contact in series with the temperature relay contact T in the control loop, so that the contactor remains open when the temperature relay contact T is opened and closed, see Figure 2.
Figure 2 Improved temperature relay controlled by time-delay closed contacts
ABC-three-phase power supply; HK-automatic air switch; J-contactor coil; DR-load resistor; RDB, RDc-fuse; K-switch; T-temperature relay contact; QA-start button; SJ-time Relay coil; SJ1-time relay delay breaker normally open contact.
The working principle is as follows: the switch K is closed, the time relay coil SJ is energized, the armature is pulled, and the delay starts. When the time-delay normally open contact SJ1 is closed, the contactor coil J is energized, the armature is pulled in, the main contact is closed, power is supplied to the drying furnace, and heating is started. At the same time, the contactor normally opens the auxiliary contact J1 to close, the contactor keeps self-locking, the contactor normally closes the auxiliary contact J2, the time relay coil SJ loses power, the armature is released, and the normal state is restored, waiting for the next delay. When the temperature rises to the preset temperature, the temperature relay contact T in the regulator opens. The contactor coil J loses power, the armature is released, the main contact is disconnected, the power supply to the drying furnace is stopped, the common auxiliary contact J1 of the contactor is opened, the normally closed auxiliary point J2 is closed, the time relay coil J is energized, and the armature is pulled. The delay begins. At this time, if the temperature relay contact T appears to open and close the oscillation, the time relay delay time can escape the oscillation (generally 20-30 seconds). After the delay is over, the time relay closes the normally open contact SJ1, the contactor coil J is electrically attracted to the armature, the main contact is closed to supply power to the drying furnace, and heating begins.
The improvement of this temperature control circuit has been successful. From November last year to the present, the equipment is operating normally and no original fault has occurred. Therefore, we have come up with another form, that is, the contactor delay-breaking form after the temperature relay contact is disconnected, as shown in Figure 3. The working principle is as follows: when the switch K is closed, the time relay coil SJ is electrically attracted to the armature, the time-breaking normally open contact SJ1 is closed, the contactor coil J is electrically attracted to the armature, the main contact is closed, and the beginning is The furnace is powered. When the temperature rises to the preset temperature, the temperature relay contact T is disconnected, and the time relay coil SJ is de-energized to release the armature. If the temperature relay contact opens and closes at this time, since the time relay delays the normally open contact SJ1 in the delay state, the contactor coil J has not lost power, and continues to supply power to the drying furnace, and the temperature continues to rise. At the end of the delay, the temperature relay contact opening and closing oscillation has been avoided and is in a stable open state. When the temperature drops below the preset temperature, the temperature relay contact is closed, the time relay coil SJ1 is closed, the contactor coil J is electrically attracted, the main contact is closed, the power supply to the electric furnace is started, and heating is started.
Figure 3 Improved temperature relay indirect delay-breaking control
A. B, C-three-phase power supply; HK-automatic air switch; J-contactor coil; DR-loading power; RDB, RDC-fuse; K-switch; T-temperature relay contact; SJ-time relay coil; SJ1-time surface relay delay breaker normally open contact.
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