One of the resistance furnace relay temperature control circuits

The power of the resistance furnace is relatively large, generally 30-75kW, the power supply voltage is three-phase 380V, and the maximum operating temperature is 900-1100
°C. (1) One of the
resistance furnace relay temperature control circuits
.     Figure 11-74 shows the power frequency resistance furnace heat treatment temperature automatic control circuit. The temperature measuring element is a thermocouple potentiometer KP, and the holding
time is controlled by a time relay KT. The temperature and holding time required by the heat treatment process
are set on the potentiometer and time relay respectively .
    Close the furnace door and the limit switch sQ is closed. At the beginning, the furnace temperature is low, contacts 3 and 1 of the potentiometer KP are closed, the
relay KAI is closed, the contactor KM is closed, and the electric furnace is energized to heat up. When the furnace temperature reaches the set value. KP contacts 3 and 1 are disconnected
, KAi and KM are released, and heating stops. At the same time, the time relay KT coil is energized and starts timing and heat preservation. When the furnace temperature
drops below the set value, contacts 3 and 1 of KP are closed again, KAI is electrically closed again, and the electric furnace is energized
again Repeat the above process to keep the furnace temperature in the heat preservation stage at the set value. within the temperature range. When the holding time reaches the time,
the KT delayed closing normally open contact closes, and the bell HA sounds an alarm.