Midea LSBLG320/MZ - Time Relay and Intermediate Relay; Compressor Thermal Overload Relay; Current Transformer and Transducer; Electronic Expansion Valve

DX type water-cooled screw chiller (PCB Control) MCAC-CTSM-2014-03
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2) Time Relay and Intermediate Relay
The time relay is an automatic switch device which performs delayed control based the
electromagnetic or mechanical principles. It controls the star delta switching time for the compressor
contactor. The preset time is star operation time (6s). The coil voltage is AC 220V. The rated contact
current is usually low and is used for controlling the loop only.
The intermediate relay delivers intermediate signals among control circuits to increase the number
and capacity of contacts. Normally, the main control board output controls starting and stopping of loads
such as motor and water pump by using the intermediate relay to drive the contactor coil. The coil power
supply can be DC or AC. Our standard screw compressor model uses the AC220V coil.
3) Compressor thermal overload relay
The thermal overload relay works based on the principle of heating effect of electric current. With
inverse time limit action feature which is similar to the permissible overload feature of the motor, it is
used to provide overcurrent protection for the compressor and fan. For compressor overload protection,
the major loop current of the motor is converted to an AC 0-5 A current signal by the current mutual
inductor. Then the thermal overload relay performs overload protection. For fan overload protection, the
thermal overload relay is connected in series with the major loop.
4) Current transformer and Transducer
The current transformer transforms primary current with a larger value to secondary current with a
smaller value for the purpose of protection or measurement. A current transformer with transformation
ratio of 400/5 can transform 400A current to 5A.
A transducer transduces the measured current to DC voltage or DC current. After the current
transformer transforms the current to AC 0-5 A current signals, the transducer outputs 4-20 mA analog
signal based on the linear scale to the main control board.
5) Electronic Expansion Valve
The electronic expansion valve is equipped with a stepping motor which controls the valve status. A
special-purpose electronic expansion valve control module is required to drive the electronic expansion
valve.