Written by: Ian Jonas Yam

Sunday, September 25, 2011

B/W Floatless Liquid Level Detection Induction Control Relay Switch

While liquid level detection float devices applies the practical approach of utilizing the principle of buoyancy which depends mainly on a floating object to detect the liquid level in a reservoir that mechanically actuates a switching mechanism to manipulate an electrical switch output to control a water pump, a floatless counterpart which does not rely on mechanical float mechanism makes use of metallic rods immersed in the liquid which serves as electrode probes to detect the depth of the liquid level in the reservoir.

The distinct function associated with the floatless liquid level detection and control device involves the process of electrical conduction between the electrodes when completely submerged in the liquid, resulting in completing the electrical pathway in the secondary circuit of the induction control relay that energizes its electrical contacts to provide a switch output to control an external circuit of a water pump.

B/W floatless liquid level detection induction control relay switch
B/W Induction Control Relay for Floatless Liquid Level Detection for Water Pump Control

The drawing above illustrates the function of the floatless liquid level detection induction control relay for water pump application. As shown on the drawing, the principle of operation of the floatless liquid level detection device features the use of three corrosion resistant rods which serves as electrodes to determine the level of the water contained in the reservoir. The ground electrode is placed on the bottom of the reservoir which serves as the common ground for the other two electrodes mounted above the reservoir. This ground electrode should not be confused with a potential earth ground and should not be connected to the earth ground as this will cause the circuit to not work at all.

The other two electrodes aside from the ground electrode that are both mounted on the top portion of the water reservoir are the start electrode and the stop electrode. The stop electrode is longer in length than the start electrode to reach the depleting level of water in the reservoir. The longer length stop electrode is intended to detect the lower level of the water where the pump should stop running to prevent drying up the reservoir and to protect the water pump from continuous prolonged exhaustive use. To prevent the repetitive cycling of running and stopping of the pump system, an allowable distance between the stop and the start electrode is provided so that a holding span is achieved that maintains the pump in stop mode in order to allow the accumulation of water with sufficient level in the reservoir. Then as the reservoir is replenished with water, the water level slowly rises until finally reaching the shorter length start electrode, only then will the water pump be restarted to pump water out of the reservoir.

The induction relay switch consists of a primary coil (terminals 3-4) and a secondary coil (terminals 7-8) along with two normally open contacts (terminals 5-6 and terminals 9-10). This device is fitted with stationary laminated core which produces a magnetic flux when an alternating current supply voltage is connected to terminals 3 and 4 for the primary coil. The magnetic flux flows through the shortest path in the laminated core awaiting for the secondary coil to energize which happens once all of the three electrodes connected to terminals 8, 9 and 10 becomes shorted together to provide a complete circuit to terminals 7 and 8 for the secondary coil, which comes into effect when all electrodes are submerged in the water, this in turn would extend the path of the magnetic flux towards the fullest extent of the stationary core to cause the movable plunger to be pulled in upward towards the stationary laminated core. When the movable plunger is attracted by magnetism that causes it to fasten together in a holding clasp with the stationary core, its associated normally open contacts, which are internally fitted in the stem of the plunger, will be switched to a close contact to provide a complete circuit connected to the external start control circuit to run the water pump.

When the water level falls below the start electrode, the water pump continues to run since the secondary contact is retained at closed state, hence still providing a complete circuit that maintains the secondary coil energized due to the continuous current flow through terminal 8 on one end of the secondary coil and the presence of current flow path crossing between terminals 9 and 10 due to the retained closed secondary contact, then finally connecting to terminal 7 for the other end of the secondary coil, which is made possible with both the stop electrode and the ground electrode still actively submerged in the water.

When the water pump continues to run, the water level in the reservoir falls until it is no longer in contact with the stop electrode, this removes current flow to one end of the secondary coil connected to terminal 8 which renders an incomplete circuit to the secondary coil of the induction relay, this results in the absence of magnetic flux for the movable plunger which causes it to dismount away from the stationary core, hence opening the contact on terminals 5 and 6, which also opens the external start control switch of the water pump, thereby placing the pump in stop mode to discontinue the pumping out of water from the reservoir.

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