Alternate Operation of Two Motor Pumps

If the frequency of operation of pumping liquid into a tank is to frequent, too much stress is subjected to a single motor pump. In such case the life span of the motor may not last long. To avoid to much stress to a single motor pump, it is wise to add another motor pump. The two motor are design to operate alternately. Figure (A) illustrate two motor pumps that alternately pumps liquid into tank.

Control Operation

Figure (B) illustrate the schematic diagram of the control circuit of the “Alternate Operation of a Two Motor Pump”. Closing the toggle switch SW will place the control circuit ready for operation.

As the liquid level in the tank drops to the lower limit LL as shown in figure (A). The float switch* contact FS (1-2) will close, see figure (B). Contactor M1 (A-B) is energize by the current that flows from the line L1, toggle switch SW, float switch FS (1-2), contact M2 (11-12), contact R (11-12), contactor M1 (A-B) and to line L2. contact M1 (13-14) will close to maintain contactor M1 (A-B) continuously energized. Contact M1 (11-12) will open to prevent contactor M2 (A-B) from being energized. Contact M1 (1-2) will close to energize relay R (A-B).

*For the detail operation of the float switch FS (1-2), refer to Article 10.0 entitle “Float Switch Motor Pump Control”.

With relay R (A-B) energize, contact R (1-2) will close to maintain relay R (A-B)  continuously energized. Contact R (13-14) will close and contact R (11-12) will open. At this condition, the motor pump M11, controlled by contactor M1 (A-B), will pump liquid into the tank. The float switch FS (1-2) remains close as the liquid in the tank rises up.

When the liquid reaches the upper limit UL, the float switch contact FS (1-2) will open. This de-energize contactor M1 (A-B) to stop the motor pump M11 from pumping liquid into the tank. Also contacts M1 (13-14) and M1 (1-2) will open. Contact M1 (11-12) will open. As liquid is drain from the tank and its level drops, the float switch FS (1-2) remains open. Relay R (A-B) is still energized.

(A) Liquid tank with two motor   pump.

When the level of the liquid reaches the lower limit LL, the float switch FS (1-2) will close again. Contactor M2 (A-B) is energized by the current that flows from line L1, toggle switch SW, float switch FS (1-2), contact M1 (11-12), contact R (13-14) which is now closed, contactor M2 (A-B) and to line L2. contact M2 (11-12) will open to prevent contactor M1 (A-B) from being energized. Contact M2 (13-14) will close to maintain contactor M2 (A-B) continuously energized. Contact M2 (21-22) will open to de-energized relay R (A-B). 

With relay R (A-B) ed-energized, contact R (11-12) will close, and contacts R (13-14) and (1-2) will open. At this condition the motor pump M22 controlled by contactor M2 (A-B) will pump liquid into the tank. The float switch FS (1-2) remains close as the liquid in the tank rises up.

(B) Control circuit diagram of the “Alternative Operation of Two Motor Pumps”

When the liquid level reach again the upper limit UL, the float switch contact FS (1-2) opens. This de-energize contactor M2 (A-B) and stops the motor pump M22 from pumping liquid into the tank. Contacts M2 (11-12) and M2 (21-22) will close and contact M2 (13-14) will open. The condition of the circuit is brought back to the shown in figure (B). Alternate operation of the motor pump M11 and M22 is repeated as the liquid alternately rises up and drop down.

Note: If the float switch FS in the laboratory are insufficient, they can be substituted with an ordinary ON-OFF toggle switches or micro switches.