WASTEWATER

By Rick Lallish

Which method to turn pumps on or off uses air pressure?

A. Mercury switch (mercroid style)

B. Bubble systems

C. Electrode systems

D. Float switch

Answer: B. Bubble systems used to be a very common method of turning pumps on or off in lift stations. The method uses a system of switches and a small tube in the lift station. The system uses an air compressor to pump air in the tube, and the pressure of the water above the tube is sensed by the pressure switches. A low-pressure signal will turn the pumps off, and high pressure turns them on. Understanding various means of pump or motor controls is vital for the daily operator duties and advancement of certifications.  More information may be found in the Electrical Fundamentals for Water and Wastewater textbook from Skeet Arasmith, Third Edition, 2015.

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DRINKING WATER

By Drew Hoelscher

In an emergency, the operator bypassed a failed variable-frequency drive installed on a 2 hp three-phase pump motor by hardwiring the pump’s motor directly to the power source. Afterward, the operator noticed that the shaft of the pump was rotating in the wrong direction. How will this affect the equipment?

A. The overloads will trip due to excessively high electric current

B. The pump’s discharge flow rate will be drastically reduced

C. The pump will have an increased flow rate and develop a suction cavitation condition

D. The pump impeller and cooling fan will lock up or break from being over-tightened

Answer: B. Electric motors are designed to rotate clockwise, counter-clockwise, or both. Typically, the direction of rotation is indicated by arrows and is viewed from the driven end or where the load is applied. In this scenario, the operator should swap any two phases of power to correct the direction of rotation, which will reestablish the correct discharge flow rate. Larger high-speed motors with incorrect rotation could be affected more so by improper cooling. In some cases, cooling fans have angled blades to ensure sufficient cooling during operation.

About the authors

Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.