WTO Pumps and Motors 3 — Questions and Answers
Question 1: Which motor protection device opens the circuit when current draw exceeds a set threshold for an extended period?
- Fuse
- Thermal overload relay (Correct answer)
- Surge arrester
- Capacitor
Correct answer: Thermal overload relay
A thermal overload relay monitors current continuously and trips the circuit when sustained overcurrent generates excessive heat, protecting the motor windings.
Question 2: A vertical turbine pump is typically used in which water treatment application?
- Chemical feed dosing
- Raw water intake from deep wells (Correct answer)
- Backwash water supply
- Sludge dewatering
Correct answer: Raw water intake from deep wells
Vertical turbine pumps are specifically designed to lift water from deep wells or sumps where the water surface is well below the pump motor.
Question 3: What happens to pump flow rate when two identical centrifugal pumps are operated in parallel?
- Flow rate doubles, head doubles
- Flow rate doubles, head remains approximately the same (Correct answer)
- Flow rate remains the same, head doubles
- Flow rate and head both remain the same
Correct answer: Flow rate doubles, head remains approximately the same
Parallel pump operation approximately doubles the flow rate at the same system head, as both pumps contribute flow to the same discharge header.
Question 4: What is the typical power factor range for an unloaded induction motor?
- 0.95–1.00
- 0.80–0.90
- 0.50–0.70 (Correct answer)
- 0.10–0.30
Correct answer: 0.50–0.70
Unloaded induction motors have poor power factors (0.50–0.70) because they draw significant reactive magnetizing current relative to useful work output.
Question 5: A pump's efficiency drops significantly after several years of service. What is the most common cause?
- Motor aging and insulation breakdown
- Impeller wear and increased clearances (Correct answer)
- Clogged discharge valves
- Incorrect motor rotation direction
Correct answer: Impeller wear and increased clearances
Wear on the impeller vanes and increased clearances between the impeller and casing allow water to recirculate internally, reducing volumetric efficiency.
Question 6: Which parameter is used to select the correct pump size for a given application?
- Motor horsepower rating only
- Required flow rate and total dynamic head (Correct answer)
- Pipe diameter and material
- Suction pipe length only
Correct answer: Required flow rate and total dynamic head
Pump selection requires knowing the required flow rate (GPM) and total dynamic head (TDH) to match a pump that operates near its best efficiency point.
Question 7: In a variable frequency drive (VFD) application, reducing motor speed by 50% reduces pump power consumption by approximately what factor?
- 50% (half the power)
- 75% (one-quarter less)
- 87.5% (one-eighth less)
- 12.5% (one-eighth of original power) (Correct answer)
Correct answer: 12.5% (one-eighth of original power)
By the affinity laws, power varies as the cube of speed, so halving the speed reduces power to (0.5)³ = 12.5% of the original power demand.
WTO Pumps and Motors 3
Which motor protection device opens the circuit when current draw exceeds a set threshold for an extended period?