Pump Motor Selection Calculation

Pump Motor Selection Formula:

\[ HP = \frac{GPM \times Head\ (ft) \times SG}{3960 \times Eff} \]

Flow Rate (GPM):

GPM

Head:

Specific Gravity (SG):

unitless

Efficiency (Eff):

unitless

Calculated Horsepower (HP):

Required Motor Size:

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1. What is Pump Motor Selection Calculation?

The Pump Motor Selection Calculation determines the required horsepower for a pump motor based on flow rate, head, specific gravity, and efficiency. Proper motor sizing is crucial for optimal pump performance and energy efficiency.

2. How Does the Calculator Work?

The calculator uses the pump motor selection formula:

\[ HP = \frac{GPM \times Head\ (ft) \times SG}{3960 \times Eff} \]

Where:

\( HP \) — Horsepower (required motor power)
\( GPM \) — Flow rate in gallons per minute
\( Head \) — Total dynamic head in feet
\( SG \) — Specific gravity of the fluid (water = 1.0)
\( 3960 \) — Conversion constant
\( Eff \) — Pump efficiency (typically 0.7-0.9)

Explanation: The formula calculates the theoretical horsepower required to move fluid against the specified head, then applies efficiency to determine actual motor requirements.

3. Importance of Proper Motor Sizing

Details: Correct motor sizing ensures efficient operation, prevents motor overload, reduces energy costs, and extends equipment lifespan. Undersized motors can overheat and fail, while oversized motors operate inefficiently.

4. Using the Calculator

Tips: Enter flow rate in GPM, head in feet, specific gravity (1.0 for water), and pump efficiency (typically 0.7-0.9). The calculator provides both calculated horsepower and recommended motor size with safety margin.

5. Frequently Asked Questions (FAQ)

Q1: Why is a safety margin added to motor size?
A: Safety margins (typically 10-20%) account for system variations, future demands, and ensure reliable operation under all conditions.

Q2: What is specific gravity and when does it matter?
A: Specific gravity is the ratio of fluid density to water density. It significantly affects power requirements for fluids denser than water.

Q3: How do I determine pump efficiency?
A: Pump efficiency varies by type and size. Consult manufacturer curves or use typical values: centrifugal pumps 70-85%, positive displacement pumps 80-90%.

Q4: What is total dynamic head?
A: Total dynamic head includes static head (vertical lift), friction losses, and pressure differences the pump must overcome.

Q5: Can this calculator be used for all pump types?
A: This formula works best for centrifugal pumps. Positive displacement pumps may require different calculations based on pressure and viscosity.