Calculating Horsepower Required for Wanner Hydra-Cell Pro^® Pumps

We undersized a motor on a Model D35EKSTNNECA. The customer used a VFD for 3.4 to 20 GPM at up to 1,200 PSIG. This article describes what we missed and how we fixed it.

The Wanner^® horsepower formula for a D35:

Maximum pressure on this job is 1,200 PSI. RPM and GPM are needed at both ends of the flow band:

Three variables give the fourth. They wanted a direct-drive couple and a VFD to swing 3.4–20 GPM.

Rated GPM and rated RPM are the two constants. On a D35 “E” cam that is 34 GPM at 1,150 RPM.

Put in target GPM and solve for target RPM.

At 3.4 GPM the E-cam D35 wants 115 RPM:

At 20 GPM the target RPM is 676.5.

Plug those targets into the horsepower formula at 1,200 PSIG:

• 3.4 GPM @ 1,200 PSIG → 2.98 HP
• 20 GPM @ 1,200 PSIG → 17.5 HP

So a 20 HP motor, right?

Wrong. The limit is shaft torque. Most motors hold constant torque across a 10:1 turndown, but that “constant” is the torque at the motor’s rated RPM. This shaft will run well below rated RPM.

Calculating Torque

Maximum motor torque in inch-pounds:

20 HP at 1,150 RPM is 1,096 in-lb. That is the ceiling a standard 20 HP motor can hold from 115 to 1,150 RPM.

Is 1,096 in-lb enough?

Use the lowest operating RPM in the same formula. At 115 RPM the duty wants 1,643 in-lb. That is above 1,096. Always check torque at the slowest shaft speed.

Solve the same relation for horsepower at 1,643 in-lb:

Result: 30 HP to deliver 1,643 in-lb and keep it across a 10:1 turndown. Direct-drive plus VFD on this duty is a 30 HP motor.

Torque vs. Horsepower

Torque is the twist needed to beat resistance. On a Wanner Hydra-Cell Pro^® pump that resistance is discharge pressure.

Wanner’s HP formula tracks rotational speed for displacement. When the motor is not at rated RPM, check torque. The horsepower that makes that torque is larger than the flow-and-pressure HP, and that larger number is the motor you buy.

RPM sits in both formulas. Drop RPM and calculated horsepower falls while required torque rises.

Problem

At high pressure and cut RPM the VFD tripped on amps. On a constant-torque load the motor still has to make torque at low speed, so current stays up (or climbs a little) while horsepower falls. That current can exceed the drive limit.

The skid, VFD, wire and floor space were all built around 20 HP. Stepping to 30 HP meant a new motor, base, coupling and drive — expensive for both sides.

Solution

20 GPM at 1,200 PSIG still wants about 1,630 in-lb when the motor shaft is below rated RPM. What if the motor ran at 1,150 RPM and the pump shaft still saw only the RPM for 20 GPM?

That is the fix: slow the pump mechanically and let the motor run full speed. The practical answer here was belts. A 900 RPM motor or a gearbox would have worked and cost more time and money.

Sheave diameter and belt length are a separate write-up. Wanner helped size those.

Parts: a belt-guard base, sliding motor mount, sheaves and belts. Footprint stayed close. Some pipe moved. Nothing electrical changed.

Automating the Math

Wanner’s Partners Portal spreadsheet (Technical Information → Calculation Tools) does flow/pressure HP and torque HP at reduced RPM. It only covers six models, and you still have to figure operating RPM by cam. After this job we posted a fuller sheet:

https://innovativepumps.com/calculators/Hydra-Cell-Pro-Pump-Horsepower-Calculator.htm

It covers every Hydra-Cell Pro model, sets RPM from a cam dropdown, and also shows maximum pressure available at a chosen motor HP — useful for PRV selection when a customer can live with a little less pressure on a smaller motor.

We are not a catalog house. The sealless PD design is how we solve the odd job. Use us on the next one that is not simple.

Related Articles

• Motor sizing for high-temperature liquid
• Components of a high-pressure pump skid
• Sealless pressure regulating valves
• Assembling pump to motor with an adapter
• D35 Hydra-Cell Pro