Vertical multistage pumps are the standard solution for applications demanding high pressure, such as high-pressure water supply, building pressurisation (booster) systems, reverse-osmosis feed and industrial washing lines. In these pumps, performance is determined not only by the pump body but by the electric motor that powers it. A wrong motor means either insufficient pressure, or overheating, or installation problems due to flange and speed mismatch. In this guide we cover how to select a vertical multistage pump motor, with critical topics such as flange type and speed matching, power calculation, IP55 protection and NPSH, focused on high-pressure and booster applications. Our aim is to help you source the motor that fits your pump exactly, in the right power and with the right connection.

Vertical multistage pump and flange-mounted electric motor

Why Does a Vertical Multistage Pump Need a Different Motor?

In multistage pumps each stage (impeller) raises the pressure a little; as stages are stacked, the total head (pressure) increases. In the vertical design the pump stands compactly on a vertical axis and the motor is usually on top, connected directly to the pump by a flange. This arrangement makes it mandatory that the motor be both the right flange type and the right speed, because the motor is coupled directly to the pump shaft. Although the basic logic of pump motor selection is similar to that of centrifugal pumps, the power and speed sensitivity is higher because of the pressure and the stage count. For the basics of centrifugal pump motor selection, our article on centrifugal pump motor selection is a good start.

These pumps are also the heart of booster (pressurisation) systems. To keep the water pressure of a building or facility constant, several vertical pumps run in parallel and the motor of each must be reliable and suited to continuous operation. For comparison with booster and in-line systems, our articles on in-line and circulation pump motor and, for booster applications, booster motor replacement are complementary.

Flange Type and Speed Matching

In vertical multistage pumps the motor is connected directly to the pump by a flange. Therefore the motor flange type (B5 large flange, or the special flange specified by the pump manufacturer) must fit the pump connection face exactly. If the flange bolt circle, diameter and shaft dimension do not match, the motor cannot be fitted mechanically. For flange type selection, our guide on B5 or B14 mounting type selection shows the way. In vertical mounting the oil seal and shaft-up/down position also matter; our article on vertical mounting (V1/V5) and oil seal covers this topic.

Speed matching is one of the most critical points. Vertical pumps requiring high pressure usually run with a 2-pole (around 2900 rpm) motor, because pressure is directly related to the impeller peripheral speed and higher speed produces higher pressure. In applications requiring lower pressure and higher flow, a 4-pole (around 1450 rpm) motor may be preferred. Whatever speed the pump operating point (flow-pressure) requires, the motor is selected accordingly. For the pole-speed relationship, see our articles on 2, 4, 6 pole selection and, for actual speed, slip and actual speed.

Power Calculation: Flow, Pressure and Efficiency

In a multistage pump the required motor power is calculated from the flow (Q), the total head (H) and the pump efficiency. As the stage count rises, the pressure and therefore the required power increase. The motor should be selected to meet the power need at the pump operating point and leave a reasonable margin; an oversized motor means efficiency loss at part load, an undersized motor means overload and overheating. For the logic of power calculation, our article on motor power calculation and, for right sizing, motor load ratio and right sizing are core resources.

Vertical pump motors in a booster pressurisation system

IP55 Protection and Operating Environment

Pump motors mostly operate in environments with a risk of moisture, water splash and dripping. Therefore at least IP55 protection class is standard in a vertical multistage pump motor; in tougher (outdoor, washdown) environments a higher IP class may be needed. For IP protection class selection, our guide on IP protection class selection (IP55, IP65, IP66) offers a clear decision framework. HEM Motor pump motors are offered in IE4 efficiency class, in the 0.25 kW – 355 kW range, with F-class insulation and S1 continuous duty, suitable for vertical and horizontal systems.

Duty type also matters in a continuously running pump motor; booster systems generally run at S1 continuous duty. See our article on duty type (S1-S6) selection. For the savings a high-efficiency motor provides in pumps and fans, our article on the IE4 threshold in pumps, fans and compressors is valuable.

NPSH and Cavitation: What Does It Mean on the Motor Side?

NPSH (Net Positive Suction Head) is the minimum pressure condition required for the pump to operate without cavitation on the suction side. If NPSH is insufficient, cavitation occurs in the pump; this both wears the pump and creates irregular load and vibration in the motor. Although NPSH is not set by the motor directly, the irregular load caused by cavitation affects the motor life. Therefore the pump-motor assembly must be selected as a whole at the correct operating point. For vibration and balanced operation, our article on vibration and balance (ISO 10816/20816) is useful.

Driving the vertical pump motor with a VFD (frequency inverter) is a common solution to keep the pressure constant and save energy; by the affinity law, reducing speed yields large energy gains in a pump. On this, our articles on VFD energy savings in pumps and fans and VFD with an asynchronous motor show the way.

Difference Between Vertical Pump and Submersible Pump Motors

Two solutions often confused in water applications are the vertical multistage surface pump and the submersible (deep-well) pump. In a vertical multistage pump the motor runs above the water surface, in a dry environment, and connects to the pump by a flange; this provides ease of maintenance and replacement. In a submersible pump the motor is immersed in the water and has a completely different structure. In booster and surface pressurisation applications the vertical pump stands out, while for drawing deep water from a well the submersible solution is preferred.

This distinction directly affects motor selection: the vertical pump motor is a standard dry-type surface motor and IP55 protection is generally enough; the submersible motor is specific to underwater conditions. To compare the two solutions and select the right one, our articles on deep-well pump motor selection and, for wastewater applications, submersible drainage and sewage pump motor show the way. If salty and corrosive water is involved, the guide on seawater and saltwater pump motors is also valuable.

Continuous Operation, Heating and Efficiency

Booster and pressurisation systems often run all day, even without interruption. Therefore a structure suited to continuous operation, F-class insulation and correct cooling are critical in a vertical multistage pump motor. Under continuous full load the motor temperature settles at an equilibrium point; the insulation class must be selected to handle this temperature safely. For temperature-rise class, our article on temperature-rise class and temperature rise explains how it affects life.

In a continuously running pump motor, the energy cost makes up a large part of the total cost of ownership. Therefore selecting a high-efficiency (IE3/IE4) motor provides significant savings in the long run. For the effect of efficiency and right sizing on savings in a pump, our articles on efficiency class and right sizing and, for total cost, total cost of ownership (TCO) show the way. Power factor and reactive draw also matter at part load; you can see our guide on power factor (cos phi) and correction.

Sourcing the Right Motor for an Existing Pump

When renewing the motor of an existing vertical pump, matching the old motor nameplate data (kW, speed, flange type, shaft diameter) exactly is the safest path. A motor with the wrong flange or wrong speed means a return during installation. For correct matching from the nameplate, our articles on exact matching with nameplate data and direct replacement of an old brand motor are practical inspection tools.

For all your pump, booster and pressurisation motor needs you can review our efficient electric motors product group and the HEM Motor home page. If you want to drive the pump at low speed, our worm gear reducers group can also be considered. For fire and circulation lines, our article on boiler room and circulation pump motors offers additional information.

Frequently Asked Questions

Does a vertical multistage pump need a 2-pole or 4-pole motor?

In applications requiring high pressure, a 2-pole (around 2900 rpm) motor is usually preferred, because higher speed produces higher pressure. In applications requiring lower pressure and high flow, a 4-pole (around 1450 rpm) motor may be suitable. The right choice is made according to the pump operating point (flow-pressure).

What happens if the motor flange does not fit the pump?

Because the motor is connected directly to the pump by a flange in a vertical pump, the flange type, bolt circle, diameter and shaft dimension must match exactly. If they do not, the motor cannot be fitted mechanically and the installation is returned. Therefore matching the flange type and shaft dimensions with the existing pump before ordering is essential.

Which IP protection class is needed in a pump motor?

At least IP55 protection is standard in vertical multistage pump motors. In outdoor, high-pressure washdown or heavily humid environments a higher IP class may be needed. The right IP class is determined by the water and dust exposure of the environment in which the motor operates.

Get a Quote

Contact us to source the motor that fits your vertical multistage pump exactly, in the right power, with the right flange and the right speed. Share your pump operating point and the existing motor nameplate; let us determine the motor suited to high-pressure and booster applications. Phone: +90 (532) 345 49 86 — or request a fast quote via our contact page.

Purchasing and Selection Checklist

  • Is the pump operating point (flow Q, head H) determined?
  • Is the required power calculated from the operating point and pump efficiency?
  • Does the speed (2-pole ~2900 rpm or 4-pole ~1450 rpm) suit the pressure need?
  • Do the flange type, bolt circle, diameter and shaft dimension match the pump?
  • Is the protection class at least IP55 (higher if needed)?
  • Is the duty type S1 (continuous) and the insulation F-class?
  • Is the operating point correct against NPSH and cavitation risk?
  • If constant pressure/energy saving with a VFD is needed, is the motor compatible?