Deep Well Pump Motor Selection: A Calculated Buying Guide

The most expensive mistake made when buying a deep well pump motor is choosing the motor "approximately." An undersized motor stalls in the field, burns the winding and takes your well out of service in the middle of the irrigation season; while an oversized motor inflates both the purchase price and the electricity bill you will pay for years. The right selection is made by calculating three fundamental quantities together — flow, pressure (total head) and speed. As HEM Motor, we have been manufacturing electric motors in Turkey since 1979; we have supplied motors to thousands of deep well applications, from agricultural irrigation cooperatives to drinking water unions, from geothermal facilities to industrial wells. In this guide, we explain the calculation an engineer makes before ordering in the buyer's language, step by step and with example scenarios. By the end of the article, you will have a clear technical specification in hand to send to your supplier.

Step 1: Clarify Your Flow Requirement

Flow (Q) is the amount of water the pump must deliver per unit time, and is usually expressed in m³/hour or litres/second. This is the first question of the purchasing process, because it directly determines both the pump stage count and the motor power. Pay attention to three points when determining the flow requirement:

The well's yield: The flow the pump can deliver must not exceed the safe yield of the well. The safe yield value in the well test report (pumping test) is the upper limit of your selection. A system that tries to draw more water than the well can give drops the dynamic level below the pump's suction level, and the motor runs dry and is damaged.

Usage requirement: In irrigation the plant water consumption and the area to be irrigated, in drinking water the population and the tank volume, in industry the process requirement determine your flow. You can find the hourly flow by dividing the total daily water requirement by the daily operating hours.

Future growth: If the flow requirement will increase, it is much more economical to plan this not with a pump stage but with the right motor and pump selection from the outset.

Step 2: Calculate the Total Pressure (Head)

The quantity used on the pressure side is the total head (Hm), expressed in metres of water column (mwc). In a deep well application it consists of four components:

1. Dynamic level: The depth from the surface of the water level in the well while the pump is running. Not the static level, but the dynamic level that drops during pumping, is taken as the basis.

2. Above-surface height: The geometric height difference from the wellhead to the elevation of the tank or hydrant to which the water will be pumped.

3. Pipe friction losses: The friction loss in the column pipe and the transmission line. In a practical preliminary calculation, 5-10% of the line length can be taken; the exact calculation is made according to the pipe diameter and flow.

4. Operating pressure: The pressure desired at the outlet, such as drip irrigation, sprinkler or mains pressure (1 bar = approximately 10 mwc).

The sum of these four items gives you the Hm value. For example, if you are pumping water from a well with an 80 m dynamic level to a tank 20 m higher, with 10 m of friction loss, Hm = 80 + 20 + 10 = 110 mwc.

Step 3: Move from Flow and Pressure to Motor Power

The pump shaft power is calculated with the following formula widely used in the sector:

P (kW) = Q (m³/h) × Hm (mwc) / (367 × η)

Here η is the total efficiency of the pump (typically in the range of 0.60-0.75 in deep well pumps). Let us continue with the example above: for 40 m³/h flow, 110 mwc pressure and 0.70 pump efficiency, the shaft power comes out as P = 40 × 110 / (367 × 0.70) ≈ 17.1 kW. The motor is selected above this shaft power with a safety margin; leaving a 10-15% reserve is common engineering practice in deep well applications. In this example the right selection would be an 18.5 kW motor.

Let us give a critical warning here for buyers: when the operating point on the pump curve shifts to the left or right, the power requirement changes. When the well's dynamic level falls with the season, the pump shifts to higher pressure; when the level rises, to higher flow. Ask your supplier to select the motor not according to a single point but according to the entire realistic operating band of the pump curve. Because HEM Motor produces from 0.55 kW to 355 kW, it offers a solution from stock at intermediate powers too; you are not forced to buy an unnecessarily large motor with a "we have 22 kW on hand, it'll do" approach.

Step 4: 1500 rpm or 3000 rpm?

Speed selection is one of the most frequently questioned topics in pump motor purchasing. In asynchronous motors on a 50 Hz mains, 2-pole motors rotate at about 3000 rpm (2900-2950), and 4-pole motors at about 1500 rpm (1450-1480). Both options have clear advantages:

The advantages of the 3000 rpm motor

According to the pump laws, pressure is proportional to the square of the speed; at the same impeller diameter, a 3000 rpm pump produces much higher pressure. Thanks to this, for the same pressure, fewer stages, a shorter pump and a smaller-frame, more economical motor suffice. In narrow-diameter wells (for example 6"-8" wells), high speed is often the only sensible option due to the space constraint. In deep well vertical-shaft pumps and submersible applications, 3000 rpm is a common standard.

The advantages of the 1500 rpm motor

Low speed means less wear, lower noise and, especially in sandy, sediment-laden waters, a markedly longer pump life. Because impeller and bearing wear increase rapidly with speed, choosing 1500 rpm in wells with sand intake lowers the total operating cost. In high-flow, relatively low-pressure applications (large-diameter wells, channel pumping), 1500 rpm stands out both hydraulically and economically.

The practical rule is this: if it is a narrow well + high pressure + clean water, 3000 rpm; if it is a wide well + high flow + sandy/abrasive water, 1500 rpm prevails. On projects where you are undecided, you can consult us with the well log and water analysis; as a manufacturer we keep a wide stock at both speeds.

Step 5: Vertical or Horizontal? Motor Selection by Mounting Type

In deep well applications, the position of the motor determines the system architecture:

Vertical-shaft (lineshaft) deep well pumps

The pump is at the bottom of the well and the motor at the wellhead, with the power transmitted by a long shaft. In this system the motor must be suitable for vertical operation; it must have a bearing arrangement that can carry the axial load and a suitable mounting form. Because the maintenance of the motor can be done easily at the wellhead, it is a common choice in drinking water unions and large irrigation facilities. For this type of project, you can review the models suitable for vertical operation on our deep well pump motor page.

Horizontal centrifugal and booster sets

In systems where the water is taken into an intermediate tank and pumped from there by a horizontal centrifugal pump, standard B3 foot-mounted or B5 flange horizontal motors are used. Horizontal mounting is the simplest solution in terms of coupling alignment, bearing life and ease of service. You can find our wide horizontal pump motor options on our pump electric motors page.

A critical purchasing note on mounting type selection: be sure to state in the order if a motor will operate vertically. If a motor bearing-arranged for horizontal operation is mounted vertically, the axial load exhausts the bearings early, and this situation may fall outside the warranty scope.

Three Example Purchasing Scenarios by Well Type

Scenario 1: Agricultural irrigation well (8", 90 m dynamic level)

A business irrigating its 60-decare garden by drip irrigation calculates Hm = 122 mwc with 30 m³/h flow, a 90 m dynamic level, 12 m friction loss and 2 bar (20 mwc) operating pressure. The shaft power is 30 × 122 / (367 × 0.68) ≈ 14.7 kW; the right motor, with a 12% reserve, is in the 16.5-18.5 kW band. Due to the narrow well and clean water, 3000 rpm is chosen. Because the energy cost stands out in intensive seasonal use, the IE4 efficiency class should be considered instead of IE3.

Scenario 2: Drinking water union (12" well, lift to tank)

On a lift line running 18 hours a day, with 80 m³/h flow and 70 mwc pressure, the shaft power comes out as 80 × 70 / (367 × 0.72) ≈ 21.2 kW, and a 25 kW class motor is chosen. Because the annual operating hours are very high, the return of the efficiency class is large: the few-point efficiency difference provided by the IE4 motor pays back the price of the motor in a few years as energy saving. You can find the detail of this calculation in our article on the payback period of replacing an old motor with IE4.

Scenario 3: Industrial facility process well (sandy water)

A facility drawing 50 m³/h of water from a well with sand intake chooses a 1500 rpm multistage pump and a suitable 4-pole motor to limit wear. When the speed is halved the pump stage count increases, but the impeller life lengthens manifold; instead of a pump overhaul every three years, the total cost falls with an overhaul every six to seven years. On the motor, IP55 protection and class F insulation should be a standard requirement for the dusty and humid wellhead conditions.

The 4 Most Common Mistakes in Purchasing and Their Costs

1. Calculating according to the static level: When the well starts to operate, the water level falls. A motor selected according to the static level becomes insufficient at the real dynamic level; the pump cannot deliver the desired flow, or the motor runs constantly in overload. The solution is simple: the calculation must definitely be made over the dynamic level in the pumping test.

2. Assuming the pump efficiency higher than it is: The best efficiency point in the catalogue and the real operating point in the field are different. Taking the efficiency in the realistic 0.65-0.70 band instead of 0.75 protects you from the mistake of selecting the motor one power step too small.

3. Leaving the starting method to the end: In deep wells, the use of star-delta or a soft starter is common; in drive (frequency converter) systems, the motor's suitability for the drive supply must be confirmed in advance. Incompatibilities experienced on the panel side after the motor is bought can delay commissioning for weeks.

4. Choosing the efficiency class only by price: On every pump motor running over 2000 hours a year, the energy cost exceeds the purchase price of the motor several times each year. The few-point efficiency difference between IE3 and IE4 pays for itself in a short time on intensively running wells. On a small garden well running a few hundred hours per season, however, IE3 may be economically sufficient. The right decision is made according to the operating hours; as a company that produces both classes, we make this calculation with your data at the quotation stage.

Pre-Purchase Technical Specification Checklist

When requesting a quote, send your supplier the following information; this list ensures both that you get the right price and that you zero out the risk of the wrong product: the required flow (m³/h) and total head (mwc), the well diameter and dynamic level, the water quality (sand, corrosive matter), the speed preference (1500/3000), the mounting position (vertical/horizontal) and the mounting form (B3, B5, B35), the mains voltage and starting method (direct, star-delta, drive), the protection class (IP55) and insulation class (F), the efficiency class (IE3/IE4) and daily operating hours, the delivery location and deadline expectation. As HEM Motor, we provide a clear quote within the same day with this information; you can reach all the high-efficiency models from our IE4 electric motors category.

A significant proportion of businesses investing in a pump motor also operate a fire pump set in the same facility. We addressed the questions that need to be asked when selecting a motor for critical systems in detail in our guide on the 10 questions to ask when buying a fire pump motor.

Frequently Asked Questions

How many kW motor should I choose for a deep well pump motor?

The motor power is calculated according to the flow, total head and pump efficiency with the formula P = Q × Hm / (367 × η), and a 10-15% safety margin is added on top. For example, for 40 m³/h flow and 110 mwc pressure, an 18.5 kW motor is typically required. If you send us your well test report and pump curve, we check the calculation free of charge and determine the right power together.

How quickly is a deep well pump motor delivered from stock?

Thanks to its production facility in Turkey and its strong stock structure, HEM Motor ships pump motors in common powers and speeds usually the same day or the next business day. Configurations such as a special shaft, vertical-operation bearing arrangement or a different voltage are completed on our own production line in short times committed to in writing.

My motor burned out during the irrigation season; how do I supply an urgent equivalent motor?

It is enough to convey to us, by telephone or photo, the power, speed, body and mounting details on the old motor's nameplate. Because we are the manufacturer, we ship standard equivalents from stock immediately, and on motors whose nameplate is unreadable we identify an equivalent from the shaft and flange dimensions. To prevent a loss of season, we can organise shipment by courier or vehicle the same day.

Get a Quote

Choose your deep well pump motor with calculation, not estimation. Send us your well information, your pump curve or the nameplate of your existing motor; let HEM Motor's engineering team — a manufacturer since 1979 — determine the right power, speed and mounting type together with you. Let us give a price the same day with the advantage of strong stock and fast delivery. You can reach us right away on +90 (532) 345 49 86 or send your quote request via our contact us page.