Dosing chlorine, a coagulant or a pH-adjusting chemical drop by drop in exactly measured amounts at a water treatment plant, or adding a precise additive according to a recipe at a chemical plant, is a small but critical task. The metering and diaphragm (dosing) pumps that do this work are usually driven by relatively low-power motors; but selecting this small motor requires at least as much care as a large crusher motor for the system to work correctly. In this article we discuss the selection of dosing and diaphragm pump motors conceptually under the headings of low power, intermittent duty, precise flow, the right flange and corrosive-environment protection.

Low-power diaphragm pump motor and B14 flange for chemical dosing in water treatment

How Do Dosing and Diaphragm Pumps Work?

A diaphragm (membrane) pump is a positive-displacement pump that draws in and pushes out liquid through the back-and-forth motion of a flexible diaphragm. When the diaphragm moves forward it opens the discharge valve and pushes the liquid out; when it moves back it opens the suction valve and takes in new liquid. Because each stroke carries a specific volume of liquid, flow is directly proportional to the number of strokes and the stroke length. This is exactly the purpose in dosing pumps: to dose a specific and repeatable volume precisely. So the motor turns the mechanism (cam, eccentric or crank) that drives the diaphragm at a certain speed; flow adjustment is usually done by changing the stroke length or the motor speed.

This positive-displacement and intermittent operating character separates the dosing pump motor from a continuous-flow centrifugal pump motor. We covered the flow-head relationship and power matching in centrifugal pumps in centrifugal pump motor selection; in a dosing pump, the priority is not high flow but small and fully controlled flow. You can find how the required pump power is calculated in motor power calculation for pump, fan and conveyor.

Low Power: Correct Selection in the ≤1.5 kW Band

Dosing and diaphragm pumps usually run on low-power motors; most applications are 1.5 kW and below, and very small powers such as 0.12-0.55 kW are common. In this power band, compact aluminium-body motors are preferred; when used with a reducer, a low output speed is easily achieved. Even in low-power motors the correct power margin matters: an oversized motor takes up space and runs inefficiently, while an undersized motor struggles and heats up. We covered the importance of running the motor at the right load in load ratio and correct sizing.

At these small powers, HP-kW conversion and nameplate reading can also be confusing; to understand the power of small motors correctly, the article on HP-kW power understanding and ordering helps. For output speed and reduction ratio selection in geared low-power applications, see 220V geared motor output speed and ratio selection.

Intermittent Duty (S3) and the Importance of Duty Type

Dosing pumps often run not continuously but intermittently: they dose for a while, then stop, then start again. This operating profile is defined in motor duty types as S3 (intermittent periodic duty). Duty type directly affects the thermal design of the motor; the heating behaviour of a continuously running S1 motor differs from a frequently starting-stopping S3 motor. Frequent starting produces extra heat in the motor; so correctly determining the duty type of a motor that will run intermittently prevents early failure.

In intermittent and frequently starting operation, the starting current also gains importance; although this is usually manageable in small motors, in very frequent starting the motor and drive must be selected accordingly. We covered the logic of starting current in starting current and its reduction, and the effect of duty type on motor temperature in frequent starting in temperature monitoring. We also examined the contribution of insulation class (F/H) to thermal endurance in insulation class F and H.

Speed control with a frequency drive and reducer for precise flow in a dosing pump

Precise Flow: Control With VFD and Reducer

The heart of a dosing application is precise flow, because too little or too much chemical degrades both process quality and safety. There are two basic ways to adjust flow precisely. The first is to change the motor speed with a frequency drive (VFD); as speed drops the pump stroke count decreases and flow falls. This lets a single pump work precisely over a wide flow range. We covered how an asynchronous motor is controlled with a VFD in frequency drive with asynchronous motor.

The second way is to place a reducer between motor and pump to lower the output speed; this works especially when very slow and steady dosing is required. At small powers, worm gear reducers are a compact and economical solution; the desired output speed is achieved by selecting the reduction ratio. We covered motor matching to the reducer by IEC frame and flange in motor matching to the reducer, and the comparison of a geared motor with a separate motor + reducer in geared motor or separate motor + reducer. You can reach our products via our efficient electric motors and product pages.

The Right Flange: B14 and B5 Connection

In dosing and diaphragm pumps the motor is usually connected directly to the pump body with a flange; so flange type selection is critical. At small powers the two most common options are the B14 and B5 flanges. B14 (small flange) connects directly with the threaded holes on the front flange and is ideal for compact, low-power applications; frame types are between 56 and 160L. B5 (large flange) connects with larger flange holes and is preferred at higher powers. Selecting the right flange ensures an exact fit to the pump; the wrong flange type makes assembly impossible.

We covered the details of B14 and B5 flange connections and the mounting type codes, and the incompatibility between B5 and B14, in B5 or B14 mounting type selection. Connecting small-frame (56-71) motors to a worm gear reducer with a B5/B14 flange is common; you can find this matching again in IEC frame and flange matching.

Repeatability and Dosing Accuracy

The most sought-after property in dosing applications is repeatability: every time the pump runs it should deliver the same volume under the same conditions. This accuracy depends not only on the pump mechanics but also on the stable speed of the motor driving it. The more constant the motor speed under load, the more consistent the stroke count and therefore the flow. In asynchronous motors some slip (speed drop) occurs as load increases; keeping this slip small and predictable is important for dosing accuracy. We covered the relationship between speed, slip and actual speed in detail in speed, slip and actual speed.

When a frequency drive is used, the drive's ability to keep the speed constant directly affects dosing accuracy; a well-tuned VFD tries to hold the speed at the target value even when the load changes. When a reducer is used, the efficiency and ratio stability of the reducer gain importance. In both cases, selecting the motor at the right power is a precondition for stable and repeatable dosing, because a motor that is strained, overheating or overloaded cannot hold its speed and dosing accuracy degrades. So in dosing pump motor selection, "low power" never means "unimportant selection"; on the contrary, the accuracy of the system depends directly on the stability of this small motor. We gathered common mistakes and the right approach in motor selection in mistakes when buying a motor.

Corrosive Environment: IP55/IP65 Protection

Dosing pumps often work in corrosive and humid environments where chemicals are present: chlorine rooms, acid-base dosing stations, water treatment and chemical plants. In these environments the protection class (IP) of the motor is vital. IP55 provides protection against dust and water jets from any direction and is sufficient for most dosing applications. In more aggressive, washdown or heavily corrosive environments, IP65 (fully dust-tight, resistant to low-pressure water jets) is preferred. In a corrosive environment, the motor paint, terminal box sealing and, if necessary, special coating should also be evaluated.

We covered the need to consider blowers, mixers and pumps together in water and wastewater plants in water treatment and wastewater plant motors, and mixer, reactor and dosing motors in chemical-pharma-detergent plants in motor selection in chemical, pharma and detergent factories. You can find the correct cable gland and IP protection at the terminal box in terminal box and cable connection, and the general logic of IP protection classes in food factory motors: hygiene and IP protection. We gathered dosing and pump motors in biogas and treatment plants in biogas and treatment plant motors.

Frequently Asked Questions

What power motor is needed for a dosing pump?

Most dosing and diaphragm pumps are low-power; the majority of applications are 1.5 kW and below, often in the 0.12-0.55 kW band. The required power depends on the head (back pressure), the flow and the resistance of the mechanism driving the diaphragm. The key is to select a motor with power suited to the real need of the pump, with the right duty type and flange; an oversized motor means lost efficiency and space, while an undersized one means overheating.

Is a VFD or a reducer better for precise flow adjustment?

The two suit different jobs and are often used together. A frequency drive (VFD) changes the motor speed electronically and provides precise flow adjustment over a wide range; it is ideal where flow changes frequently. A reducer lowers the output speed mechanically and works when very slow, steady dosing is needed. If a constant but very low flow is required, the reducer stands out; if a variable and adjustable flow is required, the VFD stands out.

Is IP55 enough in a corrosive environment, or is IP65 needed?

In most dosing applications IP55 provides sufficient protection against dust and water jets. But if the environment is heavily corrosive, constantly humid or washed with pressurised water, IP65 should be preferred. Besides the protection class, the motor paint, terminal box sealing and, if necessary, special coating should also be evaluated, because in a corrosive environment the motor is protected not only by the IP class but also by the material and the coating.

Get a Quote

If you want to select a motor with the right power, duty type, flange and protection class for your dosing, diaphragm or metering pump, our expert team is at your side. Share your pump's power, flow, back pressure and environment data; let us determine the right motor and, if needed, the reducer solution together. Call +90 (532) 345 49 86 now or request a quote through our contact page. For the product range, visit our efficient electric motors page and our hemmotor.com homepage.

Dosing Pump Motor Selection Checklist

  • Have you determined the real power need of the pump (by flow and back pressure)?
  • Did you correctly define the duty type (continuous S1 or intermittent S3)?
  • Did you select a VFD, a reducer, or both together for precise flow?
  • Did you select the flange type (B14 / B5) to fit the pump body exactly?
  • Did you determine whether IP55 or IP65 is needed by the corrosive and humidity level of the environment?
  • Have the motor paint, terminal sealing and coating been evaluated for the corrosive environment?
  • Even at low power, has the correct power margin (neither large nor small) been chosen?
  • In frequent-start operation, have the insulation class and temperature monitoring been considered?