How to select a BLDC micropump

What a BLDC micropump is, and where it fits

A BLDC micropump is a small centrifugal pump with a brushless DC motor built in. In the wet-can (canned or wet-rotor) design, the motor’s permanent-magnet rotor carries the impeller and runs in the pumped liquid inside a sealed can. The stator and its drive electronics sit outside the can, dry. With no rotating shaft seal, there is nothing to leak or wear at the shaft; the liquid cools and lubricates the rotor bearings.

That combination gives long life, low noise and easy speed control from a DC supply, which is why these pumps are used for liquid cooling of batteries, power electronics, lasers and computers, solar and heating circulation, medical and laboratory devices, and beverage and dispensing equipment.

They are centrifugal pumps, so they move liquid against modest pressures at relatively high flow. They are not metering pumps: for precise dosing or high pressure, use a positive-displacement pump (diaphragm, gear or peristaltic) driven by a DC or stepper motor.

The decisions, in order

1. Design flow the process needs.
2. System pressure drop at that flow, converted to head.
3. Pump curve that passes above the duty point, with margin for speed trimming.
4. Fluid and temperature against the wetted materials.
5. Priming and dry running: how the pump stays flooded.
6. Life and duty in operating hours.
7. Noise and vibration.
8. Supply voltage, current and control: PWM, voltage, on/off, tacho feedback, protections.
9. Ports, size and mounting.

Step 1: design flow

Start from what the liquid has to do. For cooling, the flow comes from the heat to remove and the temperature rise you allow across the load:

Q = P / (ρ × c × ΔT)

with heat P in W, density ρ in kg/m³, specific heat c in J/(kg·K) and ΔT in K. Q comes out in m³/s; multiply by 60,000 for L/min.

For water (ρ ≈ 1,000 kg/m³, c ≈ 4,180 J/(kg·K)), removing 1 kW with a 5 K rise needs about 0.048 L/s, or 2.9 L/min. Water-glycol mixtures have a lower specific heat, so they need more flow for the same rise; use the coolant supplier’s property data.

For circulation, dispensing or filling, take the flow from the process: litres per minute delivered, or volume per cycle divided by the time available.

Step 2: system pressure drop and head

Add up the pressure drop of every component at the design flow: cold plates, heat exchangers or radiators, filters, quick-disconnects, valves, pipes and fittings. Component datasheets usually give pressure drop against flow.

In a closed loop there is no height to lift: the liquid goes up and comes back down. Pressure drop is all friction, and in most practical loops it rises roughly with the square of flow. In an open system (filling, fountains, transfer), add the static head: the height difference between the liquid surface at the inlet and the outlet.

Convert pressure to head, which is how pump curves are drawn:

H = Δp / (ρ × g) (m), with Δp in Pa and g = 9.81 m/s²

For water, 1 bar ≈ 10.2 m.

Step 3: match the pump curve to the system curve

A centrifugal pump’s curve falls from maximum head at zero flow (“shut-off”) to maximum flow at zero head. The system curve rises from zero (closed loop) or from the static head (open system). The pump runs where the two cross; Grundfos defines the operating point as where the system curve intersects the performance curve (Grundfos).

Do not choose on the headline “maximum flow” and “maximum head”. A pump listed at 10 L/min and 5 m will deliver neither at the same time: at 5 m head it delivers zero flow, and it reaches 10 L/min only against no resistance.

Choose a pump whose curve passes above your duty point at nominal voltage, then use speed control to bring it down. That margin covers component tolerances, fouling, cold, viscous glycol at start-up and filters clogging.

Speed control and the affinity laws

Grundfos summarises the affinity laws for centrifugal pumps: flow varies linearly with speed, head with the square of speed, and power with the cube, assuming constant efficiency (Grundfos). In a closed loop with no static head, the system curve and the affinity curves have the same shape, so flow scales almost directly with speed. That makes PWM a simple, effective way to set flow and to cut power and noise at part load.

Step 4: fluid and temperature

List the fluid exactly: deionised water, tap water, water with corrosion inhibitor, a specific water-glycol concentration, or another liquid. Then check every wetted part:

• Pump body and impeller plastics, against the fluid at the maximum temperature.
• Rotor can and magnet encapsulation.
• Bearings and shaft, often ceramic or carbon in wet-rotor pumps.
• Static seals, typically O-rings. Parker’s handbook, for example, rates EPDM for hot water and glycol-based fluids but not for mineral oil products, and nitrile for mineral oils but not glycol-based brake fluids (Parker).

Check the liquid temperature limit as well as ambient. The MP Series in our range is rated to 85 °C; TOPSFLO rates its TL-C01 for 100 °C hot water circulation in one variant and below 50 °C in its submersible variant (TOPSFLO). For food and beverage, ask for wetted-material approvals; TOPSFLO, for example, states its TL-C01 wetted materials are FDA approved.

Particles are a life issue. The rotor bearings run in the liquid, so abrasive debris wears them. Filter the loop and flush new systems before commissioning.

Step 5: priming and dry running

Small centrifugal pumps are generally not self-priming and must start flooded. The liquid lubricates and cools the bearings, so running dry damages them. TOPSFLO, for example, offers dry-running protection only as a customisable option on its TL-C01 (TOPSFLO).

Design for it:

• Mount the pump below the reservoir so the inlet is flooded by gravity.
• Bleed air from the loop at filling, and put the reservoir or air separator at the high point.
• Keep the inlet line short and generous; restriction on the suction side encourages cavitation.
• Use the pump’s tacho (FG) output or current signature to detect a stalled or dry pump and stop it.

Step 6: life and duty

Wet-can BLDC pumps are typically rated in operating hours. The MP Series in our range and TOPSFLO’s TL-C01 are both rated at 20,000 hours of continuous duty.

Convert that to the product’s real use:

• 8 hours a day, 250 days a year: 2,000 h/year, so about 10 years.
• 24 hours a day, every day: 8,760 h/year, so about 2.3 years.

If the product runs continuously and must last longer, plan a replacement interval, reduce speed when full flow is not needed, or consider redundancy. Life also falls with higher liquid temperature, dirty fluid and any dry running.

Step 7: noise and vibration

Wet-rotor BLDC pumps are quiet compared with brushed or seal-type pumps, but noise still matters in medical, laboratory, consumer and office equipment:

• Run slower. By the affinity laws, a pump slightly oversized and run at reduced speed is usually quieter than a small pump at full speed.
• Isolate the pump on rubber mounts and use flexible tubing at the ports, so vibration does not reach the chassis.
• Remove air. Entrained bubbles cause gurgling and rattling.
• Avoid cavitation by keeping the inlet flooded and unrestricted.

Step 8: supply and control

• Voltage: micropumps are usually 6, 12 or 24 V DC. Check behaviour across the supply range, especially on batteries.
• Current: the start-up and maximum current set the supply and wiring. The MP Series draws up to 4 A depending on model.
• Control: simple on/off, speed by supply voltage, or a PWM speed input. A tacho or FG output lets the controller confirm the pump is turning.
• Protections: look for locked-rotor, reverse-polarity, over-current and over-temperature protection in the pump’s electronics, or provide them externally. The industrial WP Series in our range lists blocked-rotation, reverse connection, under- and over-voltage, over-current, over-temperature and idling protection.

Worked example: liquid cooling for a power electronics module

Requirement: A 48 V power electronics module dissipates 600 W into a cold plate. The coolant is 50/50 water-glycol, with the outlet allowed to rise no more than 4 K above the inlet. The loop has a radiator and fan, a 100 µm filter and 2 m of 8 mm bore tubing. The controller has a 24 V rail and a spare PWM output. The product runs 16 hours a day.

Flow: using approximate 50/50 water-glycol properties (ρ ≈ 1,060 kg/m³, c ≈ 3,500 J/(kg·K); confirm with the coolant supplier’s data):

Q = 600 / (1,060 × 3,500 × 4) ≈ 4.0 × 10⁻⁵ m³/s ≈ 2.4 L/min. Allow 3 L/min for margin.

Pressure drop at 3 L/min (from component datasheets, in this example): cold plate 0.20 bar, radiator 0.10 bar, filter 0.05 bar, tubing and fittings 0.05 bar: 0.40 bar.

Head: H = 40,000 / (1,060 × 9.81) ≈ 3.8 m.

Pump choice: choose a 24 V model whose published curve gives at least about 3.8 m at 3 L/min with headroom, say 4.5 to 5 m at 3 L/min at full speed. The MP Series covers 1.5 to 39 L/min and heads to 14.5 m across 44 models, with PWM or voltage speed control, so a model in the lower part of the range fits; the exact model is picked from its curve.

Speed trim: if the chosen pump delivers 3.6 L/min at full speed in this loop, the affinity laws suggest about 3.0 / 3.6 ≈ 83% speed for 3 L/min. Pump power falls to about 0.83³ ≈ 57% of full-speed power, and noise falls with it.

Life: 16 h/day × 365 = 5,840 h/year; 20,000 h gives about 3.4 years. If the product must last 5 years, plan a replacement or ask for the expected life at the reduced speed and the actual coolant temperature.

Installation: pump mounted below the reservoir, filter on the return side ahead of the reservoir, air bled at commissioning, and the FG output wired to the controller to trip the module if flow stops.

Products in our range to consider

• MP Series wet-can BLDC micropumps: sealless brushless micropumps, 44 models at 6 to 24 V DC, 1.5 to 39 L/min, heads to 14.5 m, liquid to 85 °C, rated 20,000 hours, PWM or voltage speed control.
• WP Series industrial wet-can pumps: larger sealless BLDC coolant pumps from 80 W to 4 kW and up to 600 L/min, IP67/IP68, with PWM, CAN or RS485 control on some models.
• DINGS ZWWC slotless BLDC motors: 10 to 42 mm slotless brushless motors to about 50,000 rpm, for driving your own pump heads or compact fluid devices.
• D-Series coreless DC motors: brushed coreless motors, 13 to 40 mm frames, 3 to 48 V DC, with optional encoders and gearheads, often used on small positive-displacement pumps.
• DINGS ZYT brushed DC motors: 8 to 80 mm brushed motors at 12 or 24 V with gearbox, encoder and brake options, for peristaltic or diaphragm pump drives.

What else the build needs

• Controller or PWM output with tacho input. See motion controllers.
• Power supply sized for start-up current, with reverse-polarity protection.
• Heat exchanger, reservoir, filter and tubing sized for the design flow.
• Sensors: flow, temperature or pressure where the process needs confirmation beyond the tacho signal.
• Motor and drive if you build a positive-displacement pump around your own pump head. See servo and stepper motors and how to select a DC motor.
• Rotary union if coolant must pass into a rotating part. See how to select a rotary union.

Things that catch engineers out

• Choosing on maximum flow and maximum head, which never occur together.
• No pressure-drop data for the cold plate or radiator at the real flow.
• Glycol treated as water: lower specific heat, higher viscosity, especially when cold.
• Pump above the reservoir, so it starts dry or loses prime.
• Air left in the loop, causing noise and poor flow.
• Rated life in hours read as years on a continuous-duty product.
• Wetted materials not checked against inhibitors, glycol or cleaning agents.
• Unfiltered loops wearing the rotor bearings.
• No stall or dry detection using the tacho output.

Checklist

Send these to our engineers, or enter them in the spec builder:

• Fluid, concentration and additives; any food or medical requirements
• Liquid temperature range and ambient
• Design flow (L/min) and how you derived it
• Pressure drop at that flow, or a component list with datasheets
• Static head for open systems
• Supply voltage and range; available current
• Control method: on/off, voltage, PWM; tacho output needed
• Duty: hours per day and required product life
• Noise limits and mounting arrangement
• Port size and type, orientation, envelope
• Required protections (locked rotor, reverse polarity, over-temperature)
• Quantities and timing

For the wider family, see our servo and stepper motors overview.