Designing medical devices that use pumps takes special know-how regarding vacuums, pressures, and how motors integrate into a design. In fact, there are so many variables in pump technology that most applications probably need custom-built units. Information from manufacturers' literature can help get you in the ballpark. Then, it may be necessary to turn to a supplier that provides the best in performance, customization, qualification, value added, and price.

Pump technologies

To start, any pump must satisfy the job specifications. These might include, for example, the size envelope the pump must fit, flow at the working point, a duty cycle, and an allowable noise level. Most pumps use dc brush motors, with or without a core. A few use brushless dc motors. The major pump technologies include:

Proprietary piston pumps such as our WOB-L. It uses a one-piece rod assembly with a Teflon piston cup that rides inside an aluminum cylinder. The pumps thereby generate little heat. They work well for medical devices such as dental carts, autoclaves, O₂ concentrators, and emergency-vehicle crash carts.

These piston pumps provide moderate-to-high pressures (from 1 to 11 bar) and flows (from 5.4 to 187 l/min) in compact sizes (from 49.5 × 30 × 82 mm to 255.3 × 185.7 × 396.2 mm). Most units are smaller than the size of a beer can. Flows of more than 140 l/min or working pressures more than 5 bar require one-horsepower configurations. But, in most cases, fractional-horsepower motors, typically 1/3 hp or lower, fulfill performance and weight requirements.

The piston technology is efficient, especially compared with similarly sized diaphragm pumps. One reason: the pumps have a permanent split-capacitor motor with many motor windings and a start capacitor that supplies much of the magnetizing current for the rotor. These motors provide more starting torque than a typical shaded pole motor.

Piston pumps are also relatively quite, and, in some cases, rate at 55 db or lower. They need no oil to run, so their output air is clean. Several seal materials are available to meet demanding applications that can also be configured for a high-pressure restart capability. (Material options are determined during discussions with the OEM.) Piston pumps require filtration on the intake to keep the air free of dirt and debris, and the air must be dry. It should be no surprise that dirt, heat, and humidity have a negative effect on service life.

Diaphragm pumps, often used in nebulizers, air mattresses, and drug-delivery devices, also run dry, and are efficient and quiet. The pumps tolerate aggressive media such as dyes, alcohols, ammonia, and formaldehyde, and can be configured to move fluids in chemical and standard PH versions.

The pumps can be used to generate compressed air or a vacuum. Flows range up to 88 l/min, pressures to 2.8 bar, and end vacuum to 10 mbar absolute. Motor options for diaphragm pumps include shaded pole, permanent split capacitor, permanent magnet, ironless core, and brushless dc. These pumps can be configured in twin-head designs for the highest flow (parallel) or deepest vacuum (series).

Rotary-vane pumps work well in devices such as blood-pressure cuffs and compression-therapy equipment. They run dry, are self-lubricating, and have the highest air-flow to physical-size ratio. They also have the smoothest air flow of any pump because they do not pulsate and have a simple design that contributes to a long life. However, rotary-vane pumps are less efficient than piston or diaphragm pumps, and do not suit applications that need over 1 bar.

Miniature rotary vane pumps have a characteristic whine when running at speeds up to 6,000 rpm. Exhaust and intake ports require filtration to trap vane debris which can contaminate the downstream air. The pumps feature flows to 283 l/min, pressures up to 1.0 bar, and end vacuum to -878 mbar. Motor options include split phase, capacitor start, ironless-core dc and three-phase (polyphase).

Articulated piston pumps usually work in settings that require a long life, especially with full-pressure restarts. Examples might include supplying oxygen and vacuum in hospitals and air for dental tools. The pumps have high-pressure capabilities and can either be oil-lubricated or oil-less.

These large, heavy pumps can be noisy, so they often get installed in basements on various-size tanks. Intake air must be filtered and dry, and because of the pump's connecting rod-design, the device tolerates running in dusty and dirty environments. Flows are up to 227 l/min, pressures up to 12 bar, and vacuums to -928 mbar. Motor options include split phase, capacitor start, capacitor run, permanent magnet, and polyphase.

Linear pumps are basically a dry running and oil-free diaphragm design. They actuate with either a linear or vibrating armature. A linear actuator works by applying alternating ac current to the electromagnet, which moves the actuating rod to force the diaphragm up and down. A vibrating actuator passes the main ac to the electromagnet by a rectifier diode. It suppresses one of the half-waves, while a spring returns the vibrating arm to its initial position during this interval.

The pumps deliver flows up to 270 l/min, pressures up to 1.6 bar, and vacuums to -878 mbar. Motor options include ac and dc voltages. These designs have a long life, low noise level (>45 db), and high efficiency.

Pump Listing
As the chart indicates, no one single technology is good across the board.

Typical pump motors

Most major pumps use dc brush motors, with or without a core, or brushless dc motors. Brush (made of carbon) dc motors with an iron core are relatively inexpensive, but last only about 500 to 3,000 hours. That's because the motor's spinning (in most cases from 3,000 to 6,000 rpm) wears the brushes against the commutator. It is possible to replace the brushes, but the second set only gets half the life of the first set due to commutator fatigue and wear.

Brush dc motors without a core offer significant advantages over their core counterparts. Benefits range from improved efficiency to higher power density in a smaller frame size. For example Portescap's (portescap.com) self-supporting coil technology makes for light motors with low inertia. They suit high-efficiency applications requiring many starts and stops.

For any motor, Joule loss associated with a motor's heating tends to degrade its performance over time. This is governed by the coil resistance, R, and the torque constant, k. The lower the motor-regulation factor (R/k²), the better and more efficiently a motor performs. Thus, given a similar motor length, a 16-mm diameter Portescap model 16G motor with R/k² (103/Nms) in the 50 to 70 range has better performance as compared to a Portescap model 16N motor with R/k² (103/Nms) in the 300 range.

The motors have special precious metal or carbon brushes with small contact surfaces and pressures for low electrical resistance and less friction. This minimizes brush erosion and extends the motor's life.

For a diaphragm pump running at 18 l/min open flow at 2,800 rpm, one model of Portescap motor supplies a continuous torque of 2 mNm. To find the supply voltage, first find the required drive current with:so the supply voltage is 8.8 V. Thus, a 9 V battery pack will work for a portable pump. Or, the battery pack acts as a reserve in case of power failure for critical equipment such as infusion pumps.

Brushless dc motors, on the other hand, have a starting torque and dynamic response equal to or better than brushed motors. Other advantages include rapid acceleration and deceleration, no mechanical wear from brushes, and precise speed regulation. The motors comprise a rotor containing a series of permanent high-energy rare-earth magnets. The rotor sits in a housing surrounded by the stator, which is made up of several coils of wire. It requires an encoder to sense motor positioning and velocity.

Brushless dc motors require a controller that can be either built into the motor or supplied separately. The controller determines the rotor rotation by energizing the motor coils in a sequence. The controller and sensor combination act as an electrical commutator, as opposed to a mechanical commutator in a brushed dc motor. However, linear, vibrating armature, and articulated-piston pumps are not available with brushless dc motors.