Power Resistors Put the Brakes on Medical Motors

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Feedback loops are the bread and butter of control systems. To prevent something from spiraling into larger and larger oscillations or spiked values, it’s critical to bring them back into normal ranges.

Not only is that the case for electrical circuitry, but for physical systems. Any moving device under the power of a motor of any sort  typically needs a braking system to control speed and bring something to a complete halt.

In the medical field, that is a surprisingly wide set of equipment and devices. Hospital beds have motors. So do CT, MRI, and X-Ray machines. And physical therapy equipment. Booms and even robotic surgical systems are motorized. Medical pumps, lab automation, biosafety cabinets, and ventilators.

Medical devices that use motors, whether rotor, linear, or any other form, need to control the movement activity. Forego a braking function and the result is disastrous, with malfunctions and errors on the lighter side and injuries and death potential outcomes.

Some form of electrical braking, rather than depending on mechanical friction alone, is generally called for, which means eliminating kinetic energy from the system, typically in the form of heat.

Power resistors, with their ability to handle and dissipate high transient currents are an intrinsic part of dynamic braking systems. The same underlying technologies and component designs that can slow and stop objects as large as trains and elevators can handle the energy loads in medical devices and equipment.

For example, Ohmite’s type 270 have wattage ratings from 12 to 1000 watts with an overload capability of 10 times the rated wattage for five seconds. The 280 is for low resistance values, from 0.1 ohm for a 300-watt unit, with special order units available for up to 1,500 watts and the ability to withstand frequent start-stop cycles.

HS aluminum housed resistors are designed for direct heatsink mounting with thermal compound with power dissipation from 15- to 300-watts. PFE and PFR series edgewound and round wire resistors can handle from 700 to 1,000 watts. The 14984 series is a round edgewound resistor for high currents.

BA series aluminum cased resistors provide a rugged wirewound design on ceramic cores and are ideal for high power and great stability in dynamic braking. Ohmite S series shunts can handle currents up to 1,200 amps and are ideal for laboratory environments. SHA precision manganin copper alloy shunts are designed for such applications as motor controls and battery management systems with currents up to 1,000 amps.

The SGM series carries a NEMA 3R enclosure rating, so is good for exterior or interior use and protects against falling dirt, accidental contact, splashing water, rain, snow, and external ice formation. WLRH heliohm wirewound resistors with nickel-copper, nickel-chrome or stainless-steel wire wound in a double helix around the center core, depending on size and model. Among other uses, they are popular in motor starting and speed control.

Medical devices generally come under the International Electrotechnical Commission set of standards broadly called IEC 60601. These are separate from the regulatory process and compliance that countries require from vendors to show the efficacy and soundness of medical devices but are every bit as important.

Various parts of IEC 60601 apply to broad categories of medical devices, in medical settings, at homes, or in special environments like military or industrial facilities. The standards cover use of equipment by virtually anyone, whether used by medical personnel, industrial workers, or patients and their caregivers. Also, the standards have undergone multiple revisions, and so familiarity with the most recent is important.

In other words, the quality of parts is important as designs will have to prove themselves compliant, so work with a vendor who can deliver quality. Ohmite, for example, has decades of experience in the needs and requirements of the medical industry and is well-known for its expertise in power resistors of all types, including those used in braking.

While descriptions and specs on Ohmite’s website will be helpful, they aren’t the only resource the company offers. Contact Ohmite and communicate with an expert who at no charge can help you specify the best type of filter for your application and requirements.