As the world's only manufacturer of standup MRI, Fonar is accustomed to forward thinking and problem solving. Its revolutionary equipment allows doctors to scan patients in a weight-bearing state, in a recumbent position, or both -- then compare the results. At the heart of the system are rugged and reliable electromagnets, yielding excellent image clarity. Providing a stable power supply to the electromagnets is vital to ensuring this high degree of reliability and image clarity.

In fact, controlling the voltage to a few parts per million is required, and is achieved using several stages of regulation. Regulating the incoming line voltage is the critical first step. Incoming line voltage, which is normally supplied at +/- 10%, is regulated to +/- 1%. Output from the voltage regulator is sent to a power supply where it is converted to dc with a stability of 1 ppm.

How important is the initial stage of the process? In areas that are prone to brownouts, voltage fluctuations can have disastrous effects. "In a best-case scenario, it would require the technologist to repeat a scan," comments Fonar's director of electrical systems engineering Mark Gelbein; "in the worst case it would turn the scanner off. A technician would then have to restart the system after the voltage dip had passed." Although the time required for the magnet to restabilize is brief, it could seem like an eternity to an anxious patient.

Because of the high (225) kVA required to power the electromagnets, the Fonar design team originally considered tap switchers, ferroresonant type voltage regulators, direct sine wave synthesis, and the Staco variable transformer controlled voltage regulator.

Three Key Considerations

When considering technology options, three parameters were vital to the final decision. From a performance perspective, the input line voltage, which is typically +/- 10%, has to be regulated to +/- 1%. Each of the considered technologies is capable of achieving this level.

The second consideration is reliability. The unit has to perform reliably on a daily basis with Fonar's low-impedance power source requirements. Linear rather than stepped regulation was required, since any transit voltage jumps would disturb the power supply, translating to output disturbance -- which is unacceptable.

Ferroresonant constant voltage transformers use a capacitor in series with the transformer coil and tend to be high impedance devices. Additionally, their sensitivity to load changes and inability to handle high inrush load well compounded the reliability problem. Further, because they can interact with switch mode power supplies to produce transients and electrical (output) noise, their resonant circuits make them particularly sensitive to frequency changes. The compound effect of these multiple reliability issues ruled out ferroresonant technology.

Other technologies such as tap-switchers use solid-state switching circuits for changing taps on transformer for regulation. Although these regulators are fast, their fast response time can often create instability when powering equipment with switch mode power supplies. Their output waveform tends to produce harmonics and radio frequency. The combination of these undesirable attributes made tap switchers a less than optimal choice.

The voltage regulator suggested by Staco Energy Products utilizes a variable transformer/buck-boost transformer design. The buck-boost transformer reduces the work the variable transformer must perform, and multiplies the transformer's power rating. The buck-boost reduces the current per-coil requirement of the variable transformer, resulting in increased MTBF -- a key measure of reliability.

Additional reliability-oriented advantages include the ability to withstand substantial current overloads, no additional harmonics or waveform distortions, a low impedance (less than 1%), high overload capacities (1000% for 30 cycles), attenuation of electrical noise, and imperviousness to frequency changes. Taking all reliability factors into consideration, Staco voltage regulators best fulfilled these criteria.

The final parameter in the decision-making process was cost. While each of the technologies can regulate voltage to the +/- 1% design specification, at the 225 kVA operating level that Fonar requires, all but Staco were cost-prohibitive: competing technologies were two to four times the cost of the voltage regulators provided by Staco Energy Products. Often this heightened price was accompanied by reductions in reliability (as noted previously). "The Staco Energy product really met our criteria," continues Gelbien. "It is the most cost effective solution at our kVA levels."

Redesigning the Product To Meet Customer Need

Many Fonar installations are used in 480 V service, so the standard model 225 kVA voltage regulator suited their needs and met the budget requirements. For 208 V installations, however, the task of voltage regulation was disproportionately expensive.

In its simplest terms, kVA or watts is volts times amps. So at a fixed kVA level (in Fonar's case, 225), as volts decrease, amps increase. At 208 volts, you essentially have twice the amps -- which means that the magnetic components are twice as large -- making the unit bigger, heavier, and more expensive.

"We had a 208 V unit that was 50% more expensive than the 480 V model," comments Gelbein. "So we asked Staco what they could do to meet our cost parameters without impacting the performance."

Staco Energy Products R & D custom engineered a solution at the customer's required price point. By including an autotransformer to step up the line voltage from 208 to 480 V, Staco Energy engineers were able to use standard 480 V components. This significantly reduced the cost of the 208 V version, while providing proven performance of the larger model.

"The Staco Energy team has been great to work with," continues Gelbein. "We had a situation early on where we customized the unit, name, and part number, which meant it was not technically UL approved. Staco Energy Products sent someone from the inspection agency into the field to get those units approved -- and they did so very quickly." Staco Energy Products did this while working with ETL and now have the regulators ETL test to UL standards.

Site-Specific Considerations

Although the doctors operating the standalone imaging centers are the ultimate end users of Fonar products, Staco Energy Products are generally made by contractors during the installation process. To accommodate varied installation parameters, Staco voltage regulators are available in NEMA 1 indoor or NEMA 3R outdoor enclosures. Regardless of the enclosure chosen, Staco regulators are 100% front-serviceable, and fit through a 25 in. door opening. No cooling fans are required. Extremely short lead-times are achieved through cellular production and streamlined manufacturing.

In addition to voltage regulators, Fonar uses a Staco Energy single-phase UniStar II UPS system to ensure uninterrupted power to the patient table -- allowing technicians to remove a patient from the gap of the magnet in the event of a power failure. This double-conversion, regenerate power, true online UPS provides clean, regulated, harmonic free, sinusoidal voltage and frequency. In addition to maintaining compliance with IEC601-1, this design makes good sense as it allows technicians to remove the patient quickly (far quicker and more professionally than a manual crank might).

As is often the case with complex technologies, the success of Fonar's standup MRI hinges on several components working together. Tight regulation of the incoming line voltage provides a stable input to the power supply, which in turn produces a very tightly controlled dc output, which powers the electromagnets. The Staco Voltage Regulator provides linear control with no reflected or added harmonic distortion. When faced with options for the control of the incoming line voltage, Staco Energy Products was the clear choice.