Maintaining electrical power quality within automated systems
Electrical filters and surge suppression: Surge suppression removes transients and swells, protecting electrical equipment from the effects of these overvoltage conditions. In contrast, electrical filters smooth the system voltage to remove noise and harmonics. Read about the filters on industrial power supplies used in large aircraft (with 400 Hz electrical sources) in the digikey.com article Power Supply Operation on a 400 Hz Source. Or consider another electrical-filter type that’s especially common in automated installations near the point of use — LC filters – to complement motor drives. LC filters are a type of tank or resonant circuit (also called a tuned circuit) with an inductor L and a capacitor C to generate output at a set frequency. LC filters for motors usually serve the purpose of converting a drive’s rectangular PWM output voltage into a smooth sine wave with low residual ripple. Benefits include the extension of motor life through avoidance of high dv/dt, overvoltage, overheating, and eddy-current losses. Surge protectors work by either blocking or shorting current — or combining surge-blocking and shorting measures. Surge protecting via blocking: Current can be blocked with inductors that damp sudden current changes. However, most surge protectors short when overvoltage occurs, diverting current back into the power
load isn’t directly connected to the mains supply but is always drawn from the system battery, which is continuously charged by the mains supply. The mains ac power is transformed to battery voltage and rectified to dc, so it can charge the battery. Power from the battery is then inverted to produce ac and stepped up by another transformer to mains voltage. This means that power quality issues in the supply do not affect the output and very high levels of power quality and protection are provided. However, it also results in considerably lower energy efficiency and higher upfront UPS cost. For all but the most sensitive and critical loads, an offline UPS coupled with a power supply with sufficient holdup time is a better choice. Conclusion Determining a design’s requirements for power quality is the first step
seconds. For applications where continuous power is required, a UPS must be combined with a generator to supply power while the generator starts up. UPSs protect equipment from power outages. Offline or voltage and frequency-dependent UPSs are the most cost effective but have two major shortcomings: ■ Under normal conditions, offline UPSs pass current directly past the battery to the output. When the UPS circuitry detects a power outage, a switch connects the battery to the output via an inverter. This means that the power may be interrupted by as much as 25 msec. ■ Offline UPSs also provide little to no protection against other power-quality issues such as surges and noise. In contrast, a line-interactive or voltage-independent (VI) UPS works in essentially the same way as a voltage and frequency-dependent UPS, but it has an additional voltage stabilizer to improve power-output quality under normal operation. Such systems still exhibit a switchover time during which power is interrupted — but it’s usually just 5 msec or so, which is well within the holdup time of most power supplies. Taking power-supply sophistication one step further to provide the greatest protection are online UPSs, also known as voltage and frequency independent UPSs. In UPSs, the
UPSs complement generators in critical applications
UPSs and generators for backup power ensure continuity of operations during blackouts and brownouts. UPSs use batteries and are typically designed to provide power for periods of a few minutes to a few hours. Generators use an engine to generate power for prolonged periods, limited only by the fuel available. UPSs provide an instant response to a power outage, ensuring that the power supply is uninterrupted. Generators on the other hand have a startup time of at least several
Figure 5: This is a Schaffner EMC Inc. LC sine-wave filter to help motor drives deliver smooth sine waves into attached motor windings without voltage peaks. The filter also allows for installations with longer motor-cable lengths. (Image source: Schaffner EMC Inc.)
The energy rating (typically in joules) is the maximum power that can be absorbed before components within the surge protector burn out and fail. An important but often overlooked specification for surge protectors is what happens when the surge protector fails. If a surge exceeds the protector’s energy rating and internal subcomponents fail, that protector will no longer be able to protect against further surges. But this doesn’t mean that power is cut off: some surge protectors (such as some designed to protect server or other electronic memory) will continue to supply power after failure. The only indication that surge protection no longer exists may be a warning light. Other surge protectors do indeed cut power or reduce power transmission when they fail.
distribution lines where it’s dissipated by resistance in the circuit’s wires. Surge protecting via shorting: Rapid shorting (triggered when voltage exceeds a set level) is done with a spark gap, a discharge tube, or a semiconductor device. Only rarely (during large or very prolonged surges) do surges melt the surge protector’s power lines or internal components. Capacitors may also damp out sudden voltage changes. Key specifications for surge protectors include clamping voltage, response time, and energy rating. The clamping voltage — also known as let-through voltage — is the maximum voltage allowed to pass through the surge protector. It’s typical for 120 V devices to have a clamping voltage of 220 V.
to preventing downtime and maintenance costs from dirty
utility power, electrical noise, and harmonics. These requirements significantly vary depending on the machine design and its functions. However, once these parameters are defined, design engineers can properly specify power supplies with filters, surge suppression, backup power, and power conditioning. This can profoundly improve the reliability of automated equipment.
Figure 6: This 24 V DC 5 A uninterruptible power supply (UPS) mounts on DIN rail and provides up to 25 minutes of backup power at full load. (Image source: Phoenix Contact)
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