Maintaining electrical power quality within automated systems
There is always some noise in the electrical supplies of machinery caused by the resistance of the conducting wires involved. Such noise is called thermal noise and is generally a negligible disturbance. More significant and potentially detrimental noise is caused by local loads such as welders and electric motors. Noise from such components and systems can often be difficult to quantify — and pose the most risk of causing affected equipment subcomponents to overheat, wear, and even fail. Electrical harmonics are voltage or current disturbances at frequencies that are integer multiples of the system ac
frequency. They are caused by nonlinear loads such as rectifiers, computer power supplies, fluorescent lighting, and certain types of variable-speed electric motors. Current harmonics tend to be larger than voltage harmonics and actually tend to drive the latter. These electrical harmonics (due to the way they induce heat generation) can dramatically degrade the efficiency and life of electric motors. They may also cause vibrations and torque pulsations in the mechanical output of electric motors, which shortens the life of the power-transmission subcomponents integrated into the motors — especially the shaft- supporting bearings.
Grounding: Proper grounding is essential for a power supply to function correctly. It provides a reference voltage (from which all other voltages are measured) and a return path for electrical current. Read the Digi-Key article What You Need to Know about Ground Fault Sensing and Protection for more on this topic. Isolation: Although non-isolated power supplies can be more energy efficient and compact, isolation between the input and output voltage protects against dangerous voltages passing to the output in the event of a component failure. Isolation may also be required to protect operators from dangerous voltages or to protect equipment from transients and swells. Forms of isolation include: ■ Physical insulation between components ■ Inductive coupling through a transformer — power converters that change the voltage of a power system ■ Optical couplings — which are most suitable for signal transfer between different parts of a power system while ensuring a very high level of isolation
Figure 4: Power supplies often function as power converters to either 1) change an ac source’s voltage or frequency or 2) rectify or otherwise convert ac power into dc. Case in point: This 48-V 400-W AC- to-DC pulse-frequency-modulated (PFM) converter from Vicor Corp. has integrated filtering and transient surge protection. One caveat: The Vicor Integrated Adapter (VIA) converter only accepts input from an external rectified sinusoidal ac source — with a power factor maintained by the module. Harmonics conform to IEC 61000-3-2 and internal filtering enables compliance with applicable surge and EMI requirements. (Image source: Vicor Corp.)
Key power-system parameters
Two important specifications for power supplies include the power factor and holdup time . Power factor is a dimensionless ratio used to describe the difference between true power and apparent power in ac systems. Apparent power is the combination of the true power and the reactive power. Reactive power in turn is drawn from the network, stored momentarily, and then returned without being consumed. This is typically caused by inductive or capacitive loads, which leads to the current and voltage being out of phase. Reactive power increases the load on distribution systems, reduces power quality, and leads to higher energy bills. Ideally a system will have a power factor of one — meaning there’s no reactive power in the system. Designs with power factors below 0.95 cause increased load on the distribution system and may incur reactive-power charges. Holdup time is how long a power supply can continue to supply power within its specified voltage after a power outage. Consider the case of uninterruptible power supplies (UPSs) and generators — types of backup power used to ensure continuity of automated operations during blackouts and brownouts. As detailed more fully
in this article’s final section, a UPS must supply power for any significant period. But depending on the UPS design, these may introduce a delay of up to 25 msec between a utility-power failure and the UPS initiation of power delivery. Power-supply holdup time allows the power supply to bridge this gap, largely using power stored in capacitors. In fact, switch- mode power supplies tend to have longer holdup times than linear power supplies due to their higher-voltage capacitors. Other features to address machine-induced power problems Grounding, isolation, and filtered power converters provide the foundation for a quality power supply. Figure 3: Shown here is a TML 100C Series, 85-100 Watt AC-to-DC power module from Traco Power. Active power-factor correction (PFC) ensures a power factor of better than 0.95 (for 230 VAC) and better than 0.99 (for 115 VAC). (Image source: Traco Power)
Figure 2: Harmonic waveforms are frequency integer multiples of some fundamental waveform that (in electrical power systems) can combine with the fundamental waveform and cause problems. Harmonics typically originate from some electrical load or within an attached piece of machinery. (Image source: Design World)
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