Power Conditioning for Large Inrush and High Overload Current Applications
Many power conditioners are incompatible or marginally compatible with a common load type: devices or systems that require a large inrush and/or high overloads. Loads that contain devices such as motors, transformers, magnets, etc. fall in this category. Following is a brief discussion on the application of various power conditioner types in large inrush or high overload applications.
Large Inrush/High Overload Applications
Many electrical loads exhibit large inrush or overload currents which are necessary to establish magnetic fields or to overcome transient process conditions in applications such as manufacturing, processing, medical imaging, municipal services, etc. To further complicate matters, the widespread integration of sensitive electronics in virtually every type of modern equipment often makes power conditioning a necessity. High inrush current profiles of devices like motors and transformers nearly shoot up to 5 to 15 times full load current (FLA) within the first half cycle and then slowly decay over the next 5 to 60 cycles to a steady state current. It is important to consider the duration of the overload as well as the magnitude.
Power Conditioners – Voltage Regulators
The typical power conditioner or voltage regulator takes a broad range of input voltage and outputs a regulated voltage within a narrow band. While actual performance varies by product, most power conditioners (except the UPS) will accept +10% to -25% of the nominal input voltage and regulate this to ±3% of the desired output voltage. For many UPS units these numbers are ±10% on the input and ±1% on the output.
Inrush/Overload Capacity of Power Conditioners
Ferroresonant Transformer (Constant Voltage Transformer)
The ferroresonant transformer (ferro) has a significant limitation in high inrush/overload situations. When the load current exceeds about 150% of rated current, the output voltage collapses until the current level is reduced. This voltage collapse is so severe the ferro essentially turns into a current-limiting inductor preventing the load from starting or operating. To overcome this, the ferro is typically oversized by 2 to 3 times the rated load.
Electronic Voltage Regulator
Electronic voltage regulators use power semiconductors to accomplish voltage regulation is one of two ways:
1. By switching taps on a transformer, or
2. By converting the incoming AC power to DC, manipulating the DC power and then converting the DC power back to AC.
While power semiconductors afford electronic voltage regulators their very high speed of voltage correction, they can also be the Achilles heal when it comes to high inrush/overload applications. A power semiconductor will fail prematurely when subjected to the cumulative effects of overcurrent and/or overheating caused by loads requiring high inrush or overload currents.
Many electronic voltage regulators will have an overload capacity rating limited by their power semiconductors while some have current-limiting designs that can accept very high load currents without compromising the power semiconductors.
It is possible to oversize electronic voltage regulators to get more overcurrent tolerance, but this may not guarantee that the power semiconductors are not overstressed in a high inrush/overload application. The better choice is to select an electronic voltage regulator with a high overload capacity.
Mechanical Voltage Regulator
Mechanical voltage regulators for power quality applications typically have very high overload capacities, however their use is limited and rapidly dwindling for a number of reasons:
1. Their correction speed is too slow to support electronic equipment,
2. They are often limited to smaller kVA ratings, and
3. Frequent voltage fluctuation results in high maintenance.
In high inrush/overload applications where the voltage fluctuations are infrequent and correction speed is unimportant, mechanical voltage regulators might be a good choice.
Uninterruptible Power Supply (UPS)
The typical UPS has a low tolerance to high inrush/overload currents and is often substantially oversized to accommodate peak currents. Power semiconductors and/or ferroresonant transformers found in UPS units have the problems similar to those discussed above.
When a high inrush/overload application is so critical that it demands the power interruption protection provided by a UPS, it is important to generously oversize the UPS for the peak current.
Conclusion
It is possible to use any type of power conditioner – voltage regulator in high inrush/overload applications, however some special considerations are needed if the power conditioner has limited overload capacity:
1. Determination of frequency of occurrence and magnitude of peak current loads, and
2. Sizing the power conditioner to avoid excessive stress on power semiconductors and/or keeping maximum currents below the critical limits for ferroresonant transformers.
Electronic voltage regulators with high overload capacity and mechanical voltage regulators (where applicable) are the best choices for high inrush/overload applications since they eliminate the need for oversizing, provide higher reliability and are less likely to be affected by downstream faults. High overload capacity power conditioners are generally specified as accepting at least 1,000% of rated current for 60 cycles or more.
The Sure-Volt™, like all power conditioners and voltage regulators from Utility Systems Technologies, is designed for use in high inrush/overload applications - having an overload rating of 1,000% for 1 second.