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The Operation, Application and Comparison of Automatic Voltages Regulators in AC Power Applications
6 The Mechanical (Type) Voltage Regulator
The mechanical type voltage regulator has the greatest number of different names often reflecting their means of operation and/or purpose:
· Mechanical tap changing regulator
· Electromechanical voltage regulator
· Tap changer
· Electromechanical voltage stabilizer
· Tap-switching voltage regulator
· Motorized variable transformer
· Servo voltage regulator
· Magnetic induction voltage regulator
· Servo voltage stabilizer
· Magnetic induction voltage stabilizer
· Motorized variac
· Motor-driven variable autotransformer
· Variable autotransformer
· Line voltage regulator
· Line drop compensator
· On load tap changer
· LDC
· OLTC
· Step voltage regulator
· Auto-boost regulator
This type of voltage regulator has been in use for more than 50 years and has seen refinements throughout the years primarily with advances in control technology and servo drive systems. This type of voltage regulator dominates the electric utility market and is the most popular type outside of North America for power quality applications. As more electronics are deployed in industrial settings worldwide, the mechanical voltage regulator is being displaced by the electronic voltage regulator.
While the mechanical voltage regulator comes in a broad variety of designs they all share the characteristics of having one or more servomotors to physically move some component(s) within the unit. The purpose of this movement is to affect voltage regulation by changing the turns ratio or magnetic coupling of a transformer. Included in this type of voltage regulator are devices for power quality control as well as devices used by electric utilities for correcting voltage (line) drop in distribution lines (see Power Quality versus Line Drop Compensation below).
The two primary methods of operation used by mechanical voltage regulators are: (A) by changing the turns ratio, or (B) by altering the magnetic induction between the primary and secondary coils of a transformer by physically changing their orientation.
[h=2]The Operation, Application and Comparison of Automatic Voltages Regulators in AC Power Applications[/h][h=4]6.1 Tap Changing Voltage Regulator Operation[/h]
Simply said, when a transformer has an equal number of winding turns on the primary (input) and secondary (output), the input and output voltage should be equal. By adding or subtracting the number of turns on the secondary, the output voltage can be changed – up or down, respectively. Rather than physically changing the number of turns, the turns ratio can be altered by changing the location of the physical connection to the secondary. Transformers often have multiple locations (known as “taps”) for connection to the secondary to adjust the output voltage.
The mechanical tap changing voltage regulator utilizes contactors or brushes along with some type of motorized drive system to change the taps on the secondary of the transformer. A simplified diagram is shown at the right. The controller monitors the output voltage and operates the drive system to change taps until the output voltage is within the proper range. There are many variations on this type of arrangement.
[h=2]The Operation, Application and Comparison of Automatic Voltages Regulators in AC Power Applications[/h][h=4]3.3 Sensitivity to Voltage Levels and Voltage Fluctuation[/h]Every piece of electrical equipment will operate within a range of voltage levels, however not necessarily with optimal performance. When the voltage level falls outside of its operational range, a device may be unable to start or operate, it may malfunction or the device may be damaged. The width of the voltage level range within which a device will operate is a measure of its sensitivity to voltage level.
A device that will operate fairly well within a range of +/-10% of nominal voltage would be considered to have a relatively low sensitivity to voltage level or voltage fluctuations. A device that operates properly only when the voltage level is within +/-5% (or less) of nominal voltage would be considered to be sensitive to voltage level or fluctuations.
Three phase motors are very tolerant of voltage level fluctuations while the electronic controls for the same motor might be quite sensitive.
[h=4]3.5 Voltage Too High, Too Low[/h]Voltage that is too high can cause premature failure of electrical and electronic components (e.g. circuit boards) due to overheating. The damage caused by overheating is cumulative and irreversible. Frequent episodes of mild overheating can result in the same amount of component damage as a few episodes of severe overheating. Like slicing a loaf of bread – you can have many thin slices or a few really thick slices – but when you get to the end, you’re done.
Motors can, on the other hand, often benefit from voltages that tend to be a little bit high. The reason is fairly simple. As the voltage level goes up, the current is reduced and lower current usually equates to less heat generation within the motor windings. There is a point where the voltage level supplied can be so high as to damage a motor but this level is far higher than that for electronics.
Keeping electrical and electronic components cool tends to insure their longevity. Slight reductions in voltage levels may permit many electronics to perform perfectly well while minimizing their temperature. Of course, the same is not true of motors.
Just as higher voltages can help reduce motor operating temperatures, low voltage is a major cause of motor overheating and premature failure. A low voltage forces a motor to draw extra current to deliver the power expected of it thus overheating the motor windings. The rule of thumb for motors is “for every 10 degrees C (50 degrees F) a motor is operated above its rated temperature, motor life will be decreased by 50%”.
More than motors and circuit boards are at risk for damage when voltage levels are bad, but chronic problems with either is often an indication of a voltage problem.
[h=4]AVR: Phases and Phase Regulation[/h]Automatic voltage regulators can be designed for single phase or three phase AC applications.
It is common for utilities to use single phase automatic voltage regulators ganged together to provide voltage regulation for three phase. These are often “can-type” units pole-mounted outdoors.
Single phase automatic voltage regulators may also be used where a three phase source is used to supply three single phase loads. Most three phase AVRs may also be used to feed single phase loads.
For three phase loads, it is usually more cost effective to use a three phase AVR.
A three phase automatic voltage regulator might regulate all three phases together or it might regulate each phase independently, depending on the design of the AVR.
When dealing with three phase power, it is not uncommon to find that one phase has a “high” voltage level while another has a “low” voltage level. In this situation, being limited to regulating the voltage level of all three phases together, up or down the same amount, may not produce satisfactory results.
Independent phase regulation is often the preferred method since it typically provides better phase-to-phase voltage level balance. Large differences in voltage levels from phase-to-phase can cause premature failure of electrical devices due to overheating or vibration.[h=2][/h]
[h=3]Automatic Voltage Regulators and Power Conditioners[/h]An AVR is at the heart of devices often called power conditioners or power stabilizers. The typical power conditioner is an automatic voltage regulator combined with one or more other power-quality capabilities such as:
- Surge suppression,
- Short circuit protection (circuit breaker),
- Line noise reduction,
- Phase-to-phase voltage balancing,
- Harmonic filtering, etc.
Power conditioners are typically used only in low voltage (< 600V) applications and sizes below 2,000 kVA. Since there is no official definition of a power conditioner, there are some devices marketed as power conditioners that do not provide automatic voltage regulation. This fact and the wide variation in capability between products make it imperative the buyer does his or her homework to match product functionality and application needs.
