The AC Voltage Regulator - Types and Comparison
The AC Voltage Regulator
The AC voltage regulator, as the name implies, is a device intended to regulate voltage that is to provide a constant voltage or nearly constant voltage level. Voltage regulator devices come in sizes that can be mounted on printed circuit boards to those that will fill a good sized room. Here, we look at voltage regulators typical for AC applications of about 1,000 VA (roughly the capacity of a hand-held hair dryer) and larger.
Voltage Regulators and Power Conditioners
A voltage regulator of some type provides the basic functionality of devices labeled as power conditioners or power stabilizers. The typical power conditioner is a voltage regulator combined with one or more other power-related capabilities such as:
- Surge suppression,
- Short circuit protection (circuit breaker),
- Line noise reduction,
- Phase-to-phase voltage balancing, and
- Harmonic filtering
Since there is no official definition of a power conditioner, there are some devices marketed as power conditioners that do not provide voltage regulation. This fact and the wide variation in capability between products make it imperative the buyer does his or her homework.
The Need for Voltage Regulation
Utility Voltage Levels
Anyone getting power from an electric utility will see the incoming voltage level change over the course of a day. The degree of voltage level fluctuation in a specific location is a function of many variables including: 1) location on the local distribution line, 2) proximity to large electricity consumers, 3) proximity to utility voltage regulating equipment, 4) seasonal variations in overall system voltage levels, 5) load factor on local transmission and distribution system, etc.
Voltage levels are often higher during the nighttime hours and weekends when the electrical demand is lowest and reach their lowest points weekday afternoons when the demand for electricity peaks. Most electric utilities try to maintain voltage to its users within plus or minus 5% of the nominal voltage level, however for short periods the voltage level may be as much as 6% high or 13% low (according to ANSI Standard C84.1). Momentary voltage levels deviating even further from nominal are also permissible or unavoidable.
Voltage Drop in a Facility
It is expected and accepted that there will be a voltage drop of 3 to 5% from the point where the electric utility delivers power (the meter) to the end user to the point within a facility where the electricity is finally consumed in an electrical device (the load). Unlike utility voltage levels which may be high or low, the voltage drop due to wiring impedance within a building will always drive voltage levels lower. So, if the incoming utility voltage is, for example, 5% low, the voltage drop within a building might take the voltage down to 8 to 10% low at the point of usage.
Sensitivity to Voltage Levels
Electrical devices will operate within a certain range of voltage levels, however not necessarily with optimal performance. Outside of this voltage range, the device may be unable to operate or may malfunction. The width of the voltage range within which a device will operate is a measure of the devices sensitivity to voltage level. A device that will operate fairly well within a range of plus or minus 10% of nominal voltage would be considered to have a low sensitivity to voltage fluctuations.
The means to keep voltage levels at the point of usage from affecting sensitive devices is the voltage regulator.
Voltage Regulator Function
Voltage Regulator Regulation
A voltage regulator takes in a range of voltage levels and automatically outputs a voltage with a much narrower range of voltage levels. For example, a typical voltage regulator may have an input voltage range of +10% to -25% of the nominal input voltage and convert this to a regulated voltage range of +3% to -3% of the nominal output voltage as shown the graphic for a 480V input and output. A voltage regulator may have a symmetrical input voltage range (e.g. +10% to -10% of nominal voltage) or an asymmetrical input voltage range as shown in the example. The choice of symmetrical versus asymmetrical input voltage range is dictated design and purpose of the voltage regulator. The output voltage regulation range is almost universally symmetrical (e.g. +3% to -3% of nominal output voltage).
A voltage regulator may also perform a voltage step-up or step-down function whereby the nominal incoming voltage is transformed to a different output voltage level (e.g. a step-up from a 208V input to a 400V output). For a voltage regulator with step-up or step-down capability, the input and output voltage ranges are usually applied to the input and output voltages (e.g. the example would become 208V +10/-25% on the input voltage range and 400V +3%/-3% for the regulated output range).
Voltage Regulator: Phases and Phase Regulation
Voltage regulators can be designed for single phase or three phase AC applications.
In some situations, single phase voltage regulators may be ganged together to provide three phase voltage regulation. This is common for voltage regulators used by electric utilities.
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Single phase voltage regulators may also be used where a three phase source is used to supply three single phase loads. For three phase applications, it is usually more cost effective to use a three phase voltage regulator.
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A three phase voltage regulator might regulate all three phases equally or it might regulate each phase independently, depending on the design of the voltage regulator. Independent phase regulation is often the preferred method since it may also provide 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. |
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Voltage Regulator Types
Ferroresonant Transformer Voltage Regulator
The ferroresonant transformer voltage regulator is also known as the constant voltage transformer, the CVT or simply the “ferro”. The ferro voltage regulator has a very long history having been invented in 1938 by Joseph Sola and selling tens of thousands of units still today. The ferro voltage regulator of today has had some minor improvements but remains fundamentally unchanged from the original design.
Ferro Voltage Regulator Operation
The ferro voltage regulator uses the unique principles of ferroresonance or the operation of a transformer in the realm of magnetic saturation. When the iron core of a transformer is in saturation, relatively large changes in winding current results in very small changes in magnetic flux. Since winding current and magnetic flux are proportional to input and output voltage, respectively, this means relatively large changes in input voltage result in small changes in output voltage: this being the fundamental nature of a voltage regulator.
The graphic on the right shows a simplified version of a magnetization curve to demonstrate this concept. In the saturation region (red) of the curve, a large change in input voltage results in a small change in output voltage. Operation in the saturation region has the disadvantage of very poor electrical efficiency. This is the reason that standard power transformers are designed to operate in the normal range (blue) where electrical efficiency is much higher. While standard power transformers have some minimal capacity for voltage regulation their primary purpose is to transform voltage from one level to another with high electrical efficiency being the most important characteristic.
Operation in the saturation region also produces undesirable sinewave distortion. For this reason, ferro voltage regulators incorporate an LC (inductor-capacitor) circuit tuned to the AC frequency to effectively filter out these distortions.
Ferro Voltage Regulator Characteristics
The typical ferro voltage regulator has the following characteristics:
Sizes: |
50 VA up to 25 kVA |
Voltages: |
Common AC voltages - 120, 208, 240 & 480 at 60 Hz |
Phase: |
Single phase application only |
Input voltage range: |
+10% to -20% (varies by manufacturer) |
Output regulation: |
+/-1% to +/-3% (varies by manufacturer) |
Efficiency: |
Up to 92% (see comment below) |
Ferro Voltage Regulator Advantages
The ferro voltage regulator, by its unique design, provides good isolation of load equipment from line noise and surges. The output voltage regulation is also smooth as long as the manufacturer has taken steps to minimize the distortion that is caused by operation in the saturation region. Ferro voltage regulators can be very durable with some manufacturers offering warranties in excess of 10 years.
One unusual operating characteristic of the ferro voltage regulator is that its design can limit the amount of current to the load. When is load current exceeds about 150% of rated current of a ferro voltage regulator, the unit is “pulled” out of the saturation region, the output voltage collapses to a very small value and the current is thereby limited. In some applications, this characteristic is intentionally used to protect loads from high currents, however in other applications this characteristic becomes a substantial disadvantage.
Ferro Voltage Regulator Disadvantages
The principle of magnetism that affords the ferro voltage regulator its voltage regulation capability is also its Achilles heal. Ferro voltage regulators can have electrical efficiencies up to 92% at full load but at lighter loads