EXCITATION SYSTEM VOLTAGE AND CURRENT SPECIFICATION TUTORIALS

Rated Current

The continuous current rating should be specified to be equal to or larger than the maximum required by the synchronous machine field under any allowed continuous operating conditions. Note that some machines have a continuous overload rating.

In addition, ANSI C50.12-1982 [2], ANSI C50.13-1989 [3], and ANSI C50.14-1977 [4] allow all machines to operate at rated MVA and within ± 5% of rated terminal voltage. Some machines may require an even wider operating range.

The need for off-frequency operation must also be considered in establishing the rating. Some machines, such as combustion-turbine-drive units, have a variable rating depending on ambient air temperature.

The excitation system for these machines may require a variable rating based on ambient air temperature. In the past, some excitation systems have had a small continuous negative current rating. This was utilized when the machine was operated as a synchronous condenser and the negative field current allowed a slightly greater transmission line charging capability.

The negative current was relatively easily supplied from commutator-type exciters, with little additional complexity. Modern exciters employing solid-state rectifiers do not normally have inherent capability for negative currents.

It could be obtained with great complexity, which cannot justify the associated minimal performance improvement (except for synchronous condensers). Some modern exciters will generate a transient negative voltage to force the decay of field current toward zero.

This should not be confused with the concept of a continuous negative current rating, which is mentioned above. For exciters that are specified with redundant current paths or cooling elements, the continuous rating should apply with the redundant parts out of service. The exciter efficiency and losses should be measured at the rated current and voltage point with all redundant parts in service, since this is the normal operating mode.

Rated Voltage

The continuous voltage rating of a system should be such that the voltage is sufficient to supply the necessary continuous current to the synchronous machine field, with the field at its maximum temperature under rated load conditions.

In addition, the continuous voltage capability should allow operation of the synchronous machine at rated MVA and within ±5% of rated terminal voltage unless otherwise specified.

In determining the required voltage for the continuous as well as the transient ratings mentioned later, all voltage drops, including interconnecting bus or cable run voltage drop, up to the field winding terminals should be considered. Any brush drop voltage should be considered part of the synchronous machine field circuit.

D'ARSONVAL MOVEMENT - HOW AMMETERS WORK? BASIC DEFINITION AND TUTORIALS

Getting back to electromagnetic deflection, and the workings of the galvanometer, you might have thought by now that a magnetic compass doesn’t make a very convenient type of meter. It has to be lying flat, and the coil has to be aligned with the compass needle when there is no current.

But of course, electrical and electronic devices aren’t all turned in just the right way, so as to be aligned with the north geomagnetic pole. That would not only be a great bother, but it would be ridiculous.

 Imagine a bunch of scientists running around, turning radios and other apparatus so the meters are all lying flat and are all lined up with the earth’s magnetic field. In the early days of electricity and electronics, when the phenomena were confined to scientific labs, this was indeed pretty much how things were.

Then someone thought that the magnetic field could be provided by a permanent magnet right inside the meter, instead of by the earth. This would supply a stronger magnetic force, and would therefore make it possible to detect much weaker currents.

It would let the meter be turned in any direction and the operation would not be affected. The coil could be attached right to the meter pointer, and suspended by means of a spring in the field of the magnet.

This kind of meter, called a D’Arsonval movement, is still extensively used today. The assembly is shown in Fig. 3-4. This is the basic principle of the ammeter.

A variation of this is the attachment of the meter needle to a permanent magnet, and the winding of the coil in a fixed form around the magnet. Current in the coil produces a magnetic field, and this in turn generates a force if the coil and magnet are aligned correctly with respect to each other.

This meter movement is also sometimes called a D’Arsonval movement. This method will work, but the inertial mass of the permanent magnet causes a slower needle response.

This kind of meter is also more prone to overshoot than the true D’Arsonval movement; the inertia of the magnet’s mass, once overcome by the magnetic force, causes the needle to fly past the actual current level before finally coming to rest at the correct reading.

It is possible to use an electromagnet in place of the permanent magnet in the meter assembly. This electromagnet can be operated by the same current that flows in the coil attached to the meter needle. This gets rid of the need for a massive, permanent magnet inside the meter.

It also eliminates the possibility that the meter sensitivity will change in case the strength of the permanent magnet deteriorates (such as might be caused by heat, or by severe mechanical vibration). The electromagnet can be either in series with, or in parallel with, the meter movement coil.

The sensitivity of the D'Arsonval meter, and of its cousins, depends on several factors. First is the strength of the permanent magnet, if the meter uses a permanent magnet.

Second is the number of turns in the coil. The stronger the magnet, and the larger the number of turns in the coil, the less current is needed in order to produce a given magnetic force.

If the meter is of the electromagnet type, the combined number of coil turns affects the sensitivity. Remember that the strength of a magnetomotive force is given in terms of ampere turns. For a given current (number of amperes), the force increases in direct proportion to the number of coil turns.

The more force in a meter, the greater the needle deflection, and the smaller the amount of current that is needed to cause a certain amount of needle movement.

The most sensitive ammeters can detect currents of just a microampere or two. The amount of current for full scale deflection (the needle goes all the way up without banging against the stop pin) can be as little as about 50 uA in commonly available meters.

Thus you might see a microammeter, or a milliammeter, quite often in electronic work. Meters that measure large currents are not a problem to make; it’s easy to make an insensitive device.

Sometimes, it is desirable to have an ammeter that will allow for a wide range of current measurements. The full-scale deflection of a meter assembly cannot easily be changed, since this would mean changing the number of coil turns and/or the strength of the magnet. But all ammeters have a certain amount of internal resistance.

If a resistor, having the same internal resistance as the meter, is connected in parallel with the meter, the resistor will take half the current. Then it will take twice the current through the assembly to deflect the meter to full scale, as compared with the meter alone.

By choosing a resistor of just the right value, the full-scale deflection of an ammeter can be increased by a factor of 10, or 100, or even 1000. This resistor must be capable of carrying the current without burning up.

It might have to take practically all of the current flowing through the assembly, leaving the meter to carry only 1/10, or 1/100, or 1/1000 of the current. This is called a shunt resistance or meter shunt.

Meter shunts are frequently used when it is necessary to measure very large currents, such as hundreds of amperes. They allow microammeters or milliammeters to be used in a versatile multimeter, with many current ranges.

THE AMPERE - UNIT OF CURRENT BASIC DEFINITION AND TUTORIALS

Current is a measure of the rate at which charge carriers flow. The standard unit is the ampere. This represents one coulomb (6,240,000,000,000,000,000) of charge carriers per second past a given point.

An ampere is a comparatively large amount of current. The abbreviation is A. Often, current is specified in terms of milliamperes, abbreviated mA, where 1 mA 0.001 A or a thousandth of an ampere.

You will also sometimes hear of microamperes (μA), where 1 μA 0.000001 A 0. 001 mA, a millionth of an ampere. And it is increasingly common to hear about nanoamperes (nA), where 1 nA 0. 001 μA 0.000000001 A (a billionth of an ampere).

Rarely will you hear of kiloamperes (kA), where 1 kA 1000 A. A current of a few milliamperes will give you a startling shock. About 50 mA will jolt you severely, and 100 mA can cause death if it flows through your chest cavity.

An ordinary 100-watt light bulb draws about 1 A of current. An electric iron draws approximately 10 A; an entire household normally uses between 10 A and 50 A, depending on the size of the house and the kinds of appliances it has, and also on the time of day, week or year.

The amount of current that will flow in an electrical circuit depends on the voltage, and also on the resistance. There are some circuits in which extremely large currents, say 1000 A, flow; this might happen through a metal bar placed directly at the output of a massive electric generator.

The resistance is extremely low in this case, and the generator is capable of driving huge amounts of charge. In some semiconductor electronic devices, such as microcomputers, a few nanoamperes will suffice for many complicated processes. Some electronic clocks draw so little current that their batteries last as long as they would if left on the shelf without being put to any use at all.


The name of the unit is a tribute to the French scientist Andr´e Marie Amp`ere. The electrical engineering convention states that the positive direction of current flow is that of positive charges.

In metallic conductors, however, current is carried by negative charges; these charges are the free electrons in the conduction band, which are only weakly attracted to the atomic structure in metallic elements and are therefore easily displaced in the presence of electric fields.

WHO IS ANDRE MARIE AMPERE?

(1775–1836)
Born: Lyon, France

Ampere is best known for his pioneering work in the field of Electrodynamics. During his emotionally troubled life, he held several professorships: at Bourg, Lyon, and at the Ecole Polytechnic in Paris.

While Ampere worked in several sciences, the work of the Danish physicist Hans Christian Oerstad on the electric deflection of a compass needle, as demonstrated to him by Dominique Arago, caused Ampere’s great interest in electromagnetism.

His seminal work, Notes on the Theory of Electrodynamic Phenomena Deduced Solely from Experiment, established the mathematical formulations for electromagnetics including what is now known as

Ampere’s Law. It can be said that Ampere founded the field of electromagnetics. He is honored for this by the naming of the unit of electric current as the ampere.

VOLTAGE – CURRENT AND POWER IN A CIRCUIT WITH COMBINED RESISTANCE AND INDUCTANCE BASIC AND TUTORIALS

Apparent Power

The practical significance of apparent power is as a rating unit. For example, an ac generator supplies apparent power at essentially constant voltage and frequency. Its output capacity is then described in Mega Volt Amps (MVA).

Transformer and motor capacities are also rated in MVA or KVA for similar reasons. Although the utility must provide apparent power, it receives direct compensation only for active power utilised by its customers.

The ratio of active to apparent power is therefore, an important quantity and is defined as the power factor. This number can range from zero to one but good economics requires it to be as close to unity as possible.

It can be thought of as a measure of the system’s effectiveness in using apparent power to do useful work. The terminology can be in any one of the following forms:

Power Factor = PF

= Real Power/Apparent Power

= Watts/ Volt Amps

Note: There is a time lag between the apparent power and real power. This corresponds to the time lag between voltage and current for reactive loads.

For capacitive loads, we have a leading power factor.

For inductive loads, it is lagging. Power Factor Angle is a measurement that describes how close the apparent power is to being totally real or supplying a pure resistive load. The Power Factor rating can be summarized in two main areas:

• Efficiency: The lower the power factor, demanded by the load which requires a given amount of active power, the greater the size of line current that has to be supplied by the generator and sent through the transmission system. This means higher winding and line losses and reduced efficiency;

• Voltage Regulation: The lower the power factor and the greater the generator current, the greater the reactance voltage drops along the line. This means a lower voltage at the load and, consequently, poorer system voltage regulation.

The relationship between apparent, active and reactive power is that of a right-angled triangle.

The following relationships exist between voltage, current, apparent power (U), active power (P) and reactive power (Q).

U^2 = P^2+Q^2

U = VI

P = VI cosθ

Q = VI sinθ

The power factor is equal to cosθ