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The best way to understand electricity and its terminology is to develop some analogies between electricity and topics that are easy to understand. Even though electricity and water don't mix, the concepts are actually fairly similar. By comparing electrical terminology to water flowing in a pipe, we should be able to with a simpler understanding of electricity.
For assistance in identifying some of these terms, go to our electrical definitions (for your convenience, this link will open in a new window).
The pressure in a pipe can be compared to electrical voltage across a wire. If the pressure on both ends of a pipe is the same, then no water will flow. If you took two water tanks of the same size, where one was full and the other was empty, and connected them together with a hose at their bottoms, water would flow from the full tank into the empty tank. The water would stop flowing when the depth of the water in each tank was the same. The full tank has a higher pressure at the bottom (where the hose is connected) than the empty tank. When the depth of the water is equal in each tank, then the pressure at the bottom of both tanks is equal.
If both ends of a wire are connected to the same voltage (for example, the positive terminal of a battery) then no current will flow either. In either case, it is the difference in pressure or voltage that causes the water or electricity to flow.
Regardless whether we are referring to water flow or electrical current, current is the movement of water or electricity.
When discussing the flow of water, we are referring to how many gallons per minute are passing though a hose. For electricity, we are looking at how many electrons per second are passing a point. Literally, 1 Ampere is equal to 6.24x1018 electrons per second. (The way this number is written is called scientific notation, and is used for very large numbers. This number, if written out would be 6,240,000,000,000,000,000.) Since it is too difficult to work with numbers this large on a daily basis; we use the much simpler term of Amperes, or Amps for short.
The flow of water through a pipe, or electrical current through a wire, is directly related to the pressure or voltage difference across the pipe or wire.
Going back to the example of our two tanks. If you were to fill one tank with a couple of inches of water, the flow of water wouldn't be very fast filling the empty tank. If you then filled the first tank with several feet of water, the speed at which the water flowed out of the hose into the second tank would be much higher. The same is true with electricity--the greater the difference in voltage from one end of the wire to the other, the higher the current.
Pipe Diameter and Resistance
The more resistance in a circuit, the lower the current will be. Similarly, the smaller the diameter of a water pipe, the less water can flow through the pipe. Looking at the tank example, it should be obvious that if we connect the two tanks with a small hose, the time it takes to fill the second tank will be longer. If we make the hose much longer, the additional resistance to flow from the inside of the hose will also slow down the flow (increase its’ resistance).
Therefore, resistance is related to not only the size of a wire, but also the length of a wire. Consider the windings of a motor…they are made up of a very long and very thin piece of copper wire. The reason this wire doesn't just melt is because it is long enough and thin enough to act as a resistor, which slows down the flow of electricity
In electrical systems, there is a relationship between current, voltage, and resistance. This is known as ohms law, and can be written in many different forms, but always boils down to V=IR, where V is voltage, I is current, and R is resistance.
This equation holds true whether we are dealing with AC, DC, Capacitive, Inductive, Three Phase, or any other type of circuit. However, it should be noted that sometimes the values for current and/or voltage are no longer simple values. The V and I of Ohms' Law can be replaced by complex mathematical expressions, but they still represent the current and voltage.
In fact, it isn't that the equations change, it is the values of current and voltage, which become complex. For example, we may replace the simple term "I" with the complex term "I*cos(p)",I*cos(p)",I*cos(p)",I*cos(p)",I*cos(p)", where p represents a shift in the phase angle, or timing, of the current.
Ohm's law can be written in different forms, but are still the same equation. The three common forms of Ohms law are:
AC versus DC
A battery is direct current (DC). The polarity of a battery is always the same--positive on one side and negative on the other. In an AC system, the polarity is constantly changing every 1/60th of a second (60 times per second, or 60 Hz).
If you had a very small (and very fast) person sitting on that 9-volt battery inside your TV's remote control, and he was switching the positive and negative wires back and forth 60 times every second, you would have a 9-volt AC power source.
In the AC system in our homes, this switching between positive and negative is a little smoother, and if you could look at it, it would look like a sine wave.
The electrons traveling through a wire aren't actually moving up and down like the picture, this is just a mathematical representation of their movement. The electrons are actually moving forward, then backward in the wire, where their speed is represented by the height of the sine wave.
Ground, Neutral, and Hot
These are terms we use to describe the parts of an electrical wiring system. These are just relative terms, and are the names we have given to the wires used in a standard electrical system. They are kind of like nicknames.
If you had a really big voltmeter, and placed one probe way out in space and one probe on the Earth, you would show a voltage between the Earth and Space. I don't know what this voltage would be, it could be one volt or it could be a million volts. In simple terms, we use the Earth as a reference point (we say the Earth is at zero volts, even though we know it is not).
The Ground wire in your home or shop is literally connected to an eight foot copper rod driven into the Earth. Therefore, we say the Ground wire is at zero volts. (Believe it or not, the Earth is a conductor of electricity. Not as good as copper, but it does conduct.)
If our equipment were not grounded (electrically free-floating), it would have a voltage difference with respect to Earth, just as the free-floating Earth has a voltage difference with respect to outer space. The equipment chassis is connected to the Ground wire, which of acts like an anchor to keep the chassis' voltage at zero. In short, the Ground wire is a safety device that anchors our equipment to zero volts, but is not supposed to carry any current unless something with the appliance is malfunctioning. (An appliance is a generic term for any device, such as a lamp, saw, oven, motor, and so on. It is not limited to the typical home appliance.)
If something does go wrong with the appliance, then the ground wire will, and should, carry current. But the main purpose of the ground wire is to always ensure that the chassis of the appliance remains at zero volts.
Neutral and Hot
The only difference between the Neutral wire and Hot wire(s) of a modern electrical system is that the Neutral wire is forced to be at zero volts (anchored) by connecting it to Ground back at the circuit breaker panel. If we did not anchor Neutral to Ground, then both the Neutral wire and the Hot wire would be at some intermediate voltage (both would be free-floating). This is done as a safety issue. It is much easier to work on a system when we only have one wire with a non-zero voltage. Unlike the Ground wire however, the Neutral wire is designed to carry current during normal operation.
Since the Neutral wire is at zero volts though, there is no voltage difference between it and Ground, and that means there is little chance for a user to get electrocuted by touching the Neutral. This is why it is normal electrical procedure to have the Neutral wire pass directly to an appliance without going through a switch or circuit breaker. Switches and circuit breakers are placed on the Hot leg of a system.
The purpose of the circuit breaker is to protect the wires between the breaker and the load, although it can also serve as a service disconnect (a means of disconnecting power from the circuit).
A circuit breaker is not intended to protect the appliance, only the wire between the breaker and the outlet. In your home, you will have 15 or 20 amp breakers, but the motor that you plug into the outlet may self-destruct if the current exceeds 10 amps. The motor is responsible to protect itself if the current goes over 10 amps, not the circuit breaker.
National Electric Code mandates that ALL Hot wires going to a load must, not only have a circuit breaker, but ALL circuit breakers feeding that device must trip together. Therefore, a 240-volt tool must use a two-pole breaker, and a three-phase tool must use a three-pole breaker.
You should of course already know to always turn the power off before you do any electrical work, but you should take this concept a little further.
You should remove all possibility for someone else to turn the power back on. If a tool has a plug, then unplug it and place the cord within your line of sight, so that you can see if someone goes to plug it back in. If the tool only has a circuit breaker, and it is out of your line of sight, find a way to lockout the breaker in the off position.
Most breakers have a small hole through the trip handle, and this can be used with a small lock, or similar object to prevent the breaker from being turned on. At the same time, you should label the lockout with a tag to indicate the circuit is being serviced.
There may be times when there is no way of turning the power off. In these instances, only qualified persons with experience working with this condition should have anything to do with the circuit.
Whether a circuit is energized or not should make no difference in the way you work. If you always work on the equipment under the assumption that is energized, you will not be injured in the event someone reapplies power to the circuit.
Current passing through their body electrocutes people, not voltage. Voltage can kill, but it is the difference in voltage which causes the problem, and the difference in voltage is what causes current to flow through a person's body.
A bird does not get electrocuted when it lands on a power line, because its entire body is elevated to that voltage (free-floating). If the wingtip of the bird touched a different voltage source, like Ground or another wire, it completes a path to a different voltage potential and the result would be electrocution.
In order for current to flow, there must be a path from a higher voltage to a lower voltage. If there is no path, current cannot flow. This path can include a wire, a metal water pipe, the chassis of an electrical panel, or waterlogged shoes on earth-ground
Apply this principle whenever performing wiring. Assure that you allow no part of your body to come into contact with a ground or other source of voltage.
Whenever possible, perform tasks with only one hand to ensure that the other hand does not inadvertently touch somewhere it shouldn't. In the event you do inadvertently complete a circuit with your body, current will pass through your single hand instead of traveling across your body to get to ground.
Back Fed Voltage/Current
What makes the above mentioned approach all the more important is the unlikely occurrence of a back fed voltage. This situation has killed and maimed many professional electrical workers. This doesn't apply to a situation where you can unplug the entire system, like a tool with a plug. It applies to working on a system, or part of a system that is not completely isolated from all other parts, like a wall outlet.
You may have disconnected the Hot wire from the source, and maybe even the Neutral too, but there could be a circuit path somewhere downstream from your location that you don't consider, or are unaware of.
Capacitors and Inductors
Capacitors and inductors are two types of devices that store energy, like a battery does. Each of these stores different types of energy in different ways. It is this ability to store energy that makes capacitors and inductors somewhat complicated when evaluating electrical systems.
In very simple terms, a capacitor is made from two parallel plates of metal, which are separated by an insulating material. Since this insulating material separates the plates of a capacitor, no current actually flows through the capacitor, although it does sometimes appear to. Each plate of the capacitor will hold an electrical charge kind of like a battery, where one plate will have a negative charge and the other plate will have a positive charge [you can picture this as static electricity, like when you rub a balloon over your hair. Your head will have a positive charge, and the balloon will have a negative charge (or vise versa)].
Since no current can flow across the insulating material, energy is stored in the capacitor in the form of electric charge between the two plates. When you put voltage to a capacitor and then remove the wires, the capacitor will hold that voltage until it is discharged. (This is why capacitors can pose a grave safety hazard: They can seriously shock a person long after a tool is unplugged.)
An inductive device is any coil of wire, which includes motors, transformers, and generators. Every time electricity flows through a wire, it creates a small magnetic field around the wire. (This is the same type of magnetism that holds a refrigerator magnet to the refrigerator, except that it is only present when current is flowing.) This magnetic field forms circular lines of flux around the wire. When we coil up a wire, we not only concentrate the wire itself into a small area, but we also concentrate the wire's magnetism into a small area too.
An inductor stores energy in the magnetic field around the coils. It takes energy to develop the magnetic field around the coils, and the magnetic field gives off energy as it collapses (it collapses when the current is stopped or reversed.)
In an AC circuit, remember that the voltage is changing from positive, through zero, to negative 60 times every second. When we connect an inductor, like a motor or transformer, to an AC circuit, the magnetic field around the wires are also constantly changing as a result. They are continually expanding and contracting as the current is reversing.
Effects of Capacitive and Inductive Devices
Capacitors will store voltage, while inductors will store current. When we put a voltage to a motor, the effect of storing this current will delay the flow of current by a fairly small amount of time. This is referred to as a phase shift in the current.
Types of Electricity in Commercial Applications
There are three common terms used to describe the electricity used in commercial applications. Single-phase 120 volt, Single-phase 240 volt, and three-phase voltage (which can be supplied in varying amounts, usually expressed as 120/208, 120/240, or 277/480).
Don't be confused if you hear the terms 110 volts instead of 120 volts, or 220 volts instead of 240 volts. These are out of date terms which people still refer to, but all public utilities in the US deliver 120 volts and 240 volts for consistency and load sharing. Most tools and motors use these other terms (110/220) just to indicate that they will still perform if the voltage drops to that level.
Single Phase 120/240
Single phase 120 volt and 240 volt lines, are just different parts of the same system. This is actually a 240-volt system, but we split it in half to get two, 120-volt systems. This is the reason why it is called a single-phase system.
It is just one phase of power at 240 volts. To get the 120 volts, we use what is called a center-tap. Standard outlets use the Neutral wire (the center tap) and one Hot wire, where the voltage between the Neutral and Hot is 120 volts. The 240 outlets use both Hot wires, where one wire is 120 volts above the Neutral and the other is 120 volts below the Neutral (as before, we anchor the Neutral to Ground, and let the two Hot lines "float" above and below). It is said that each of the Hot legs (called poles) of a single phase system are 180° out of phase.
It can be confusing that this system is called single phase, but it might be helpful to refer to this as a two pole system. (Using the term two pole is correct, but calling it a two-phase system is incorrect.)
Three Phase Systems
Where the single phase system has two poles 180° out of phase, the three phase system has three poles which are 120° out of phase (note 3*120° = 360° = full circle). Just as before, the voltage between the Hot and Neutral is 120 volts, but because of the phase angle, the voltage between any two Hot wires is 208 volts, which is 40*(0.866) = 208 volts. (Where 0.866 is the cosine of 120°.)
The majority of three phase motors don't use the Neutral wire. This is called a Delta Connected system. When the Neutral is used, it is called a wye-connected system. The majority of power sources are "wye- connected". A delta-connected load (motor) can always be connected to a wye source by just ignoring the Neutral wire, but the reverse is virtually never true. (It can be done, but it requires a center tap, three-phase, transformer to artificially create the Neutral.)
Current in the Neutral Wire
This is a question I get asked by even very experienced people. "If the current through the Neutral wire is the sum of the currents through each of the Hot wires, then shouldn't the maximum current in the Neutral be three times that of any one leg (at maximum power)?" That is, if 20 amps of current are flowing through each of the three Hot wires, then shouldn't the Neutral have 60 amps flowing through it? The answer is NO. (The following explanation is based on the three-phase system, but it also holds true for the single phase, two-pole system.) The current flowing through the Neutral wire is the sum of the currents flowing though each of the phases; yes, but what complicates this, is that each of the phase currents has both a magnitude and direction. Any time we have an expression which has both magnitude and direction, it can be expressed as a vector (the arrows I have drawn in the above diagram are vectors). We can't just add the magnitude of vectors without considering the direction as well.
In order to explain the current in the Neutral wire, we need to understand some principles of vector mathematics. Vectors can be used to describe anything that has a magnitude and direction. One example deals with travel, where the length of travel is the magnitude, and the direction is just that, direction. If we walk 10 feet East then turn around and walk 10 feet West, it is said that our net travel is zero (we are at the same point we started at.) If we walk 10 feet East and 10 feet North, then we could have accomplished the same travel by walking 14 feet Northeast (@45°). In determining the 14 feet Northeast, we could either draw a picture, or use trigonometry. The drawing method is called tip-to-tail because we redraw the vectors such that the second one starts where the first one stops.
To prepare for the examples discussed in the section below: if we walk 20 feet at 120°( 120° from north), and then turn and walk 20 feet at 240° from north, we could accomplish the same travel by walking 20 feet due south. (By the way, the reason for the 20 feet due South, and not 24 feet or some other number, is because 120° and 240° make up what is called a perfect triangle. If it weren’t for these nice angles, it would have resulted in some other length, not 20 feet.
One Phase at Max Current, Two at Zero
To understand how the current through the Neutral is determined, we will examine three worst-case situations. First, when one phase is at twenty amps, and the other two phases are at zero, the current in the Neutral will of course be 20 amps.
Two Phases at Max Current, One at Zero
When the current in two phases is 20 amps and the third is zero (the one pointing straight up is zero), the current in the Neutral will be 20 amps at 180°. The sum of 20 amps at 120° plus 20 amps at 240° is 20 amps at 180°.
All Three Phases at Max Current
When all three phases carry the maximum current, they all cancel out, and the net current through the Neutral is zero amps. In short, the current through the Neutral can never exceed the maximum current in any one of the Hot legs.
Power is the ability of a system to perform work. Water performs work when it turns the turbine blades of a hydroelectric plant. Electricity performs work when it heats up a heating element or turns a motor. It takes power to store energy, like in capacitive or inductive devices, while these devices then release some energy, or power, at a later time. (These devices can both expend power and deliver power.)
For DC systems, power is the product of Current times Voltage, and will take on the form P=I*V. For AC systems with only resistive loads, the same holds true. But in capacitive or inductive circuits on an AC system, the device will momentarily store some power (or delay it), and so the issue becomes slightly more complicated. We need to compensate for this delay in power transmission, and this is where the term Power factor comes in.
When we use any capacitive or inductive device on an AC circuit, the current or voltage flowing through the circuit will be slightly delayed, or out of phase. A motor is an inductive element, and the current lags behind the voltage (remember, the inductor had the ability to store current). In a capacitor the voltage lags behind the current (the capacitor stores voltage).
You may sometimes hear the phrase: the current leads voltage in a capacitor, but this is just a matter of convention where the voltage is assumed to always be the same and the current either leads or lags. This will now be rephrased to the standard convention: Current LAGS voltage in an Inductor and Current LEADS voltage in a Capacitor.
For a purely capacitive or inductive circuit with zero resistance, the angle of lead/lag is 90°. Adding resistance to the circuit will decrease the leading/lagging angle.
The term "Power factor", is the cosine of the phase angle. For a purely inductive circuit, the lag angle is 90°, and the power factor is zero [cosine(90)=0]. A common power factor for electric motors is 0.8, which gives us a lagging angle of 36° (This is because there is some resistance inside the motor windings).
Apparent power is defined as the power that is "apparently" absorbed by a system. That is, the product of current times voltage tells us a device appears to be using a certain amount of power. However, this does not take into account the fact that the device can store (or delay) current or voltage, and this results in the calculations being slightly skewed.
Apparent power is useful when we have a device like a diesel-electric generator, where the wires inside have a limited capacity to pass current, and we may not know in advance what will be connected to the generator. In other words, it doesn't matter what the delay (or phase angle) is, the generator can only allow a limited amount of current to pass through its wires.
Because of this, many generators (and most transformers) are rated in volt-amperes (VA), or thousand-volt-amperes (KVA). A 25 KVA generator (or transformer) can deliver no more than 70 amps per phase @ 208 volts before it burns out the windings. This can therefore power 25 kilowatts of heaters, but only 20 kilowatts for motors (assuming 80% power factor), because both of these loads will use 70 amps. Since the manufacturer does not know what the generator will ultimately be used for, they rate it in KVA because this indicates the maximum current regardless of the load's power factor.
The real amount of power a device is using, or results in actual work performed, is called the "real power". Real power takes into account the fact that current or voltage is stored, or delayed. The real power tells us how much actual work can be performed, or how many horsepower our motor is delivering. For a resistive and/or DC circuit, the apparent power and the real power are the same, but for a capacitive or inductive circuit, the real power is heavily dependent on the amount that the current or voltage is delayed. Real power is presented in Watts. There is mathematically no difference between watts and volt-amperes, except that we use one term for apparent power, and one for real power, but they are both units of power. We use the power factor to go from apparent power to real power.
The real power of a system is equal to the apparent power times the power factor. In every day use, this boils down to P=I*V*pf.
Regardless of the type of system, Efficiency is the difference between power in and power out. If you are peddling a bike, your legs are Power in, and the tire against the road is Power out. The difference between these two is the efficiency of power transmission. For a bike, this loss of power, or efficiency, would be primarily the friction of the chain (even the friction of your trousers against your legs), wind resistance in the spokes, and even small frictional losses between the tire and the road, but it is not due to the steepness of the hill or wind resistance against you and the bike's frame, as this is a portion of the work the bike is performing (the load).
In a motor, the loss of power is due to the resistance of the windings, friction in the bearings, air resistance inside the motor, and what is known as "hysteresis losses" in the iron core of the motor.
All magnets, regardless of type or origin, will have a north and south pole. This is very similar to a battery always having a positive and negative terminal. If you have two magnets, the poles with opposite polarity will attract one another, while poles with the same polarity repel one another. These attraction and repulsion forces can be quite strong, and this is what will make a motor turn.
If you have a magnet, and you are physically moving a wire near this magnet, it will create a current in the moving wire. The faster you move the wire, the larger the current. Furthermore, the bigger the magnet, the larger the current. If you change the direction the wire moves, the current will also change direction. This is the basic premise for a simple generator, where we use a diesel engine to move wires past a magnetic field.
Any flowing electric current creates a magnetic field. When this current is flowing through a wire, the magnetic field forms circular rings around the wire. We can concentrate the magnetic field by coiling the wire into tight loops, thereby making an electromagnet. We can concentrate the magnetic field even more, by wrapping the wire around an iron bar. This electromagnet also has both north and south poles like any other magnet, but the polarity of the poles changes as the electricity changes. If we send 60hz line power through an electromagnet, the polarity of the magnetic poles will alternate sixty times per second.
A motor is made up of electric and/or permanent magnets that are constantly attracting and/or repelling one another. This creates movement of the spinning rotor. The only thing that differs from one type of motor to another is how these magnets are created and controlled.
This is the stationary magnetic component in motors, and constitutes the chassis in some cases. On most motors, the stator's magnetic field is created from electromagnets. One notable exception is small DC motors found in such items as toy trains etc., where these use small permanent (bar-type) magnets. Permanent magnets are not normally used in larger motors because they can loose their magnetism if the magnetic field in the windings is too strong. This would saturate the permanent magnet, and re-magnetize the stator in reverse polarity.
The rotor is the component that makes up the spinning shaft of the motor. It is almost always electromagnetic in nature (coils).
These are the coils of wire that make up the electromagnet. They are usually wrapped around a laminated stack of iron sheets. The reason for the laminations is too complex to get into, but for those already familiar with the basic concepts, it is to reduce hysteresis losses in the iron core.
This is found in universal and DC motors. These devices, along with the brushes serve to switch the polarity of the windings as the motor makes a revolution. (A forward and reversing switch, in short)
These are typically carbon/graphite bars which carry the current from the incoming wires to the commutator, and then to the rotor windings. The brushes are soft such that they will form to the commutator contacts as it spins.
Due to the principles of Conservation of Energy (energy can neither be created nor destroyed, only converted from one form to another.), we can say that the power into the motor, as electricity, is equal to the power out of the motor, as horsepower, minus any losses or inefficiencies in this conversion process.
A motor is nothing more than a converter of energy. It converts electrical energy into mechanical energy plus a little heat as a byproduct. (Note that losses or inefficiencies do not violate the physical law of "conservation", these losses result in heat or other forms of energy.)
The overall equation for converting electrical power to mechanical horsepower is: HP=W/745. Where HP is horsepower, W is watts, and 745 is a conversion factor. We know from our previous discussion that power, in volt-amperes, is given by the following equation: P=I*V. For an inductive device like a motor, we also need to take into account the phase angle between current and voltage by adding the power factor term (pf). Our equation for power becomes P=I*V*pf. Our final equation then becomes: HP=I*V*pf/745.
Shared Ground System
A shared ground system is one where there is a ground wire or path that is common to all panels. In a shared ground system, only the main load center can be bonded (neutral tied to ground). At no other point in the system can there be a connection between ground and neutral.
Any, and all, sub-panels must be wired with separate wires for neutral and ground which originate back at the main load center. By default, all electrical systems within the same building are of the shared ground type. Because of this, if you add a sub-panel in your basement or attached garage, then you must carry a ground wire from the main panel to the sub-panel, and you must remove the bonding screw from the sub-panel.
Split Ground System
In a split-ground system, you have two totally separate grounding systems. The electrical systems cannot be contained in the same building, and there must be no path to ground which they have in common (this must also include water pipes). When there is a completely separate grounding system between two electrical systems, then each system must be bonded.
One simple example of a split system is where incoming power from the utility enters a distribution panel (a splitter panel), and passes to two separate buildings. In each of these buildings, there is a separate load center, and a separate ground rod. There is no ground wire that connects the two buildings. In this case, there is no sub-panel, as each panel is considered a main load center.
PHONE: (678) 546-6780 FAX: (678) 546-6782
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Buford, Georgia 30518
PHONE: (678) 546-6780
Last Updated: 07 Jun 2004
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