gust electric power

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Power and Electric Energy

Power and Electric Energy

Voltage(Work) * Current(Charge) = Power(Work)Charge Time Time

Power(Watts) = Potential(Volts) * Current(Amperes) = EI

Energy Calculation

What is the energy required to operate a 3000 W heater for 20 minutes?

Energy = Power * TimeE = 3000 W * 1200 sE = 3,600,000 J

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Resistance

The relative difficulty with which current can be transmitted in a material is defined as the electrical resistance of the material.Two quantities of voltage and current can be related through the physical parameter, resistance. The voltage supplies the potential force in an electrical system. Flow of charge or current is the desired result.

Ohm’s Law

I = E / R– Where I = current in amperes, A– E = potential in volts, V– R = resistance in ohms, ohm

R = ρ(L / A)– Where R = resistance, ohm– L = length– A = cross-sectional area– ρ = resistivity

Gustafson, Fundamentals in Electricity for Agriculture, 1988

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Resistance of a Conductor

Rt = Ri (1 + α T) ohm– Where Rt = resistance at specified temperature,

ohm– Ri = resistance at reference temperature, ohm– α = temperature coefficient of resistance, 1/oC– T = difference temperature between specified and

reference, oC

Direct and Alternating Current

Electrical systems are generally classed into two categories by the form of the current. Direct current (dc) is characterized by current flow in only one direction at all times. Batteries, thermocouples, solar cells, and rotating dc generators are all examples of sources for direct current systems.Alternating current (ac) is characterized by alternating flow in two directions.


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Direct and Alternating Current con’t

An alternating current system is one in which the direction of flow changes periodically.

e = Em sin θWhere e = instantaneous voltageEm = maximum voltageθ = angle

Amplitude of Sine Waves

One of the most frequently measured characteristics of the sine wave is its amplitude.Peak voltage is the maximum amplitude of either the positive or negative part of the cycle.The effective or rms (root mean square) of a sine wave is the value equivalent to the constant dc magnitude that would provide the same amount of power. For example, many conventional residential wiring systems operate at 120 V effective or rms. This means that a light bulb would glow at the same brightness on 120 V rms AC or connected to 120 V DC source.

Amplitude of Sine Waves con’t

Erms = EPeak / 20.5

Irms = IPeak / 20.5

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Phase Relations and Power in AC Circuits

When a sine wave voltage is imposed on a load, a sine wave current will result. For the case of a resistive load, the voltage and the current waves are “in-phase”with each other. The term “in-phase” means the current and the voltages go through zero and through their peak values at the same time.


Power Curve

True or effective power as shown in the diagram is the product of the rms voltage or current.


Power Curve con’t

When a circuit contains elements with other than pure resistance (capacitance or inductance), a phase shift will occur between the voltage and current waves. That is, the waves will no longer cross zero or reach peaks at the same time. The amount of shift measured in degrees is called the phase-shift angle or the phase shift. This phase shift will affect the shape of the power curve.

– Ex: motor circuit

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Power in AC Circuit

Power can be calculated by P = EI cos φ– Where E = voltage (RMS), V– I = current (RMS), A– φ = phase-shift angle

Note that when the phase-shift angle is zero as in a purely resistive circuit or a dc circuit, we return to the form

Power = Voltage * Current = EI (lights, electric heaters, etc)


Apparent Power

Apparent power is always greater than true power when the voltage and current are not in-phase.

Apparent Power = E*I (VA)

The ratio of true power to apparent power for a circuit is defined as the power factor. The power factor is also equal to the cosine of the phase-shift angle. Power factor can vary from a value of one, (in-phase, φ = 0, cos 0° = 1), to a value of zero, (90° out of phase, cos 90° = 0).

Power Factor = cos φ = true power = wattsapparent power volts*amperes

Power Factor

An electrical system can be loaded to operate with a power factor of 1.0. The system will distribute energy at a maximum efficiency when the power factor equals one. However, this is not always economical or feasible in real systems. Power generators are concerned with power factor since it effects their line losses and capacities.

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120/240 V Single-Phase Service System

By far the most common service system for farms and residences in North America is the 120/240 V three-wire single phase system. The system originates at a step-down transformer from the local distribution system.The distribution system at a higher voltage feeds the primary of the transformer and the secondary side is the origin of the service drop to the user.

Transformer for Origin of 120/240 V System


120/240 V Single-Phase Service System con’t

If a voltmeter is placed between the neutral and either hot wire, a voltage of 120 V will be measured. If the voltmeter is placed across the two hot conductors, a voltage of 240 V is obtained. The 240 V potential difference arises from the addition of two 120 V ac sources which are 180 electrical degrees out of phase with each other.

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Voltage Waveforms for 120/240 V Service


Voltage for 120/240 V Service in Phasor Form



Consideration must be given as to how loads are connected to this system and how they effect current level in each conductor.120 V loads may be connected between either Hot conductor (commonly called legs) and the neutral. Loads at 240 V are connected between the two Hot conductors. Note the neutral wire is not connected to the 240 V loads. Therefore, the 240 V loads can not place current on the neutral wire.

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120/240 V Load Connections



Loads requiring 120 V potential will be connected between the neutral and one of the hot conductors. In this case, current may be carried in the neutral wire.The amount of current flowing in the neutral can be minimized by balancing the loads; that is, by having equal 120 V loads connected to each of the hot wires.


Particularly in livestock facilities, it is desirable to minimize the neutral current as much as possible by the balancing of loads and use of 240 V rather than 120 V equipment. Ground currents can stress animals.As a general rule, the neutral will carry only the net difference or imbalance in current between the 120 V loads on the two legs.

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Three-Phase Systems

Where large quantities of electrical power are being transmitted or used three-phase ac power is generally used. Such currents are generated by an alternator having three identical armature coils spaced 120 degrees from each other. Since each coil has two connections, it at first appears that 6 lines are needed to transfer currents to the loads.

Three-Phase Systems con’t

It is possible to connect one end of each coil to another coil at the alternator and then transmit the current over 3 lines, one for each phase. Two basic considerations, called “wye” and “delta,” are used.

Three-Phase Delta

For the delta configuration, the ends of each winding are joined to the ends of the other 2 windings to get the characteristic triangle, or delta configuration.In this figure, the loads on the system are assumed to be balanced. That is, all the loads are of the same magnitude.


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Three-Phase Delta con’t

For the three-phase system the terminology of phase voltage, line-to-line, or phase-to-phase voltages, and phase and line currents are widely used.The voltage across one winding or a single load is called the phase voltage.The voltage between two of the conductors between the source and the load is termed the line-to-line voltage, phase-to-phase voltage, or simple line voltage.For the three-phase (3φ) delta, the phase voltage and line-to-line voltage are the same.


The current through any winding or load segment is called the phase current.The current through one of the conductors from the source to the load is termed the line current.In a delta configuration, a line current is the vector sum of two phase currents. Because of the 120o difference between each of the phase current, the vector sum of any two, the line current, is equal to 30.5 (1.732) times the phase current.


To obtain single phase from a delta system, connections are made across one of the three phases.A single-phase 240 V system can be obtained by connecting across one phase of the three-phase delta, as shown across phase B.If a single phase 120/240 V system is needed, a neutral wire is connected to a center tap as shown in phase C.In practically all installations, the neutral wire would be grounded.


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Three-Phase Wye

A common application of the wye system is known as the 3-phase, 4-wire system. Is has a grounded neutral connected to the common junction of the three transformers.In the 4-wire system, single-phase power can be obtained by connecting between the neutral and any one of the phase conductors.

Three-Phase, Four-Wire System


Three-Phase Wye con’t

If, for example, the three-phase voltage is 208 V, a single phase at 120 V could be obtained by connecting phase to neutral. Thus by using 4 wires, we can have a 3-phase system at 208 V for motors, water heaters, and similar large loads and a single-phase system at 120 V for lighting and small appliances.

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Three-Phase Power

The total power output for a three-phase system is a constant value.The total instantaneous power, which is the sum of the three curves, is a constant over time. Total true power output for the three-phase system is expressed by: Gustafson, Fundamentals in Electricity for Agriculture, 1988

Power in a Three-Phase System


Building Service Entrances

Building service equipment are those components of the system needed to carry the current from the feeder conductors to the branch circuits serving the loads within the building.The function of the service entrance is to supply power to each of the branch circuits within the building from the service conductors. In doing so it must:

– Supply a main switch or disconnect means– Supply overcurrent protection for branch circuits– Supply terminals for attachment of branch circuit conductors

including equipment grounding conductors– Supply a point of connection to earth or ground the system.

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Single-Phase Service Entrance


Electrical Grounding

The term grounding, in electrical terminology, means connected either directly or indirectly to earth. The purpose of grounding is for safety.Grounding can be divided into two sections; system grounding, which is grounding of current-carrying portions of the system and equipment grounding, which is grounding of equipment not intended to be at a voltage potential difference from the earth.

System Grounding

The principle reason for system grounding is to limit the voltage between any conductor and the earth to a minimum value for the system being used.A ground can be a grounding electrode or any conductive material connected directly or indirectly to the earth. For example, metal pipes (water, gas, drain) in a building are connected to other pipes, which in turn are buried in the ground.Voltage to ground will be the voltage between any point in the electrical system and any object that is grounded.

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System Grounding con’t

The typical 120/240 V single-phase system has the neutral wire grounded. If we measure from any grounded object to either hot conductor, the voltage will be 120 V. Thus the voltage to ground is the minimum possible for a 120/240 V system, and the voltage from any ungrounded conductor to ground is the same.



Having a minimum voltage and the same voltage to ground for all ungrounded conductors are requirements of the National Electric Code for safety.System grounding provides a measure of safety (a) for equipment or persons if an unintentional contact between a hot conductor and earth is made by limiting the voltage to a minimum value or (b) if a fault outside the building should arise.Good system grounding is dependent on establishing low resistance paths to earth at each system grounding point.


The NEC (250-81) states that if available on the premises, at each building or structure, each of the following items could be bonded together to form the grounding electrode system:

– Metal underground water pipe with at least 10 ft (3 m) in the earth– Metal frame of the building where effectively grounded– Concrete-encased electrode – and electrode encased in at least 2

in (51 mm) of concrete, located within or near the bottom of a concrete foundation

– Ground ring encircling the building at least 2.5 ft (76 cm) below the surface.

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If none of the electrodes specified above are available, one or more fabricated electrodes are required (NEC 250-83). The most common electrodes are a rod or pipe electrode. These rods are to be not less than 8 ft (2.44 m) in length. If a single electrode does not have a resistance to ground of 25 ohms or less, one additional electrode must be added.

Equipment Grounding

The NEC requires that all metal likely to become energized by an electrical short be grounded to the same ground as the system ground.To meet this requirement, equipment grounding supplies a low-resistance path from all of the metal objects to the system ground at the service entrance.In many instances, an extra conductor (green or uninsulated) will be used, or in others metal conduit may create the low resistance path.Equipment grounding is necessary to prevent electric shock to persons or animals coming in contact with metallic objects, which due to some fault, have come in contact with a hot conductor.

Equipment Grounding con’tTo illustrate the potential problem, if a fault develops in the motor windings such that the hot conductor comes in contact with the ungrounded frame, a 120 V potential exists between the motor frame and earth.


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Equipment Grounding con’tThis type of hazard can be avoided by equipment grounding the frame. If the hot conductor were to come in contact with the frame, a short circuit would exist, and the overcurrent protection for the circuit would then “blow” or open the circuit due to the high current flow.

Gustafson, Fund. in Electricity for Agriculture, 1988

Equipment Grounding con’t

In the circuit the neutral conductor is grounded at the service entrance as is required by the NEC. The NEC also requires that the neutral conductor not be grounded at any other point beyond the service entrance. This is to assure that all load current returns via the insulated neutral conductor.


Equipment Grounding con’tIf the equipment grounding system is not properly installed and maintained, it is possible that the overall resistance of the grounding path will increase to the point where insufficient fault current will flow.Two problems include: obvious shock hazard and, if the conduit is making poor contact, there may be some arcing at the bad contact, which could ignite a fire. Even if arcing does not occur, danger may exist from heating.


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Equipment Grounding con’t

Use of grounding electrodes at remote equipment can not substitute for equipment grounding. If a fault occurs to a piece of equipment which is not equipment grounded, the fault current through the grounding electrode will not be sufficient to open the overcurrent protection and a hazard remains. Gustafson, Fundamentals in Electricity for Agriculture, 1988

Polarity and Switching

In most wiring systems, conventions have been established for the color of the covering for each conductor in the system.In a single-phase system, a white wire is always used as the neutral, black or red are used as the “hot”conductors, and green or bare wires as the grounding conductors.Devices such as switches and outlets are also polarized by the use of brass colored terminals for hot contacts, silver-colored terminals for neutral contacts, and green terminals for grounding contacts.

Polarity and Switching

If a circuit is correctly polarized, all switches will be placed in the hot (red or black) conductors.When a switch is improperly placed in a neutral conductor a voltage exists between the electrical equipment and the ground even when the switch is open.If for example, the device is a light bulb socket, the screw shell would be a potential of 120 V. This could make changing the light bulb hazardous if the shell is touched.

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Polarity and Switching con’t

It should be apparent why the grounded neutral is never to be interrupted by a fuse, circuit breaker, switch, or other device unless both the hot and neutral are switched together.Under no condition should switches or over current protection be placed in a grounding conductor.


Overcurrent Protection

Any electrical system needs safeguards to assure that safe levels of current for conductors and equipment are not exceeded. Two basic classes of devices are used for this purpose, fuses and circuit breakers.Both are installed in series with the hot conductors and are designed to open the circuit if a specified current level is exceeded.Fuses are overcurrent devices of which a portion is destroyed when interrupting the circuit. They are made with low melting point links which are calibrated to melt when a specific current is reached.

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Overcurrent Protection con’t

A standard fuse will carry a 10% overload for a few minutes, a 20% overload for less than a minute, and a 100% overload for only a fraction of a second.Often in circuits for control of motors, fuses which will allow a temporary overcurrent during motor starting are needed. Time-delay fuses are designed for this need.

Plug-Type Time-Delay Fuse


Time-Delay Fuse

A time-delay fuse can open in either of two ways. During a continuous overload, a solder connection at one end of the link will melt, and the spring will pull the link away, breaking the circuit. During a short circuit with a large current, the link itself will melt almost instantaneously.An overload created by a motor start will not be large enough to melt the link or of long enough duration to melt the solder connection.

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Time-Delay Fuse con’t

The most common fuses for residential wiring are the screw or plug type fuse. They have ratings up to 30A and are used with voltages up to 125 V.When the circuit exceeds 30 A, it is necessary to use a cartridge-type fuse and fuse holder.


Circuit Breaker

A circuit breaker is a device designed to open a circuit automatically at a predetermined overload current without damage to itself. Most circuit breakers have a bimetallic strip connected with the contacts.Current passes through the bimetallic strip causing it to heat up. Since the two metal expand at different rates, it also bends. If the current level is too high, the bend will be large enough so that the contact points will open. After the element has cooled, the circuit breaker can be reset.

Circuit Breaker con’t


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Ground Fault Circuit Interrupters

In a complete electrical circuit there must be at least two wires; one to carry the current to the load and one to return the current to the outlet or source.If the insulation of the wiring or load is faulty or breaks down, all or a portion of the current may follow an alternate path through the grounding system or earth back to the source. This is called ground fault.A ground fault current may not be large enough to cause a fuse or circuit breaker to trip, but still be fatal to a person.


This table shows the current levels for human response including levels at which a person can no longer let go of a device and at which fibrillation occurs. These currents are much lower than those required to activate overcurrent protection.


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Protection

Three ways exist to attempt to protect persons from ground faults.One method is to use double-insulated tools which insulate the operator from the tool.The second is the use of three-wire (grounded) cords which extend the equipment grounding to the tool. However, all too often the third or grounded wire is not used, thereby rendering the ground wire useless.The third method is to use a device known as a ground fault interrupter.

Ground Fault Interrupter

A ground fault interrupter measures the current in the two load conductors. Whenever the difference exceeds a specified value (like 5 mA) the device opens the circuit.


Ground Fault Interrupter con’t

Ground faults can occur almost anywhere but are most serious in wet or damp areas. With this in mind, the NEC requires GFI’s in all circuits in new wiring supplying receptacles in the bathroom and garages of dwellings. They also require them on all outdoor receptacles accessible from the ground level or associated with swimming pools.Fuses and circuit breakers protect the system from excessively large currents while GFI’s protect persons from small leakage currents.

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