Showing posts with label Electrical Power. Show all posts
Showing posts with label Electrical Power. Show all posts

Friday, July 21, 2017

July 21, 2017

How to Size a Portable Generator for Home Use

Portable Generators are a reliable source of power in the absence of utility power. They provide electrical power to supply our critical power needs when the utility company is unable to supply us electrical power due to fault in their transmission system or during maintenance interventions in their power infrastructure or in the worst case of a natural disaster such as earthquake or a hurricane that has destroyed section of the power grid.

What Size of Portable Generator Do I need ?
The size of portable generator you need depends on your power requirement when the need arises to use the generator. Do you require the generator to power all of your electrical appliances at once? Or do you require the generator to power some critical electrical load during power outage? The bigger your power needs, the bigger the size of your generator and the more expensive your portable generator will be!

Running Watts of an Electrical Appliance
The running watts of an electrical appliance is the power it can draw continuously with rated voltage and current. It is usually calculated as:

Running Watts = Rated Voltage x Rated Current.

Note that the above formula will give power in volts-amps or VA but assuming a power factor = 1 which is rarely the case, we get power in watts. This approximation is done to enable easy sizing of a portable generator for home use.

The running watt can easily be calculated by using the rated voltage and current on the name plate of the appliance. Generators are also rated for their running watts. It is the power the generator can deliver continuously at rated voltage, current and frequency. A generator must not be made to continuously carry load beyond its running watts for a very long time otherwise the generator�s life will be shortened and the device becomes damaged in a short time.

Surge Watts or Start up Power of an Electrical Appliance
Certain devices and appliances have an electric motor or compressor in them. They require additional watts to start them. This additional watt may also be referred to as the surge watt of the device. The surge watts required by these devices may sometimes be two or three times the watts required to run the device. Heat producing devices also called resistive loads such as light bulbs, toasters or coffee makers do not require surge watts at start up. A generator must have enough surge watts capacity to handle devices that require surge watts at start up to prevent a nuisance tripping of the main power breaker in the generator.
As shown above, a generator must have sufficient surge capacity to carry loads requiring additional power during start up. Consider a refrigerator that works for one third of the time within a given time cycle. Each time the refrigerator compressor starts, a generator powering the refrigerator must have sufficient surge power for the compressor each time it comes on!

How to Calculate the Size of Portable Generator Required
To properly size a generator, care should be taken to analyse the load the generator is to power so that both running watts and surge watts can be correctly calculated. To calculate the size of generator:

Add up the total running and surge watts for each appliance. Multiply the total sum gotten by a contingency of 15 � 20 % to get the capacity of your generator.

As a guide during the sizing calculation for domestic application;
Surge watts for refrigerators and air conditioners = 2 x running watts 
Surge watts for motors (surface or submersible pumps) = 3 x running watts
Microwave Oven = 1.5 x running watts

Sample Sizing Calculation
Suppose the following loads are to be powered by a portable generator:

Electrical Load
Number
Running Watts
(W)
Refrigerator 1 800
Submersible pump 1 600
Lighting loads lot 150
Air Conditioner (1hp) 1 800
Deep Freezer 1 500
Microwave 1 600
Computer 1 300
TV 1 400



Determine power rating for generator  as shown below:
Electrical Load
Number
Running Watts
(W)
Surge Watts
(W)
Refrigerator 1 800  2 x 800 = 1,600
Submersible pump 1 600 3 x 800  = 2,400
Lighting loads lot 150 0
Air Conditioner (1hp) 1 800 2 x 800 = 1,600
Deep Freezer 1 500 2 x 500 = 1,000
Microwave 1 800 1.5 x 800 = 1200
Computer 1 300 0
TV 1 400  0
Total
4,350 7,800
Total Power Required = 4,350 + 7,800 = 12,150W

Add 15% contingency = 12,150 x 1.15 =13,972.5W

Size of Generator needed 

= 15,000W or 15KVA   standard Size







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Saturday, July 8, 2017

July 08, 2017

How UPS (Uninterruptible Power Supply) Systems Works

UPS stands for Uninterruptible Power Supply. A UPS system is an autonomous source of alternate power that is used to supply sensitive electronic loads such as computer centers, telephone exchanges and many industrial-process control and monitoring systems. These applications require power that is availability and of good quality.

A UPS solution for sensitive electrical loads is used to provide a power interface between the utility and the sensitive loads, providing voltage that is:
1. Free of all disturbances present in utility power and in compliance with the strict
        tolerances required by loads.
2. Available in the event of a utility outage, within specified tolerances

UPS systems satisfy requirements in 1 & 2 above in terms of power availability and quality by:
1. Supplying loads with voltage complying with strict tolerances, through use of an
        inverter
2. Providing an autonomous alternate source, through use of a battery
3. Stepping in to replace utility power with no transfer time, i.e. without any interruption in the               supply of power to the load, through use of a static switch.

These characteristics make UPS units the ideal power supply for all sensitive applications because they ensure power quality and availability, whatever the state of utility power.

Basic Parts of a UPS System
A UPS comprises the following main components:
1. Rectifier/charger, which produces DC power to charge a battery and supply an inverter
2. Inverter, which produces quality electrical power free of all utility-power disturbances, notably           micro-outages and that is within tolerances compatible with the requirements of sensitive                     electronic devices.
3. Battery, which provides sufficient backup time to ensure the safety of life and property by                   replacing the utility as required
4. Static switch, a semi-conductor based device which transfers the load from the
        inverter to the utility and back, without any interruption in the supply of power

Types of Static UPS Systems
Types of static UPSs are defined by standard IEC 62040. The standard distinguishes three operating modes for UPSs which are:
1. Passive standby (also called off-line)
2. Line interactive
3. Double conversion (also called on-line)

These definitions concern UPS operation with respect to the power source including the distribution system upstream of the UPS. IEC Standard 62040 defines the following terms:
a. Primary power: power normally continuously available which is usually supplied by
        an electrical utility company, but sometimes by the user�s own generation
b. Standby power: power intended to replace the primary power in the event of
        primary-power failure
c. Bypass power: power supplied via the bypass

UPS Operating in Passive Standby Mode

Operating Principle:
The inverter is connected in parallel with the AC input in a standby as shown below:
UPS in Passive Standby Mode. Photo Credit: Schneider Electric

Normal Mode Operation
In normal mode operation, the load is supplied by utility power via a filter which eliminates certain disturbances and provides some degree of voltage regulation (IEC 62040 specifies some form of power conditioning). The inverter operates in passive standby mode.

Battery Backup Mode Operation
In battery backup mode operation, when the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a very short less than 10 ms transfer time. The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which causes transfer of the load back to the AC input (normal mode).

Application
This configuration is a compromise between an acceptable level of protection against disturbances and cost. It can be used only with low power ratings less than 2 kVA.

Limitations
This UPS operates without a real static switch, so a certain time is required to transfer the load to the inverter. This time is acceptable for certain individual applications, but
incompatible with the performance required by more sophisticated, sensitive systems
(large computer centers, telephone exchanges, etc.). Furthermore, the frequency is not regulated and there is no bypass.

UPS Operating in Line-interactive Mode
The inverter is connected in parallel with the AC input in a standby configuration, but also charges the battery. It thus interacts with the AC input source as shown below:
UPS in Line-interactive Mode. Photo Credit: Schneider Electric

Normal Mode Operation
In normal mode operation, the load is supplied with conditioned power via a parallel connection of the AC input and the inverter. The inverter operates to provide output-voltage conditioning and/or charge the battery. The output frequency depends on the AC-input frequency.

Battery Backup Mode Operation
In this mode of operation, when the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a transfer without interruption using a static switch which also disconnects the AC input to prevent power from the inverter from flowing upstream. The UPS continues to operate on battery power until the end of battery backup time or the utility power returns to normal, which provokes transfer of the load back to the AC input (normal mode).

Bypass Mode Operation
This type of UPS may be equipped with a bypass. In the bypass mode, If one of the UPS functions fails, the load can be transferred to the bypass AC input (supplied with utility or standby power, depending on the installation).

Application and Limitation
This UPS configuration is not well suited to regulation of sensitive loads in the medium to high-power range because frequency regulation is not possible. For this reason, it is rarely used other than for low power ratings.

UPS Operating in Double Conversion (On-line) Mode

Operating Principle:
In this type of UPS, the inverter is connected in series between the AC input and the application as shown below:
UPS in Double-Conversion Mode. Photo Credit: Schneider Electric

Normal Mode Operation
During normal operation, all the power supplied to the load passes through the rectifier/charger and inverter which together perform a double conversion (AC to DC to AC), hence the name.

Battery Backup Mode Operation
In battery backup mode, When the AC input voltage is outside specified tolerances for the UPS or the utility power fails, the inverter and the battery step in to ensure a continuous supply of power to the load following a transfer without interruption using a static switch. The UPS continues to operate on battery power until the end of battery backup time or utility power returns to normal, which causes transfer of the load back to the AC input (normal mode).

Bypass Mode Operation
This type of UPS is generally equipped with a static bypass, sometimes referred to as a static switch. The load can be transferred without interruption to the bypass AC input (supplied with utility or standby power, depending on the installation), in the event of UPS failure, load current transient (inrush or fault currents) or load peaks. The presence of a bypass assumes that the input and output frequencies are identical and if the voltage levels are not the same, a bypass transformer is required.

For certain types of load, the UPS must be synchronized with the bypass power to ensure load-supply continuity. Furthermore, when the UPS is in bypass mode, a disturbance on the AC input source may be transmitted directly to the load because the inverter no longer steps in. Another bypass line, often called the maintenance bypass, is available for maintenance purposes. It is closed by a manual switch.

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Tuesday, January 31, 2017

January 31, 2017

How to Size a Photocell for a Lighting Installation

Now that you know how to install and wire a photocell in a lighting installation. The next step would be to know how to determine the current rating of the photocell for a given application so that when installed, it does not burn off easily and create problems for the lighting installation.

Consider the discharge lighting installation below where a photocell has been used.


Let LP 1 =LP2 = LP3 = LP4 = 250W
Power supply Voltage, V = 240V
Power Factor = 0.5 (discharge lamps see Typical power factor for common electrical loads)

Power in a single phase circuit is given by:


Where I is the rated current of the photocell.

Now from the above formula for power, we get : 

$I = \frac{250}{(240 * 0.5)} = 2.0833 Amps$ 

Now the photocell should be able to withstand the inrush current of a discharge lamp which is about 1.6 times nominal current.

Hence actual current rating of photocell = 1.6 x 2.0833 = 3.33 Amps

A photocell rated 5 Amps should just do for the above application with four (4) discharge lamps.

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January 31, 2017

How to Install and Wire a Photocell Switch in a Lighting Installation

A photocell switch is essentially a light dependent resistor, LDR. Its resistance decreases with increasing incident light intensity. They are used in many applications for on-off control especially in lighting installations.

In lighting applications, Photocells are placed in streetlights to control when the lights are ON or OFF. During daylight, light falling on the photocell causes the streetlights to turn off and during night hours or darkness to turn on. Thus energy is saved by ensuring the lights are only on during hours of darkness.

How to Wire a Photocell 
A photocell used in lighting application has three terminals labelled as:
1. Load line (Lo)
2. Neutral line (N)
3. Supply or live line (LI)
In most photocells, the load line wire is RED, the neutral wire is WHITE and the Supply line is black. 

This colour code is not universal. It may change for some other brand of photocell.
The picture of the terminals of a brand of photocell is shown below:

Photocell Terminal Markings

Wiring and installing a photocell is pretty straight forward as shown below:
How to Wire a Photocell Switch in a Lighting Installation

As shown above, the load wire (Lo) goes to the lighting installations connected in series while the neutral (N) wire through a breaker is looped to all the lights. The supply line through a breaker supplies the photocell electrical power.

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Monday, January 30, 2017

Voltage Drop and Power Formulas for Electrical Engineers
January 30, 2017

Voltage Drop and Power Formulas for Electrical Engineers

Working with single phase, three-phase and DC (direct current circuits) and you quickly need to reference formulas for voltage drops and power calculations for a given conductor? The table below provides a quick reference for these calculations.

Voltage Drop and Power Calculation Formulas For Single Phase Circuits

Electrical Parameters Formulas
Voltage Drop $?V = 2*I*L*(rCos? + xSin?)$
% Voltage Drop% $?V  =  \frac{?V}{V_r}*100$
Active Power $P = V*I*Cos?$
Reactive Power $Q = V*I*Sin?$
Apparent Power $S =  V*I = \sqrt{{P^2} +{Q^2}}$
Power Factor $Cos? = \frac{P}{S}$
Power Loss $P_L = 2*L*r*I^2$


Voltage Drop and Power Calculation Formulas For Three-Phase Circuits
Electrical Parameters Formulas
Voltage Drop $?V = \sqrt{3}*I*L*(rCos? + xSin?)$
% Voltage Drop% $?V = \frac{?V}{V_r}*100$
Active Power $P = \sqrt{3}*V*I*Cos?$
Reactive Power $Q = \sqrt{3}*V*I*Sin?$
Apparent Power $S = \sqrt{3}*V*I = \sqrt{{P^2} +{Q^2}}$
Power Factor $Cos? = \frac{P}{S}$
Power Loss $P_L = 3*L*r*I^2$



Voltage Drop and Power Calculation Formulas For Direct Current (DC) Circuits
Electrical Parameters
Formulas
Voltage Drop
$?V = 2*I*L*r$
% Voltage Drop
% $?V = \frac{?V}{V_r}*100$
Active Power
$P = V*I$
Reactive Power
$ - $
Apparent Power
$ - $
Power Factor
$ - $
Power Loss
$P_L = 2*L*r*I^2$


Meaning of symbols used in the formulas above:
$L$       =   Total length of conductor
$r $       =    Resistance of conductor per unit length
$x$       =    Reactance of conductor per unit length
$?V$    =   Voltage drop 
$P$       =   Active power
$Q$      =    Reactive power
$I$        =   Current

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January 30, 2017

Formulas For Star - Delta Transformations in Three - Phase Electrical Circuits

In three - phase electrical circuits, there is often the need to transform from a Delta configuration to a Star configuration and vice versa. The formulas for making these conversions are detailed below:
For a Star  to Delta Transformation, as shown in the formula above:



$Z_{12} = Z_1 + Z_2  + \frac{Z_1 Z_2}{Z_3}$

$Z_{23} = Z_2 + Z_3  + \frac{Z_2 Z_3}{Z_1}$

$Z_{13} = Z_3 + Z_1  + \frac{Z_3 Z_1}{Z_2}$




For a Delta to Star Transformation, we have:
$Z_1 = \frac{Z_{12} Z_{13}}{Z_{12}+Z_{13}+ Z_{23}}$

$Z_2 = \frac{Z_{12} Z_{23}}{Z_{12}+Z_{13}+ Z_{23}}$

$Z_3 = \frac{Z_{23} Z_{13}}{Z_{12}+Z_{13}+ Z_{23}}$


With the above formulas, you can easily transform from a Delta configuration to a Star configuration or vice versa.




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How to Size the Neutral Conductor in an Electrical Installation
January 30, 2017

How to Size the Neutral Conductor in an Electrical Installation

In a balanced three phase systems, the current in the neutral conductor is theoretically zero. However in a practical electrical installation, this is not the case. In fact there is always some current flow in the neutral although small if the loads in the three phase are sufficiently balanced. However increasing current will flow through the neutral in an installation with high harmonics necessitating the need to appropriately determine the minimum cross sectional area of the neutral that will be safe for the installation.

Given the implications of under sizing the neutral conductor, the neutral conductor, shall have the same cross section as the line conductor:
1. in single-phase, two-wire circuits whatever the section;
2. in poly-phase and single-phase three-wire circuits, when the size of the line conductors is less             than or equal to 16mm2 in copper, or 25mm2 in Aluminium.

The cross section of the neutral conductor can be less than the cross section of the phase conductor when the cross section of the phase conductor is greater than 16mm2 with a copper cable, or 25mm2 with an aluminium cable, if both the following conditions are met:
1. The cross section of the neutral conductor is at least 16mm2 for copper conductors and 25mm2          for aluminium conductors;
2. There is no high harmonic distortion of the load current. If there is high harmonic distortion (the          harmonic content, THD, is greater than 10%), as for example in equipment with discharge                  lamps, the cross section of the neutral conductor cannot be less than the cross section of the                phase conductors.

The table below shows the minimum cross sectional area of the neutral conductor in a given electrical installation under different types of circuits:

Type of Circuit
Phase Conductor Cross Section, S, (mm2)
Minimum Neutral Conductor Cross Section, SN (mm2)
Single Phase/Two Phase Circuits - Copper/Aluminium
Any
S
Three-Phase Circuits - Copper
S = 16
S
S > 16
16
Three - Phase Circuits - Aluminium
S = 25
S
S > 25
25
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Saturday, January 28, 2017

January 28, 2017

Basics of Harmonics in Electrical Systems:

Harmonics are essentially distortions in the electrical power systems. An electrical power system can be represented by a sinusoidal waveform which varies with time. The harmonic with frequency corresponding to the period of the original waveform is called fundamental and the harmonic with frequency equal to �n� times that of the fundamental is called harmonic component of order �n�. The presence of harmonics in an electrical system is an indication of the distortion of the voltage or current waveform and this implies such a distribution of the electric power could result in the malfunctioning of equipment and protective devices.

The harmonics are nothing less than the components of a distorted waveform and their use allows the analysis of any periodic non-sinusoidal waveform through different sinusoidal waveform components. 
Harmonics Distortions Waveform. Photo Credit: ABB

Causes of Harmonics in Electrical Systems
Harmonics are generated by nonlinear loads. When we apply a sinusoidal voltage to a load of this type, we shall obtain a current with non-sinusoidal waveform.

The main equipment generating harmonics are:
1. personal computer
2. fluorescent lamps
3. static converters
4. continuity groups
5. variable speed drives
6. welders.
7. Transformers (mostly third harmonics which becomes insignificant with increasing loading of the transformer).

In general, waveform distortion is due to the presence, inside of these equipment, of bridge rectifiers, whose semiconductor devices carry the current only for a fraction of the whole period, thus originating discontinuous curves with the consequent introduction of numerous harmonics.

Effects of Harmonics in Electrical Systems
The effects of harmonics can be felt in both the current and voltage.
The main problems caused by harmonic currents are:
1) overloading of neutrals
2) increase of losses in the transformers
3) increase of skin effect.

The main effects of the harmonics voltages are:
4) voltage distortion
5) disturbances in the torque of induction motors (since Torque is proportional to supply voltage)

Total Harmonic Distortion (THD) in an Electrical System
If the rms values of the harmonic components are known, the total rms value can be easily calculated by the following formula:

Total Harmonic Distortion (THD)
The total harmonic distortion is defined as:











The harmonic distortion ratio is a very important parameter, which gives information about the harmonic content of the voltage and current waveforms and about the necessary measures to be taken should these values be high.

For THDi < 10% and THDu < 5%, the harmonic content is considered negligible and such as not to require any provisions

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Wednesday, December 28, 2016

Common Characteristics of Lamps Used in Electrical Installations
December 28, 2016

Common Characteristics of Lamps Used in Electrical Installations


Below are the characteristics of the common lighting technology in use in different electrical installations. Also stated are their place of application, advantages and disadvantages:

Lighting Technology
Application
Advantages
Disadvantages
Standard Incandescent
  • Domestic use
  • Localized decorative lighting
  • Direct connection without intermediate Switchgear.
  • Reasonable purchase price.
  • Compact size
  • Instantaneous lighting
  • Good colour rendering
  • Low luminous efficiency and high electricity consumption.
  • Significant heat dissipation.
  • Short service life.
Halogen Incandescent
  • Spot lighting.
  • Intense lighting
  • Direct connection
  • Instantaneous efficiency
  • Excellent colour rendering
  • Average luminous efficiency
Fluorescent tube
  • Shops, offices, workshop.
  • Outdoors
  • High luminous efficiency.
  • Average colour rendering
  • Low light intensity of single unit.
  • Sensitive to extreme temperatures
Compact fluorescent lamp
  • Domestic use.
  • offices.
  • Replacement of incandescent lamps
  • Good luminous efficiency.
  • Good colour rendering
 High initial investment compared to incandescent lamps
HP Mercury Vapor
  • Workshops, halls, hangars.
  • Factory floors
  • Good luminous efficiency.
  • Acceptable colour rendering.
  • Compact size
  • Long service life
  • Lighting and relighting time of a few minutes
High-Pressure Sodium
  • Outdoors
  • Large halls
  • Very good luminous efficiency
  • Lighting and relighting time of a few minutes
Low-Pressure Sodium
  • Outdoors
  • Emergency lighting
  • Good visibility in foggy weather
  • Economical to use
  • Long lighting time (5 mins).
  • Mediocre colour rendering.
Metal halide
  • Large areas
  • Halls with high ceilings
  • Good luminous efficiency.
  • Good colour rendering.
  • Long service life.
  • Lighting and relighting of a few minutes
LED
  • Signaling (3-colour traffic lights, exit signs and emergency lighting
  • Insensitive to the number of switching operation.
  • Low energy consumption.
  • Low temperature
  • Limited number of colors.
  • Low brightness of single unit


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Typical Power Factors for Common Electrical Loads
December 28, 2016

Typical Power Factors for Common Electrical Loads

Power factor is very critical for calculating or measuring the electrical power consumed by an electrical device on an alternating current supply. To be able to determine electrical power on alternating current (AC) systems, you need to know the power factor of the electrical load. Below is listed the  typical power factors for common electrical loads:


Electrical Load Power Factor (Cos?) Reactive Demand Factor (Tan?)
Transformers (No load condition)
 0.1 - 0.15 9.9 - 6.6
Motor (Full load)
 0.7 - 0.85 1.0 - 0.62
Motor (No load)
 0.15 6.6



Metal Working Apparatuses:

  • Arc Welding
 0.35 - 0.6 2.7 - 1.3

  • Arc Welding  Compensated
 0.7 - 0.8 1.0 - 0.75

  • Resistance Welding
 0.4 - 0.6 2.3 - 1.3

  • Arc Melting Furnance
 0.75 - 0.9 0.9 - 0.5
Fluorescent Lamps:

  • Compensated
 0.9 0.5

  • Uncompensated
 0.4 - 0.6 2.3 - 1.3
Mercury Vapor Lamps
 0.5 1.7
Sodium Vapor Lamps
 0.65 - 0.75 1.2 - 0.9
AC DC Converters
 0.6 - 0.95 1.3 - 0.3
DC Drives
 0.4 - 0.75 2.3 - 0.9
AC Drives
 0.95 - 0.97 0.33 - 0.25
Resistive Load
 1 0


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Saturday, January 30, 2016

How To Select Batteries for Any Application
January 30, 2016

How To Select Batteries for Any Application

The selection of batteries for any application is a critical exercise. A number of factors must be considered in selecting the best battery for a particular application. The characteristics of each available battery must be weighed against the equipment requirements and one selected that best satisfy these needs. The considerations that are important and influence the selection of the battery are tabulated below
:


Battery Factors to Consider Remarks
Type of Battery Is battery required, primary, secondary, or reserve system?
Electrochemical System Compare the advantages and disadvantages of the selected battery characteristics with major equipment requirements
Voltage Consider the nominal or operating voltage, maximum and minimum permissible voltages, voltage regulation, profile of discharge curve, start-up time, voltage delay
Load Current & Profile Consider whether the load profile of the selected battery is a constant current, constant resistance, or constant power; or others; value of load current or profile, single-valued or variable load, pulsed load
Duty Cycle Is the duty cycle of the battery required continuous or intermittent, cycling schedule if intermittent?
Temperature Requirements What temperature range over which operation of battery is required?
Service Life Length of time operation is required
Physical Requirements Consider the size, shape, weight; terminals of battery
Shelf Life Active/ reserve battery system; state of charge during storage; storage time as function of temperature, humidity and other conditions.
Charge � Discharge Cycle (For Rechargeable Batteries) Float or cycling service; life or cycle requirement; availability and characteristics of charging source; charging efficiency.
Environmental conditions Consider vibration, shock, spin, acceleration, etc.; atmospheric conditions (pressure, humidity, etc.) of selected battery and determine if suitable for your application
Safety and Reliability Permissible variability, failure rates; freedom from out gassing or leakage; use of potentially hazardous or toxic components; type of effluent or signature gases or liquids, high temperature, etc.; operation under severe or potentially hazardous conditions; environmentally friendly.
Unusual or Stringent Operating Conditions Very long-term or extreme-temperature storage, standby, or operation; high reliability for special applications; rapid activation for reserve batteries, no voltage delay; special packaging for batteries (pressure vessels, etc.); unusual mechanical requirements, e.g., high shock or acceleration, nonmagnetic
Maintenance & Re-Supply Ease of battery acquisition, accessible distribution; ease of battery replacement; available charging facilities; special transportation, recovery, or disposal procedures required
Cost Initial cost; operating or life-cycle cost; use of critical or exotic (costly) materials
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Thursday, January 14, 2016

January 14, 2016

How to Determine Voltage Drop in an Electrical Conductor

Owing to the effect of voltage drop on electrical equipment, determination of the voltage drop of an electrical conductor is very important in helping to predict voltage drop level and ensure that it is in line with the relevant standard.
For an electrical conductor with impedance Z, the voltage drop is calculated by the following formula:

Where:
K   =     a coefficient equal to 2 for single phase and two phase systems.
             SQRT(3) for three phase systems
IB   =     load current in Amps. if no information are available, the cable carrying   
             Capacity of the conductor shall be considered.
L    =     Length of conductor in Km
n     =     is the number of conductors in parallel per phase
R    =     is the resistance of the single cable per kilometre in (O/km)
X    =     is the reactance of the single cable per kilometre in (O/km)
Cos? = power factor of the load


Resistance and reactance values
per unit of length of Copper and Aluminium conductors at 50Hz are available in tables for easily calculating voltage drop. In the case of 60 Hz, the reactance value can be determined by multiplying that at 50Hz by 1.2

Percentage of Voltage Drop in Electrical Installation
The percentage voltage drop can be calculated in relation with the rated voltage of the installation Vr as :


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How Voltage Drop Affect Electrical Equipment and Installations
January 14, 2016

How Voltage Drop Affect Electrical Equipment and Installations

In an electrical installation, determination of voltage drop from the point of supply to the load is very important. Excessive voltage drop on the supply line impacts most electrical equipment but not to the same degree. The effect of voltage drop on some key electrical equipment is given below.

Effects of Voltage Drop
Electric Motors
In an electric motor, the torque is proportional to the square of the supply voltage. Therefore, if the voltage drops the starting torque will also decrease, making it more difficult to start up motors. The maximum torque will also decrease .

Incandescent Lamps
The more the voltage drops the weaker the beam becomes and the light takes on a reddish tone.

Discharge Lamps
In general, they are not very sensitive to small variations in voltage, but in certain cases, great variation may cause them to switch off.

Electronic Appliances
They are very sensitive to
variations in voltage and that is why they are often fitted with internal stabilizer circuitry.

Electromechanical Devices
Devices such as contactors and auxiliary releases have a minimum voltage below which their performances cannot be guaranteed. For a contactor, for example, the
holding of the contacts becomes unreliable below 85% of the rated voltage.

Voltage Drop Limits for Electrical Installations
To limit the problems of voltage drop on electrical equipment, the IEC 60364-5-52 �Electrical installations of buildings. Selection and erection of electrical equipment - Wiring systems� Annex G states that the voltage drop between the origin of an installation and any load point should not be greater than certain limits specified in the table below expressed with respect to the value of the nominal voltage of the installation :

Type of Installation Lighting (%) Other Uses (%)
A - low voltage installation supplied directly from a public low voltage distribution system 3 5
B - Low voltage installation supplied from a private low voltage supply. 6 8
Notes:
1. As far as possible, it is recommended that voltage drop within the final circuits do not exceed those recommended for type A installations.
2. When the main wiring systems of the installation are longer than 100m, these voltage drops may be increased by 0.005% per meter of wiring system beyond 100m without this supplement being greater than 0.5%
3. Voltage drop is determined from the demand by the current using equipment, applying diversity factors where applicable or from the values of the design current of the circuits

Exceptions to the IEC 60364-5-52 standard
A greater voltage drop may be accepted for
  • Electric motors during starting period
  • For other electrical equipment have high in-rush current
Provided that in both cases it is ensured that the voltage variations remains within the limits specified in the relevant equipment standard

The following temporary conditions are excluded from the voltage drop restrictions in the table above:
  • Voltage transients
  • Voltage variation due to abnormal operation
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Wednesday, December 23, 2015

Classes of Explosive Atmospheres � EU Classification
December 23, 2015

Classes of Explosive Atmospheres � EU Classification

An understanding of this classification is required because often motors and other electrical equipment are required to be installed in these hazardous or explosive atmospheres. A clear understanding of the classes of explosive atmospheres will help in the selection and specification of electrical equipment in such environment.

Classes of Explosive Atmospheres
The EU (European Union) ATEX directive 99/92/EC distinguishes between two types of explosive atmospheres: GAS and DUST. Areas subjected to these two kinds of explosive atmospheres are each divided into three zones. Each zone characteristics are identical for gas and dust, but their numbering is different. Zones 0, 1, 2 refer to gas and zones 20, 21, 22 refer to dust. 

The various classes of explosive atmospheres are listed below
with a description of the characteristics of each one of them including the level of danger present and what category of equipment are required to be installed in such explosive atmospheres.



Zones
Description
Level of Danger to User
Type of Equipment Required
0/20 Permanent presence of explosive gasses or combustible dust. Minimum category 1 equipment required for use

 Constant Danger Category 1
1/21 Occasional presence of explosive gasses or combustible dust during normal duty. Minimum category 2 equipment required for use

 Potential Danger Category 2
2/22 Presence of explosive gasses or combustible dust not likely to occur or only for a shorter period of time. Minimum category 3 equipment required for use

 Minor Danger Category 3

A category 1 equipment is an equipment that can safely work in a zone  0 or 20 explosive atmosphere.
A category 2 equipment is an equipment that can safely work in a zone  1 or 21 explosive atmosphere.
A category 3 equipment is an equipment that can safely work in a zone  2 or 22 explosive atmosphere.

The EU classification of hazardous or explosives atmosphere is somewhat different from the North America classification but the zones (EU) and class (North America) categorization have similar characteristics. See Hazardous Area Classification
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Basics of Explosive Atmospheres
December 23, 2015

Basics of Explosive Atmospheres

An explosive atmosphere is an atmosphere that develops explosively because of an uncontrollable combustion. Explosive atmosphere consists of air and some sort of combustible material such as gas, vapours, mists or dust in which the explosion spreads after ignition. Typical examples of productions where combustible dust is of major concern, is the handling of cereals, animal feed, paper, wood, chemicals, plastics and coal.

Sources of Ignition
Sources of ignition that can cause the atmosphere to explode are listed below:

Common Sources of Ignition
Electrical sparks
Flames
Hot surfaces/ spots
Static electricity
Electromagnetic radiation
Chemical reaction
Mechanical forces
Mechanical friction
Compression ignition
Acoustic energy
Ionizing radiation

Fire Triangle
For an explosion to take place, three elements have to be present at the same time: fuel (such as explosive gas) , an oxidizer (such as the oxygen in the air) and a source of ignition (such as electrical sparks). The combination of these three elements is generally referred to as the Fire Triangle.

Explosion Limits for Typical Combustible Gases
For an explosive atmosphere to form, a certain concentration of combustible material must be present. When the concentration of combustible material is too low (lean mixture) or too high (rich mixture), no explosion will take place. In that case only a slow combustion or none at all will occur. It is only within the range of the upper and the lower explosion limit that the mixture of fuel and oxidizer reacts explosively when exposed to a source of ignition and becomes very devastating in terms of the scale of destruction.

Below are the lower and upper explosion limits of some common gases which may exist in the atmosphere where electrical or electronic equipment are installed:

Substance Designation
Lower Explosion Limit (Vol. %)
Upper Explosion Limit (Vol. %)
Acetylene
2.3
78.0 (self decomposing)
Ethylene
2.3
32.4
Gasoline
0.6
8.0
Benzol
1.2
8.0
Natural gas
4.0 - 7.0
13.0 - 17.0
Heating Oil/Diesel
0.6
6.5
Methane
4.4
16.5
Propane
1.7
10.9
Carbon Disulphide
0.6
80.0
Town gas
4.0 - 6.0
30.0 - 40.0
Hydrogen
4.0
77.0
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Saturday, December 19, 2015

How to Identify Three Phase Windings
December 19, 2015

How to Identify Three Phase Windings

A procedure has been adopted by the IEC for identifying three phase winding connections. Letters and numbers are used as follows. The high voltage (HV) terminals have upper-case letters e.g. A-B-C, R-Y-B, U-V-W, L1-L2-L3 and the low voltage (LV) terminals have lower-case letters e.g. a-b-c, r-y-b, u-v-w, l1-l2-l3. Each winding has a start numbered 1 and a finish numbered 2.
The choice of letters and numbers tends to be a national preference, see the table below for a rule of thumb guide:

Geographical Area Letters and Numbers Used
 USA L1   L2   L3      or       1   2    3
Europe U     V    W       or       R   S   T
United Kingdom   R     Y     B        or       A   B    C
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Saturday, December 12, 2015

Types of Protection for Hazardous Areas
December 12, 2015

Types of Protection for Hazardous Areas

Most electrical equipment consists of live or active static parts, and in some cases such as motors, solenoid valves and relays have moving mechanical parts, encased in an enclosure. The electrically live conductors are kept out of touch to prevent electric shock hazards. The detrimental effects of the environment e.g. rain, sprayed water, fine dust and particles are kept out of contact with the conductors, insulation, bearings and the like.

The design of the enclosure of electrical equipment with regard to hazardous area applications is defined by several lower case letter codes, mostly single digits for electrical power equipment but occasionally two digits for very low energy electronic equipment. The most frequently encountered codes are d, e, n, p and i. The lesser used codes are o, m, s and q. The table below gives a brief description of each code:

Ex or EEx Code Meaning of Code
d Flameproof Enclosure
e Increased Safety
i Intrinsic Safety. Two types ia & ib
m Encapsulated Enclosure
n Basically a UK concept that is similar to type �e�,
but only for use in Zone 2 areas
o Oil-immersed enclosure
p Pressurization and continuous dilution by
non-hazardous air or inert gas such as nitrogen
q Sand-filled enclosure
s Special designs of enclosure or system of
components

The codes are usually embraced with double or single quotation marks or less often single round brackets ( ). The code is prefixed with the letters Ex or EEx. Occasionally two letters are combined for special designs of equipment e.g. Ex �de� for some types of motors
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How to Convert KVA to Amperes for Single and Three Phase Circuits Using Tables
December 12, 2015

How to Convert KVA to Amperes for Single and Three Phase Circuits Using Tables

The following tables list the current in amperes for different KVA values for both single and three phase circuits for 120, 240, 480 and 600V circuits. To get the corresponding value of AMPS for a given KVA, simply read off the current value at the required voltage for the given KVA rating.

KVA Rating to AMPS for Single Phase Circuits


Single Phase Circuits
KVA Rating
Current in Amps at:

120V
240V
480V
1 8.33 4.16 2.08
1.5 12.5 6.24 3.12
2 16.66 8.33 4.16
3 25 12.5 6.1
5 41 21 10.4
7.5 62 31 15.6
10 83 42 21
15 124 62 31
25 208 104 52
37.5 312 156 78
50 416 208 104
75 624 312 156
100 830 415 207
167 1390 695 348
200 1660 833 416

KVA Rating to AMPS for Three Phase Circuits

Three Phase Circuits
KVA Rating
Current in Amps at:
120V 240V 480V 600V
3 8.3 7.2 3.6 2.9
6 16.6 14.4 7.2 5.8
9 25 21.6 10.8 8.7
15 41.6 36 18 14.4
30 83 72 36 28.8
45 125 108 54 43
75 208 180 90 72
112.5 312 270 135 108
150 415 360 180 144
200 554 480 240 192
225 625 540 270 216
300 830 720 360 288
400 1110 960 480 384
500 1380 1200 600 480
750 2080 1800 900 720
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