Basic Electrical Theory for Ottawa Electricians
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Rule #1: |
The same current flows through each part of a series circuit. |
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Rule #2: |
Total Resistance of a series circuit is equal to the sum of the individual resistances. |
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Rule #3: |
The total voltage across a series circuit is equal to the sum of the individual voltage drops. |
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Rule #4: |
The voltage drop across a resistor in a series circuit is proportional to the size of the resistor. |
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Rule #5: |
The total power dissipated in a series circuit is equal to the sum of the individual power dissapations. |
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AMPERES
= |
VOLTS RESISTANCE |
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RESISTANCE
= |
VOLTS AMPERES |
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VOLTS
= |
AMPERES x RESISTANCE |
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Rule #1: |
The same voltage exists across each branch of a parallel circuit and is equal to the source voltage. |
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Rule #2: |
The current through a branch of a parallel network is inversely proportional to the amount of resistance of the branch. |
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Rule #3: |
The total current of a parallel circuit is equal to the sum of the currents of the individual branches of the circuit. |
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Rule #4: |
The total resistance of a parallel circuit is equal to the reciprocal of the sum of the reciprocals of the individual resistances of the circuit. |
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Rule #5: |
The total power dissipated in a parallel circuit is equal to the sum of the individual power dissapations. |
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TOTAL VOLTAGE
= |
E(1) = E(2) = E(3) ...etc. |
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TOTAL RESISTANCE
= |
VOLTS AMPERES |
TO DETERMINE THE TOTAL RESISTANCE IN A PARALLEL CIRCUIT WHEN THE TOTAL CURRENT AND TOTAL VOLTAGE ARE UNKNOWN USE EITHER OF THE FOLLOWING FORMULAS:
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RT
= |
1 ___________________ 1 + 1 + 1 + ......etc R1 R2 R3 | |
| FOR TWO RESISTORS IN PARALLEL USE THIS FORMULA CALLED THE "PRODUCT OVER THE SUM" | ||
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RT
= |
R(1) *
R(2) R(1) + R(2) | |
POWER IN SINGLE PHASE RESISTIVE
CIRCUITS
WHERE POWER FACTOR IS 100 PERCENT
(THESE
FORMULAS ARE COMMONLY USED TO SOLVE MOST CIRCUIT POWER PROBLEMS ON
TESTS)
TO
DETERMINE THE POWER CONSUMED BY AN INDIVIDUAL RESISTOR IN A SERIES CIRCUIT USE
THIS FORMULA:
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POWER
= |
I2 x R |
TO
DETERMINE THE POWER CONSUMED BY AN INDIVIDUAL RESISTOR IN A PARALLEL CIRCUIT USE
THIS FORMULA:
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POWER
= |
E2 R |
TO DETERMINE THE TOTAL POWER CONSUMED BY AN INDIVIDUAL CIRCUIT USE THIS FORMULA:
POWER = E
(TOTAL VOLTAGE) x I (TOTAL CURRENT)
RULES OF THUMB:
- THE TOTAL RESISTANCE OF RESISTORS IN PARALLEL IS ALWAYS LESS THAN THE VALUE OF ANY ONE RESISTOR.
- THE TOTAL RESISTANCE OF PARALLEL RESISTORS THAT ARE ALL THE SAME VALUE IS THAT VALUE DIVIDED BY THE NUMBER OF RESISTORS.
- ALWAYS USE THE PRODUCT OVER SUM RULE TO BREAK DOWN TWO PARALLEL RESISTORS INTO ONE RESISTOR. THIS IS MUCH EASIER THAN TRYING TO SOLVE LARGE ALGEBRAIC EXPRESSIONS.
- 746 WATTS IS EQUAL TO ONE HORSEPOWER
- EFFICIENCY IS EQUAL TO OUTPUT DIVIDED BY INPUT
- IN INDUCTIVE CIRCUITS CURRENT LAGS VOLTAGE.
- IN CAPACITIVE CIRCUITS CURRENT LEADS VOLTAGE.
- POWER FACTOR IS A MEASURE OF HOW FAR CURRENT LEADS OR LAGS VOLTAGE.
POWER IN ALTERNATING CURRENT CIRCUITS WHERE POWER FACTOR IS NOT 100 PERCENT
POWER = E x I x POWER FACTOR (FOR SINGLE PHASE)
POWER = E x I x 1.732 X POWER FACTOR (FOR THREE PHASE)
THIS POWER IS ALSO CALLED TRUE POWER OR REAL POWER AS OPPOSED TO APPARENT POWER FOUND BY CALCULATING VOLT-AMPERES.
VOLT-AMPERES = E x I (FOR SINGLE PHASE)
VOLT-AMPERES = E x I x 1.732 (FOR THREE PHASE)
IT CAN READILY BE DETERMINED BY ALGEBRA THAT
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POWER FACTOR
= |
TRUE POWER
APPARENT POWER |
MOTOR APPLICATION FORMULAS
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HORSEPOWER
= (for three phase motors) |
1.732 x VOLTS x
AMPERES x EFFICIENCY x power factor
746 |
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THREE PHASE AMPERES
= (for three phase motors) |
746 x HORSEPOWER 1.732 x VOLTS x EFFICIENCY x POWER FACTOR |
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SYNCHRONOUS RPM
= |
HERTZ x 120 NUMBER OF POLES |
MOTOR MARKINGS AND CONNECTIONS
CONNECTIONS FOR NINE LEAD
THREE PHASE MOTORS
THREE PHASE STAR OR Y
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THREE PHASE DELTA
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DELTA WYE HOOKUP FOR TRANSFORMER
MOTOR CONTROLLER WITH THREE
START STOP STATIONS
(HOLDING CONTACTS
NOT SHOWN)
TRANSFORMER TURNS RATIO
Ep = Tp
Es Ts
Where
Ep is primary voltage
Es is secondary voltage
Tp is number
of turns in primary
Ts is number of turns in secondary
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NEMA RATING FOR ENCLOSURES
NEMA and other organizations have established standards of enclosure construction for control equipment. In general, equipment would be enclosed by an Ottawa Electrician for one or more of the following reasons:
- Prevent accidental contact with live parts by an Ottawa Electrician.
- Protect the control from harmful environmental conditions.
- Prevent explosion or fires which might result from the electrical arc caused by the control.
NEMA I - GENERAL PURPOSE
The general purpose enclosure is intended primarily to prevent accidental contact with the enclosed apparatus by an Ottawa Electrician. It is suitable for general purpose applications indoors where it is not exposed to unusual service conditions. A NEMA I enclosure serves as protection against dust and light indirect splashing, but is not dusttight.
NEMA 3 - DUSTTIGHT, RAINTIGHT
This enclosure is intended to provide suitable protection against specified weather hazards. A NEMA 3 enclosure is suitable for application outdoors, on ship docks, canal and construction work, and for application in subways and tunnels by an Ottawa Electrician. It is also sleet-resistant.
NEMA 3R - RAINPROOF, SLEET RESISTANT
This enclosure protects against interference in operation of the contained equipment due to rain, and resists damage from exposure to sleet. It is designed with conduit hubs and external mounting by an Ottawa Electrician, as well as drainage provisions.
NEMA 4 - WATERTIGHT
A watertight enclosure is designed to meet the hose test described in the following note: "Enclosures shall be tested by subjection to a stream of water. A hose with a one inch nozzle shall be used and shall deliver at least 65 gallons per minute. The water shall be directed on the enclosure from a distance of not less than 10 feet and for a period of five minutes. During this period it may be directed in any one or more directions as desired. There shall be no leakage of water into the enclosure under these conditions."
A NEMA 4 enclosure is suitable for applications outdoors on ship docks and in dairies, breweries, etc.
NEMA 4X - WATERTIGHT, CORROSION-RESISTANT
These enclosures are generally constructed along the lines of NEMA 4 enclosures except they are made of a material that is highly resistant to corrosion. For this reason, they are ideal in applications such as paper mills, meat packing, fertilizer and chemical plants where contaminants would ordinarily destroy a steel enclosure over a period of time.
NEMA 7 - HAZARDOUS LOCATIONS - CLASS I
These enclosures are designed to meet the application requirements of the National Electrical Code for Class I hazardous locations. In this type of equipment, the circuit interruption occurs in air.
"Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures."
NEMA 9 HAZARDOUS LOCATIONS - CLASS II
These enclosures are designed to meet the application requirements of the National Electrical Code for Class II hazardous locations.
"Class II locations are those which are hazardous because of the presence of combustible dust."
The letter or letters following the type number indicates the particular group or groups of hazardous locations (as defined in the National Electrical Code) for which the enclosure is designed. The designation is incomplete without a suffix letter or letters.
NEMA 12 - INDUSTRIAL USE
The NEMA 12 enclosure is designed for use in those industries where it is desired to exclude such materials as dust, lint, fibers and flyings, oil see page or coolant see page. There are no conduit openings or knockouts in the enclosure, and mounting by an Ottawa Electrician is by means of flanges or mounting feet.
NEMA 13 - OILTIGHT, DUSTTIGHT
NEMA 13 enclosures are generally of cast construction, gasketed to permit use in the same environments as NEMA 12 devices. The essential difference is that, due to its cast housing, a conduit entry is provided as an integral part of the NEMA 13 enclosure, and mounting by an Ottawa Electrician is by means of blind holes, rather than mounting brackets.