Electrical Instrumentation

Direct Current (DC)

Current that does not reverse its direction of flow. current that flows from positive to negative poles.

increasing resistance in a series:

Decreases Current

Stator

A stationary loop of wires around the rotor in which electricity is produced. the stationary part of the motor that consists of individual coil windings inside which the rotor will rotate when voltage is applied.

Limit Switch

A touch sensor used to limit the motion of a moving device.

hand-off-automatic switch

The starter coil is energized manually when the switch is turned to the 'hand' position, and is energized automatically by the control device's contacts when the switch is in the 'auto' position.

Voltage

electrical pressure, the force which causes current to flow through a conductor.

Ohm's Law

explains the relationship of voltage, current, and resistance in a circuit. The current flowing through an electrical circuit is proportional to the voltage across it and inversely proportional to the resistance.

Single phase power

power supply that allows the potential difference to drop to zero with every change in the direction of current flow. has a voltage of 120 volts nominal and a frequency of exactly 60 Hertz.

Volt

the unit of electric potential (pressure)- the difference of electric potential existing between two points in an electrical circuit.

voltage drop

the voltage that is dropped across a load or resistance.

amplitude of a wave

the wave's height from zero to the crest

open circuit

when any part of the path is open or broken and there is no continuity in the conducting path.

Peak value

measurement of the voltage from zero to the maximum positive peak.

Auxiliary contact interlock

nother method of interlocking is provided by using auxiliary contacts on the motor starter or by using a control relay's contacts. also referred to as electrical interlocking. Thecontacts that are on the motor starter provide a secondary interlock backup, even if mechanical interlock is already provided. These interlocking circuits are not only used in motor control circuits but also in machine control circuits.

Control Relay

switches one or more power circuits

Three phase power

Multiple voltage waveforms are superimposed on one another, resulting in a waveform that maintains a nearly constant high voltage. alternators using 3 separate coils to generate 3 different voltages, 120 degrees apart. Delivered to the user on 3 or 4 wires.

Total resistance in series

R1+R2+R3= Resistor Total is

Kirchhoff's Laws

[circuits] 1. the sum of current flowing into a junction must equal the sum of the current leaving at any junction, and 2. the sum of voltage sources = the sum of voltage drops around a closed-circuit loop. the algebraic sum of all the voltage rises and voltage drops equals zero. This means that the total applied source voltage is equal to the sum of all the voltage drops around any one path of an electrical circuit.

Ground or Common

a conducting connection between an electrical circuit and the earth, or some other conducting body. The point of reference that every electrical circuit has.

Overload

a higher current flow than the circuit's normal full load current up to 600%.

relay

an electrical device, typically incorporating an electromagnet, that is activated by a current or signal in one circuit to open or close another circuit. A magnetically operated switch.

Push button interlock

common technique of protecting two circuits from being on at the same time. provided by a double circuit push button, having one set of contacts normally open and another one normally closed. In a forward/reverse motor control circuit, these push buttons are connected so they are interlocked at the push button control level. This provides instantaneous change from forward to reverse and vice versa without having to stop the motor, whereas the mechanical interlock circuit would require the operator to first stop the ongoing direction of the motor before reversing it. In many instances, motors and equipment cannot withstand a sudden reversal of direction.

peak to peak value

easurement of the voltage from the maximum positive peak to the maximum negative peak.

Mechanical interlock

generally found in magnetic reversing starters. I.e. motor operated valve in which the valve can be driven open or closed. Made up of a mechanical linkage placed in such manner & position that it makes it impossible to close both starter controls in the forward and reverse state at the same time. usually installed at the factory, is represented in the ladder diagram by dotted lines between contactors.

Overload Relays

provided to protect motors, motor control apparatus and motor-branch circuit conductors against excessive heating due to motor overloads and failure to start.

source

provides the voltage and current that the circuit needs.

current

the movement of electrons through a conductor. Measured in amps. In a parallel circuit, it takes two or more paths.

Resistance

the opposition to the flow of current and limits the amount of current in an electrical circuit.

load

the part of an electrical circuit in which work is accomplished.

cycle

the rotation of an alternator through a complete revolution of 360 degrees.

Switches

turn on or off the flow of electricity to the load. Switches are classified as normally open (N.O.) or normally closed (N.C.).

Ohm

unit of electrical resistance

Resistors in parallel

-Current splits into parallel branches so they add - same voltage drop in each branch b/c all pathways start and end at a common point I1 = I2 + I3 V = V1 = V2 -Reciprocal resistances add Requiv = (R1*R2)/(R1+R2 ) -Equivalent resistance decreases as more resistors are added in parallel -Sum of the currents going into each division must equal the total current going out

Total voltage in a series circuit

...is shared between each component. When current flows through a resistor there is a voltage that is dropped across that resistor. If there is only one resistor in the circuit, all of the applied voltage will be dropped across it. If there is more than one resistor, the total voltage will be split among the resistors proportionally depending on the values of each resistor. The sum of all the individual voltage drops will equal the applied source voltage. (Vt=V1+V2). if there are two resistors at 4 Ω each, (R1 = 4Ω and R2 = 4 Ω) and your supply voltage is 12 volts, 6 volts will be dropped across R1 (V1 =6v) and 6 will be dropped across R2 (V2 = 6 v).

series circuit

A circuit in which all parts are connected end to end to provide a single path of current.

AC generator (alternator)

A coil of wire is rotated in a magnetic field. As the coil cuts through the magnetic field, the relative motion causes a current to be generated in the coil. an electromechanical device capable of converting mechanical power to electrical power.

short circuit

A connection that allows current to take the path of least resistance. an overcurrent that greatly exceeds the normal full-load current of the circuit.

transformer

A device that increases or decreases the voltage of alternating current. It changes AC voltage from one level to another by magnetic induction. It has no moving parts and steps up or steps down voltage as needed. Also used to transmit electricity over long distances at higher voltages

pressure switch

A device that opens or closes a set of contacts when a certain pressure is applied to the diaphragm of the switch.

Watt

A measure of electrical power equal to one joule of work per second.

Rotor

A rotating electrogmagnet inside the stator. the rotating part of the motor constructed from either coils of wire wound on laminated steel or from all laminated steel without any wire coils.

Motor Control Center

An assembly of one or more enclosed sections having a common power bus and principally containing motor control units. contains control logic, motor starter and circuit breakers for one or more motor.

THE EFFECTS OF AN OPEN

In parallel circuits there is a difference between an open in the main line and an open in a parallel branch. An open in the main line prevents any current from flowing in the line to all the branches. The current is zero in every branch. If the open is in only one of the branches, no current will flow in that branch, but the current in all the other parallel branches will be normal. This is because they are still connected to the voltage source.

Voltage in a parallel circuit

Same across each component. In parallel circuits, the electric potential difference across each resistor is the same. In a parallel circuit, the voltage drops across each of the branches is the same as the voltage gain in the battery. Thus, the voltage drop is the same across each of these resistors.

Current in a series circuit

Same at all points. In all parts of a series circuit the current cannot differ in any way because there is just one path for the current to flow. Remember that current is measured at a single point by breaking the circuit and inserting a current meter. Total current (It) in a series circuit depends on the total circuit resistance (Rt) and the source voltage or total voltage (Vt) applied to the circuit (It=Vt/Rt). The current in a series circuit is the same through each component and is the same throughout the circuit. (It=I1=I2). Since the current is the same throughout the circuit, the total current (It) is the same amount that passes through each resistor.

Two basic ways of wiring cells together

Series connection & parallel

Ampacity

The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. the smaller the wire the greater the resistance for any given length. When current is carried by a conductor it must pass through the electrical resistance of the conductor. Power dissipated in a wire manifests itself in the form of heat. Watts law says that power is current squared times the resistance of the wire or (P=I2*R).

open circuit

The resistance of an open circuit is infinitely high. When an open circuit occurs in a series circuit, the current is zero and there is no voltage drop across any of the series resistances, although the source still maintains its output voltage and the full source voltage will appear across the open part of the circuit. Consider the 120 volts from your power line to your home. The 120 v source is across the two terminals of the wall outlet even with nothing plugged in, an open circuit. When nothing is connected, the voltage is still there. If you connect a lamp or appliance, current will flow in the circuit. The fact that the open terminals have the entire applied voltage indicates a good way to find an open component in a series string. If you measure the voltage across each good component, zero voltage will be normal. However, the component that is open will have the full source voltage across it.

Resistance in a series circuit

Total resistance in a series circuit is the sum of all resistances. When a series circuit is connected across a voltage source, the current must flow through all the resistances. This path is the only way the current can return to the battery. With two or more resistors in the same current path, the total resistance across the battery or voltage source is the opposition of all the resistances. The total resistance in any series circuit is equal to the sum of all the individual resistances. (Rt=R1+R2). To find the total resistance (Rt) of a string of resistors (R1, R2) simply add the resistor values. As additional resistors are wired into the circuit, the total resistance increases.

Watt's Law in parallel circuits

Watt's Law states that power is the product of voltage times current (P = V x I). If the voltage in a parallel circuit is held to a constant value, adding another branch or decreasing a load will increase the current drawn from the source and will also increase the total power in that circuit. Another way to find total power in a parallel circuit is the sum of the power in each branch of that circuit.

Resistors

a device having a designed resistance to the passage of an electric current. They are used in circuit design to limit the flow of current to a desired value.

Ampere

a unit of electric current equal to a flow of one coulomb per second.

Phase sensing relay

aka voltage monitor): protects against low voltages, voltage unbalance, loss of phase and phase reversal.

fuse

an overcurrent protective device with a circuit opening fusible part that is heated and severed by the passage of overcurrent.

Overcurrent

any current in excess of the rated current of equipment or ampacity of a conductor. It may result from overload, short circuit, or ground fault

Increasing the voltage in a series circuit :

increases current

Cells wired in parallel:

is used to increase the amount of current available. When sources are wired in parallel, the current is added together to determine the total current available. However, the voltage is limited to the capability of the smallest cell in the chain. If four 1.5 volt cells capable of producing 2 amps each are connected in parallel, the output voltage would be 1.5 volts with a total of 8 amps.

A cell wired in a series connected:

is used to increase the amount of voltage available. When sources are wired in series, all voltage sources are added together to determine the total voltage. For example, an automobile battery is made up of six cells producing 2.1 volts each. The cells connected in series inside the battery give a terminal voltage of 12.6 volts (6 x 2.1=12.6). However, the current producing capability of the series wired battery is limited to the current produced by a single cell. Current flow must travel through each cell that is wired in series, therefore, the maximum current flow is limited to the capability of the smallest cell in the chain. If the six 2.1 volt cells in series each had a capacity of 30 amperes, the terminal voltage would be 12.6 volts with a current capacity limited to 30 amps. If one cell in this battery were to fail, the whole battery would fail.

Watt's Law

tells how much power is being used and is equal to volts times the current flow in the circuit

series circuits

the components (resistors or loads) in a series circuit are connected in successive order with the end of one joined to the end of the next

Interlocking circuits

when one part of the control circuit is energized and doesn't allow another part to be energized. most frequently used in motor control circuits to prevent a command for a motor to go forward and reverse at the same time. Or only allow a certain number of pumps to run at a station. Most pump stations have a spare pump and motor and the electrical service is not big enough to run all of the pumps. There are three types of commonly used interlocking techniques: 1) mechanical interlock, 2) push button interlock, and 3) auxiliary contact or control relay interlock.

phase displacement

when the current and voltage rise and fall and do not cross the zero line at the same point, the waves are said to be out of phase.