Electrical Measuring Instruments

• Electrical measuring instruments are essential for maintaining modern devices
• Technicians must measure current, voltage, resistance, and power
• Electromagnetism is the most common measurement principle

Fundamental Assumptions of Electromagnetism

• The strength of an electromagnetic field is proportional to the current in the coil
• Voltage, resistance, and power relate to current flow; other values can be found if the current is known

Electrical Measuring Instruments

• Common instruments: ammeter, voltmeter, ohmmeter, and wattmeter
• Two types of meters: digital and analogue

D'Arsonval Meter Movement

• The basic DC meter movement is used in ammeters, voltmeters, and ohmmeters
• Pointer deflection is proportional to current through the coil
• A reference magnetic field is created by a horseshoe-shaped permanent magnet
• The measured current flowing through the coil creates a magnetic field
• The two fields oppose each other, causing the coil to rotate on low-friction bearings
• Rotation continues until the calibrated hairspring balances the magnetic force
• Utilizes a slotted screw on the front for pointer zero adjustment

Permanent Magnet Moving Coil (PMMC) Meter

• Designed based on the D'Arsonval meter movement
• A coil of wire is wound on an aluminum frame or bobbin
• The bobbin is supported by jeweled bearings to allow free movement
• Hairsprings attached to the coil's ends return it to its original position without current
• Hairsprings also make electrical connections to the coil
• They resist coil movement when current flows
• As current increases, the magnetic field around the coil strengthens, moving the coil farther
• A pointer attached to the coil indicates the amount of current on a scale

Accuracy Enhancements

• An iron core inside the coil concentrates magnetic fields
• Curved pole pieces attached to the permanent magnet ensure steady turning force

Meter Ratings and Terms

• Full-scale current is the current needed for full-scale deflection
• Meter sensitivity is the reciprocal of full-scale current
• It represents total resistance per volt to produce full-scale current
• A meter needing 1 milliamp for full-scale deflection has a sensitivity of 1,000 ohms per volt
• Total resistance of a meter, or meter resistance, must be considered in calculations
• Moving coil and hairsprings have resistance
• Some meters include a temperature compensating resistor in series with the coil
• This resistor maintains constant meter resistance despite temperature changes
• Lower full-scale current rating indicates higher sensitivity

DC Ammeter

• Measures current in amperes (A)
• Smaller currents are measured in milliamperes or microamperes
• DC ammeters are sensitive to current direction and must be connected with correct polarity
• The positive terminal (red) connects to the positive side of the circuit
• The negative terminal (black or blue) connects to the negative side
• All current must pass through the load and the meter
• Always connect in series with the circuit path
• Parallel connection damages the ammeter due to excessive current

Ammeter Shunts

• Shunts must be installed in parallel with the meter if the current range is greater than the full-scale current of a meter
• They are a type of resistor that Increases the amount of current it can measure
• The load current flowing through the shunt produces a voltage drop proportional to current

Shunt Design

• Shunts are designed to carry a fixed high proportion of the current
• For example, 99% or 99.9% through the shunt and 1% or 0.1% through the meter coil

Calculating Shunt Resistance Example

• To measure 10mA with a 1mA meter: the shunt should carry 9mA, meter carries 1mA
• If the meter resistance is 50Ω and the shunt is in parallel, then Esh = Em
• Using Ohm's law: Esh = Ish x Rsh and Em = Im x Rm, then Ish x Rsh = Im x Rm
• Therefore: Rsh = (Im x Rm) / Ish = (1mA x 50Ω) / 9mA = 5.56Ω

Shunt Placement

• Shunts may be located in the ammeter case or externally.
• Remotely positioned external shunts are used in high current cables (generator feeder lines)

Multirange Ammeter

• Several shunt resistors in the meter case and a switch select the desired range
• This allows for measuring different maximum current readings or ranges
• Modern meter movements have sensitivities from 5 microamperes to 1 milliampere

Multirange Ammeter Operation

• In the 100 microampere position, all current flows through the meter movement
• With the switch at 1 milliampere, the current has parallel paths through the meter and shunt resistors
• Part of the current goes through the meter, and the rest goes through the shunt resistors
• Resistors R1, R2, and R3 shunt the meter when the meter is set to the 10-milliampere position
• Select a range to not exceed the meter movement of 100 microamperes
• Shunt resistors have close tolerances, typically less than 1 percent
• The resistance of a shunt resistor must be known accurately to ensure proper measurement and protection
• Internal shunts are used for ranges below 50 amperes
• External Shunts are used for higher current ranges above 50 amperes

Range Selection

• Use the range selection switch
• Starting with the highest range is important, if the current is larger than the scale the meter can be damaged
• Use a range that indicates near the middle of the scale

Ammeter Insertion Effects

• Ammeters affect the circuit resistance and current, often overlooked source of error
• All ammeters have internal resistance
• Inserting an ammeter increases the circuit resistance, reducing the measured current
• Error is dependent on resistances in the ammeter and circuit being measured
• A smaller resistance of the meter movement (Rm), the smaller the affect of the circuit being measured

Voltmeter

• High resistance instrument for measuring voltage between two points in an electric circuit
• Works like an ammeter but employs a different external circuit
• Analogue voltmeters use a pointer, while digital voltmeters use a digital display

Voltmeter Design

• D'Arsonval movements are sensitive devices with low full-scale deflection
• They have current ratings like 50 μA and wire resistance less than 1000 Ω
• Voltmeters need to reduce voltage to a level the meter can handle
• Connecting a multiplier (Rs) in series with the meter movement converts the basic d'Arsonval meter movement to a DC Voltmeter

The Purposes of the Multiplier (Rs)

• To extend the voltage range of the meter movements
• To limit the current through the d'Arsonval meter movement to a maximum full-scale deflection current

Multirange Voltmeter

• Multiple ranges allows electromachanical meters (such as voltmeters) to read a broad range of voltages
• A multi-pole switch and multiple multiplier resistors are used, one for each voltage range

Multirange Voltmeter Operation

• A five-position switch makes contact with only one resistor at a time
• An "off" setting is provided when the switch isn't connected to a resistor
• Each resistor can be sized to provide a full-scale range for the voltmeter
• Ranges are based upon the meters movement
• The meter movement's scale must be equipped with labels for each range
• Resistor value is determined using a known total voltage, movement full-scale deflection rating, and movement resistance

Multiplier Resistances Example

• For a 1 volt range, R4 = 500 Ω
• For a 10 volt range, R3 = 9.5 kΩ
• For a 100 volt range, R2 = 99.5 kΩ
• For a 1000 volt range, R1 = 999.5 kΩ

Voltmeter Loading Effects

• When measuring voltage across a component, the voltmeter is in parallel with it
• The parallel combination of two resistors makes the resistance seen by the source less
• The decrease in voltage depends on the voltmeter's sensitivity
• The effect is called voltmeter loading, and the resulting error is called loading error

Circuit Example

• A source of 150 volts applied to two 10-kilohm resistors in series shows each resistor has a voltage drop of 75 volts
• When connected to a 150 volt range the voltmeter will have a resistance of 10 kilohms
• The parallel combination of R2 and the meter presents a total resistance of 5 kilohms
• Voltmeter addition changes the voltage drops to 100 volts across R1 and 50 volts across R2 because of the voltmeter
• A voltmeter should have a high resistance being measured to minimize the loading effect

Voltmeter Safety Precautions

• Voltmeters require safety precautions to prevent injury and damage
• Always connect voltmeters in parallel
• Always start with the highest range of a voltmeter
• De-energize and discharge the circuit before connecting/disconnecting
• In DC voltmeters, observe proper polarity
• Never use a DC voltmeter to measure AC voltage
• Observe general safety precautions

Ohmmeter

• Mechanical ohmmeter (resistance meter) designs are rarely used today
• Digital measurements are more popular
• The ohmmeter measures resistance placed between its leads
• Leads are connected across an external resistance
• Indication is through a mechanical meter measuring electric current
• The ohmmeter must have an internal voltage source to create current
• Ranging resistors provide appropriate current for a given resistance

Ohmmeter Scale

• The ohmmeter scale is uneven and reversed compared to voltmeters and ammeters
• A very low value resistor allows a high current to flow giving large meter deflection
• A high resistance means low current flow and low deflection
• The uneven scale comes from inverse relationship of the ohms law formula: R = E/I

Ohmmeter Design

• The ohmmeter's pointer deflection is controlled by the amount of battery current
• Before measuring, test leads of the ohmmeter are first shorted together
• The meter is then is calibrated for proper selected range operation
• With leads shorted, the pointer indicates zero on the ohms scale
• The test leads should be removed when finished to ensure contact with each other to discharge the ohmmeter battery is not made
• When properly adjusted, the rheostat and shorted leads will point to zero position
• Variable resistor (rheostat) adjusted to adjust current so pointer is at exactly zero when leads shorted
• Used to compensate the internal battery aging voltage
• When the test leads are separated, the pointer will be infinity due to the interruption of current
• The ohmmeter indication is "backwards", infinity is usually on the left of the scale and voltage & current have zero at the left of scales

Using the Ohmmeter

• After adjusting to zero reading, it is ready to be connected in a circuit to measure resistance
• The power switch of the circuit to be measured should always be in the OFF position
• Prevents the voltage source of the circuit from being applied across the meter, which causes meter damage
• The ohmmeter test leads are connected in series with the circuit to be measured
• This current is produced by the 3-volt battery

Testing Connections

• Connecting leads tested at points a & b
• The amount of current that flows through the meter coil depends on the total resistance of resistors R1 and R2, and meter resistance
• The amount of combined resistance of RI and R2 raises the series resistance, the pointer comes to rest
• Ohmmeter should not be connected to an energized circuit and cause it's reading to be invalid

Multi-Range Ohmmeter

• A multi range ohmmeter usually has several operational ranges such as R x 1, R x 10, R x 100, R x 1k, R x 100k and R x 1M
• This reading on range selections are interpreted in different manners than ammeters and voltmeters
• Reading is multiplied by the factor indicated by the range setting
• Larger range means less accurate reading, being crammed up on the left hand side
• Shunt resistors are needed to measure multiple ranges, measuring values from small to very large
• The shunt resistance increases for higher ohm ranges equaling the center scale
• A battery may also be used for highest ohm range

Shunt Ohmmeter

• Ohmmeter is known as series ohmmeter, to be the measuring resistance in series with the internal resistor
• In the shunt ohmmeter, the resistance to be measured is an ohmmeter of the meter movement
• The difference is the scale that is shown on a series and shunt ohmmeter
• Rheostat use for infinity reading for a full scale
• In R x 10, and R x 100 meters indicate infinity as an indicator for current
• Switch has an OFF position to prevents a drained battery
• The series ohmmeter adjust at 0 while a shunt ohmmeter adjusts to infinity
• Shunt ohmmeter usually has 5 to 400 ohms, needs to be placed in OFF position

Safety Ohmmeter

• Used for ultra-safe resistance testing in explosive squib and detonator circuits
• Used in potentially explosive zones
• Uses very small currents up to 0.5 mA

Megohmmeter (Megger)

• Used for multi-millions of ohms, such as conductor insulation
• It is needed to use a higher potential than battery of an ohmmeter to test isolation
• Composed of elements of measurement
• A DC voltage generator for measurement and indication the value of resistance

High Resistance Circuit Testing

• Both coils A & B are mounted on the movable member with a fixed relationship
• Coil B moves pointer counter clock wise and coil A moves clockwise
• Test leads are open, the internal current flows through Coil B and it deflects to infinity

Megger Usage

• A high voltage is given on the test leads, checking normal resistance is infinity and small testing the resistance

Megger Precautions

• Only use high-resistance measurements (insulation measurements, cables)
• Never test if the handle is cranked
• Disconnect item before using
• De-energize before using

AC Meters

• When a DC meter is connected in an CA, the meter will deflect towards zero as it moves due to voltage

Using a DC Meter in AC Circuits

• Use a DC-style meter in an AC-style AC must be rectified in DC
• 2 types: half and full wave rectifier
• Meters contain built-in rectifier for AC measurements

AC DC Current

• The D'Arsonval meter uses voltage connection as current
• More often it is beneficial to use full wave rectifiers with higher receiver sensitivity
• The most frequently used current is a bridge type rectifier

Value D'Arsonal Multimeter

• With the rectifier DC is converted, the D' Arsonval movement will react to the DC pulsating
• Although the effective meter uses RMS values, a difference is used on the AC meter
• The AC gives an incorrect rating if used with DC

Multimeter Other Meter Movements

• Movement contains permanent coils and other types of meter movements, used on AC or DC measurement
• Movement in the meter is that the position of average to indicate
• Pointers are enclosed in dampening cases and are spring loaded
• There consists methods for the current passing towards a coil and the coils turn magnetic field

Electrodynamic Meter Movement

• Electrodynamic movement uses basic principles to move around
• Pointer is attached to fixed coils to move the meter, which are coils being series meter terminals
• Current flows in either direction where magnetic fields will exists between two field coils
• Some Voltmeters and Ammeters used electrodynamic operations on the Wattmeter

Moving Iron Vane Meters

• Moving iron vane is used DC voltages and movement to measure
• Repulsion of movement with equal magnitude
• Measuring with a fixed coil producing an current of magnetitude -Suspended in this field contains two iron fields which move through a magnetic force
• Moving is used for a small current flow

Hot-Wire Meters

• Hot-wire measurement both uses thermal effect that flows through meter
• The heating effect of current flows, alternating current by the scale depending on the frequency
• Movement is stretched depending on wire that shows current

Thermocouple Meters

• The amount of current will heat in terminals of the resistance wire
• The D'Arsonval allows small measurements of current through the heat generated from resistor
• Used on the resistor wire in proportion to what it heats

Multimeter

• The most common instrument that technicians use
• Multiple meter from multiple different forms, allowing DC and AC form measurement

Analog Multimeter

• Typically 20/1000 ohms and contains re-zero dial

Digital Multimeter

• Displays value which is often accurate and compact for technicians to use

Meter Safety Precautions

• Wrongfully using meter that causes injury or damage
• Remove if not using properly
• Connecting series and parallel circuit's
• Properly measuring circuit to ensure AC
• Properly adjusting "0 ohm'd" reading

Transformers

• Scale current that is easy to measure
• The measuring is used to scale current for easy connection
• Various sizes are used to measure instrumentation

CT Leads

• Connecting the measuring device
• A CT test means if a conductor carries 5amps flow flow is produced in CT leads

Importance of Transformers

• High voltage due to high currents could be destructive
• Transformers that measure flow that often is beyond meter range for measuring
• Sensed transformer magnetic field

Watt Meter

• Meters measure by Watts
• The current allows movement for different current coils
• The voltage movement is proportional to the actuating scale and potential

The Wattmeter components and function

• Actuating force will then connect to the correct area and measure voltage in circuit
• Damaging the measurement often is the most dangerous
• Pointer is depended on safe limits and both currents are high