Not to be confused with ohm metre
An analog ohmmeterAn ohmmeter is an electrical instrument that measures electrical resistance (the opposition offered by a circuit or component to the flow of electric current). Multimeters also function as ohmmeters when in resistance-measuring mode. An ohmmeter applies current to the circuit or component whose resistance is to be measured. It then measures the resulting voltage and calculates the resistance using Ohm’s law V = I R {\displaystyle V=IR} .
An ohmmeter should not be connected to a circuit or component that is carrying a current or is connected to a power source. Power should be disconnected before connecting the ohmmeter. Ohmmeters can be either connected in series or parallel based on requirements (whether resistance being measured is part of circuit or is a shunt resistance).
Micro-ohmmeters (microhmmeter or micro ohmmeter) make measurements of low resistance. Megohmmeters (also a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is the ohm (Ω).
Design evolution
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The first ohmmeters were based on a type of meter movement known as a 'ratiometer'.[1][2] These were similar to the galvanometer type movement encountered in later instruments, but instead of hairsprings to supply a restoring force they used conducting 'ligaments'. These provided no net rotational force to the movement. Also, the movement was wound with two coils. One was connected via a series resistor to the battery supply. The second was connected to the same battery supply via a second resistor and the resistor under test. The indication on the meter was proportional to the ratio of the currents through the two coils. This ratio was determined by the magnitude of the resistor under test. The advantages of this arrangement were twofold. First, the indication of the resistance was completely independent of the battery voltage (as long as it actually produced some voltage) and no zero adjustment was required. Second, although the resistance scale was non linear, the scale remained correct over the full deflection range. By interchanging the two coils a second range was provided. This scale was reversed compared to the first. A feature of this type of instrument was that it would continue to indicate a random resistance value once the test leads were disconnected (the action of which disconnected the battery from the movement). Ohmmeters of this type only ever measured resistance as they could not easily be incorporated into a multimeter design. Insulation testers that relied on a hand cranked generator operated on the same principle. This ensured that the indication was wholly independent of the voltage actually produced.
Subsequent designs of ohmmeter provided a small battery to apply a voltage to a resistance via a galvanometer to measure the current through the resistance (battery, galvanometer and resistance all connected in series). The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance is increased, the current through the meter (and hence deflection) would decrease. Ohmmeters form circuits by themselves, therefore they cannot be used within an assembled circuit. This design is much simpler and cheaper than the former design, and was simple to integrate into a multimeter design and consequently was by far the most common form of analogue ohmmeter. This type of ohmmeter suffers from two inherent disadvantages. First, the meter needs to be zeroed by shorting the measurement points together and performing an adjustment for zero ohms indication prior to each measurement. This is because as the battery voltage decreases with age, the series resistance in the meter needs to be reduced to maintain the zero indication at full deflection. Second, and consequent on the first, the actual deflection for any given resistor under test changes as the internal resistance is altered. It remains correct at the centre of the scale only, which is why such ohmmeter designs always quote the accuracy "at centre scale only".
A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through the resistance, and another circuit that measures the voltage (V) across the resistance. These measurements are then digitized with an analog digital converter (adc) after which a microcontroller or microprocessor make the division of the current and voltage according to Ohm's law and then decode these to a display to offer the user a reading of the resistance value they're measuring at that instant. Since these type of meters already measure current, voltage and resistance all at once, these type of circuits are often used in digital multimeters.
Precision ohmmeters
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For high-precision measurements of very small resistances, the above types of meter are inadequate. This is partly because the change in deflection itself is small when the resistance measured is too small in proportion to the intrinsic resistance of the ohmmeter (which can be dealt with through current division), but mostly because the meter's reading is the sum of the resistance of the measuring leads, the contact resistances and the resistance being measured. To reduce this effect, a precision ohmmeter has four terminals, called Kelvin contacts. Two terminals carry the current from and to the meter, while the other two allow the meter to measure the voltage across the resistor. In this arrangement, the power source is connected in series with the resistance to be measured through the external pair of terminals, while the second pair connects in parallel with the galvanometer which measures the voltage drop. With this type of meter, any voltage drop due to the resistance of the first pair of leads and their contact resistances is ignored by the meter. This four terminal measurement technique is called Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances. The Four-terminal sensing method can also be utilized to conduct accurate measurements of low resistances.
References
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][1] Illustration of type. Note the absence of any zero adjustment and the changed scale direction between ranges.https://www.codrey.com/electrical/ohmmeter-working-and-types/
Capacitance: is the property of an electrical circuit that opposes a change in voltage, the units for capacitance are Farads. Capacitance stores energy in an electrical circuit in the form of charges stored on electrical plates separated by an insulating material. As the voltage in a circuit increases more electrons are stored on the plates, when the voltage decreases the stored electrons will discharge into the circuit attempting to maintain the voltage at a constant level. Since the voltage in an AC circuit is periodically changing any capacitance in the circuit will create a reactance or (opposition to the changing voltage) known as capacitive reactance (XC). The units of XC are ohms and cause the voltage to lag the current by 90°.
The units of XC are ohms and are dependent on the amount capacitance in the circuit and the frequency of the applied voltage.
XC = 1/(2ΩfC)
Frequency: is a measure of the number of events occurring in a set period of time.
F= # events/time
Phase Angle: A common use of phase angles is to measure the time delay between 2 or more periodic events that have the same period. Since the inverse of time (T) is frequency (F), periodic events that have the same frequency take the same amount of time to complete the event.
T=1/F
However, just because they take the same amount of time to perform the event and have the same frequency doesn’t mean that they begin and end at the same time.
The phase angle presents the delay between these events in degrees. For example, a 90° phase angle means the events are separated by ¼ of a cycle. Since inductance causes the current to lag the voltage by 90° if the period of the wave is 4 seconds the frequency would be .25 hertz. Therefore, the current would be delayed by 1 second or 90°.
Basic Electrical theory states:
In a purely Resistive circuit, current & voltage are in phase this means that both the voltage and current waveforms reach their maximum positive and maximum negative peaks and the 0 crossing at the same time.
In a purely Inductive circuit voltage leads current by 90° meaning the voltage reaches its maximum and minimum values 90° before current.
In a purely Capacitive circuit current leads voltage by 90° meaning the current reaches its maximum and minimum values 90° before voltage.
So, how does MCA™ use phase angle?
if the phase angle is 0° the circuit being tested is purely resistive. However, since a motor uses stator coils to create the magnetic field, they are inductive. But the coils are constructed of conductors which are resistive, and they are coated with an enamel film which is capacitive. So, the phase angle of each phase will be dependent on the relationship of these three electrical properties.
In 3 phase motors all coils should be identical and have the same phase angle. If the insulation between conductors begins to degrade either the inductance or capacitance will change. The phase angle or the time delay between the current and voltage will be one of the first measurement to change with even very slight changes of L or C.
Experience has shown that an early indication of any winding systems insulation degradation will be if the phase angle of any phase deviates from the average phase angle of all three phase by more than 2 degrees.
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