Indicating Instruments MCQ Quiz - Objective Question with Answer for Indicating Instruments - Download Free PDF

Permanent Magnet Moving Coil (PMMC)

Characteristics of the PMMC instrument

• PMMC is used to measure only DC or the average value of an electrical quantity. 
• The PMMC instrument has a linear or uniform scale, hence, the deflecting torque produced is directly proportional to the current.
• Either spring or gravity control can produce the controlling torque in PMMC instruments.
• For spring control: Tc ∝ θ, and for gravity control: Tc ∝ sin θ 
• In Permanent Magnet Moving Coil (PMMC), Electromagnetic damping is used to quickly settle the pointer without oscillation. It is achieved using eddy currents induced in a conducting coil or aluminum former moving in the field of a permanent magnet.

Permanent Magnet Moving Coil (PMMC)

Characteristics of the PMMC instrument

• PMMC is used to measure only DC or the average DC value of an electrical quantity. It cannot measure AC values.
• The PMMC instrument has a linear or uniform scale, hence, the deflecting torque produced is directly proportional to the current.
• The controlling torque in PMMC instruments can be produced by either spring control or gravity control.
• For spring control: Tc ∝ θ, and for gravity control: Tc ∝ sin θ 
• The damping torque in the PMMC instrument is provided by eddy current damping.
   

Advantages

• The PMMC has a high torque-to-weight ratio and, hence, has great accuracy.
• It produces no losses due to hysteresis.
• It has efficient damping characteristics and is not affected by a stray magnetic field.
   

Disadvantages

• Moving coil instruments are expensive.
• The moving coil instrument can only be used on the D.C. supply as the current reversal produces a torque on the coil.
• It may show an error due to the loss of magnetism of the permanent magnet.

Electrodynamometer instruments

• Electrodynamometer instruments are a type of measuring device used to measure both AC (alternating current) and DC (direct current) electrical quantities such as current, voltage, and power.
• Electrodynamometer-type wattmeters are widely used as standard reference instruments in laboratories because of their high accuracy and precision.
• They work on the principle of the interaction between the magnetic fields produced by fixed and moving coils.

Principle of Operation:

• Fixed coils produce a magnetic field. Moving coil interacts with the magnetic field of the fixed coils, resulting in a deflecting torque proportional to the product of the current in both coils.
• For current measurement: Both coils (fixed and moving) are connected in series to carry the same current. The torque generated is proportional to the square of the current, making the instrument respond to both AC and DC.
• For power measurement: One set of coils is connected in series with the load (to measure current), and the other set is connected across the load (to measure voltage). The deflecting torque in this case is proportional to the product of voltage, current, and the power factor, which allows it to measure true power in AC circuits.

Concept

The impedance for the series RL circuit is given by:

\(Z=\sqrt{R^2+X_L^2}\)

where, Z = Impedance

R = Resistance

X_L = Reactance

Calculation

Given, R = 4000Ω

V_DC = 200 V

V_AC = 200 V

I_AC = 0.04A

AC impedance, \(Z={V_{AC}\over I_{AC}}\)

\(Z={200\over 0.04}=5000\space \Omega\)

Since the impedance is higher than the DC resistance, it suggests the presence of an inductive reactance X_L.

\(5000=\sqrt{(4000)^2+X_L^2}\)

XL = 3000 Ω

A moving iron voltmeter measures the RMS voltage but is calibrated based on the assumption of pure resistance. The actual voltage it registers is:

V_read = I_AC × R

Vread = 0.04 × 4000 

Vread = 160 V

% error = \({V_{true}-V_{read}\over V_{true}}\times 100\)

% error = \({200-160\over 200}\times 100\)

% error = 20%

Concept

The torque in the MI instrument is given by:

\(T_d={1\over 2}I^2{dL\over dθ}\)

In a moving iron (MI) instrument, the torque is proportional to the square of the current.

T α I²

\({T_2\over T_1}=({I_2\over I_1})^2\)

Calculation

Given, I₂ = 3 A 

I₁ = 1 A

\({T_2\over T_1}=({3\over 1})^2\)

\(T_2=9T_1\)

The torque increased to 9 times of initial value.

Ammeter:

• It is used to measure the current.
• An ideal ammeter has zero internal resistance and thus it provides the path for maximum current.
• It is always connected in series as it measures current.
• The range of ammeter can be extended by using a low shunt resistance.

Voltmeter:

• It is used to measure the voltage.
• An ideal voltmeter has infinite resistance and thus it provides the path for minimum current.
• It is always connected in parallel as it measures voltage.
• The range of voltmeter can be extended by using a high series resistance.

The essential features are possessed by an indicating instrument deflecting, controlling, and a damping device.

• Deflecting device: The deflection device produces deflecting torque which causes the moving system to move from its zero position.
• Controlling device: The controlling device produces the controlling torque (Tc) which opposes the deflecting torque and increases with the deflection of the moving system. It also brings the pointer back to zero when the deflecting torque is removed.
• Damping device: This device produces damping torque this torque is necessary to bring the pointer to rest quickly. This damping torque (Td)  is used to reduce the oscillation.

Moving iron meter has large magnetic reluctance as compared to PMMC meter. That’s why more power is required to operate the moving iron meter.

Advantages of moving iron:

• It is a universal instrument which can be used for the measurement of AC and DC quantities
• These instruments can withstand large loads and are not damaged even under severe overload conditions
• It is very cheap due to the simple construction

Disadvantages of moving iron:

• These instruments suffer from error due to hysteresis, frequency change and stray losses
• The reading of the instrument is affected by temperature variation

Note: In terms of accuracy PMMC meter has the highest accuracy. The order of accuracy is given below.

Induction < Moving iron < PMMC instruments

Null type instrument: An instrument in which zero or null indication determines the magnitude of measured quantity, such type of instrument is called a null type instrument.

It uses a null detector which indicating the null condition when the measured quantity and the opposite quantity are same.

The permanent magnet moving coil (PMMC) instrument uses two permanent magnets to create a stationary magnetic field. These types of instruments are only used for measuring the DC quantities.

Electrolytic meters are exclusively DC ampere-hour meters, measuring an electric quantity directly and electric energy only indirectly, on the assumption that the pressure of the supply is constant.

Dynamometer type instrument is used for the measurement of A.C. as well as D.C. quantity. It is equally accurate on AC and DC circuits.

Shaded-pole type and induction type instruments are only used for AC measurements.

• The instruments which use the heating or thermal effect of the current for knowing their magnitude such type of instrument is known as the hot wire instrument.
• The hot wire instrument is used for both the AC and DC current.
• Hotwire instrument works on the principle of the thermal effect, that the length of the wire increases because of the heating effect of the current flow through it.
• When the current is passed through the fine platinum-iridium wire it gets heated up and expands.
• The sag of the wire is magnified, and the expansion is taken up by the spring.
• This expansion causes the pointer to deflect, indicating the value of the current.
• This expansion is directly proportional to the heating effect of the current and hence directly proportional to the square of the RMS value of the current.
• Therefore, the meter may be calibrated to read the rms value of the current.​

The permanent magnet moving coil instrument uses two permanent magnets to create a stationary magnetic field. These types of instruments are only used for measuring the DC quantities.

If we apply AC current to these types of instruments the direction of current will be reversed during the negative half cycle and hence the direction of torque will also be reversed which gives the average value of torque zero.

Important Points:

• Permanent Magnet Moving Coil (PMMC) is only used for DC measurements.
• Moving Iron (MI) type instruments can be used for both AC & DC measurements.
• Rectifier-type instruments are used for AC measurements.
• Induction-type instruments are only used for AC measurements.
• Thermocouple meters can be used for both DC as well as AC quantities.

Concept:

PMMC ammeters use spring controlling and deflecting torque varies directly as current

In spring control instrument, the controlling torque is given by,

\(Tc = {K_c}\theta\)

\({T_d} \propto I\)

In a PMMC instrument deflecting torque is directly proportional to current.

The final steady state deflection of PMMC instrument is directly proportional to current flowing through the coil.

At equilibrium position, TC = TD

⇒ I ∝ θ

• In case of over damping, the instrument will become slow and lethargic and it rises very slowly from its zero position to final position
• An over damped system would never allow the system to reach the desired end state since it is over damped and that is why they are never used.