Indicating Instruments MCQ – Indicating Instruments Questions and Answers

Answer:1. Air friction damping

Explanation: 

Moving-iron instruments are generally used to measure alternating voltages and currents. In moving-iron instruments, the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in the coil.

The moving iron instrument uses air friction damping.

Answer:2. Highly resistive

Explanation: 

• An electrodynamometer instrument can be used as a wattmeter to measure power
• Two fixed coils are connected in series with the load are called current coils; These will carry a full load current I_L as shown in the figure
• The moving coil is connected across the supply is called a voltage coil (or) pressure coil, which will carry a current I_P as shown in the figure
• Both coils are connected in parallel so that two different currents will flow through the coils I_L and I_P respectively.

In order to measure the AC power using this meter, we have to consider two assumptions inside the meter.

• The supply voltage is equal to the load voltage which means a drop in the current coil is neglected
• The pressure coil is assumed to be purely and highly resistive in nature

Answer: 2. Controlling torque

Explanation: 

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.

Answer:3. -4 A and 5 A

Explanation: 

i(t) = -4 + 3√2 sin (ωt + 30°) A

Reading of PMMC instrument = Average value of i(t) = -4

Reading of moving iron meter = RMS value of i(t)

\( = \sqrt {{{\left( { – 4} \right)}^2} + {{\left( {\frac{{3\sqrt 2 }}{{\sqrt 2 }}} \right)}^2}} = 5\;A\)

Answer:1. Moving coil

Explanation: 

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.

Answer:4. Braking torque

Explanation: 

The braking torque is not essential for the working of an indicating instrument.

The essential  torques for the working of an indicating instrument are given below:

Deflecting torque: The torque needed to move the pointer over a calibrated scale is known as deflecting torque and it can overcome the inertia of the moving system, controlling torque and damping torque.

Controlling torque: It is to control the pointer to a definite value that is proportional to the quantity being measured. In absence of controlling torque, the pointer will swing beyond its final steady-state position and the deflection will be indefinite.

Damping torque: Deflecting torque is used for deflection, the controlling torque acts opposite to the deflecting torque. So before coming to rest the pointer always oscillates due to inertia, so to bring the pointer to rest within a short time by reducing oscillations, we use damping torque without affecting controlling torque (or) inertia.

Answer:2. Reluctance

Explanation: 

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 that 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.

Answer: 1. Higher

Explanation: 

The moving-iron instrument (soft-iron instrument) uses the opposing forces on equally polarised, magnetized soft-iron vanes in the magnetic field of a coil with a measurement current. By suitably shaping the air gap the scale can cover a wide range.

• The deflection of the moving-iron instrument is independent of the current direction.
• It is thus equally suitable for DC and AC (for low frequencies such as the mains frequency).
• The moving-iron instrument is an RMS meter.
• The moving-iron instrument has a high internal power consumption.
• The moving-iron instrument can by its nature withstand high overloads.

Answer: 2. 2.4 kΩ

Explanation: 

If a voltmeter at internal resistance 1.2 kilo-ohms is connected across a 600-ohm resistor. Then the circuit will be

Voltmeter reads 5 V

35 × 400/(400 + R) = 5

R = 2.4 kΩ

Answer: 2. θ

Explanation: 

In spring control instrument, the controlling torque is given by

T_c = K_cθ

T_d ∝ I

At equilibrium position, T_C = T_D

I ∝ θ

Hence in spring control, the scale is uniform

Answer: 2. 5, √125 and √125

Explanation: 

PMMC measures dc or average value, and hot-wire and moving iron instruments measures RMS value.

If a current i is given as:

i = a₀ + a₁ sin (ω₁t + θ₁) + a₂ sin (ω₂t + θ₂) + a₃ sin (ω₃t + θ₃) +………..

Where,

a₀ = DC value = average value of current

Rms value is:

\(i_{rms}= \sqrt {a_0^2 + \frac{1}{2}\left( {a_1^2 + a_2^2 + a_3^2 + \ldots } \right)} \)

Calculation:

Given current i,

i =  5 + 14.14 sin (314 t + 45°)

PMMC meter reading = i_dc = average value = 5 A

Hot wire meter reading = Moving iron meter reading = i_rms

\( = \sqrt {{5^2} + \frac{1}{2}{{\left( {14.14} \right)}^2}} \)

i_rms = √125 A.

Answer:3. Both 1 and 2

Explanation: 

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.

Answer:1. Eddy current

Explanation: 

Eddy current damping:

• When a conductor moves in a magnetic field an emf is induced in it and if a closed path is provided, a current (known as eddy current) flows
• This current interacts with the magnetic field to produce an electromagnet torque which opposes the motion
• This torque is proportional to the strength of the magnetic field and the current produced

There are two common forms of eddy current damping devices:

1) A metal former that carries the working coil of the instrument

2) A thin aluminum disc attached to the moving system of the instrument, this disc moves in the field of a permanent magnet; The disc should be of conducting and non-magnetic material.

Answer:3. PMMC type

Explanation: 

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

In spring control instrument, the controlling torque is given by

T_c = K_cθ

T_d ∝ I

At equilibrium position, T_C = T_D

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

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

At equilibrium position, TC = TD

⇒ I ∝ θ

Hence in spring control, the scale is uniform

Answer: 2. Attraction type and repulsion type

Explanation: 

Moving iron type instruments are of mainly two types. Attraction type and repulsion type instrument.

Attraction type moving iron instrument:

• Whenever a piece of iron is placed nearer to a magnet it would be attracted by the magnet. The force of this attraction depends upon the strength of the said magnetic field. If the magnet is an electromagnet then the magnetic field strength can easily be increased or decreased by increasing or decreasing the current through its coil.
• Accordingly, the attraction force acting on the piece of iron would also be increased and decreased. Depending upon this simple phenomenon attraction type moving iron instrument was developed

Answer:4. Maximum current

Explanation: 

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 the 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 the voltmeter can be extended by using a high series resistance.

Answer:3. To protect against external magnetic fields.

Explanation: 

Shielding of Electro dynamometer type instrument:

• The magnetic field produced by the fixed coils in the Electro dynamometer type instruments is weaker than in other types of instruments such as PMMC. It is nearly 0.005 to 0.006 Wb/m².
• In d.c. measurements even the earth’s magnetic field may affect the readings. Thus it is necessary to shield an electrodynamometer-type instrument from the effect of stray magnetic fields.
• The stray magnetic field is around 0.0005 Wb/m², which has a significant effect in the case of an electrodynamometer-type instrument.
• Air-cored electrodynamometer-type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy.
• This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication.

Answer:4. Low sensitivity

Explanation: 

Disadvantages of Electrodynamic instruments:

• Air-cored coils are used, the operating magnetic field of these instruments is so weak that considerable errors may be introduced due to stray magnetic fields and in order to protect them, they must be shielded with cast-iron cases.
• Energy must be used to create two magnetic fields, such instruments are relatively insensitive or have low sensitivity.
• The power required is generally greater than that required by the permanent-magnet type owing to the greater weight of the moving parts.
• Dynamometer-type ammeters and voltmeters have uneven scales. However, the dynamometer wattmeter has a uniform scale.
• They are more expensive than the permanent-magnet-type instruments.

Answer: 2. Thermal effect

Explanation: 

• An instrument that uses the heating or thermal effect of the current for knowing its 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.​

Answer:4. 44

Explanation: 

The deflecting torque is given by

\({T_d} = {I^2}\frac{{dL}}{{dθ }}\)

Where,

I is operating current

L is self-inductance

θ is deflection

T_d ∝ I²

When the current through a moving iron instrument is increased by 20%, then deflection torque is

T’_d ∝ (1.2I)²

∴ T’_d = 1.44 T_d

Hence deflection torque is increased by 44%