Test: Measuring Instruments - Electrical Engineering (EE) MCQ

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

Absolute error (ε):

The difference between the indicated or measured value and the true or actual value is called absolute error. Also known as a static error.
Absolute error (ε) = A_m - A_t
Where
A_m =  measured or indicated value
A_t = true or actual value
Gross error:

• Gross errors are the observational errors that happen due to the lack of observation of the observer.
• These errors vary from observer to observer.
• The gross errors may also occur due to improper selection of the instrument.

Relative error:

The relative error is the absolute error over the true or actual value.

Relative static error R.S.E = ε / A_t = 

% R.S.E =  × 100

Probable error is a quantity formerly used as a measure of variability which is equal to 0.6745 times the standard deviation.

The correct answer is 1 KWh.

Concept:

Electric bulb: It is an electric device that converts electric energy into heat and light energy.

• Resistance of the bulb 
• If we consider the bulb as a resistor then, we can easily find the current and voltage drops.
• Power consumed = i²R
• Power consumed ∝ Brightness

The rate of work done by the electric current is called as electric power.

The difference of potential between two points is called a potential difference.

• Electric energy (E) = electric power (P) × time (t)

Unit of electric energy = unit of electric power × unit of time = kilowatt × hour = kilowatt-hour

A Galvanometer is used to detect the direction of the current.
This device is used for detecting and measuring a small amount of electric current.
Johann Schweigger invented the device in 1820.

A megohmmeter is an electric meter that measures very high resistance values by sending a high-voltage signal into the object being tested.

Measurement of resistance, inductance, and capacitance

Power transformer

• A power transformer is a static machine used for transforming power from one circuit to another without changing the frequency.
• A power transformer can increase or decrease the voltage and current, hence can be used as either a step-up or step-down transformer.

Current transformer

• A current transformer is a type of transformer that is mainly designed to reduce the high currents to a lower value for measurement and protection purposes.
• Basically, a current transformer is a voltage step-up and current step-down transformer which means when it reduces the current, it increases the voltage in the same ratio.
• A power transformer and a current transformer are always connected in opposite phase connection.

If a power transformer has a star-connected primary and a delta-connected secondary, the CT connections on its primary and secondary sides should be connected in delta and star respectively.

In order to nullify the phase shift of the Power transformer (PT), The Current transformers (CT) are connected in a reverse configuration.

Concept:

A galvanometer can be converted into an ammeter by connecting a low resistance in parallel.

The effective resistance of device = R_sh in parallel with R_g

Where R_g is galvanometer resistance

A galvanometer can be converted into a voltmeter by connecting a high resistance in series.

The effective resistance of device = R_se in series with R_g

Where R_g is galvanometer resistance

Application:

The voltmeter has high resistance and ammeter has low resistance.

Voltmeter with high full-scale range has high resistance.

So, a voltmeter of range 10 V has higher resistance.

Energy meter:

Energy meter or Watt-hour meter is used to measure the energy in kWh.

It is an integrating type instrument.

Its working principle is similar to the transformer.

There are three essential mechanisms required in the energy meter named Driving torque, Braking torque, and registered mechanism.

Driving torque:

This torque is required to revolve the disc or rotate the disc.

It is obtained by the electromagnetic induction effect.

Braking torque:

It is required to rotate the disc at a constant speed.

It is obtained by using a permanent magnet placed inside the energy meter near the Aluminum disc.

Eddy currents have induced in the magnet due to the movement of the rotating disc through the magnetic field. This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk. The speed of the disk can be controlled by changing flux.

Breaking torque of induction type single-phase energy meter is:

K = constant

ϕ = flux

N = speed in rpm

R = radius of the disc

R_e = resistance in path of current (i.e. disc)

The braking torque of induction type single-phase energy meter is directly proportional to the square of the flux.

Registered mechanism:

It registers the no. of rotations or revolutions of the disc which is proportional to the energy consumed in kWh.

Meter constant = (No. of revolutions / kWh)

Points to remember:

Creeping:

Sometimes the disc of the energy meter makes the slow but continuous rotation at no load i.e. when the potential coil is excited but with no current flowing in the load called creeping error

This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields, etc

It can be reduced by making two opposite holes on the disc.

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.

Concept:

Absolute Error: The deviation of the measured value from the true value (or) actual value is called error. It is also known as a static error.

Static error (E) = A_m – A_t

Am = Measured value

At = True value

Relative Static Error: The ratio of absolute error to the true value is called relative static error.

Limiting Error:

The maximum allowable error in the measurement is specified in terms of true value, is known as limiting error. It will give a range of errors. It is always with respect to true value, so it is a variable error.

Guaranteed Accuracy Error:

The allowable error in measurement is specified in terms of full-scale value is known as a guaranteed accuracy error. It is a variable error seen by the instrument since it is with respect to full-scale value.

Application:

Given-

A_m = 125 V, A_t = 125.5 V

∴ Static error (E) = 125 - 125.5

E = 0.5 V