Electro Dynamometer Type Instruments MCQ Quiz - Objective Question with Answer for Electro Dynamometer Type Instruments - Download Free PDF

Advantages of Electrodynamic instruments:

• Air cored coils are used, they are generally free from hysteresis and eddy current errors when used on AC circuits.
• These instruments can be used for both DC. and AC measurements.
• It is the most accurate instrument for measuring AC signals with frequencies lower than 200 Hz.
• It has precision grade accuracy when used as a wattmeter.
• Less power consumption.

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 having 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

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.

Electrodynamometer instrument as a wattmeter:

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

Important Point:

To reduce the error in the wattmeter reading due to pressure coil inductance, the wattmeters are compensated by connecting a capacitor in parallel with the swamping resistor.

The value of capacitance at which the pressure coil is made purely resistive is

\(C = \frac{{0.41{L_p}}}{{R_s^2}}\)

The correct answer is option '2'

Concept:

Dynamometer type wattmeter:

• It is a type of moving coil instrument used for the measurement of power.
• It consists of two coils - fixed and moving coil.
• The fixed coil is connected in series with the load, hence known as the current coil.
• The moving coil is connected in parallel with the load, hence known as the potential or pressure coil.
• The fixed coil is divided into two parts.
• The two fixed coils are air-cored to avoid hysteresis effects when used on AC.

Additional Information

Advantages of Dynamometer type instrument:

• As the instrument has Square Law response so can be used on both the dc as well as on AC.
• These instruments are free from hysteresis and Eddy current errors. It is because of absence of iron in the operating part of the instrument.
• Ammeter up to 10A and voltmeter up to 600V can be constructed with precision grade accuracy.
• Dynamo type voltmeter are useful for accurate measurement of rms value of voltage irrespective of waveform.
• Because of Precision grade accuracy and same calibration for DC and AC measurement instruments are used as transfer and calibration instruments.

Disadvantage of Dynamometer type instrument:

• The scale is not uniform as the instrument uses Square Law response.
• These instruments have small torque-weight ratio so the friction error is considerable.
• Owing to heavy moving system friction losses in these instruments are somewhat more than those in other instruments.
• As a result of measures taken to reduce the frictional errors, their cost is more in comparison to moving iron and PMMC instruments.
• They are more sensitive to overload and mechanical impact and are to be handled with care.
• Adequate screening of the movements against the stray magnetic field is essential.
• The sensitivity of the instrument is typically very low due to poor deflecting torque. The sensitivity of dynamo type wattmeter is 10 to 30 per volt in comparison to the sensitivity of 20-kilo-ohm per volt in case of D’Arsnoval movement.
• The power consumption of this instrument is comparatively high because of their construction.

Advantages of Electrodynamic instruments:

• Air cored coils are used, they are generally free from hysteresis and eddy current errors when used on AC circuits.
• These instruments can be used for both DC. and AC measurements.
• It has precision grade accuracy when used as a wattmeter.
• Less power consumption.

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 having 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

Electrodynamometer instrument has two fixed coils and one moving coil. For using this instrument as a wattmeter to measure the power, the fixed coils acts as a current coil and must be connected in series with a load. The moving coil acts as a pressure coil and must be connected across the supply terminals.

The correct answer is option 4):(There are two pressure coils and two current coils).

Concept:

  Dynamometer type three-way wattmeter:

• Trikala wattmeter is a dynamometer type and consists of two separate wattmeters connected together.
• There are two current coils (CC) and two pressure coils (PC). In two wattmeter system, the total power consumed will be the sum of the readings of both the wattmeters, which has to be calculated manually.
• Whereas, in a tricycle electrodynamometer wattmeter, the total power consumption is directly represented, as the power consumption observed by both the elements is being integrated internally.
• In Trikala wattmeter, torque is developed on both the elements.
• The value of the torque on each element is proportional to the power that passes through it. The total torque on a three-watt wattmeter is the sum of the torques on an individual wattmeter.

Electrodynamometer type instrument utilized the principle of Electro-dynamic Effect or force between two current-carrying coils.

Additional Information

Type of Measuring Instrument:

Important Points

Electro-Dynamometer Type Instruments (EMMC):

• These instruments are the modified form of permanent-magnet type.
• Here a magnetic field is produced by two air-cored fixed coils placed on either side of the moving coil.
• These instruments are based on the principle that mechanical force exists between the current-carrying conductors.
• It essentially consists of a fixed coil and a moving coil.
• The fixed coil is split into two equal parts (F, F) which are placed close together and parallel to each other.
• The moving coil (M) is pivoted in between the two fixed coils and carries a pointer as shown in Fig.

• The current is led into and out of the moving coil by means of two spiral hair-springs which also provide the controlling torque.
• Air friction damping is provided by means of the aluminum vanes that move in the sector-shaped champer at the bottom of the instrument.

Deflecting torque Equation (T_d):

The force of attraction or repulsion between the fixed and moving coils is
directly proportional to the product of ampere-turns of fixed coils and the moving coil i.e.,

T_d ∝ N_f I_f × N_m I_m

Since N_m and N_f are constant,

T_d ∝ I_f I_m

Since the instrument is spring-controlled, the controlling torque is proportional to the angular deflection θ i.e.,

T_C ∝ θ

In the steady position of deflection, T_d = T_C
∴ θ ∝ I_f I_m

Thus deflection (θ) is directly proportional to the product of currents in the fixed coils and the moving coil.

If, I_f = I_m = I

∴ θ ∝ I²

It has a square scale.

Construction of Dynamometer type three-Phase Wattmeter:

• Three-phase Wattmeter is a dynamometer type and consists of two separate wattmeters mounted together in the same case.
• There are two current coils (CC) and two pressure coils (PC).
• In two wattmeter method, the total power consumed will be the sum of the readings of both the wattmeters, which has to be calculated manually.
• Whereas, in a 3-phase electrodynamometer wattmeter, the total power consumption is indicated directly, as the power consumption noticed by both the elements is being integrated internally.
• In a three-phase Wattmeter, the torque develops on both the elements.
• The value of torque on each element is proportional to the power that passes through it. The total torque on the three-phase Wattmeter is the sum of the torque on an individual Wattmeter

In all instruments except the Moving coil, the measured value is directly proportional to the square of response. They exhibit square law.

Additional Information

Where B is flux density in Tesla.

N is the number of turns

A is a cross-section area.

I is the current flowing through the instrument.

L is self-inductance and M is mutual-inductance.