Test: Electrodynamometer Type Instruments - Electrical Engineering (EE) MCQ

Dynamometer type instruments:

• Dynamometer-type instruments working on the principle of " Changing the mutual inductance."
• It responds to both AC as Well as DC quantity.

The dynamometer-type instrument is shown below.

• There are two types of coils in this instrument. they are named as Fixed and Moving coil.
• The fixed coil also called the Current coil and the moving coil named the Pressure coil.
• Dynamometer develops deflected torque (Td) by the interaction of magnetic fields.
• Dynamometer instrument can be used as Ammeter, Voltmeter, and Wattmeter.
• But it is more effective in the Wattmeter type because power is proportional to the square of the current.
• The scale is uniform in the Wattmeter type.
• While using it as Ammeter its scale is square-law scale (non-uniform) because of the deflecting torque.​
• Air friction damping is employed for these instruments and is provided by a pair of aluminium vanes, attached to the spindle at the bottom.

Deflecting torque developed in DC Power Measurement:
T_d ∝ V I₁ 
Where,

• V is the voltage applied
• I₁ is the current through the wattmeter
• The deflecting torque is directly proportional to the power delivered to the load.

Deflecting torque developed in AC Power Measurement:
T_d ∝ V I₁ cos ϕ 
The deflecting torque is directly proportional to the real power of the circuit.
Therefore, In an ac dynamometer-type wattmeter, the instantaneous value of developed deflecting torque is proportional to the product of voltage and current in the respective coils at the corresponding instant, the constant of proportionality is the same as in the case of dc use.

• Thus, the instantaneous value of the deflecting torque is proportional to the instantaneous power. 
• Owing to the inertia of the moving system, the pointer reads the average power.
• In dc circuits, the power is given by the product of voltage and current, and hence the torque is directly proportional to the power.
• Thus, the instrument indicates the power.

Electrodynamometer Instruments:

• The electrodynamometer is a transfer-type instrument.
• A transfer-type instrument is one that may be calibrated with a dc source and then used without modification to measure AC.
• This requires the transfer type instruments to have the same accuracy for both DC and AC.
• These instruments are also called Electrodynamic or Dynamometer Type Instruments.
• A schematic diagram of  Electrodynamic or Dynamometer Type Instruments is shown below:
  
• An electrodynamic instrument is a moving-coil instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil.
• This instrument can be used either as an ammeter or a voltmeter but is generally used as a wattmeter.
• The electrodynamic or dynamometer-type instrument is a moving-coil instrument but the magnetic field, in which the coil moves, is provided by two fixed coils rather than by permanent magnets (eg: PMMC instruments).

Construction:
Different parts of an Electrodynamometer instrument are briefly explained below
Fixed Coils:

• The field is produced by a fixed coil.
• This coil is divided into two sections to give a more uniform field near the center and to allow passage of the instrument shaft.
• The instrument as shown in the figure by a milliammeter, or may become a voltmeter by the addition of series resistance.
• The fixed coils are wound with fine wire for such applications.
• Field (fixed) coils are usually wound with a heavy wire carrying the main current in ammeters and watt meters.
• The wire is stranded where necessary to reduce eddy current losses in conductors.
• The coils are usually varnished and baked to form a solid assembly.
• These are then clamped in place against the coil supports, This makes the construction rigid so that there is no shifting or change in dimensions that might affect the calibration.
• The mounting supports are preferably made out of ceramic, as metal parts would weaken the field of the fixed coil on account of eddy currents.

Moving Coil:

• A single-element instrument has one moving coil.
• The moving coil is wound either as a self-sustaining coil or else on a non-metallic former.
• A metallic former cannot be used as eddy currents would be induced in it by the alternating field.
• Light but rigid construction is used for the moving coil.
• It should be noted that both fixed and moving coils are air-cored.

Control:

• The controlling torque is provided by two control springs.
• These springs act as leads to the moving coil.

Moving System:

• The moving coil is mounted on an aluminum spindle.
• The moving system also carries the counterweights and truss-type pointer.
• Sometimes a suspension may be used in case high sensitivity is desired.

Damping:

• Air friction damping is employed for these instruments and is provided by a pair of aluminum vanes, attached to the spindle at the bottom.
• These vanes move in sector-shaped chambers.

Shielding:

• The field produced by the fixed coils is somewhat weaker than in other types of instruments.
• It is nearly 0.005 to 0.006 Wb/m2.
• In DC 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 (0.0005 - 0.00075 Wb/m2).
• Air-cored electrodynamometer-type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy.

Cases and Scales:

• Laboratory standard instruments are usually contained in highly polished wooden cases.
• These cases are so constructed as to remain dimensionally stable over long periods.
• The glass is coated with some conducting material to completely remove the electrostatic effects.
• The case is supported by arable leveling screws.
• A spirit level is also provided to ensure proper leveling.

Concept:
The error in the wattmeter at the VA reading is given by:
E_r = sinϕ × tanβ × true power
where β = Impedance angle of pressure coil
ϕ  = Power factor angle
The impedance angle of the pressure coil is given by:

Calculation:
Given, cosϕ = 0
∴ sinϕ = 1
True power = 1000W

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 IL 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 IP as shown in the figure
• Both coils are connected in parallel so that two different currents will flow through the coils IL and IP 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
   

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

Electrodynamometer instrument:

• Electrodynamic type instruments are similar to the PMMC-type elements except that the magnet is replaced by two serially connected fixed coils that produce the magnetic field when energized.
• The fixed coils are spaced far enough apart to allow passage of the shaft of the movable coil.
• The movable coil carries a pointer, which is balanced by counterweights. Its rotation is controlled by springs.
• The motor torque is proportional to the product of the currents in the moving and fixed coils. If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same time, and the turning force continues in the original direction.

Measurement using Electrodynamometer instrument:

• This type of instrument can be used to measure AC or DC current, voltage, or its major application as a wattmeter for power measurement.
• For voltage and current measurement, the moving and fixed are serially connected. For power measurement, one of the coils (usually the fixed coils) passes the load current and other coil passes a current proportional to the load voltage.
• Similar to moving iron vane instruments, the electrodynamic instruments are true RMS (effective value) responding meters.
• When electro dynamic instruments used for power measurement its scale is linear because it predicts the average power delivered to the load and it is calibrated in average values for AC.

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

Construction of Electrodynamometer Wattmeter:
Fixed Coils: 

• The reason for using fixed coils as current coils is that they can be made more massive and can be easily constructed to carry considerable current since they present no problem of leading the current in or out.
• The fixed coils are wound with heavy wire. This wire is stranded or laminated especially when carrying heavy currents in order to avoid eddy current losses in conductors.

Moving Coil: 

• The moving coil is mounted on a pivoted spindle and is entirely embraced by the fixed spindle & is entirely embraced b the fixed current coils.
• The use of moving coil as pressure coil is a natural consequence of design requirements.
   

Scales and Pointers: 

• They are equipped with mirror type scales and knife-edge pointers to remove reading errors due to parallax.

Damping: 

• The damping is the effect which reduces the movement of the pointer.
•  In this Wattmeter Air friction damping is used.
• The other types of damping are not used in the system because they destroy the useful magnetic flux.
  

Dynamometer 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.