Electrodynamometer Instruments

• An electrodynamometer type instrument is a moving coil instrument in which the

operating field is produced by another coil which is fixed. This type of instrument
can be used either as an ammeter or as a voltmeter, but is generally used as a
wattmeter.
• The instrument whose working depends on the reaction between the magnetic
field of moving and fixed coils is known as the Electrodynamometer Wattmeter. It
uses for measuring the power of both the AC and DC circuits.
• The working principle of the Electrodynamometer Wattmeter is very simple and
easy. Their working depends on the theory that the current carrying conductor
placed in a magnetic field experiences a mechanical force. This mechanical force
deflects the pointer which is mounted on the calibrated scale.
• Electrodynamometer instruments are also capable of functioning as transfer
instruments. Besides, their use as an ammeter, voltmeter, and wattmeter; they are
also used to transfer calibration of working instruments.
Construction & Working
Construction
The following are the important parts of the Electrodynamometer Wattmeter.
• Fixed coil – The fixed coil connects in series with the load. It is considered as a current coil
because the load current flows through it. For making the construction easy the fixed coil
divide into two parts. And these two elements are parallel connected to each other. The fixed
coil produces the uniform electric field which is essentials for the working of the instruments.
• Moving Coil – The moving coil consider as the pressure coil of the instruments. It connects in
parallel with the supply voltage. The current flows through them is directly proportional to the
supply voltage. The pointer mounts on the moving coil. The movement of the pointer controls
with the help of the spring.
• Control – The control system provides the controlling torque to the instruments. The gravity
control and the spring control are the two types of control system used. Out of two, the
Electrodynamometer Wattmeter uses spring control system. The spring control system is used
for the movement of the pointer.
• Damping – The damping is the effect which reduces the movement of the pointer. In this
Wattmeter the damping torque produces because of the air friction. The other types of
damping are not used in the system because they distort the useful magnetic flux.
• Scales and pointers – The instruments use a linear scale because their moving coil moves
linearly. The apparatus uses the knife edge pointer for removing the parallax error which
causes because of oversight
Operating Principle
• The Electrodynamometer Wattmeter has two
types of coils; fixed and the moving coil.
• The fixed coil connects in series with the
circuit whose power consumption use to be
measured.
• The supply voltage applies to the moving coil.
The resistor controls the current across the
moving coil, and it is connected in series with
it.
• The pointer is fixed on the moving coil which
is placed between the fixed coils.
• The current and voltage of the fixed and
moving coil generate the two magnetic fields.
• The interaction of these two magnetic fields
deflects the pointer of the instrument.
• The deflection of the pointer is directly
proportional to the power flows through it.
 Electrodynamometer Instruments

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 Torque Equation

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 Torque Equation

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 Advantages
• As the coils are air cored, these instruments are free from hysteresis and
eddy current losses.
• These instruments can be used on both a.c. and d.c. They are also used as a
transfer instruments.
• Electrodynamometer voltmeter are very useful where accurate r.m.s values
of voltage, irrespective of waveforms, are required.
• Low power Consumption.
• Light in weight.

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 Disadvantages
• These instruments have a low sensitivity due to a low torque to weight ratio. Also it
introduces increased frictional losses. To get accurate results, these errors must be
minimized.
• They are more expensive than other type of instruments.
• These instruments are sensitive to overload and mechanical impacts. Therefore care
must be taken while handling them.
• They have a non-uniform scale.
• The operation current of these instruments is large due to the fact that they have
weak magnetic field.

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Errors
Pressure Coil Inductance – The pressure coil of the Electrodynamometer has some
inductance. Because of the inductance, the current of the pressure coils lags behind the
voltage. Thus, the power factor of the wattmeter becomes lagging, and the meter reads high
reading.
Pressure Coil Capacitance – The pressure coil has capacitances along with the inductance.
This capacitance increases the power factor of the instrument. Hence causes the error in the
reading.
Error due to Mutual Inductance Effect – The mutual inductance between the pressure
and current coil produces an error.
Eddy Current Error – The eddy current induces in the coil creates its own magnetic field.
This field affects the main current flows through the coil. Thus, the error occurs in the
reading.
Stray Magnetic Field – The stray magnetic field disturbs the main magnetic field of
the Electrodynamometer. Thus, affect their reading.
Temperature Error – The variation in temperature will change the resistance of the
pressure coil. The movement of the spring, which provides the controlling torque also
affected because of the temperature change. Thereby, the error occurs in the reading.
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 Numerical

1. In an electrodynamometer instrument the total resistance of the voltage coil circuit is

8200 Ω and mutual inductance changes uniformly from -173µH at zero deflection to +
175µH at full scale, the angle of full scale being 95 degree. If a potential difference of
100V is applied across the voltage circuit, and a current of 3A at a power factor of 0.75
is passed through the current coil, what will be the deflection , if the spring control
constant is 4.63 x 106 𝑁 − 𝑚/𝑟𝑎𝑑
2. For a dynamometer type ammeter the mutual inductance M varies with deflection θ
(expressed in degrees) as
M = -6 cos (θ+300) mH
Find the deflecting torque produced by a DC current of 50 mA corresponding to a
deflection of 600.
3. A dynamometer ammeter is having two fixed coils having a total resistance of 3.0 Ω and
a total inductance of 0.12 H, and a moving coil of resistance 30 Ω and an inductance of
0.003 H. Calculate the error in reading when the instrument is calibrated with d.c. and
used in a.c. 50 Hz with moving coil shunted directly across the field coils.

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1. In an electrodynamometer instrument the total resistance of the voltage coil circuit is 8200 Ω and
mutual inductance changes uniformly from -173µH at zero deflection to + 175µH at full scale,
the angle of full scale being 95 degree. If a potential difference of 100V is applied across the
voltage circuit, and a current of 3A at a power factor of 0.75 is passed through the current coil,
what will be the deflection , if the spring control constant is 4.63 x 10−6 𝑁 − 𝑚/𝑟𝑎𝑑
1. For a dynamometer type ammeter the mutual inductance M varies with deflection θ (expressed
in degrees) as
M = -6 cos (θ+300) mH
Find the deflecting torque produced by a DC current of 50 mA corresponding to a deflection of
600.
• A dynamometer ammeter is having two fixed coils having a total resistance of 3.0 Ω and a total
inductance of 0.12 H, and a moving coil of resistance 30 Ω and an inductance of 0.003 H.
Calculate the error in reading when the instrument is calibrated with d.c. and used in a.c. 50 Hz
with moving coil shunted directly across the field coils.