The electrodynamometer is a moving-coil instrument that uses a fixed coil to produce a magnetic field, rather than a permanent magnet. It can be used as an ammeter, voltmeter, or wattmeter for both AC and DC measurements up to 125Hz. It provides very high accuracy and is used in laboratories for calibrating other instruments. The moving coil is subjected to a torque based on the current in the fixed and moving coils and their mutual inductance. While expensive, it has advantages of being usable for both AC and DC and being free from hysteresis and eddy current losses due to its air-cored coils. However, it has low sensitivity and torque.
2. Electrodynamometer type Instrument
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.
It can be used for measurement of current, voltage & power for both AC
and DC up to 125Hz and with some modifications as power factor &
frequency meters.
Accuracy is very high and hence can be used in laboratory for calibration
of other instruments.
3. 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.
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. Since the reversing the current direction does not
reverse the turning force, this type of instruments can be used to
measure AC or DC current, voltage, or its major application as a
wattmeter for power measurement.
7. 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 centre and to allow passage of the instrument shaft.
• Fixed coils are usually wound with heavy wire carrying the main current
in ammeters and watt meters.
• The wire is stranded where necessary to reduce eddy current losses in
conductors.
Moving Coil:
• A single element instrument has one moving coil (pressure or voltage
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 current would be induced in it
by the alternating field.
• It should be noted that both fixed and moving coils are air cored.
8. 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 counter weights and truss type pointer.
• Sometimes a suspension may be used in case a 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.
• Eddy current damping cannot be used in these instruments as the
operating field is very weak (on account of the fact that the coils are air
cored) and any introduction of a permanent magnet required for eddy
current damping would distort the operating magnetic field of the
instrument.
9. 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/m.
• In D.C. Measurements even the earth magnetic field may affect the
readings.
• Thus it is necessary to shield an electrodynamometer type instrument
from the effect of stray magnetic fields.
• 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.
10. Working Principle
• Electro-dynamometer type instruments are very similar to PMMC type
instrument in which the operating field is produced, not by a permanent
magnet but by another fixed coil (usually two fixed air cored coils are used).
• The PMMC instrument cannot be used on A.C currents or voltages. If A.C
supply is given to these instruments, an alternating torque will be
developed. Due to moment of inertia of the moving system, the pointer will
not follow the rapidly changing alternating torque and will fail to show any
reading.
• In order that the instrument should be able to read A.C quantities, the
magnetic field in the air gap must change along with the change in current.
This principle is used in the electro-dynamometer type instrument.
11. Torque Equation
Let,
i1 = instantaneous value of current in the fixed coils, (A)
i2 = instantaneous value of current in the moving coils, (A)
L1 = self-inductance of fixed coils, (H)
L2 = self-inductance of moving coil, (H)
M = mutual inductance between fixed and moving coils (H)
12. • Flux linkage of Coil 1, ψ1 = L1 i1 + Mi2
Flux linkage of Coil 2, ψ2 = L2 i2 + Mi1
Electrical input energy = e1 i1 dt + e2 i2 dt = i1 dψ1 + i2 dψ2
As e1 = dψ1/dt and e1 = dψ2/dt
Electrical input energy = i1 d( L1 i1 + Mi2 )+ i2 d(L2 i2 + Mi1)
= i1L1di1 + i1
2dL1 + i1i2dM + i1Mdi2 + i2L2di2 + i2
2dL2 + i1i2dM + i2Mdi1
Since L1 and L2 are constant, therefore dL1 = 0 and dL2 = 0
= i1L1di1 + i1i2dM + i1Mdi2 + i2L2di2 + i1i2dM + i2Mdi1 …………(1)
Energy stored in the magnetic field = ½ i1
2L1 + ½ i2
2L2 + i1 i2M
Change in energy stored = d(½ i1
2L1 + ½ i2
2L2 + i1 i2M)
= i1L1di1+ i2L2di2+ i1Mdi2 + i2Mdi1+ i1i2dM+(i1
2/2)dL1 + (i2
2/2)dL2
But L1 and L2 are constant, therefore dL1 = 0 and dL2 = 0
= i1L1di1+ i2L2di2+ i1Mdi2 + i2Mdi1+ i1i2dM ……(2)
13. From the principle of conservation of energy,
Total electrical input energy = Change in energy in energy stored + mechanical energy
The mechanical energy can be obtained by subtracting above equations
Therefore, mechanical energy = i1 i2dM
suppose Td is the deflecting torque and dθ is the change in deflection, then,
Mechanical energy = work done = Td dθ
Thus we have,
Td dθ = i1 i2dM
Td = i1 i2 (dM/dθ)
controlling torque Tc = kθ where k = spring constant
At final steady position Td = Tc
I1 I2 (dM/dθ) = kθ
Deflection, θ = (I1 I2/k)(dM/dθ)
14. Advantages
1. Can be used for both a.c. as well as d.c. measurements (transfer
instrument)
2. Accuracy is very high (can be used for calibration of other instrument).
3. Free from hysteresis and eddy current loss (coils are air cored)
4. As the instrument exhibit square law response the angular deflection of
these instrument is directly in terms of RMS value of the a.c parameter
under measurement.
15. Disadvantages
1. Low torque to weight ratio hence have low sensitivity.
2. The scale is non-uniform.
3. Highly expensive as compared other instrument for ammeter, voltmeter.
therefore used for precision measurement.
4. Frictional losses are high.
5. Higher power consumption as compared to PMMC instrument.
16. Sources of Errors
• Error due to low torque to weight ratio.
• Error due to temperature change
• Error due to eddy current.
• Error due to external magnetic field
• Error due to frequency
