1. EC 217 MEASUREMENTS AND INSTRUMENTATION
Chapter #4 Alternating-Current (AC) Indicating Instruments.

2. Introduction
It is well known that the moving coil instrument responds to the average value or the dc value of the current through its coil.
If it carries an alternating current with +ve and –ve half cycles, the driving torque would be in one direction for the +ve alternation and in other direction for the –ve direction.
If the frequency of the AC is very low, the pointer swing back and forth around the zero. If the frequency is high the pointer can not follow the rapid reversals of the driving torque.

3. Electrodynamometer (cont.)
The electrodynamometer is one of the most AC movement instruments. It is used in:
i- Accurate AC voltmeter and ammeter in wide range of frequency.
ii- As a wattmeter.
iii- Power factor meter.
iv- Frequency meter.

4. Electrodynamometer (cont.)
Scale
Fixed coil
Pointer
Moving
coil
Measured current i

5. Electrodynamometer (cont.)
The electrodynamometer uses the current under measurement to produce the necessary field flux (B) in the fixed coil. The fixed coils are spaced for enough apart to allow the moving coil to rotate.
The instrument is surrounded by a laminated shield to protect it from stray magnetic fields which may effect its operation.
The torque T is given by, where: T*B*I*A*N
B = magnetic flux density in the fixed coils
A =Area of the moving coil
I =measured current
N = no of moving coil turns.

6. Electrodynamometer (cont.)
But B α I, then Tα I2, therefore T is proportional to the instantaneous value of I2, this value is always +ve and torque pulsation’s are therefore produced.
But the movement can not follow the rapid variations of the torque and takes up a position in which the average torque is balanced by the torque of the control springs.
The meter deflection is therefore a function of the mean of the squared current.

7. Electrodynamometer (cont.)
The scale of the electrodynamometer is usually calibrated in terms of the square root of the average current squared, this meter reads the r.m.s or the effective value of the ac i.e.

8. D’ Arsonval meter movement used with half-wave
Ip (peak value)
Irms t I

9. Electrodynamometer (cont.)
Disadvantages:
i- High power consumption (due to its construction).
ii- The magnetic field B is weak compared to PMMI.
iii- Low sensitivity.
iv- Large current is bad to measure here.