5.2. Electrodynamic or Dynamometer Instruments
In an electrodynamic instrument the operating field is produced by another fixed coil and not by permanent magnet. This instrument can be used as an ammeter or as voltmeter but is generally used as a wattmeter.
Fig. 19. Electrodynamic or dynamometer instrument.
These instruments essentially consist of fine wire moving coil placed in the magnetic field produced by another fixed coil when carrying currents. The coils are usually air cored to avoid hysteresis, eddy currents and other errors when the Instrument is used on A.C. The fixed coil FC is divided into two halves placed close together and parallel to each other in order to provide a fairly uniform field within the range of the movement of the moving coil.
The upper diagram in Fig. 19 (a) shows a sectional elevation through fixed coil FC and the lower diagram represents a sectional plan on XX. The moving coil MC is carried by a spindle S and the controlling torque is exerted by spiral hair springs A, which may also serve to lead the current into and out of MC.
Deflecting torque. The deflecting torque is due to interaction of the magnetic fields produced by currents in the fixed and moving coils.
- Fig. 20 (a) shows the magnetic field due to current flowing through FC (I1) in the direction indicated by the dots and cross.
- Fig. 20 (b) shows the magnetic field due to current (I2) in MC.
- Fig. 20 (c) shows the combined effect of the above magnetic fields. By combining these magnetic fields it will be seen that when currents (I1 and I2) flow simultaneously through FC and MC, the resultant magnetic field is distorted and effect is to exert a clockwise torque on MC.
Fig. 21. Measurement of small currents.
Fig. 22. Measurement of heavy currents.
Thus whether the instrument is used as an ammeter or voltmeter its scale is uneven through the whole of its range and is cramped or crowded near the zero in particular.
Note. When the dynamometer instrument is used to measure an alternating current or voltage, the moving coil-due to its inertia-takes up a position where the average deflecting torque over one cycle is balanced by the restoring torque of the spiral springs. For that position, the deflecting torque is proportional to the mean
value of the square of current or voltage, and the instrument scale can therefore be calibrated to read the r.m.s. value
- In these instruments the damping is pneumatic (i.e. air damping). Eddy current damping is admissible owing to weak operating field.
Ammeters. (i) With fixed and moving coils in series …. 0/0.01 A-0/0.05 A
(ii) With moving coil shunted or parallel connections …. upto 0/30 A.
Voltmeters. Upto .0-750 volts.
Advantages and Disadvantages
(i) Can be used on both D.C. as well as A,C. systems.
(ii) They are free from hysteresis and eddy current errors.
(iii) It is possible to construct ammeters upto 10 A and volt-meters up to 600 V with precision grade accuracy.