11.

Electrodynamometer Type Instruments

The electrodynamometer type instrument is a transfer instrument. A transfer
instrument is one which is calibrated with a d.c. source and used without any
modifications for a.c. measurements. Such a transfer instrument has same accuracy for
a.c. and d.c. measurements. The electrodynamometer type instruments are often used
in accurate a.c. voltmeters and ammeters, not only at the powerline frequency but also
in the lower audiofrequency range. With some little modifications, it can be used as a
wattmeter for the power measurements.

Construction

The Fig. 3.3 shows the construction of the electrodynamometer type instrument.
The various parts of the clectrodynamometer type instrument are :

Fixed Coils : The mecessary
field required for the operation of
the instrument is produced by the
fixed coils. A uniform field is
obtained near the center of coil due
to division of coil in two sections.
These coils are air cored. Fixed coils
are wound with fine wire for using
as voltmeter, while for ammeters
and wattmeters it is wound with
heavy wire. The coils are usually

varnished. They are clamped in Fixed coils
place against the coil supports. This
makes the construction rigid. Fig. 3.3 Electrodynamometer type instrument

Ceramic is usually used for mounting supports. If metal parts would have been
used then it would weaken the field of the fixed coil.

Moving coil : The moving coil is wound either as a self-sustaining coil or else on
a non-metallic former. [f metallic former is used, then it would induce eddy currents
in it. The construction of moving coil is made light as well as rigid. It is air cored.

Controlling : The controlling torque is provided by springs. These springs act as
12005 to the moving coil.

Moving system : The moving coil is mounted on an aluminium spindle. It consists
of counter weights and pointer. Sometimes a suspension may be used, in case a high
accuracy is desired.

Damping : The damping torque is provided by air friction, by a pair of aluminium
vanes which are attached to the spindle at the bottom. They move in sector shaped
chambers. As operating field would be distorted by eddy current damping, it is not
employed.

Shielding : The field produced by these instruments is very weak. Even earth's
magnetic field considerably affects the reading. So shielding is done to protect it from
stray magnetic fields. It is done by enclosing in a casing of high permeability alloy.

Cases and Scales : Laboratory standard instruments are usually contained in
polished wooden or metal cases which are rigid. The case is supported by adjustable
levelling screws.

A spirit level may be provided to ensure proper levelling.
For using electrodynamometer instrument as ammeter, fixed and moving coils are

connected in series and carry the same current. A suitable shunt is connected to these
coils to limit current through them upto desired limit.