Construction of PMMC Instruments
The constructional features of this
instrument are shown in Fig.
The moving coil is wound with many
turns of enameled or silk covered
The coil is mounted on rectangular
aluminum former, which is pivoted
on jeweled bearings.
The coils move freely in the field of a
Most voltmeter coils are wound on
metal frames to provide the required
Most ammeter coils, however, are
wound on non-magnetic formers,
because coil turns are effectively
shorted by the ammeter shunt.
The coil itself, therefore, provides
electro magnetic damping.
Old style magnet system consisted of
relatively long U shaped permanent magnets
having soft iron pole pieces.
Owing to development of materials like
Alcomax and Alnico, which have a high co-
ercive force, it is possible to use smaller
magnet lengths and high field intensities.
The flux densities used in PMIMC instruments
vary from 0.1 Wb/m to 1 Wb/m.
When the coil is supported between two jewel
bearings two phosphor bronze hairsprings
provide the control torque.
These springs also serve to lead current in
and out of the coil. The control torque is
provided by the ribbon suspension as shown.
This method is comparatively new and is
claimed to be advantageous as it eliminates
RECTANGULAR IN SHAPE
WOUND ON ALUMINIUM
FORMER WITH LARGE
WIDTH OF RECTANGLE
IS LESS THAN DISTANCE
b/w POLES OF PM WITH
AN AIR GAP.
PIVOT AT TOP AND
BOTTOM IS MADE
TO FIT OVER THE
AXLE OF MOVING
COIL AND ACTS AS
The pointer is carried by the
spindle and moves over a
The pointer is of lightweight
construction and, apart from
those used in some
inexpensive instruments has
the section over the scale
twisted to form a fine blade.
This helps to reduce parallax
errors in the reading of the
scale. When the coil is
supported between two jewel
bearings two phosphor
bronze hairsprings provide
the control torque.
These springs also serve to
lead current in and out of the
ENCLOSED IN A
DUST PROOF CASE
THE SHAPE AND
SIZE OF THE CASE
DEPENDS UPON THE
CAPACITY OF THE
IT WORKS ON THE
PRINCIPLE OF DC
PRODUCES FLUX OF
THE FLUX DENSITY AT ONE SIDE INCREASES
WHILE OTHER SIDE DECREASES
THIS IMBALANCE EXERTS A FORCE ON THE
CONDUCTOR IN THE DIRECTION OF LEAST
>Torque, moment or moment of force is the
tendency of a force to rotate an object about an
axis, fulcrum, or pivot.
>DEFLECTING TORQUE=TOTAL FORCE*DISTANCE
A=AREA OF CROSS SECTION
>AT FINAL DEFLECTION Td=Tc
=>Tc PROPORTIONAL TO I
HERE DAMPING IS EDDY CURRENT DAMPING
- The PMMC consumes less power and has
- It has uniformly divided scale and can
cover arc of 270 degree.
- The PMMC has a high torque to weight
- It can be modified as ammeter or voltmeter
with suitable resistance.
- It has efficient damping characteristics and
is not affected by stray magnetic field.
- It produces no losses due to hysteresis.
-The moving coil instrument can only be
used on D.C supply as the reversal of current
produces reversal of torque on the coil.
- It’s very delicate and sometimes uses ac
circuit with a rectifier.
- It’s costly as compared to moving coil iron
- It may show error due to loss of
magnetism of permanent magnet.
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The basic sources of error in PMMC
instruments are friction, temperature and
aging of various parts. To reduce the
frictional errors ratio of torque to weight is
made very high.
The most serious errors are produced by the
heat generated or by changes in the
temperature. This changes the resistance of
the working coil, causing large errors. In case
of voltmeters, a large series resistance of
very low temperature coefficient is used. This
reduces the temperature errors.
The aging of permanent magnet and control
springs also cause errors. Opposite errors in
PMMC is caused by weakening of magnet and
spring cause. The weakening of magnet
causes less deflection while weakening of
control springs cause large deflection, for a
particular value of current. The proper use of
material and pre-ageing during
manufacturing can reduce the errors due to
weakening of control springs.