Slovak University of Technology
Faculty of Material Science and Technology in Trnava
ELECTRICAL ENGINEERING
AND ELECTRONICS
Unit 4 - Electrical measurements
Electromechanical Instruments
‰Analog instruments
Permanent magnet moving coil (PMMC) instruments: deflecting
torque, connection of PMMC v-meter and A-meter, PMMC meters
with rectifiers (basic connection, which quantity they measure,
frequency dependence)
- Moving iron (Iron-vane) instruments: principle, measured
quantity, V-meter, A-meter, changing of measurement ranges,
frequency dependence
- electrodynamic instruments: principle, what they measure, Wmeter, changing of measurement ranges, frequency dependence.
- Vibrating reed frequency meter
Electromechanical Instruments
„
Permanent-Magnet Moving-Coil Instruments
Deflection Instrument Fundamentals
„ Deflecting force
causes the pointer to move from its zero position
when a current flows
is magnetic force; the current sets up a magnetic
field that interacts with the field of the permanent
magnet (see Figure 3.1 (a))
Electromechanical Instruments
„
Controlling force
is provided by spiral springs (Figure 3.1 (b))
retain the coil and pointer at their zero position when no
current is flowing
When current flows, the springs wind up as the coil
rotates, and the force they exert on the coil increases
The coil and pointer stop rotating when the controlling
force becomes equal to the deflecting force.
The spring material must be nonmagnetic to avoid any
magnetic field influence on the controlling force.
Electromechanical Instruments
„
Since the springs are used to make electrical
connection to the coil, they must have a low
resistance.
Damping force
is required to minimize (or damp out) the
oscillations
must be present only when the coil is in motion,
thus it must be generated by the rotation of the
coil
In PMMC instruments, the damping force is
normally provided by eddy currents.
Eddy currents induced in the coil former set up a magnetic flux that
opposes the coil motion, thus damping the oscillations of the coil (see
Figure 3.2 (b)).
Electromechanical Instruments
„
Two methods of supporting the moving system
of a deflection instrument
Jeweled-bearing suspension
ƒ Cone-shaped cuts in jeweled ends of pivots
ƒ Least possible friction
ƒ Shock of an instrument ⇒ spring
supported to absorb such shocks
Taut-band method
ƒ Much tougher than jeweled-bearing
ƒ Two flat metal ribbons (phosphor bronze or
platinum alloy) are held under tension by
spring to support the coil
Electromechanical Instruments
„
„
„
„
Because of the spring, the metal ribbons
behave like rubber under tension.
The ribbons also exert a controlling force as
they twist, and they can be used as electrical
connections to the moving coil.
Much more sensitive than the jeweled-bearing
type because there is less friction
Extremely rugged, not easily be shattered.
Electromechanical Instruments
PMMC
„
„
Construction
D’Arsonval or horseshoe magnet
Core-magnet
Torque
„
Equation and Scale
When a current I flows through a one-turn coil
situated in a magnetic field, a force F is exerted
on each side of the coil
F
= BIl
newtons
„
Since the force acts on each side of the coil,
the total force for a coil of N turns is
F = 2BIlN
„
The force on each side acts at a radius r,
producing a deflecting torque:
T D = 2BlINr = BlIN (2r )
= BlIND
= BAIN
„
The controlling torque exerted by the spiral
springs is directly proportional to the
deformation or windup of the springs. Thus,
the controlling torque is proportional to the
actual angle of deflection of the pointer.
T
C
= K θ
where
K
is a constant
„
For a given deflection, the controlling and deflecting
torque are equal:
Kθ = BlIND
θ = CI where C is a constant
Example 3.1 A PMMC instrument with a 100-turn coil has a
magnetic flux density in its air gaps of B = 0.2 T. The coil
dimension are D = 1 cm and l = 1.5 cm. Calculate the
torque on the coil for a current of 1 mA.
= (0 . 2 T )(1 . 5 × 10
)(1 × 10 )(100 )(1 × 10 )
Solution T = BlIND
− 2
d
= 3 × 10
− 6
Nm
− 3
− 2
DC Ammeter
„
Galvanometer
is
a PMMC instrument designed to be
sensitive to extremely low current levels.
The simplest galvanometer is a very sensitive
instrument with the type of center-zero scale.
The torque equation for a galvanometer is
exactly as discussed in the previous section.
The most sensitive moving-coil galvanometer
use taut-band suspension, and the controlling
torque is generated by the twist in the
suspension ribbon.
With
the moving-coil weight reduced to the
lowest possible minimum for greatest
sensitivity, the weight of t he pointer can
create a problem. The solution is by mounting
a small mirror on the moving coil instead of a
pointer.
The
mirror reflects a beam of light on to a
scale. This makes light-beam galvanometers
sensitive to much lower current levels than
pointer instruments
Current sensitivity galvanometer
Voltage sensitivity galvanometer
Galvanometers are often employed to detect
zero current or voltage in a circuit rather than
to measure the actual level of current or
voltage.
„
DC Ammeter
is
always connected in series
low internal resistance
maximum pointer deflection is produced by a
very small current
For a large currents, the instrument must be
modified by connecting a very low shunt
resister
Extension of Ranges of Ammeter
„
Single Shunt Type of Ammeter
Electromechanical Instruments
Vsh = Vm
I sh Rsh = I m Rm
Rsh =
I m Rm
I sh
I sh = I − I m
∴ Rsh =
I m Rm
I − Im
„
Swamping Resistance
The moving coil in a PMMC instrument is wound with
thin copper wire, and its resistance can change
significantly when its temperature changes.
The heating effect of the coil current may be enough to
produce a resistance change, which will introduce an
error.
To minimize the error, a swamping resistance made of
manganin or constantan is connected in series with the
coil (manganin and constantan have resistance
temperature coefficients very close to zero.
„
The ammeter shunt must also be made of
manganin or constantan to avoid shunt
resistance variations with temperature.
Multirange Ammeters
Make-before-break switch
ƒ The instrument is not left without a shunt in
parallel with it.
ƒ During switching there are actually two
shunts in parallel with the instrument.
DC Voltmeter
„
Voltmeter Circuit
Extremely
high resistance
Always connected across or in parallel with the
points in a circuit at which the voltage is to be
measured
The voltmeter range is increased by connecting a
multiplier resistance with the instrument (single or
individual type of extension of range).
=
V
R
s
I
=
R
m
1
I
The
the
+ I
× V
V
s
reciprocal
total
R
m
− R
m
= Range
1
=
× Range
I m
current
∴
R
m
m
Given
R
s
s
of
sensitivit
− R
full
scale
current
y of
= S × Range
voltmeter
m
resistance
− R
the
meter
 1

 I m
(S )
m
= S × Range

 is

Swamping
„
Resistance
The change in coil resistance (Rm) with
temperature change can introduce errors in a
PMMC voltmeter.
„
The presence of the voltmeter multiplier
resistance (Rs) tends to swamp coil
resistance changes, except for low voltage
ranges where Rx is not very much larger
than Rm.
Multi-range
Voltmeter
In Figure 3.16(a), only one of the three
multiplier resistors is connected in series with
the meter at any time.
„ The range of this voltmeter is
V = Im(Rm+R)
„
where R can be R1, R2, or R3
In Figure 3.16(b), the multiplier resistors are
connected in series, and each junction is
connected to one of the switch terminals.
„ The range of this voltmeter can also be calculated
from the equation
V = Im(Rm+R)
„
where R can now be R1, R1+R2, or R1+R2 +R3.
„
Of the two circuits, the on in Figure 3.16(b) is the
least expensive to construct. This is because all of
the multiplier resistors in Figure 3.16(a) must be
special (nonstandard) values, while in Figure
3.16(b) only R1 is a special resistor.
Ohmmeter
„
Series Ohmmeter
Basic
„
„
„
Circuit
is normally part of a volt-ohm-milli-ammeter (VOM) or
multifunction meter, do not exist as individual instruments.
The simplest circuit consists of a voltage source connected
in series with a pair of terminals, a standard resistance,
and a low-current PMMC instrument.
The resistance to be measured (Rx) is connected across
terminal A and B.
The
meter current
Im =
Eb
R x + R1 + R m
Figure 6.1 Basic series ohmmeter circuit consisting of a PMMC
instrument and a series-connected standard resistor R1. When the
ohmmeter terminals are shorted (Rx=0) meter full-scale deflection occurs.
At half-scale deflection Rx = R1, and at zero deflection the terminals are
open-circuited.