Chapter 3.1
Meter Movements
FORCE
Analog Meters
• Analog meters are based on the fact that a current carrying conductor generates a
magnetic field.
– Discovered by Oersted
• By making this field interact with another one, a force is exerted on the conductor.
• The force can be made proportional to the current in the conductor
• If the conductor is attached to a pointer, then a visual indication of the current can be
produced.
• Recall the right hand rule from high school physics:
FIELD
S
CU
RR
EN
T
N
Current carrying
conductor
• For a conductor of length l m (perpendicular to field), carrying a current I Amps in a
magnetic field B Tesla, the force on it is given by:
F = BIl Newtons
Basic meter movement
• Consider the following arrangement:
Pointer
Moving
coil
N
S
• The current to be measured is applied to the moving coil
– through the phosphor-bronze coil springs.
• Current in coil creates a torque T, given by:
– where N = no. of turns in coil; T = BNIA sin α
EE11A Handouts Chapter 3
Prepared by: Mr. Fasil Muddeen
1
© 2001
– I = current
– A = CSA of coil
– α = angle between the normal plane of the coil and the field.
•
•
•
•
B, N and A are constant
By appropriately shaping the pole pieces, α is kept constant
Therefore T∝ I
T causes rotation of coil which is opposed by the tension in the coil spring until a
balance is obtained.
• If we project the pointer movement onto a scale, we can obtain a direct indication of
the current reading.
• When current is removed, coil spring tension causes pointer to return to zero.
• Is the pointer movement linear or nonlinear?
• This type of meter mechanism or meter movement is called the D’Arsonval
movement or the Permanent Magnet Moving Coil (PMMC) movement
• Know both terms!
• Figure 4-3 pg 86 in the text shows the main features of a PMMC meter movement
Sensitivity of PMMC movement
• Recall our definition of sensitivity
– A measure of how much of an input is required to produce a specific output.
Sensitivity =
Incremental Change in Output
Incrementa l Change in Input
• In the case of analog meters the sensitivity is the amount of current required to
give a FSD
• Since T = BNIAsinα
α , to maximise T; either increase N or A. Either choice means
more weight and more wire resistance and therefore a compromise has to be reached
between weight, resistance and sensitivity.
• Typical PMMC movement sensitivities and the associated coil resistances are given
in the following table.
IFSD
Resistance
50µ
µA
1000-5000 Ω
500µA 100-1000 Ω
1mA
30-120 Ω
10mA
1-4 Ω
• Therefore as sensitivity increases so too does the coil resistance.
EE11A Handouts Chapter 3
Prepared by: Mr. Fasil Muddeen
2
© 2001