PMMC - Permanent magnet moving coil instrument
In the PMMC instrument, we have two pivots, a spindle and two springs, a permanent magnet, and an aluminum frame with the copper coil wound with N turns. The terminals of the copper coil are connected to the two springs attached to the spindle.
Whenever there is a current flow, the deflection torque is produced because the force acts on it so that the springs attached to the spindle get compression thereby the energy stored will be more in the spring. When the spindle rotates the aluminum frame also rotates. Initially the current flows into the copper coil wound on the aluminum frame, according to Lorentz law, whenever a current-carrying conductor is placed in a magnetic field it experiences a force. The eddy current is developed on the aluminum frame as the permanent magnet is used in the instrument which in turn makes the pointer move smooth and reduces the number of oscillations.
F = Bil sinθ
PMMC instrument works on the principle of Lorentz law.
Relationship between input current and the deflection output:
F = Bil sinθ
where B is the magnetic flux density in the meter and I is the current in the conductor and l is the length of the conductor.
According to Fleming's left-hand rule, the magnetic field lines and the current direction is perpendicular so that the angle between them is 90.
where sin 90 = 1
Therefore we get, F = Bil
Torque is defined as the force * perpendicular distance between them.
Td/ conductor = F/conductor * perpendicular distance for one conductor
= Bil/conductor * d/2
Td/ turn = Bild
Td/ turn = BiA
Td/coil (deflection torque) = BiAN
Spring control in PMMC:
Tc = Kc * θ
At one instant, Tc = Td
Kc * θ = BiAN
θ = BiAN / Kc
θ ∝ i
The deflection angle is proportional to the input current.
Hence it is linear.
Advantages of PMMC:
- Linear scaling will be there on the output.
- Because of the eddy current damping it is highly accurate.
Disadvantages of PMMC:
- AC current cannot be measured using a PMMC meter.
- Hysteresis error due to Aluminum frame in the instrument.
Moving Iron instrument:
A moving iron instrument consists of two pivots, two springs a soft iron disc at the center with a spindle, an air friction damping, and a copper coil wound on the electromagnet.
Whenever there is an AC supply in the moving iron instrument, there will be current flow in the copper coil an emf is produced according to Faraday's laws of electromagnetic induction.
E = - N dΦ / dt
There will be a flow of magnetic field lines from the north pole to the south pole in the electromagnet and as well as from the south pole to the north pole. So that the net transfer flux is zero. The attraction force acts on the soft iron disc so that it starts rotating. When the current flow is high then the magnetic field developed will be high as it is an electromagnet. In the positive half cycle of the AC, the electromagnet acts based on the magnetic field direction, and in the negative half cycle of the AC the magnetic direction changes accordingly. But the iron disc will have attraction force on the electromagnet and still conducts so that it rotates.
The deflection torque will be produced based on the current flow so that there will be a deflection angle. The controlling torque opposes the deflection torque as it is a spring control based on the energy stored in the springs. There will be an air friction damping that is connected to the spindle and this will reduce the number of oscillations before the steady-state of the deflection.
In a moving iron instrument, the copper coil is the fixed coil as it is stationary where the iron disc rotates based on the supply of the current in the current-carrying coil.
As
per the law of conservation of energy, the electrical energy supplied is equal to
the energy stored in the inductor and the mechanical output produced.Sping control in moving iron instrument,
As the deflection angle is proportional to the square of the current the moving iron instrument is non-linear.
Advantages of EMMI(Electromagnet moving iron):
- This instrument is used to measure both AC as well as DC.
Disadvantages of EMMI:
- As the deflection angle is proportional to the square of the current it is non-linear so accuracy will be low when compared to PMMC.
- It is a less sensitive instrument.
Moving iron instrument as induction ammeter:
In the case of moving iron ammeter, the time constant of the basic meter is equal to the time constant of the shunt then the meter is independent of the frequency.
Moving iron instrument as voltmeter:
The capacitor has to be connected in parallel to reduce the frequency error.
