The Permanent Magnet Moving Coil (PMMC) Meter movement was invented by D’Arsonval. Hence, it is also called D’Arsonval movement. The power consumption in.
Permanent Magnet Moving Coil or PMMC Instruments are the most accurate type for the measurement of DC current or voltage. The working principle of PMMC instruments are same as that of d’Arsonaval type of Galvanometer, difference being that a direct reading instrument is provided with pointer and scale.
Permanent Magnet Moving Coil or PMMC Instrument Definition: The instruments which use the permanent magnet for creating the stationary magnetic field between which the coil moves is known as the permanent magnet moving coil or PMMC instrument. A Permanent Magnet Moving Coil (PMMC) meter – also known as a D’Arsonval meter or galvanometer – is an instrument that allows you to measure the current through a coil by observing the coil’s angular deflection in a uniform magnetic field. AC electromechanical meter movements come in two basic arrangements: those based on DC movement designs, and those engineered specifically for AC use. Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of. (DOC) PMMC Instrument: DC Voltmeter and Ammeter Mohamad Hafizzudin Bin Hamdan - Academia.edu —The permanent-magnet moving-coil (PMMC) instrument consist basically of a.
Construction of PMMC Instruments
Pmmc Meter
Permanent Magnet Moving Coil or PMMC Instruments consists of following components:
- Moving Coil
- Magnet System
- Control Spring
- Damping
- Pointer and Scale
Moving Coil:
The moving coil made up of copper is wound with many turns on the rectangular Aluminum former. This Aluminum former is pivoted on the jewelled bearing. The coil can move freely in the magnetic field produced by the Permanent Magnet System. In case of PMMC voltmeters, coil is generally wound on metallic frame to have the required electromagnetic damping. But in case of Ammeter, coils are wound on non magnetic former because coil turns are effectively shorted by Ammeter Shunt. Thus we observe that in PMMC instruments, coil itself provides the electromagnetic damping.
Magnet System:
Simple U shaped permanent magnet made of Alcomax or Alnico is widely use in PMMC instruments. Theses magnets have high coercive force and can produce field of the order of 0.1 to 1 Wb/m2. A soft iron end cylinder is bored in between the poles to make the field radial and uniform. In addition, this soft iron also reduced the reluctance of the air path between the poles and hence increases the magnetic field strength.
The controlling torque in PMMC Instruments is provided by two control spring mounted on the jewel bearing. Theses control springs are phosphor bronze hair spring either helical or spiral, coiled in opposite direction. Control Spring also serves to lead current in and out of the moving coil.
Damping:
Damping torque in PMMC instruments are produced by the movement of Aluminum former in the magnetic field of Permanent Magnet. Due to movement of Aluminum former an eddy current is developed on the former which produces a damping torque.
Pointer and Scale:
The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light weight construction and has a section over the scale twisted to form a fine blade. This helps in reducing the Parallax Error in reading the scale. In many instruments this parallax error can be further reduced by careful alignment of pointer blade and its reflection in the mirror adjacent to scale.
Torque Equation for PMMC Instruments:
The deflecting torque equation for Permanent Magnet Moving Coil or PMMC Instruments is given as
Deflecting Torque Ƭd = NBLdI = GI
Where G = a constant = NBLd
Where N = Number of turns in the moving coil
B = magnetic flux density between the magnetic poles
L = Length of moving coil
d = Breadth of moving coil
As the controlling torque is provided by the spring, therefore
Ƭc = KƟ
Where K = Spring constant
Ɵ = Angular movement of coil
At steady state condition, deflecting and controlling torque shall be equal,
Ƭd = Ƭc
Pmmconline
⇒ GI = KƟ
⇒ Ɵ = (G / K)I ……………………(1)
Thus from the above equation (1), we observe that deflection in Permanent Magnet Moving Coil or PMMC Instruments is directly proportional to the current flowing in the moving coil. Because of this the meter scale of such instrument for the measurement of current / voltage is linear.
Figure 1 shows the basic construction of a PMMC instrument. A moving-coil instrument contains principally of a permanent magnet to offer a magnetic field and a small lightweight coil pivoted within the field. A soft iron core is included between the poles of the magnet so that the coil rotates in the narrow air gap between the poles and the core. When a current is passed through the coil windings, a torque is exerted on the coil by the interaction of the magnet’s field and the field set up by the current in the coil. The resulting deflection of the coil is indicated by a pointer that moves over a calibrated scale.
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Pmmc Volt Meter
Fig.1: PMMC Basic Construction
Pmmc Meter Working Principle
In addition to a deflecting force provided by the coil current and the field from the permanent magnet, a controlling force is needed. This is the force that returns the coil and pointer to the zero position when no current is flowing through the coil. The controlling force also balances the deflecting force, so that the pointer remains stationary for any constant level of current through the coil. The controlling force is usually provided by spiral springs as indicated in figure 2. The springs are also employed as connecting leads for conducting current through the coil.
Fig.2: PMMC Coil Springs
Permanent-magnet Moving Coil (pmmc) Meter
One other force, known as a damping force, is required for correct operation of a deflection instrument. When no damping force is present, the pointer swings above and below its final position on the scale for some time before settling down. In the case of the PMMC instruments, the damping force uses eddy currents. To facilitate this, the coil is wound on an aluminum frame or coil former in which eddy currents are generated by any rapid movements of the coil in the magnetic field. The eddy currents set up a magnetic flux that opposes the original movement that generates them. Thus, oscillations of the meter pointer are damped out.
Pmmc Meter Diagram