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Chapter 2 : Dc Meters

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Chapter 2 : Dc Meters EE 101 MEASUREMENT DC Meter / 1 CHAPTER 2 : DC METERS 2.1 BASIC PRINCIPLE OF ANALOG METER This permanent magnet moving coil meter movement is the basic movement in most analog (meter with a pointer indicator hand) measuring instruments. It is commonly called d'Arsonval movement because it was first employed by the Frenchman d'Arsonval in making electrical measurements. This type of meter movement is a current measuring device which is used in the ammeter, voltmeter, and ohmmeter. Basically, both the ammeter and the voltmeter are current measuring instruments, the principal difference being the method in which they are connected in a circuit. While an ohmmeter is also basically a current measuring instrument, it differs from the ammeter and voltmeter in that it provides its own source of power and contains other auxiliary circuits. 2.1.1 Basic Principle Operation Of Permanent-Magnetic Moving-Coil Movement a) Basic Construction b) The Permanent-Magnetic Moving-Coil Movement Used In A Meter. Figure 2.1 : Permanent-Magnetic Moving-Coil EE 101 MEASUREMENT DC Meter / 2 The compass and conducting wire meter can be considered a fixed-conductor moving-magnet device since the compass is, in reality, a magnet that is allowed to move. The basic principle of this device is the interaction of magnetic fields: the field of the compass (a permanent magnet) and the field around the conductor (a simple electromagnet). A permanent-magnet moving-coil movement is based upon a fixed permanent magnet and a coil of wire which is able to move, as in figure 2.2. When the switch is closed, causing current through the coil, the coil will have a magnetic field which will react to the magnetic field of the permanent magnet. The bottom portion of the coil will be the north pole of this electromagnet. Since opposite poles attract, the coil will move to the position shown in figure 2.3. Figure 2.2 : A movable coil in a magnetic Figure 2.3. : A movable coil in a magnetic field (with current). field (no current). The coil of wire is wound on an aluminum frame, or bobbin, and the bobbin is supported by jeweled bearings which allow it to move freely. This is shown in figure 2.4. To use this permanent-magnet moving-coil device as a meter, two problems must be solved. First, a way must be found to return the coil to its original position when there is no current through the coil. Second, a method is needed to indicate the amount of coil movement. The first problem is solved by the use of hairsprings attached to each end of the coil as shown in figure 2.5. These hairsprings can also be used to make the electrical connections to the coil. Figure 2.4. : A basic coil arrangement. Figure 2.5. : Coil and hairsprings. EE 101 MEASUREMENT DC Meter / 3 With the use of hairsprings, the coil will return to its initial position when there is no current. The springs will also tend to resist the movement of the coil when there is current through the coil. When the attraction between the magnetic fields (from the permanent magnet and the coil) is exactly equal to the force of the hairsprings, the coil will stop moving toward the magnet. As the current through the coil increases, the magnetic field generated around the coil increases. The stronger the magnetic field around the coil, the farther the coil will move. This is a good basis for a meter. But, how will you know how far the coil moves? If a pointer is attached to the coil and extended out to a scale, the pointer will move as the coil moves, and the scale can be marked to indicate the amount of current through the coil. This is shown in figure 2.6. Figure 2.6. - A complete coil. Figure 2.7 : Complete Construction of Permanent Magnet Moving Coil (PMMC) Two other features are used to increase the accuracy and efficiency of this meter movement. First, an iron core is placed inside the coil to concentrate the magnetic fields. Second, curved pole pieces are attached to the magnet to ensure the turning force on a coil increases steadily as the current increases. These same curved pole pieces are found in a motor. 2.1.3. Deflection Torque It has been mentioned that interaction between the induced field and the field produced by the permanent magnet causes a deflecting torque, which results in rotation of the coil. Deflection torque is controlling torque controls the deflection and tries to stop the pointer at its final position. But due to inertia, the pointer oscillates around its final position before coming to rest. Hence damping torque is provided to avoid this oscillation and bring the pointer quickly to its final position. EE 101 MEASUREMENT DC Meter / 4 Thus the damping torque is never greater than the controlling torque. In fact it is the condition of critical damping which is sufficient to enable the pointer rising quickly to its deflected position without overshooting. The deflecting torque produced is described below in mathematical form: Deflecting Torque, T d = BINA (Equation 2.1) Where B = flux density in Wb/m 2 (Tesla) I = current (A). N = number of turns of the coils. A = area ( length X wide), (m2). Example 1: Given frame of permanent moving coil is 6m 2. The number of winding around coil is 50 and flux 0.12 wb/m 2. If 1mA current through the coil, calculate the deflection torque. Solution Td = BINA = (0.12 wb/m 2)( 1mA)(50)(6m 2) = 36mNm 2.1.5 Damping A problem that is created by the use of a rectifier and d’Arsonval meter movement is that the pointer will vibrate (oscillate) around the average value indication. In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator. This oscillation will make the meter difficult to read. The process of "smoothing out" the oscillation of the pointer is known as DAMPING. There are two basic techniques used to damp the pointer of a d’Arsonval meter movement. i. The first method of damping comes from the d’Arsonval meter movement itself. In the d’Arsonval meter movement, current through the coil causes the coil to move in the EE 101 MEASUREMENT DC Meter / 5 magnetic field of the permanent magnet. This movement of the coil (conductor) through a magnetic field causes a current to be induced in the coil opposite to the current that caused the movement of the coil. This induced current will act to damp oscillations. In addition to this method of damping, which comes from the movement itself, most meters use a second method of damping. ii. The second method of damping used in most meter movements is an airtight chamber containing a vane (like a windmill vane) attached to the coil. As the coil moves, the vane moves within the airtight chamber. The action of the vane against the air in the chamber opposes the coil movement and damps the oscillations. There are two general classes of damped motion, as follows: 1. Periodic, in which the pointer oscillates about the final position before coming to rest. 2. Aperiodic, in which the pointer comes to rest without overshooting the rest position. The point of change between periodic and aperiodic damping is called "critical damping." An instrument is considered to be critically damped when overshoot is present but does not exceed an amount equal to one half the rated accuracy of the instrument. A problem that is created by the use of a rectifier and d’Arsonval meter movement is that the pointer will vibrate (oscillate) around the average value indication. This oscillation will make the meter difficult to read. The value of the damping ratio ζ determines the behavior of the system. A damped harmonic oscillator can be: i. Critical damping (ζ = 1) When ζ = 1, there is a double root γ (defined above), which is real. The system is said to be critically damped. A critically damped system converges to zero faster than any other, and without oscillating. An example of critical damping is the door closer seen on many hinged doors in public buildings. The recoil mechanisms in most guns are also critically damped so that they return to their original position, after the recoil due to firing, in the least possible time. ii. Over-damping (ζ > 1) When ζ > 1, the system is over-damped and there are two different real roots. An over- damped door-closer will take longer to close than a critically damped door would iii. Under-damping (0 ≤ ζ < 1) Finally, when 0 ≤ ζ < 1, γ is complex, and the system is under-damped. In this situation, the system will oscillate at the natural damped frequency ωd, which is a function of the EE 101 MEASUREMENT DC Meter / 6 natural frequency and the damping ratio. To continue the analogy, an underdamped door closer would close quickly, but would hit the door frame with significant velocity, or would oscillate in the case of a swinging door. Figure 2.8 : Damping Curve EE 101 MEASUREMENT DC Meter / 7 2.1.6 Common Damping System In Indicating Instrument a. Air friction damping Figure 2.9 : Air Friction Damping . b. Liquid damping Similar principle as air damping only the vane moves in a liquid chamber with a proper concentration. c. Eddy current damping Eddy currents are currents induced in conductors to oppose the change in flux that generated them. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time.
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