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.
|Published (Last):||25 September 2018|
|PDF File Size:||1.94 Mb|
|ePub File Size:||4.92 Mb|
|Price:||Free* [*Free Regsitration Required]|
It operates on the principle that the torque is exerted on the moving coil placed in the field of the permanent magnet. The parts of the PMMC instruments are explained below in details. Moving Coil — The coil is the current carrying part of the instruments which is freely moved between the stationary field of the permanent magnet. The current passes through the coil deflects it due to which the magnitude of the current or voltage is determined.
The coil is mounted on the rectangular former which is made up of aluminium. The former increases the radial and uniform magnetic field between the air gap of the poles. The coil is wound with the silk cover copper wire between the poles of a magnet.
The Alcomax and Alnico material are used for creating the permanent magnet because this magnet has the high coercive force The coercive force changes the magnetisation property of the magnet.
Also, the magnet has high field intensities. The springs are made up of phosphorous bronze and placed between the two jewel bearings. The spring also provides the path to the lead current to flow in and out of the moving coil.
The controlling torque is mainly because of the suspension of the ribbon. This damping torque is induced because of the movement of the aluminium core which is moving between the poles of the permanent magnet. The pointer notices the deflection of the coil, and the magnitude of their deviation is shown on the scale.
The pointer is made of the lightweight material, and hence it is easily deflected with the movement of the coil. Sometimes the parallax error occurs in the instrument which is easily reduced by correctly aligning the blade of the pointer.
The deflecting torque is expressed by the equation shown below. I — current through the coil. For final deflection, By substituting the value of equation 1 and 3 we get, The above equation shows that the deflection torque is directly proportional to the current passing through the coil.
The magnet, spring and the moving coil are the main parts of the instruments which cause the error. Magnet — The heat and vibration reduce the lifespan of the permanent magnet. This treatment also reduced the magnetism of the magnet. The magnetism is the property of the attraction or repulsion of the magnet. The weakness of the magnet decreases the deflection of the coil. Springs — The weakness of the spring increases the deflection of moving coil between the permanent magnet. So, even for the small value of current, the coil show large deflection.
The spring gets weakened because of the effect of the temperature. One degree rise in temperature reduces the 0. Moving Coil — The error exists in the coil when their range is extended from the given limit by the use of the shunt.
The error occurs because of the change of the coil resistance on the shunt resistance. This happens because the coil is made up of copper wire which has high shunt resistance and the shunt wire made up of Magnin has low resistance. To overcome from this error, the swamping resistance is placed in series with the moving coil.
The resistor which has low-temperature coefficient is known as the swamping resistance. The swamping resistance reduces the effect of temperature on the moving coil. The scale of the PMMC instruments is correctly divided. The power consumption of the devices is very less. The PMMC instruments have high accuracy because of the high torque weight ratio.
The single device measures the different range of voltage and current. This can be done by the use of multipliers and shunts. The PMMC instruments use shelf shielding magnet which is useful for the aerospace applications. The PMMC instruments are only used for the direct current.
The alternating current varies with the time. The rapid variation of the current varies the torque of the coil.
But the pointer can not follow the fast reversal and the deflection of the torque. Thus, it cannot use for AC. The cost of the PPMC instruments is much higher as compared to the moving coil instruments. The moving coil itself provides the electromagnetic damping.
The electromagnetic damping opposes the motion of the coil which is because of the reaction of the eddy current and the magnetic field. Related terms:.
PMMC Instrument; Construction, Working and Application
The internal construction of moving coil instruments A soft-iron yoke Y is used to complete the flux path and to provide shielding from stray external fields. Watch the video animation to understand quickly the working of a moving coil instruments. In this section, you will derive and learn the equation of deflection torque, control torque and damping torque in a PMMC instrument. Also, for a given coil l, b and n are constants and thus the product Blnb is also a constant, say k1. Control Torque The control on the movement of the pointer over the scale is provided by two spirally wound, phosphor-bronze springs S1 and S2, one at each end of the spindle S.
PMMC Instruments – Construction and Working Principle
Pin Several electrical machines and panels are fitted onboard so that the ship can sail from one port to another, safely and efficiently. The electrical machinery and system require scheduled maintenance and checks to avoid any kind of breakdown during sailing. Different instruments are used onboard for measuring several electrical parameters to analyze and keep these machines in proper running condition. A permanent magnet moving coil PMMC is one such instrument which is popularly used onboard and has several applications. Fleming left-hand rule: If the first and the second finger and the thumb of the left hand are held so that they are at right angle to each other, then the thumb shows the direction of the force on the conductor, the first finger points towards the direction of the magnetic field and the second finger shows the direction of the current in the wire. The value of control torque depends on the mechanical design of spiral springs and strip suspensions. The controlling torque is directly proportional to the angle of deflection of the coil.
Permanent Magnet Moving Coil Instrument (PMMC) – Working and Application on Ship
It is one of the simple and most used instrument on board with sophisticated names. They are used on board to assist in regular maintenance of electrical equipment or when precise measurements are required. It is a simple machine which create stationary magnetic fields with two powerful permanent magnet. This is then used with the moving coil connected to the electric source to produce deflection torque; according to the popular theory of Fleming left hand rule. These parts are explained below: 1.