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pmmc stands for permanent magnet moving coil instruments these instruments are secondary measuring instruments in these instruments a permanent magnet is installed and a moving coil in this way when current is passed through the fixed coil then a force will be developed on the moving coil and thus it moves and with this moving pointer moves on the scale and gives the corresponding reading . if someone wants to read it thoroughly then he can try ashfaq hussain , j.b.gupta of basic elelctrical engg. for b.tech first year .
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Why pmmc used for DC only?
Because in the coil DC current follow easly and pmmc meter is making with coil and AC current is not follow in coil so we used the pmmc only fpr DC current.
Is Series resistance and PMMC used as voltmeter?
yes
How would you describe the construction and working principle of PMMC instruments with its three advantages?
The permanent magnet moving coil instrument is the most accurate type for D.C. Measurements. The working principle of these instruments is the same as that of the d'Arsonval type of galvanometers, the difference being that a direct reading instrument is provided with a pointer and a scale(Fig) Permanent magnet moving coil instrumentConstruction of PMMC InstrumentsThe constructional features of this instrument are shown in Fig.The moving coil is wound with many turns of enameled or silk covered copper wire.The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings.The coils move freely in the field of a permanent magnet.Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping.Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt.The coil itself, therefore, provides electro magnetic damping.Magnet SystemsOld style magnet system consisted of relatively long U shaped permanent magnets having soft iron pole pieces.Owing to development of materials like Alcomax and Alnico, which have a high co-ercive force, it is possible to use smaller magnet lengths and high field intensities.The flux densities used in PMIMC instruments vary from 0.1 Wb/m to 1 Wb/m.ControlWhen the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension as shown.This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.DampingDamping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.Pointer and ScaleThe pointer is carried by the spindle and moves over a graduated scale.The pointer is of lightweight construction and, apart from those used in some inexpensive instruments has the section over the scale twisted to form a fine blade.This helps to reduce parallax errors in the reading of the scale. When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.These springs also serve to lead current in and out of the coil.Torque Equation.The torque equation of a moving coil instrument is given byAs the deflection is directly proportional to the current passing through the meter (K and G being constants) we get a uniform (linear) scale for the instrument.Errors in PMMC InstrumentsThe main sources of errors in moving coil instruments are due toWeakening of permanent magnets due to ageing at temperature effects.Weakening of springs due to ageing and temperature effects.Change of resistance of the moving coil with temperature.Advantages and Disadvantages of PMMC InstrumentsThe main advantages of PMMC instruments areThe scale is uniformly divided.The power consumption is very lowThe torque-weight ratio is high which gives a high accuracy. The accuracy is of the order of generally 2 percent of full-scale deflection.Using different values for shunts and multipliers may use a single instrument for many different current and voltage ranges.Since the operating forces are large on account of large flux densities, which may be as high as 0.5 Wb/m, the errors due to stray magnetic fields are small.Self-shielding magnets make the core magnet mechanism particularly useful in aircraft and aerospace applications.The chief disadvantages areThese instruments are useful only for D.C. The torque reverses if the current reverses. If the instrument is connected to a.c., the pointer cannot follow the rapid reversals and the deflection corresponds to mean torque, which is zero. Hence these instruments cannot be used for a.c.The cost of these instruments is higher than that of moving iron instruments.
What is pmmc type of instruments?
Produce of series type Ohmmeter using PMMC of Ifsd:100uA for range of 0-20kohm?
Why pmmc used for DC only?
Because in the coil DC current follow easly and pmmc meter is making with coil and AC current is not follow in coil so we used the pmmc only fpr DC current.
Is Series resistance and PMMC used as voltmeter?
yes
Operation of PMMC instrument?
aku pon tatau
Why pmmc instrument is cannot be used to measure ac?
Pmmc meter are most accurate instrument for DC measurement however they are unable to measure the AC quantity. The working principle is as follows: a moving coil is free to move in a external static magnetic field. And the pointer moves according to the interaction of two fields, field of static mag. and that of measuring quantity. Now in case of it is impossible to produce a sustained magnetic field, that's why we can not use PMMC to measure the ac.
What is the explanation for the mechanism of DC in pmmc?
a pmmc has a coil suspended in permanent magnetic field.acording to the laws of electromagnetism a current carrying coil susspended in a magnetic field experiences an electromagnetic torque.This torque is counterbalanced by the mechanical torque devoloped in the coil in equlibrium.The angle teta can be measured and it is calibrated to directly give the current
Why pmmc instrument measure only ac not dc?
Permanent magnet moving coil instruments are used for measuring d.c. quantities only. This is so because for these instruments the torque is proportional to the operating quantity either voltage or current.
How many types of plastics are there?
plastics are of 5 types[1]- polythene[2]- polyvinyl chloride (PVC)[3]- orion/polyvinyl cyanide /polyacryl nitrile (PAN)[4]- polymethyl meth acrylate (PMMC)[5]- polysterene
Definition of PMMC instrument?
An ammeter or other electrical instrument in which a small coil of wire, supported on jeweled bearings between the poles of a permanent magnet, rotates when current is carried to it through spiral springs which also exert a restoring torque on the coil; the position of the coil is indicated by an attached pointer.
What is working of PMMC?
In PMMC meter or (D'Arsonval) meter or galvanometer all are the same instrument, a coil of fine wire is suspended in a magnetic field produced by permanent magnet. According to the fundamental law of electromagnetic force, the coil will rotate in the magnetic field when it carries an electric current by electromagnetic (EM) torque effect. A pointer which attached the movable coil will deflect according to the amount of current to be measured which applied to the coil. The (EM) torque is counterbalance by the mechanical torque of control springs attached to the movable coil also. When the torques are balanced the moving coil will stopped and its angular deflection represent the amount of electrical current to be measured against a fixed reference, called a scale. If the permanent magnet field is uniform and the spring linear, then the pointer deflection is also linear
What is a moving coil instrument?
It works on DC only It consist a permanent magnet coil.
How would you describe the construction and working principle of PMMC instruments with its three advantages?
The permanent magnet moving coil instrument is the most accurate type for D.C. Measurements. The working principle of these instruments is the same as that of the d'Arsonval type of galvanometers, the difference being that a direct reading instrument is provided with a pointer and a scale(Fig) Permanent magnet moving coil instrumentConstruction of PMMC InstrumentsThe constructional features of this instrument are shown in Fig.The moving coil is wound with many turns of enameled or silk covered copper wire.The coil is mounted on rectangular aluminum former, which is pivoted on jeweled bearings.The coils move freely in the field of a permanent magnet.Most voltmeter coils are wound on metal frames to provide the required electro-magnetic damping.Most ammeter coils, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt.The coil itself, therefore, provides electro magnetic damping.Magnet SystemsOld style magnet system consisted of relatively long U shaped permanent magnets having soft iron pole pieces.Owing to development of materials like Alcomax and Alnico, which have a high co-ercive force, it is possible to use smaller magnet lengths and high field intensities.The flux densities used in PMIMC instruments vary from 0.1 Wb/m to 1 Wb/m.ControlWhen the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.These springs also serve to lead current in and out of the coil. The control torque is provided by the ribbon suspension as shown.This method is comparatively new and is claimed to be advantageous as it eliminates bearing friction.DampingDamping torque is produced by movement of the aluminium former moving in the magnetic field of the permanent magnet.Pointer and ScaleThe pointer is carried by the spindle and moves over a graduated scale.The pointer is of lightweight construction and, apart from those used in some inexpensive instruments has the section over the scale twisted to form a fine blade.This helps to reduce parallax errors in the reading of the scale. When the coil is supported between two jewel bearings two phosphor bronze hairsprings provide the control torque.These springs also serve to lead current in and out of the coil.Torque Equation.The torque equation of a moving coil instrument is given byAs the deflection is directly proportional to the current passing through the meter (K and G being constants) we get a uniform (linear) scale for the instrument.Errors in PMMC InstrumentsThe main sources of errors in moving coil instruments are due toWeakening of permanent magnets due to ageing at temperature effects.Weakening of springs due to ageing and temperature effects.Change of resistance of the moving coil with temperature.Advantages and Disadvantages of PMMC InstrumentsThe main advantages of PMMC instruments areThe scale is uniformly divided.The power consumption is very lowThe torque-weight ratio is high which gives a high accuracy. The accuracy is of the order of generally 2 percent of full-scale deflection.Using different values for shunts and multipliers may use a single instrument for many different current and voltage ranges.Since the operating forces are large on account of large flux densities, which may be as high as 0.5 Wb/m, the errors due to stray magnetic fields are small.Self-shielding magnets make the core magnet mechanism particularly useful in aircraft and aerospace applications.The chief disadvantages areThese instruments are useful only for D.C. The torque reverses if the current reverses. If the instrument is connected to a.c., the pointer cannot follow the rapid reversals and the deflection corresponds to mean torque, which is zero. Hence these instruments cannot be used for a.c.The cost of these instruments is higher than that of moving iron instruments.
