PMMC (PERMANENT MAGNET MOVING COIL)
PMMC means Permanent Magnet Moving Coil. This method belongs to direct method and for high accuracy type for DC measurement.
PRINCIPLE:
The PMMC works on the principle i.e. when a current carrying conductor is placed in a magnetic field it exerts mechanical force. The coil placed in magnetic field and current carrying operation is attached for the moving system. With the movement of the coil the pointer moves over the scale.
CONSTRUCTION:
PMMC consists of two permanent magnets with soft iron pieces and the coil is wound on aluminium former with many turns inside the iron core (Purpose of coil is to make field uniform). The controlling torque is provided with springs and eddy current damping is provided by aluminium former.
WORKING:
When PMMC fed with supply the coil carries a current. Due to electromagnet mechanism the mechanical force (Torque) is produced in coil which is connected to the pointer which slides over the scale.
*If the direction of current is reversed, the torque also gets reversed. So the torque production takes place in opposite direction. Hence the pointer shows below zero readings. The conclusion says, this instrument working only for unidirectional current i.e. DC.
*The components require for PMMC are
1. Magnetic system
2. Controlling torque
3. Damping torque
4. Pointer and scale
Errors in PMMC:
The occurrence of PMMC is listed below
1. Due to the ageing of permanent magnets
2. Due to temperature and atmospheric conditions
3. Weakening of magnets strength and Tension of spring
ADVANTAGES AND DISADVANTAGES:
Advantages:
1. Uniformly divided scale is operated
2. Power consumption is between 25μW to 200μW
3. By using shunts and multipliers, the voltage and current ratings are increased
4. By maintaining high torque ratio the accuracy changes to high.
Disadvantages:
1. In economical part of view, this instrument costs high compared to moving iron instrument.
2. This instrument only applicable for DC supply.
3. Some errors are occurred due to ageing of magnets and springs.
DYNAMOMETER TYPE
These are modified form of PMMC instruments. In this type the operating field is produced by another coil but not permanent coil. The remaining features such as moving &controlling system is as same as PMMC .These Instruments are used or both AC and DC .These meters are used as ammeters ,voltmeters but generally used as watt meters.
Principle:
The working principle states the mechanical force is existed between the current carrying conductors.
Construction:
A dynamometer consists of two coil one is fixed and another is movable. The fixed coil split into two equal parts placed parallel & closed to each other. The moving coil is placed in between two fixed coils as shown in figure:
à The fixed and moving coils are existed separately based on type of connection. The moving coil is attached to the pointer for the deflection
à The torque`s are controlled by spring control and air friction.
Working:
When an instrument is fed with the supply, the current flows through the coil and se ups a magnetic field .This field exerts mechanical force. This torque moves the pointer over the scale.
à If deflecting torque = controlling torque the pointer show zero position, by reversing the current in field coils the field also get reversed setting magnetic field but there is no change in deflecting torque due to magnetic field. These instruments can be used for both AC& DC measurements
Advantages:
à Used for DC&AC measurements
à These instruments are free from hysteresis and eddy current errors
Disadvantages:
à Non-uniform scale (in Ammeter & voltmeter)
à High cost and rarely used
à Torque /weight ratio is small hence frictional errors are obtained which reduce sensitivity.
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