A Permanent Magnet Moving Coil (PMMC) instrument is a type of electrical measuring instrument that uses a moving coil mechanism and a permanent magnet to measure the current or voltage in a circuit.
Construction of PMMC:
- The PMMC instrument consists of one pair of permanent magnets which are stationary.
- The moving system consists of a spindle attached to a rectangular aluminium frame.
- A coil made of thin copper wire is wound over this frame. The PMMC instrument operates on the principle of the interaction between a magnetic field and an electric current.
- When a current flows through the coil, a force is generated that causes the coil to rotate and the current which is to be measured is passed through the coil.
- Pointer is mounted on the spindle.
- A mirror is provided below the scale to avoid parallex errors. The spindle is supported by jewelled bearings as shown in the following figure.
Two spiral springs are mounted on the spindle to produce a control torque.
The control springs also serve an additional purpose and act as contact leads of the coil which is wound on an aluminium frame. Thus, the external current enters the coil via the control spring.
A soft iron core is placed in the space within the aluminium frame.
This core is stationary and in no way connected to the system. It is provided to reduce the reluctance of the magnetic path between the two poles of the permanent magnet.
Operations:
- Damping torque is produced by the eddy current damping method.
- The eddy currents are induced in the moving aluminum frame which damps out the oscillations of the moving system about its final mean deflected position.
- Deflecting torque(Td) = Force × perpendicular distance from center = NBiA Nm
- Thus the deflection is directly proportional to the current flow in the coil, as result, the scale will be linear.
Additional Information of PMMC:
1. Magnet system:
For more sensitivity, the magnet system can be modified. with materials like Alcomax and Alnico which have high coersive force.
It is possible to use smaller lengths and high field intensities. so the weight of the system can be reduced.
2. Control Torque:
Control torque can be provided by ribbon suspension instead of using two-phosphor bronze hairsprings.Errors in PMMC instruments:
- Change of resistance of the moving coil with temperature.
- weakening of permanent magnets due to ageing and temperature effects.
- weakening of springs due to ageing and temperature effects.
Advantages of PMMC:
- The torque weight ratio is high which gives high accuracy.
- The power consumption is very low from 25 microwatts to 200 microwatts.
- A single instrument can be used for different current and voltage ranges by using different values of shunt and series resistors.
- As deflection is directly proportional to the current flowing through the coil. we get a uniform(linear) scale for the instrument.
Disadvantages of PMMC:
- The cost of these instruments is more than that of the moving iron instrument.
- These instruments can be used only on d.c supply.
- People also ask:
- What is the accuracy of PMMC?
- The accuracy of a PMMC instrument typically ranges from 1% to 2% of the full-scale deflection. However, the actual accuracy may vary depending on the specific instrument model and the conditions under which it is being used.
What is PMMC type voltmeter?
- A PMMC-type voltmeter is an electrical measuring instrument used for measuring the voltage in a circuit.
- It is based on the PMMC instrument principle, where a moving coil mechanism is suspended in a magnetic field generated by a permanent magnet.
- When a voltage is applied to the coil, a force is generated that causes the coil to rotate, and the pointer attached to the coil moves along a calibrated scale to indicate the voltage being measured. PMMC-type voltmeters are commonly used for measuring low-level voltages with high accuracy and sensitivity.
What is a moving coil?
- Moving coil refers to a mechanism used in electrical measuring instruments, such as voltmeters and ammeters, where a coil of wire is suspended in a magnetic field and is free to rotate about its axis.
- When a current flows through the coil, it experiences a force due to the interaction between the magnetic field and the electric current, causing the coil to rotate.