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.
- A pointer is mounted on the spindle for analog reading. A mirror is provided below the pointer and scale to avoid parallax errors.
- The spindle is supported by jewelled bearings.
- 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 for 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 and working principle of PMMC Instruments:
- When a current-carrying conductor is placed in a magnetic field, it experiences a force that acts perpendicular to both the current and the magnetic field.
- According to Fleming's Left-Hand Rule, if the thumb, forefinger, and middle finger of your left hand are positioned at right angles to each other, you can determine the direction of the force. The forefinger represents the magnetic field, the middle finger indicates the direction of current, and the thumb shows the direction of the resulting force.
- When current flows through a coil on an aluminum former, it creates a magnetic field proportional to the current.
- The interaction between the electromagnetic force and the permanent magnet's fixed magnetic field generates a deflection force in the coil.
- A spring resists this deflection by providing a counteracting force, ensuring the pointer remains balanced.
- This balance between deflection torque and damping force is crucial for maintaining measurement accuracy.
- 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 about PMMC:
1. Magnetic system of PMMC:
- 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 in PMMC:
- Control torque can be provided by ribbon suspension instead of using two-phosphor bronze hairsprings.
3. Damping Torque in PMMC:
- 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.
Errors in PMMC instruments:
- Change in the resistance of the moving coil with temperature.
- weakening of permanent magnets due to aging and temperature effects.
- Weakening of the 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, between 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.
The 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 DC supply.
Important Questions on PMMC
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.
FAQs on PMMC:
1. What is a Permanent Magnet Moving Coil (PMMC) instrument?
A PMMC instrument is an electrical measuring device that uses a moving coil mechanism and a permanent magnet to measure current or voltage in a circuit.
2. What is the construction of a PMMC instrument?
A PMMC instrument consists of stationary permanent magnets, a moving coil wound on a rectangular aluminum frame, a spindle with a pointer, control springs, and a soft iron core. The coil rotates when current flows, moving the pointer for analog reading.
3. How does a PMMC instrument work?
The PMMC instrument operates on the principle that a current-carrying conductor in a magnetic field experiences a force. When current flows through the coil, the interaction between the coil’s magnetic field and the permanent magnet creates a deflection force, moving the pointer on a linear scale.
4. What is Fleming’s Left-Hand Rule in relation to PMMC instruments?
Fleming’s Left-Hand Rule helps determine the direction of the force acting on a current-carrying conductor in a magnetic field. The thumb shows the direction of force, the forefinger the magnetic field, and the middle finger the current.
5. What are the advantages of PMMC instruments?
- High accuracy due to a favorable torque-weight ratio.
- Low power consumption (25 to 200 microwatts).
- Uniform linear scale as deflection is proportional to current.
- Usable for multiple current and voltage ranges with shunt and series resistors.
6. What are the disadvantages of PMMC instruments?
- Higher cost compared to moving iron instruments.
- Usable only with DC supply, not AC.
7. What is the purpose of the control torque in a PMMC instrument?
The control torque, produced by control springs or ribbon suspension, helps counteract the deflection of the coil, ensuring the pointer remains balanced for accurate readings.
8. How is damping torque generated in PMMC instruments?
Damping torque is generated through eddy currents induced in the aluminum frame. These eddy currents help dampen oscillations and stabilize the movement of the pointer.
9. What are the common errors in PMMC instruments?
- Resistance changes in the moving coil due to temperature fluctuations.
- Weakening of permanent magnets and control springs over time and with temperature effects.
10. Can PMMC instruments measure both AC and DC currents?
No, PMMC instruments can only be used for measuring DC currents and voltages.
11. How can the sensitivity of a PMMC instrument be increased?
The sensitivity of a PMMC instrument can be enhanced by using high-coercive materials like Alcomax and Alnico in the magnet system, which allows for smaller magnets with higher field intensities.
12. How does eddy current damping work in PMMC instruments?
Eddy currents induced in the aluminum frame of the PMMC instrument provide damping, reducing oscillations and allowing the pointer to reach its final position smoothly.