PMMC stands for Permanent Magnet Moving Coil. PMMC Instruments are the most accurate type for the measurement of DC current or voltage.
Construction of PMMC Instruments
Permanent Magnet Moving Coil or PMMC Instruments consists of following components:
- Moving Coil
- Magnet System
- Control Spring
- Pointer and Scale
1. 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, the coil is generally wound on a 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. Thus we observe that in PMMC instruments, the coil itself provides the electromagnetic damping.
2. Magnet System
Simple U shaped permanent magnet made of Alcomax or Alnico is widely use in PMMC instruments. These magnets have a high coercive force and can produce a field of the order of 0.1 to 1 Wb/m2. A soft iron end cylinder is bored in between the poles to make the field radial and uniform. In addition, this soft iron also reduced the reluctance of the air path between the poles and hence increases the magnetic field strength.
3. Control Spring
The controlling torque in PMMC Instruments is provided by two control spring mounted on the jewel bearing. These control springs are phosphor bronze hair spring either helical or spiral, coiled in opposite direction. Control Spring also serves to lead current in and out of the moving coil.
Damping torque in PMMC instruments is produced by the movement of Aluminum former in the magnetic field of Permanent Magnet. Due to movement of Aluminum former, an eddy current is developed on the former which produces a damping torque.
5. Pointer and Scale
The pointer is carried by the spindle and moves over a graduated scale. The pointer is of lightweight construction and has a section over the scale twisted to form a fine blade. This helps in reducing the Parallax Error in reading the scale. In many instruments, this parallax error can be further reduced by careful alignment of pointer blade and its reflection in the mirror adjacent to scale.
When a current carrying conductor is placed in a magnetic field, it experiences a force and tends to move in the direction as per Fleming’s left-hand rule.
In PMMC type instrument, the permanent magnet is used to create a stationary magnetic field between which the coil moves. When a current flow through the coil, it generates a magnetic field which is proportional to the current. The deflecting torque is produced by the electromagnetic action of the current in the coil and the stationary magnetic field.
When the deflecting torque is equal to controlling torque, the moving coil will stop and its angular deflection represents the amount of electrical current or voltage to be measured against a fixed reference, called a scale.
Torque Equation for PMMC Instruments
The deflecting torque equation for Permanent Magnet Moving Coil or PMMC Instruments is given as
Deflecting Torque, Ƭd
Ƭd = NBLdI = GI
G = NBLd = constant
N = Number of turns in the moving coil
B = magnetic flux density between the magnetic poles
L = Length of moving coil
d = Breadth of moving coil
As the controlling torque (Tc ) is provided by the spring, therefore
Tc = Kθ
K = Spring constant
θ = Angular movement of coil
At steady state condition, deflecting and controlling torque shall be equal,
Td = Tc
⇒ GI = Kθ
Thus from the above equation, we observe that deflection in Permanent Magnet Moving Coil or PMMC Instruments is directly proportional to the current flowing in the moving coil. Because of this, the meter scale of such instrument for the measurement of current/voltage is linear.
- The PMMC instruments measure the DC current.
- If a pure AC current is passing through PMMC instrument then, it reads zero value.
Advantages of PMMC Instruments
- The scale is uniformly divided. Since current is directly proportional to the pointer.
- Power consumption is also very low.
- The PMMC instruments have high accuracy because of the high torque weight ratio.
- No hysteresis loss
- The PMMC instruments measure the different range of voltage and current by the use of multipliers and shunts.
- Errors due to stray magnetic fields are less.
Disadvantages of PMMC instruments
- The magnetic field produced is affected by the surrounding temperature which causes an error in reading.
- The instrument is used to measure DC only.
- It can measure current upto 50 mA and voltage upto 100 mV only.
- It is costly compared to moving iron instruments.