Permanent Magnet Moving Coil (PMMC) Meter

What is a Permanent Magnet Moving Coil (PMMC)?

A Permanent Magnet Moving Coil (PMMC) movement, often called a D’Arsonval meter movement or moving-coil galvanometer, converts direct current in a coil into angular deflection in a radial magnetic field. With suitable resistors, the movement can form a DC ammeter or voltmeter.

A PMMC meter places a light moving coil in the field of a permanent magnet. The linked Faraday’s Laws of electromagnetic induction describe induced emf, while PMMC deflection comes from the magnetic force on current-carrying conductors. Its direction follows Fleming’s left hand rule.

For fixed field strength and coil geometry, the deflecting torque is proportional to coil current. A pointer fixed to the moving spindle travels over a calibrated scale.

The coil settles when electromagnetic torque equals the control-spring torque. Shaped pole pieces create a nearly radial field, and a linear spring produces torque proportional to angle. Under these conditions, pointer deflection is proportional to current and the scale is uniform.

A bare PMMC movement responds to Direct Current (DC). With Alternating Current (AC), the current and torque reverse each half-cycle. At normal supply frequency, mechanical inertia prevents the pointer from following each reversal and the average torque is approximately zero. A rectifier can be added to make a PMMC-based AC instrument, with calibration that accounts for the waveform and rectifier.

Within its range, calibration and reference conditions, a PMMC movement provides a sensitive linear indication of DC current.

If you’d prefer a video explanation, here is a video on Permanent Magnet Moving Coils:

PMMC Construction

A PMMC meter, or D’Arsonval movement, has five main component groups:

• Stationary Part or Magnet System
  
• Moving Coil
  
• Control System
  
• Damping System
  
• Meter
  

Stationary Part or Magnet System

The stationary magnetic circuit uses a permanent magnet, shaped pole pieces and a soft-iron core to produce a strong radial air-gap field. Instrument designs have used stable high-coercivity alloys such as Alnico and Alcomax. The exact material and geometry depend on the movement.

Moving Coil

The moving coil rotates in the narrow air gap. Fine insulated copper wire is wound on a light rectangular aluminium former attached to the spindle and pointer. Pivot-and-jewel bearings or a taut-band suspension support the moving assembly, depending on the design.

Control System

Spiral springs provide the restoring torque in many PMMC instruments. They also carry current into and out of the moving coil. Their geometry and material affect zero position, sensitivity and temperature performance.

Damping System

Motion of the conductive aluminium former through the permanent magnet’s magnetic field induces eddy currents. Their opposing torque damps oscillation and helps the pointer settle without changing its steady indication.

Meter

The indicating assembly has a lightweight pointer and a calibrated scale. Because spring torque and electromagnetic torque are each linear under normal conditions, equal current increments produce nearly equal angular increments.

PMMC Torque Equation

For a rectangular permanent magnet moving coil, or PMMC instrument, the radial field produces the following deflecting-torque expression:

• T_d = NBl dI, where N is the number of turns,
• B is magnetic flux density in the air gap,
• l is the active length of each coil side,
• d is the coil width, so ld is its area,
• I is the moving-coil current.

For fixed geometry and field, write T_d = GI, where G = NBld. At steady deflection, electromagnetic and controlling torques are equal. If T_c is the spring controlling torque and K is the spring constant, equating the torques gives:

GI = K.x, where x is angular deflection. Therefore, current is:

Because x = (G/K)I under these assumptions, the movement uses a uniform current scale.

To extend the movement’s current range, connect a low resistance in parallel to form an ammeter. The circuit below shows the arrangement:

At point A, total current I divides into shunt current I_s and movement current I_m. Both parallel paths have the same terminal voltage. The shunt must have a stable low resistance, adequate power rating and connections that keep lead and contact effects controlled.

The shunt’s electrical resistance should remain stable with temperature, time and load. Manganin is widely used because it has low resistance drift and a low temperature coefficient near its designed operating range. The shunt carries most of the current, so I_s is much greater than I_m when the range is extended substantially. Equal branch voltage gives:

Here, R_s is the shunt resistance and R_m is the movement-coil resistance.

Combining the current and voltage relations gives:

Here, m is the current-range multiplying factor of the shunt.

Errors in Permanent Magnet Moving Coil Instruments

Important sources of indication error include:

1. Errors due to permanent magnets: Temperature and long-term magnetic change can alter flux density and torque sensitivity. Manufacturers stabilise the magnetic system and calibrate the assembled movement for its specified conditions.
2. Spring stiffness, zero position and contact resistance can change with temperature or age. Magnet and spring effects must each remain within calibration limits; their signs and magnitudes cannot be assumed to cancel.
3. Change in the resistance of the moving coil with the temperature: Copper has a positive temperature coefficient of about 0.0039 per degree Celsius near room temperature, not 0.04. Coil heating therefore raises resistance and changes current for a given applied voltage. Shunt and multiplier design, temperature compensation and rated loading limit this error.

Advantages of Permanent Magnet Moving Coil Instruments

The advantages of PMMC instruments are:

1. The radial field and linear control spring make pointer deflection proportional to DC current, which permits a nearly uniform scale.
2. A sensitive movement can reach full-scale deflection with low coil current and modest power.
3. The lightweight coil can provide a useful torque-to-mass ratio and a clear indication.
4. Shunt resistors extend ammeter ranges, while series multiplier resistors allow the same movement principle to measure DC voltage.

Disadvantages of Permanent Magnet Moving Coil Instruments

The disadvantages of PMMC instruments are:

1. A bare movement cannot directly indicate ordinary AC because its torque reverses each half-cycle. Rectifier-type PMMC instruments need waveform-specific calibration.
2. The precision magnetic circuit, moving coil and bearings can make a PMMC movement more costly and delicate than comparable moving iron instruments.