PMMC Instrument; Construction, Working and Application

PMMC stands for ‘Permanent magnet moving coil’. It is one of the simple and most used instruments on board with sophisticated names. They are used on board to assist in regular maintenance of electrical equipment or when precise measurements are required. Apart from PMMC and permanent magnet moving coil; they are also known as D’alvanometer ( A type of galvanometer based on D’Arsonval principles ).

It is a simple machine that creates stationary magnetic fields with two powerful permanent magnets. This is then used with the moving coil connected to the electric source to produce deflection torque; according to the popular theory of Fleming’s left-hand rule.

Construction of PMMC Instrument

A PMMC Equipment consists of two main parts; moving coil and a permanent magnet along with other parts. These parts are explained below:

1.Magnet System

The instrument consists of two high intensity, high coercive force magnets or a big U-shape magnet based on design. These magnets are made up of Alcomax and Alnico for higher coercive force and better field intensity. In many designs, an additional soft iron cylinder is placed in between the magnetic poles to make the field uniform; while reducing air reluctance to increasing field strength.

2.Moving Coil

It is one of the main components of permanent magnet moving coil equipment; and is made up of copper coils wounded to a rectangular block in between the magnetic poles. Made up of Aluminum; the rectangular block is called Aluminum former pivoted to the jeweled bearing. It is what allows the coil to rotate freely.

When current is passed through these coils; it receives a deflection in the field which is then used to determine voltage / Current magnitude. Non-metallic former like that of aluminum is used for current measurement; while metallic former with high electromagnetic damping is used to measure voltage.

PMMC Instrument sketch diagram

3.Control

Two spring made of phosphorus bronze acts as a control system for the permanent magnet moving coil. These springs are mounted on the jewel bearing of PMMC; providing the essential controlling torque. The controlling torque produced is mainly due to ribbon suspension. They oppose the force of deflection; so the electromagnetic force ( Of Moving Coil ) came in equilibrium with the spring tension.

This helps in keeping the pointer at a fixed position after an initial deflection. These control springs also serve the purpose of providing lead current paths in and out of the system.

4.Damping System

Damping torque is produced in the PMMC equipment by the movement of aluminum core in the magnetic field. It keeps the pointer at rest after the initial deflection. This helps in proper measurement without fluctuations. Due to the movement of the coil in the magnetic field; eddy current is produced in the aluminum former.

This produces the damping force / Torque which opposes the further motion of the coil. Slowly the pointer deflection reduces and finally stop’s at a fixed position.

5.Scale and Pointer

The pointer connected to the moving coil moves over a marked scale. The pointer moves along with the coil deflection to show readings marked on the scale. A pointer is a simple construction with light weight design and twisted section to reduce parallax error. A Parallax error can be further reduced by proper alignment of pointer blades to the initial scale.

Working Principle of a PMMC Instrument

“When a current caring conductor is placed in a magnetic field; it experiences a force perpendicular to the field and the current.” (Fleming Left Hand Rule). This force tends to move the conductor. According to Fleming left-hand rule; if your left-hand thumb, fore finger, and middle finger are at 90 degrees to each other. Then the magnetic field would be along with the fore finger, current across the middle while the force along with the thumb.

When current flows in the coil on the aluminum former; a magnetic field is produced in the coil in proportion to the current flow. This electromagnetic force along with a static magnetic field from the permanent magnet produces the deflection force in the coil. The spring then produces the controlling force to oppose further deflection; Thus helps in balancing the pointer.

Then damping force is produced in the system by the movement of aluminum core in the magnetic field. It keeps the pointer fixed to a position after it reaches equilibrium with the controlling and deflection torque; providing better precision in measurement.

Working Equation

The equation for deflection torque in PMMC is:-

TF = NBLdI

=KI

Where, K is a constant. Now Here;

N = No of Turns in coil.

L = Coil length

B = Flux Density

D = Coil Breadth

So, the following equation can also be written as;

T = NBIA

Where, A = L X D = Effective Area.

Since controlling torque produced by the springs mounted on jewel bearing is proportional to the deflecting angle.

Here;

Ts = Kθ

Where,

K = Spring Constant and θ = Angular moment of the coil.

Now, Since the controlling and deflecting torque should be in equilibrium; We have:-

TF = Ts

GI = Kθ

so, θ =

GK× I

This equation shows that the deflection ( θ ) of PMMC equipment is proportional to the current flow ( I ).

On-board Application

PMMC or Permanent magnet moving coil is one of the best instruments to measure direct current. They are accurate, sensitive, and work for a longer period of time without defects/maintenance. These instruments on board are used to measure direct current and voltage. They can also be used as a galvanometer to detect small current or change in magnetic flux.

Possible cause for error in PMMC Equipment?

1.Error due to magnetism

Permanent magnet loses their magnetism with time; this is called magnet aging. With plenty of heat and vibration on the ship (especially Engine Room ); There is a reduction of magnetism due to accelerated aging. This decrease in magnetic strength reduce the coil deflection affecting the readings.

2.Error due to Temperature Difference

Moving Coil of PMMC instrument is made up of copper wires; the temperature coefficients of copper wire is known to be 0.004 per degree Celsius. So with increase in temperature, there will be a high increase in its resistance altering the actual reading.

3.Error Due to Spring

Aging leads to weakening of spring tension; this results in decreased deflection of the moving coil. This error is opposite to that of the error due to magnetic aging and sometimes cancel each other to reduce much difference in the final readings.

Advantages of Using PMMC Equipment

  1. High weight to torque ratio.
  2. It has pointer deflection proportional to the current; which makes the scale more uniform over an arc of 270 degrees.
  3. It consumes much less power than other alternatives.
  4. No hysteresis loss.
  5. Unaffected by a stray magnetic field; perfect equipment for on-board applications.
  6. All-purpose equipment; can be used as an ammeter, voltmeter, and galvanometer.

Disadvantages of using PMMC Equipment

  1. It only works for Direct current ( D.C ).
  2. It’s costly than its other alternatives.
  3. It can show false reading due to the above stated reasons ( Cause of errors in permanent magnet moving coil instrument ).

Why PMMC is not used for A.C ?

One of the most common and obvious questions out of curiosity; its not strange that we ask why its not used for A.C and what will happen if we connect it to A.C? Actually, in Permanent magnet moving coil instrument the deflection torque is directly proportional to the current. This stat’s that, torque will reverse every 180 degree of alternating current. It happens due to the switch in magnetic polarity of electromagnet.

A PMMC equipment has two permanent magnets or a large U shaped magnet with an electromagnet in the form of a moving coil. There is a switch in magnetic polarity for this electromagnet every half cycle if we connect it to alternating current. Due to this equal and opposite deflection every 180 degrees of alternating current; we will get a net torque of zero. This is why we don’t use the PMMC instrument for Alternating current.

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Amit Abhishek

A Marine engineer turned professional blogger who had started Shipfever.com In Jan 2018 after a brief experince at sea.

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