The ship uses a variety of electrical machines to move from one location to another safely and effectively. But in order to prevent any breakdowns, these machines must be maintained while in transit. Different instruments are used to measure various electrical parameters on the ship so that we can inspect the equipment to ensure that it is operating properly. Similar to this, a device like a PMMC (permanent magnet moving coil) is frequently utilized in a variety of settings, including ships. There are two different types of this instrument: galvanometers and d’alvanometers. The PMMC instrument is described in general detail in this article.

What is a PMMC Instrument?

The abbreviation for “permanent magnet moving coil” is PMMC. This instrument is easy to use and is mostly found on ships with fancy names. These tools are used to help with electrical equipment maintenance as well as when an exact measurement is required. It is known by other names besides PMMC, such as D’alvanometer. It is a type of galvanometer that utilizes the D’Arsonval principle.

According to the Fleming left-hand rule theory, these instruments use permanent magnets to create the stationary magnetic field in the coils, which is then used with the moving coil that is connected to the electric source to generate deflection torque. According to the PMMC instrument’s working theory, when a torque is applied to a moving coil that is positioned inside a permanent magnet field, a precise result for a DC measurement is then obtained.

Working Principle of a PMMC Instrument

When a conductor carrying current is inside a magnetic field, it encounters a force that is perpendicular to both the current and the field. According to the “Fleming left hand” rule, the middle and forefinger of the left hand should be at a 90-degree angle from the thumb. Following that, the forefinger will contain the magnetic field, the middle finger will carry the current, and the thumb finger will carry the force. Once the coil on the aluminum former has current flowing through it, a magnetic field can be produced there that is inversely proportional to the current flow.

The deflection force within the coil is produced by the electromagnetic force that the permanent magnet exerts throughout the fixed magnetic field. The spring then produces the force necessary to counteract further deflection, aiding in the pointer’s balance. So the aluminum core’s movement of the magnetic field can produce damping force within the system. To a certain extent, it keeps the pointer stable. Once it reaches equilibrium, control the torque and deflection to provide accurate measurement.

PMMC Instrument Construction

The moving coils and permanent magnet are the two most important components that can be used to construct the PMCC instrument. Below is a discussion of each component of this instrument.

Moving Coil

It serves as a crucial part of the PMMC instrument. By winding copper coils to a rectangular block in between the magnetic poles, this coil can be designed. It is made of aluminum, and the rectangular block that rotates into the jeweled bearing is known as an aluminum former. As a result, it allows the coil to spin freely. When current is passed through these coils, a deflection in the field results, which can be used to determine the voltage or current magnitude. Aluminum is a non-metallic former that is used to calculate voltage while a metallic former with high electromagnetic damping is used to measure current.

Magnet System

A ‘U’-shaped magnet-based design with two high-intensity magnets makes up the PMMC instrument. For greater superior field intensity & coercive force, Alnico & Alcomax can be used in the design of these magnets. In a variety of configurations, an extra-soft iron cylinder can be placed between the magnetic poles to create an identical field while reducing air resistance and boosting the field’s strength.

Control

Due to the phosphorous bronze springs used in the PMMC device, the torque can be controlled. Between the two jewel bearings are these springs. The spring gives the lead current a path to flow into and out of the moving coil. The delay of the ribbon is primarily responsible for the torque control.

Damping Torque

The aluminum core’s motion within the magnetic field can be used to produce damping torque inside the PMMC instrument. As a result, following the early deflection, the pointer can remain at rest. It aids in accurate, fluctuation-free measurement. Eddy current may be produced inside the aluminum former as a result of the coil’s motion within the magnetic field. This produces the damping force, without which the coil would not be able to move. The pointer’s deflection will eventually stop at a fixed position after gradually decreasing.

Pointer and Scale

The moving coil in this instrument can be used to connect the pointer. It observes the deflection of the moving coil. On the scale, the magnitude of their derivation can be seen. The instrument’s pointer may be made of a lightweight material. As a result, it can be easily deflected by the rotation of the coil. Sometimes the device will experience parallax error, which can easily be reduced by strategically placing the pointer’s blade.

What are the Different Reasons that Cause an Error in PMMC?

Different errors can happen in a PMMC instrument as a result of temperature effects and instrument aging. The magnet, effects of temperature, moving coil, and spring are some of the instrument’s primary components that can lead to errors. Therefore, when the moving coil is used to connect the swamping resistance in series, these errors can be minimized. The resistor in this case with a lower temperature coefficient is what is being referred to as the swamping resistance. The moving coil’s temperature impact can be lessened by this resistance.

Torque Equation

The torque equation is the equation used in the PMCC instrument. The equation below can be used to express the deflecting torque that the movement of the coil causes.

Td = NBLdl

Where,

‘N’ is the no. of turns in the coil

‘B’ is the density of flux within the air gap

‘L’ & ’d’ are vertical as well as horizontal lengths of the surface

‘I’ is the flow of current in the coil

G = NBLd

The spring can be used to provide the moving coil with restoring torque, which has the following formula:

Tc = Kθ (‘K’ is the spring constant)

Final deflection can be done through the equation Tc = Td

Substitute the values of Tc & Td in the above equation, then we can get

Kθ = NBLdl

We know that G = NBLd

Kθ = Gl

θ= Gl/K

I = (K/G) θ

We can infer from the equation above that the deflection torque may be directly proportional to the current flowing through the coil.

Advantages of PMMC Instrument

The advantages are

• High accuracy
• It generates no losses because of hysteresis.
• The scale in the instrument can be divided properly
• It is not influenced by the stray magnetic field.
• It uses less power
• This instrument can measure the voltage & current with different ranges
• It is used as a voltmeter/ammeter with appropriate resistance.
• This instrument uses shelf shielding magnet so it is applicable in aerospace

Disadvantages of PMMC Instrument

The disadvantages are

• It is expensive compare with other alternative instruments
• It works with only DC
• Due to the permanent magnet’s loss of magnetism, an error is displayed.
• It is delicate

Applications of PMMC Instrument

The applications are

• Ammeter
• Voltmeter
• Ohmmeter
• Galvanometer