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EEE Components, PASSIVES

Solenoids and Transformers

  • Posted by doEEEt Media Group
  • On November 2, 2022
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Transformers

What is a transformer?

A transformer is a device that transfers electric energy from one alternating-current circuit to one or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage. A transformer consists of at least two windings, with the winding turning NP on the primary side and NS on the secondary side.

For the sake of simplicity, we will look at an ideal transformer with a turn ratio of 1: 1.

In the first step, we will look at a transformer with an open secondary winding NS (Figure 1.). A UP voltage pulse is created at winding NP. Due to the inductance of the winding, this pulse generates a linearly progressive current IP.

The following applies:

transformer voltage pulse eq. [1]

transformer voltage pulse eq. [1]

Winding NS also wraps around this magnetic flux. Changing the magnetic flux creates voltage.

nduced voltage in transformer by magnetic flux [2]

induced voltage in the transformer by magnetic flux [2]

Figure 1. Principle of a transformer with zero load. This ideal transformer is wound as a bifilar so as to ignore parasitic effects.

Figure 1. Principle of a transformer with zero loads. This ideal transformer is wound as a bifilar to ignore parasitic effects.

If you solve both equations after changing the voltage and then equating them, you will get the following for the voltage transformation expressed by eq. [3].

voltage transformation eq. [3]

voltage transformation eq. [3]

Current does not flow in winding NS because the winding is open. If we now connect winding NS to a load resistor RL (Figure 2.), the voltage induced in NS generates a current flow through the load resistor:

current through transformer eq. [4]

current through transformer eq. [4]

Figure 2. The same transformer as in Figure 1. but with a load

Figure 2. The same transformer as in Figure 1. but with a load

The primary current now consists of the transformed secondary current and the linearly progressive magnetization current already available without load.

transformer primary current eq. [5]

transformer primary current eq. [5]

IS* Secondary current transformed on the primary side

As no power can be generated, the transforming power is the same as the primary power put into the system. If the magnetization current is disregarded, the following applies:

magnetisation current

Currents are thus in the reversed direction as voltage. The following also applies:

Resistances are thus transformed with the transformation ratio squared. This also applies to inductances, capacitances, and impedances. So the magnetizing current is not transferred to the secondary side. It is required to generate the magnetic field. The aim of the transformer design must therefore be to keep the magnetizing current as small as possible.

There are two possibilities here:

  • Insertion of a highly absorbent core to increase the primary inductance. This causes the magnetizing current to rise less steeply and is, therefore, smaller (Figure 3.).
Figure 3. Magnetizing current of a transformer with and without a highly permeable core

Figure 3. Magnetizing current of a transformer with and without a highly permeable core

  • Shorter voltage pulses with higher frequencies are generated as the current rise stops at the voltage pulse’s end and starts again at the original point for the next pulse (Figure 4.).
Figure 4. Magnetizing current for a transformer at different driving frequencies

Figure 4. Magnetizing current for a transformer at different driving frequencies

Parasitic effects

In reality, other factors affect the behavior of transformers. The most important ones are:

  • Leakage inductance
  • Coupling capacitance (capacitance between windings)
  • Winding capacitance (capacitance within a winding)
Figure 5. Long solenoid concentric coil

Figure 5. Long solenoid concentric coil

Leakage inductance

Looking at two windings, we see that the entire flux is not coupled to the other winding. A part of the magnetic field’s streamlines closes outside the other winding. This part of the inductance is called “leakage inductance.” To minimize leakage inductance, you must know the parameters that influence it.

If you look at a long concentric coil (Figure 5.), its inductance results from:

concentric coil inductance eq. [5]

concentric coil inductance eq. [5]

lW Length of the coil
N Winding turns
A Cross section of the coil

If a second winding is wound on top of it (Figure 6.), the leakage inductance results from eq. [6]

leakage inductance on second winding [6]

leakage inductance on second winding [6]

Figure 6. Long solenoid concentric coil with second winding

Figure 6. Long solenoid concentric coil with second winding

Where A here is the surface between the two windings. It can be calculated using:

surface between two windings eq. [7]

the surface between two windings eq. [7]

MLT Mean length of turn
Hins Distance between the windings (isolation)
H1, H2 Winding height of windings 1 and 2

Figure 7. Different transformer winding structures

Figure 7. Different transformer winding structures

Coupling Capacitance

You can picture the coupling capacitance between the two windings as a plate capacitor between the two windings. From this, you can reduce this capacitance by increasing the distance or reducing the surface. Both directly increase leakage inductance.

Winding Capacitance

Each winding builds up winding capacitance because they are isolated from each other and rest on different potentials. This capacitance increases with the number of layers required within a winding. It can be reduced using various winding technologies, e.g., Z-wind (wire is returned after each layer).

Solenoids

Derived from two Greek words: Solen (pipe) and Eidos (coil), the solenoid is a type of electromagnetic device that converts electrical energy into mechanical energy. It is generally made by tightly wounding wires in a helix shape around a metal. Whenever an electric current passes through it, a magnetic field is created.

Working Principle

A solenoid works on electromagnetism and electromagnetic force. It consists of a round cylindrical coil with several wires turns and a metal rod inside the coil that is free to move. When an electric current is provided to the coil, a magnetic field is generated, due to which the metal core or rod inside the coil gets attracted due to the direction where the magnetic flux is high. This electromagnetic effect in a solenoid enables any connected plunger or armature to move as per our needs.

To increase the magnetic force produced in a solenoid coil, we will have to increase the number of turns, N, and the current, I.

Types Of Solenoid

AC Laminated Solenoid

It has a very high initial attracting force and a short closing time. It is made with laminated metal or insulated thin sheets that are individually, assembled.

DC-C Frame Solenoid

As its name states, this solenoid is constructed with a letter ‘C’ like frame cover around the coil. This type is widely used in gaming machines.

DC-C Frame Solenoid

DC-C Frame Solenoid

DC-D Frame Solenoid

As its name says, this solenoid has a coil covered by two ‘D’ frames on two sides. These types are generally used in AC power applications.

DC-D Frame Solenoid

DC-D Frame Solenoid

Linear Solenoid

This solenoid type has a freely movable steel or iron rod called a plunger inside a round cylindrical-shaped coil. Depending on the current applied, the iron rod can freely move in or out of the cylindrical coil.

Linear Solenoid

Linear Solenoid

Rotary Solenoid

It is a special type of solenoid where the magnetic force is converted into a rotational force or a rotary motion. It consists of an armature core mounted on a flat disk.
When a current is provided, the armature gets attracted toward the stator, and the flat disk rotates.

Applications

Solenoid Valve

Solenoid Valve

Solenoid Valve

The solenoid valve is a simple device in which a solenoid is used to control and regulate fluid flow. It has a coil with a free movable plunger or an iron rod with a spring inside. When we energize the coil, the plunger moves from its position due to magnetic attraction, and when we cut the power to the coil, the plunger returns to its original position with the help of a spring. As soon as the plunger comes in the path of the flowing fluid, its flow stops.

Solenoid Lock

Solenoid Lock

Solenoid Lock

Here we use the movement of the solenoid plunger for the locking and unlocking mechanism. These solenoid locks are widely used in electronic and biometric password-based locks. It consists of a strong metal plunger that can move. When the coil gets magnetized due to an electric field, the plunger moves to perform the lock and unlock mechanism.

The leakage inductance is thus independent of core material and air gap. To minimize leakage inductance, you must either increase the length of the coil (broad windings) or reduce the distance between the windings (e.g., bifilar wind).

Figure 7. shows various more or less ideal winding constructions. With existing geometry, the most commonly used means is a sandwich construction (Figure 7.d), in which the secondary winding is wound between the primary winding divided into two halves. This doubles the length of the winding.

Resources: EPCI Blog

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