Torsen Differential, How it works ?

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Torsen is a trade mark of the JTEKT Corporation. The Torsen differential has many patented components and, is the most unique and ingenious method of providing differential action while overcoming the traction difference problem. This article gives a logical introduction to the working of Torsen differential.

A detailed webpage version of the video is given below.

The internal components

The internal components of a Torsen are quite different from that of a conventional differential. An exploded view of the Torsen is given in Fig.1.

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Fig.1 An exploded view of Torsen differential

At the heart of the system lies a specially shaped gear pair assembly. Let’s see the cross sectional shape of these gears at the mating point. As can be seen, one gear is a spur gear, and the other one is a worm gear.
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Fig.2 A worm gear-worm wheel mesh lies at the heart of the Toresn; Cross sectional shape of the figure is shown in the second part

A Torsen works on the simple principle of worm gear- worm wheel; that is a spinning worm gear can rotate the wheel, but the rotating wheel cannot spin the worm gear.
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Fig.3 The worm gear- worm wheel principle lies at the heart of the Torsen operation

Throughout this discussion, just keep this principle in mind. A pair of such worm wheels are fitted with the case, so the engine power received by the case is transferred to the worm wheels. Each end of the wheels is fitted with a spur gear. As a result, a simplified Torsen differential will look as shown in the Fig.4.
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Fig.4 The complete Torsen differential

Now we will go through different driving scenarios and understand how the Torsen manages to operate the vehicle well.

The Vehicle Moves Straight

When the vehicle moves straight, the worm wheels will push and turn the worm gears. So both the drive wheels will rotate at the same speed. Please note here that, in this condition the worm wheels do not spin on its own axis. In this condition, the whole mechanism moves as a single solid unit.

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Fig.5 When the vehicle moves straight, worm wheels just push and turn the worm gears at the same speeds.

The vehicle takes a right turn

When the vehicle is negotiating a right turn, the left wheel needs to rotate at a higher speed than the right wheel. This fact is clear from the Fig.6.

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Fig.6 During a right turn the left wheel has to travel more distance

This speed differential is perfectly supported in a Torsen. Please note that the worm wheel is subjected to relative motion not the absolute motion. The worm wheel is fitted between the case and worm gear, so the relative motion between the case and worm gear is what makes the worm gear turn.

The worm gear of the faster left axle will make the corresponding worm wheel spin on its own axis. On the other side, relative to the case the slow right axle is turning in the opposite direction; thus the right worm wheel will spin in the opposite direction. The meshing spur gears at the ends of worm wheel will make sure that, the worm wheels are spinning at the same speed. Thus it guarantees a perfect differential action. Perfect differential action implies equal amount of speed loss and speed gain to the right and left wheels. With the perfect differential the vehiclce will be able to negotiate a smooth turn.

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Fig.7 The right worm wheel will spin opposite to the right worm wheel; this is due to the opposite relative motion left worm wheel is experiencing

While taking a left turn the worm wheels will spin in an exact opposite way to that shown in Fig.7.

Overcoming the Traction difference problem

Now let’s try to understand how the Torsen overcomes the drive wheel traction difference problem. As you might be aware, when your vehicle encounters a situation as shown, the slippery wheel starts to spin very rapidly and will draw the majority of the engine’s power. As a result, the vehicle will get stuck.

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Fig.8 A typical traction difference problem a vehicle is experiencing

But, if a Torsen differential is used in this case, as soon as the slippery wheel starts to spin excessively, the speed change will be transferred to the corresponding worm wheel. The right worm wheel transfers the speed change to the left worm wheel, since they are connected through spur gears. Here comes the tricky part! The left side worm wheel will not be able to turn the corresponding worm gear, because, as we said, a worm wheel cannot drive a worm gear! As a result, the whole mechanism gets locked, and the left and right wheels turn together.
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Fig.9 The excessive speed of slipping wheel make the system locked due to the 'basic principle of worm gear-worm wheel'

This allows a large amount of power to be transferred to the high-traction wheel, and the vehicle can thereby overcome the traction difference problem. To carry the load 2 more worm wheel pairs are added.

Pros and Cons of Torsen

If you are familiar with the other common technologies used to overcome the traction difference problem, you might have noticed a great advantage of the Torsen. While the other technologies allow the drive wheel to slip for a limited amount of time before it gets locked, in Torsen the locking action is instantaneous. That means as soon as the vehicle encounters a traction difference track the wheels will get locked. They are also compact compared to their counter parts.

Following are the some disadvantages of the Toresn type (T1) explained here.

  • Noisy
  • Costly
  • More difficult to assemble

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How does a Transformer work ?

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Transformers are capable of receiving AC power at one voltage and delivering it at another voltage. In this article, we will go through the working and construction of a 3 phase transformer by starting from its simplest form. We will also understand what is power transformer and how it is constructed.

A detailed webpage version of the video is given below.

Why Transformers are used ?

Transformers are ubiquitous devices. They are used to either step-up the A.C voltage or to step-down it. But, why should we do this voltage transformation ?. It is a science fact that a stepped-up voltage is associated with a reduced current. A reduced current leads to low eddy current energy loss. In this way, transformers help achieve better transmission efficiency while transferring the power over longer distances.

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Fig.1 Transformers help in step-up or step-down the voltage; this in turn increases the transmission efficiency

After the electrical power has transmitted to the desired spot, the voltage can be reduced to the desired level, using a step-down transformer.

The Basic Working Principle

The basic working principle of a transformer is simple, electromagnetic induction. According to this principle, a varying magnetic flux associated with a loop will induce an electromotive force across it. Such a fluctuating magnetic field can easily be produced by a coil and an alternating E.M.F (EP) system. A current carrying conductor produces a magnetic field around it. The magnetic field produced by a coil will be as shown in the first part of Fig.2. With the fluctuating nature of the alternating current, the magnetic field associated with the coil will also fluctuate.

This magnetic flux can be effectively linked to a secondary winding with the help of a core made up of a ferromagnetic material. The linked magnetic flux is shown in the second part of Fig.2. This fluctuating magnetic field will induce an E.M.F in the secondary coils due to electromagnetic induction. The induced E.M.F is denoted by ES.

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Fig.2 AC current in a coil produces a fluctuating magnetic field; this magnetic field can effectively linked to a secondary coil with the help of a core

Since the turns are arranged in a series, the net E.M.F induced across the winding will be sum of the individual E.M.Fs (eS) induced in each turn. Ns represents, number of turns at the secondary winding.
Since the same magnetic flux is passing through the primary and secondary coils, the EMF per turn for both the primary and secondary coils will be the same.
The E.M.F per turn for the primary coil is related to the applied input voltage as shown.
By rearraging the above equations, it can be established that, the induced E.M.F at the secondary coil is expressed as follows.
This simply means that with fewer turns in the secondary than in primary, one can lower the voltage. Such transformers are known as step-down transformers. For the reverse case, one can increase the voltage (step-up transformer).

But since energy is conserved, the primary and secondary currents have to obey the following relationship.

3 Phase Transformer

Three phase transformers use 3 such single-phase transformers, as shown in the figure below.

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Fig.3 A 3 phase transformer can be considerred as three independent single phase transformers

It is clear from Fig.3 that, independent 3 phase transformer will require a huge amount of core material and results in a bulky design. As a result practical 3 phase transformers use a slightly different coil configuration. To make it more economical the design illustrated in Fig.4 is used. Here, the primary and secondary coils sit concentrically. Three such concentric pairs are used in 3 phase transformer.
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Fig.4 HV and LV windings are placed concentrically in 3 phase transformers

The concentric windings are made to sit on three transformer core limbs as shown in the Fig.9. We will learn more about the core constriction in the coming sessions.

Power Transformer - Construction Features

The transformers which are used in high voltage applications are referred as 'Power Transformers'. They handle voltage in the range of 33 to 400 kV. The winding of a power transformer is quite different from that of a low voltage transformer (Distribution Transformer). We will explore the construction and connection details of the power transformer winding in this session.

Winding type

The power transformers generally employ a special kind of winding, known as a disc-type winding, where separate disc windings are connected in series , through outer and inner cross-overs.

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Fig.5 The separated out disks are shown in the first part of the figure; The way discs are connected together is shown in the 2nd and 3rd part of the figure.

The first part of Fig.5 shows the separated out discs. In the second and third part of the figure, the inner and outer cross-overs are shown.

Winding Connection

The low-voltage windings of a power transformer are connected in a delta configuration and the high-voltage windings are connected in a star configuration. The winding connections are shown in the Fig. 6 and Fig.7 respectively.

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Fig.6 The low voltage winding is connected in a Delta configuration

The delta connection in low voltage windings result in 3 terminals to connect the electrical power. This is marked as 'R','Y' and 'B' in the Fig.6.
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Fig.7 The high voltage windings are connected in a Star configuration

On the contrary, the star connection in high voltage transformer results in 4 terminals to connect the electric power.This is marked as 'r','y','b' and 'n' in the Fig.7. Thus, if you tap the electrical power between any pair of the phase wires the voltage further rises to root 3 times. This voltage is known as 'line voltage'. This also means that, from a 3 phase step-up transformer we can draw 4 output wires; 3 phase power wires and one neutral. If you draw power between a neutral and phase wire, that is know as 'phase voltage'.

High voltage insulated bushings are required to bring out the electrical energy. It is clear from the Fig.8 that, the bushings at the high voltage side are quite bigger compared to the low voltage bushings.

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Fig.8 Insulated bushings are required for smooth transfer of electrical power

The Core Construction

The core of the transformer is made of thin, insulted, steel laminations. Such steel laminations are stacked together, as shown in the Fig.9, to form 3 phase limbs. The purpose of thin laminations is to reduce energy loss due to eddy current formation. Pleas note here that, the separated out layer blocks in the first part of Fig.9 is a stacked layer of much thinner steel laminations. The thickness of each steel laminations varies from 0.25 - 0.5 mm.

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Fig.9 The core is made of thin insulated steel laminations; Such laminations are stacked together to form 3 phase limbs

The low voltage windings usually sit near the core. If HV windings were placed near to the cored, due to the winding's high voltage, a huge amount of insulation material would be required between the winding and core. Thus by placing the LV winding near to the core, we can save a good amount of insulation material.

The output voltage of a transformer will undergo minor fluctuations due to the reasons like load variation and change in power input supply. A tapping mechanism in the secondary coil helps in regulating the output voltage to the specified limit. The tapping mechanism simply changes the number of active coils in the transformer action, thus controls the output voltage. Since more number of turns are there in the HV windings, voltage fine tuning can be more accurately controlled by providing the tapping on the HV side. This is another reason why HV windings are not placed near to the core. If they were placed near to the core, movement of tapping mechanism would have been more difficult, causing the tapping design more complex.

Energy losses in a Transformer

Various kinds of energy loss happen while transferring power from the primary to secondary coil. Following are the major source of energy losses.

  • Eddy current loss
  • Hysteresis loss
  • I2R loss
All these energy loss are dissipated as heat, so a proper cooling mechanism is necessary to keep the core and winding temperature of the transformer below a specified limit.
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Fig.10 Coolant oil circulation in the transformer is depicted in this figure

Usually the transformer is immersed in a cooling oil to dissipate the heat. The oil dissipates the heat via natural convection. It is clear from the Fig. 10 that, hot oil at the bottom of the tank rises to the top by natural convection (Buoyancy Force). This hot fluid is passed in to the fins, which are fitted outside of the transformer, via fin top pipe. The oil liberates heat when it passes through the fins and it gets cooled down. The low temperature oil naturally sinks to the bottom and enters the transformer through fin bottom pipe. Thus a circular motion of the oil is created in the transformer.

Use of the Conservator Tank

Oil in the tank will expand as it absorbs the heat. A conservator tank helps to accommodate for this volume change. As can been seen in Fig.11, there is a free space above the oil, in the conservator tank. When the oil expands, this space shrinks and accommodate for the volume rise.

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Fig.11 The conservator tank, which is fitted on the top of the transformer helps to accommadate for the volume change of the cooling oil

To know more on different types of transformer cores and windings please check the other articles.

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