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*Metadyne* is a cross-field machine. The *cross-field machines* are special DC machines having an additional set of brushes on the direct-axis or d-axis. This arrangement of brushes enables the use of armature MMF to provide most of the excitation and achieve high power gains.

An ordinary DC generator can be converted into a metadyne by providing an additional pair of brushes on the direct-axis or d-axis (see the figure). The brushes lie on the quadrature axis or q-axis are short-circuited and the output of the machine is obtained from the d-axis brushes. The stator consists of a control field winding. A field current $𝐼_{𝑓}$ flows through the control field winding.

When the rotor of the machine is rotating at a constant speed, the MMF of the control field winding (𝐹_{𝑓}) induces an EMF 𝐸_{𝑞} between the q-axis brushes qq’. This induced EMF is given by,

$$\mathrm{𝐸_{𝑞} = 𝐾_{𝑞𝑓}𝐼_{𝑓} … (1)}$$

Where, $𝐾_{𝑞𝑓}$ is a constant.

As the brushes qq’ are short circuited, a q-axis armature current (𝐼_{𝑞}) flows and establishes q-axis MMF (𝐹_{𝑞}). Since the impedance of the short-circuited path is very low, therefore, only a small field current (𝐼_{𝑓}) in the control field winding will produce a much larger q-axis armature current. The corresponding flux density wave will be centred on the q-axis. Due to commutator action, this magnetic field is stationary in space. An EMF is induced in the armature by its rotation in the stationary q-axis flux. This generated EMF appears across the daxis brushes dd’ and is given by,

$$\mathrm{𝐸_{𝑑} = 𝐾_{𝑑𝑞}\:𝐼_{𝑞} … (2)}$$

Where, 𝐾_{𝑑𝑞} is a constant.

Now, if a load of resistance 𝑅_{𝐿} is connected across the d-axis brushes, the daxis armature current (𝐼_{𝑑}) will flow through the load. This current produces d-axis MMF (𝐹_{𝑑}). *According to the Lenz’s law,* the d-axis MMF (𝐹_{𝑑}) opposes its cause of production, that is the control field MMF (𝐹_{𝑓}).

Each stage of the generation of voltage produces a current whose magnetic field is 90° ahead of the magnetic flux wave producing the voltage. As there are two stages of the voltage generation, thus the MMF of the d-axis output current is shifted by 90° two times, i.e., 180° and hence opposes the control field MMF (𝐹_{𝑓}). Therefore, the q-axis generated EMF becomes,

$$\mathrm{𝐸_{𝑞} = 𝐾_{𝑞𝑓}\:𝐼_{𝑓} − 𝐾_{𝑞𝑑}𝐼_{𝑑} … (3)}$$

If the magnetic saturation is neglected and the speed of the machine is assumed to be constant, then 𝐾_{𝑞𝑑} is a constant.

For a given control field MMF (𝐹_{𝑓}) and load resistance, the steady state values of 𝐼_{𝑑} and 𝐼_{𝑞} are reached.

From Eqn. (3), it can be seen that any increase in current 𝐼_{𝑑} decreases the value of EMF 𝐸_{𝑑}. This in turn reduces the current 𝐼_{𝑑}. Therefore, 𝐸_{𝑑} and 𝐼_{𝑑} are decreased. Hence for a given value of control field excitation current (𝐼_{𝑓}), the d-axis output current (𝐼_{𝑑}) remains constant over a wide range of load variation. Thus, the above discussion shows that a *metadyne behaves as a constant current generator*.

Metadynes were mainly used in the following applications −

To supply DC power to process control motors

To supply the excitation systems of large AC generators

In traction systems and Ward-Leonard speed control systems, etc.

At present, metadynes are not manufactured and are replaced by *solid state power amplifiers*.

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