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The Yoke in a DC machine

The function of a Yoke in a DC machine is twofold.  The yoke acts as path for the magnetic flux.  It also provides mechanical support and shape to the dc machine.

As it is a path for the magnetic flux, the yoke should be made only of a magnetic material. 

The Yoke is usually made of cast iron, forged steel or cast steel.  In certain large machines, the yoke is made of two pieces which are bolted together.