Pole Pieces

A very quick post here on the early pole pieces vs the later, more common ones. This cropped up in the magnets article here.

So, pole pieces, what are they exactly? The pole pieces are the small triangular metal pieces which sit either side of the permanent magnet and shroud the electromeagnets (for the sake of keeping it simple, let’s just say they shroud the motor). You will probably recognise them from the picture below;

ResizeResizepole piece3 (2)


So, why are these important? The pole pieces extend the poles of the permanent magnet around the sides of the motor, so that we have opposing poles on either side of the armature. We can think of the pole pieces as an extension of the magnet. To increase the efficiency of the motor, these pole pieces can be bent inwards our outwards to give a snug fit – the closer these pole pieces hug the motor, the more efficient the motor will be. However, we must be careful not to bend the pole pieces inwards too much, so that the motor rubs against them. Some chassis have base plates or top covers with locating slots into which the tips of the pole pieces locate. This limits the amount of bending you can do with the pole piece.

Older Graham Farish Chassis:

The older chassis had pole pieces which were permanently fixed in place by way of what looks like some form of rivet or squashed lug. The picture below shows this well;

ResizeResizepole piece0.1

There is no way to remove the pole pieces like there is with the later chassis, unless you somehow remove the rivets/lugs with a rotary tool. An easier option is to leave them in place and should you wish to replace the magnet, use a modelling knife and slip the blade down the side of the magnet or at the end and tease it out. This might damage the magnet, but assuming you are replacing it because it has weakened (indeed, the fixed pole pieces were found on the early chassis, which also had the poorer magnets), you might deem this acceptable. If you have a better method then please feel free to share it via the comments form below. Once the magnet has been removed, you can either replace it with a stronger one, or insert neodymium magnets, as described here.

Later Graham Farish Chassis:

Later chassis – actually, they were not that late, since the change happened quite early on – had locating pegs or lugs (for want of a better word) onto which the pole pieces would locate. This can be better understood in the illustration below;

ResizeResizepole piece1.1


As we can see, the pegs are not inhibiting the pole pieces from being removed. A pole piece can simply be removed by finger (slot nail in between pole piece and chassis to remove) or by a modelling knife, using the knife blade to gain some leverage. Being able to remove the pole pieces gives the following advantages;

  1. Enables easier removal of the magnet
  2. Allows for the armature to be removes, should it need cleaning / servicing. This is possible with the fixed pole pieces but requires some force to get the armature out and could potentially lead to damage
  3. Allows for better cleaning of components by being able to dismantle the complete assembly

As always, if you would like to add to this article, or spot any errors, then please use the comment form below.

If you would like a more detailed explanation of how a DC motor works, then a good source of reference can be found here.

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