🔍 Key Concepts

• Interpoles (commutating poles) purpose in a DC generator: to improve commutation by neutralizing armature reaction and inducing a correcting EMF in the shorted coil. • Relationship between direction of rotation, current in the armature, and the polarity of the interpole compared to the next main pole in the direction of rotation. • Difference between generator and motor interpole polarity connections with respect to the main field poles.

💭 Think About

• Look at the direction of rotation shown and imagine which way the armature conductors under an interpole are cutting the magnetic field: what EMF (polarity) is induced in the shorted coil during commutation? • For a generator, should the EMF induced by the interpole in the commutating coil aid or oppose the armature reaction / reactance voltage to get sparkless commutation? How does that requirement fix the interpole polarity? • Compare the magnetic polarity of an interpole with the main pole it is nearest to in the direction of rotation versus the one just behind it. Which relationship gives the needed correcting EMF?

✅ Before You Answer

• Be sure you’re thinking of the machine explicitly as a generator, not a motor; the required interpole polarity relation is different for motors. • Trace the direction of rotation and identify which main pole is the following pole and which is the preceding pole for any given interpole. • Confirm whether the interpole should have the same polarity as the main pole ahead of it in rotation or the main pole behind it, and match that wording carefully to the choices.