t zero field current , the no load back emf in the DC machine is not equal zero due to :*field windingfield currentresidual magnetismCommutator

The no-load back electromotive force (EMF) in a DC machine at zero field current is not equal to zero due to residual magnetism. Here's why:

1. Residual Magnetism: Even when the field current is zero, there is still some amount of magnetism left in the iron core of the machine. This is known as residual magnetism.

2. Generation of EMF: When the armature of the DC machine rotates, it cuts through the magnetic field lines produced by the residual magnetism. According to Faraday's law of electromagnetic induction, this change in magnetic field induces an electromotive force (EMF) in the armature.

3. Back EMF: This induced EMF opposes the cause producing it, which in this case is the rotation of the armature. Hence, it is known as back EMF.

4. Non-Zero Back EMF: Therefore, even when the field current is zero, the back EMF is not zero due to the presence of residual magnetism.

The other options - field winding, field current, and commutator - are not the reasons for the non-zero back EMF at zero field current. The field winding and field current are responsible for creating the magnetic field, but they are not the reason for the back EMF when the field current is zero. The commutator is a mechanical switch that changes the direction of current in the armature, but it does not contribute to the generation of back EMF.