I've come across one thing about energy conversions: "To make an energy conversion possible, it is important that the supplied form of energy must be opposed by another force, otherwise this energy cannot be converted." Agreed! Once the motor starts generating torque, a back emf is produced in it, which in accordance with the Lenz law opposes the supplied voltage. So a supply of voltage must be maintained so that the motor keeps running. But even otherwise, just in case there were no back emf, supply voltage "has" to be there to keep the current flowing through the armature and thus rotate it in the presence of a magnetic field. So how does back emf, in this case, make the energy conversion possible, when supply voltage is anyhow a necessity, whether the back emf is generated or not?