Mon, 11 April 2011
The current drawn by a simple motor may vary in time depending on what the motor does at any given instant.
1. When a motor is connected to a battery, the current in the coil (or armature) increases dramatically but as soon as the coil starts rotating increasingly faster, the current drops and settles at a certain value. This is because of the so-called "back emf" induced in the coil as it rotates in a magnetic field (Faraday's law). The back emf has such polarity that it opposes the emf of the cell (Lenz's law).
2. When the motor spins and there is no load attached, the current stays constant. (The value of the current is I = (emf_cell - back_emf)/R where R is the total resistance of the circuit, including any internal resistance of the battery)
3. When the motor does work on an external load and it slows down, the back emf drops as its value depends on how fast the armature spins. As a result, the current through the motor increases up to a value that depends on how "hard" the motor works.