Final answer:
When power is first applied to a motor, the amount of current flow through the armature is limited by the absence of back emf. As the motor starts turning, the back emf grows, reducing the voltage across the coil and the amount of current it draws. The amount of current flow depends on the motor's angular velocity and the presence or absence of mechanical load.
Step-by-step explanation:
When power is first applied to the motor, the amount of current flow through the armature is limited by the absence of back emf. Back emf is the generator output of a motor and it is proportional to the motor's angular velocity. Initially, the motor is not turning and there is no back emf, so the coil receives the full driving voltage and the motor draws maximum current. As the motor starts turning and its angular velocity increases, the back emf grows, opposing the driving emf and reducing both the voltage across the coil and the amount of current it draws.
This effect is noticeable in many common situations. When a motor first comes on, it draws more current than when it runs at its normal operating speed. When a mechanical load is placed on the motor, the motor slows down, the back emf drops, and more current flows. If the motor runs at too low a speed, the larger current can overheat it and even burn it out. On the other hand, if there is no mechanical load on the motor, it will increase its angular velocity until the back emf is nearly equal to the driving emf, and then it will use only enough energy to overcome friction.