A DC motor draws a high inrush current when starting because the initial back EMF (electromotive force) is zero. Back EMF is generated by the rotating armature and opposes the applied voltage, reducing the net voltage across the armature. At startup, the armature is stationary, so no back EMF is generated, resulting in the full supply voltage being applied across the armature resistance. Since the resistance is typically very low, this leads to a very high initial current according to Ohm’s law.
A DC motor has high inrush current at startup due to the lack of back EMF.
As the motor begins to turn, back EMF is generated, which increases with speed and opposes the applied voltage, thereby reducing the net voltage across the armature and limiting the current.
However, at the moment of startup, when the armature is not yet rotating, the absence of back EMF means the current is limited only by the armature resistance, leading to a significant inrush current.
A DC motor draws high current at the time of starting because the initial resistance seen by the supply is just the armature resistance, which is very low.
With no back EMF to counter the supply voltage initially, the current is only limited by this small resistance.
This high starting current can be several times the motor’s rated current, which is necessary to produce the torque needed to overcome the inertia of the motor and any load attached to it.
When a DC motor is first started, the armature current is so high because the armature is not yet moving, so no back EMF is generated to oppose the applied voltage.
The result is that the entire supply voltage is applied across the armature resistance, which is typically very low. This causes a large current to flow through the armature windings until the motor speeds up and back EMF builds up to reduce the current.
The motor draws more current during starting due to the absence of back EMF and the low armature resistance. At startup, the lack of rotational motion means no back EMF is present to counteract the supply voltage. Consequently, the full voltage is applied across the low armature resistance, leading to a high inrush current.
As the motor accelerates and back EMF increases, the current gradually decreases to its normal running value.