Why do high inrush currents occur in DC motor starting and how is it mitigated in aircraft circuits?

Why do high inrush currents occur in DC motor starting and how is it mitigated in aircraft circuits?

• A. The motor windings look like a high resistance when cold.
• B. Inductive kick from the supply.
• C. The motor windings look like a low resistance when cold, causing a surge; mitigated with soft-start, sequencing, pre-charge circuits.
• D. The motor uses synchronous starting; no surge.

Answer: (C)

When a DC motor starts, there is no back electromotive force yet because the rotor isn’t turning. That means the current is governed only by the winding resistance. Copper resistance is lower when the windings are cold, so the stall (starting) current can be much higher than the running current. As the motor spins up, back EMF develops and opposes the applied voltage, so the current drops. The windings also heat up from the current, which further increases resistance and helps lower the current to the normal running value. In aircraft circuits, this surge can cause voltage dips, tripping of protection, or wear on wiring and contactors. It’s mitigated with soft-start (ramping up voltage/current gradually), sequencing (starting loads in a controlled order to avoid stressing the bus), and pre-charge circuits (bringing large capacitors up to voltage slowly to prevent a big inrush).

When a DC motor starts, there is no back electromotive force yet because the rotor isn’t turning. That means the current is governed only by the winding resistance. Copper resistance is lower when the windings are cold, so the stall (starting) current can be much higher than the running current. As the motor spins up, back EMF develops and opposes the applied voltage, so the current drops. The windings also heat up from the current, which further increases resistance and helps lower the current to the normal running value.

In aircraft circuits, this surge can cause voltage dips, tripping of protection, or wear on wiring and contactors. It’s mitigated with soft-start (ramping up voltage/current gradually), sequencing (starting loads in a controlled order to avoid stressing the bus), and pre-charge circuits (bringing large capacitors up to voltage slowly to prevent a big inrush).