SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/eng-motor-back-emf-startup

When does a simple DC motor connected directly to fixed voltage draw the most current?

At startup, before it spins, because there is no back emf yet.

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  1. At full speed with no load
  2. When it is switched on but not yet turning ✔
  3. Just as it is switched off

Why: A spinning motor also acts as a generator and makes a back emf that opposes the supply. At standstill the back emf is zero, so only the winding's small resistance limits the current.

Huh, didn't know that: A refrigerator motor can briefly dim lights when it starts because its initial current is large.

The dad joke
My motor is just like me in the morning. Starting up takes the most current.
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References

Answer: When it is switched on but not yet turning. A spinning motor also acts as a generator and makes a back emf that opposes the supply. At standstill the back emf is zero, so only the winding's small resistance limits the current.

  1. OpenStax College Physics 2e, 23.6 Back Emf openstax.org/books/college-physics-2e/pages/23-6-back-emf
    the motor draws maximum current when it is on but not turning.
    Checked 2026-10-10.

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