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What is Back EMF?

An electromotive force produced by electromagnetic induction that acts against the change of current or the rotation that caused it. Abruptly interrupting the current in a coil produces a high voltage V = L di/dt (about 1000 V when 1 A is cut in 1 µs in a 1 mH coil), which can destroy the switch, so a diode or snubber circuit diverts it. A motor generates a back EMF proportional to its speed, and in a DC motor the speed follows from applied voltage = back EMF + voltage drop across the winding resistance.

Japanese name: 逆起電力

Back EMF appears in 3 practice questions across 2 topics, including Power Electronics Circuits, PN junctions and diodes.

Field: Power Electronics Circuits

Try it: questions about Back EMF

A voltage of 24 V is applied to a DC motor with an armature resistance of 0.5 Ω and a back-EMF constant of 0.1 V/(rad/s), and the armature current is 8 A. Which of the following is the angular speed of rotation at this point?

  1. A40 rad/s
  2. B200 rad/s
  3. C240 rad/s
Show answer and explanation

Answer: B. 200 rad/s

In a DC motor the applied voltage equals the voltage drop across the armature resistance plus the back EMF (V = R × I + E). The back EMF is E = 24 − 0.5 × 8 = 20 V, and from E = K_e × ω, ω = 20/0.1 = 200 rad/s (about 1910 min⁻¹). In SI units the torque constant has the same value as the back-EMF constant, so the torque is 0.1 × 8 = 0.8 N·m. When the load and therefore the current increase, the resistive drop increases and the back EMF decreases, so the motor slows slightly.

Back EMF in the explanations

Questions that mention Back EMF (3)

  1. A voltage of 24 V is applied to a DC motor with an armature resistance of 0.5 Ω and a back-EMF constant of 0.1 V/(rad/s), and the armature current is 8 A. Which of the following is the angular speed of rotation at this point?
  2. What is the purpose of a diode (flyback diode) connected in parallel with an inductive load such as a relay coil?
  3. Which of the following correctly describes a brushless DC motor?

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Related terms

Zener diode Inverter Hall-effect sensor Boost converter Buck converter Buck-boost converter Continuous conduction mode Electrical transformer Induction motor Power factor

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