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Back emf in a motor
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A spinning motor generates opposing emf.
A spinning motor coil generates an EMF opposing the supply, so I = (V − ε_b)/R. At start-up there's no back EMF, hence large starting current.
12 V motor, R = 1 Ω, back EMF 10 V: 2 A.
If stalled the current is dangerous.
In more detail
A motor turning in a magnetic field produces a back emf that opposes the supply voltage. The net voltage drives the current, which limits how much current flows at high speed.
Real-world uses
- Electric motors slow down under load because back emf drops.
- Motor controllers account for back emf in speed control.
- A motor slows down when a heavy load is removed and its back emf rises, which is an important feature of motor design.
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Frequently asked questions
Why does a motor draw more current when stalled?
Because there is no back emf to oppose the supply, so current is limited only by resistance.
Is back emf a loss?
It is the energy converted into mechanical work, so it is useful.
Why is back emf important?
It limits the current in the motor, so the motor draws only the current needed to produce the required torque.
Related topics in Magnetism & Induction
Transformer losses · AC generator · Dynamo and alternator · LR circuit growth and decay · Maxwell's displacement current · Maxwell's equations