Back-EMF explained
Why a spinning motor fights the voltage that is driving it — and why that matters for control.
- Explain back-emf explained at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive unit
Overview
As soon as a motor's rotor starts turning, it generates its own voltage that opposes the supply — this is back-EMF, and it governs almost everything about how a motor behaves at speed.
In depth
Back-EMF is proportional to rotor speed, scaled by the motor's back-EMF constant, which itself depends on magnet flux and winding turns.
Why current falls as speed rises
The current driving torque is proportional to the difference between the applied voltage and the back-EMF, divided by winding resistance and inductance effects. As speed rises, back-EMF climbs toward the DC-link voltage, squeezing the available voltage margin and therefore the current — and the torque — that can be delivered.
If an inverter loses control of a spinning permanent-magnet motor at high speed, its uncontrolled back-EMF can exceed the DC bus rating — a real fault mode engineers must design safety systems around.
Key takeaways
- Back-EMF is proportional to rotor speed and opposes the drive voltage.
- Rising back-EMF is what limits current and torque as speed increases.
- Base speed is defined by back-EMF approaching the available bus voltage.
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