What is DC breaking?
AC Motor — DC Injection vs Dynamic Braking (Live Demo)
DC Injection → energy turns into heat *inside the motor*; bus stays flat. Dynamic Braking → energy flows to the DC bus, bus rises, chopper dumps it in the brake resistor.
What is DC braking in AC motor drives?
DC braking is used to stop a motor. Two main types exist: DC injection and dynamic braking (brake resistor). In DC injection, the drive applies direct current to the stator, creating a stationary magnetic field; as the rotor keeps spinning through that field, counter torque is produced, so the motor stops quickly—energy is dissipated as heat inside the motor. In dynamic braking, the load’s kinetic energy flows into the drive’s DC bus and raises the bus voltage; when a threshold is reached, the chopper circuit switches the brake resistor on and the energy is converted to heat in the resistor, keeping the bus under control.
Notes from the field:
+ DC injection is simple—overdo it and you just heat the windings. Short pulse is enough.
+ With dynamic braking, it’s all about DC bus voltage. If the bus swells, the drive trips. Wrong resistor sizing burns the resistor or the drive gives up.
+ “There’s already a resistor” isn’t a plan; if you don’t do the energy math, smoke is only a matter of time.
Expert tips:
+ Light load, short stop → DC injection.
+ Heavy load, high energy → brake resistor or *regenerative unit*.
+ Don’t crank parameters blindly; duration and level hit motor lifetime.
+ Frequent braking? Consider a *regenerative unit*. Healthier bus, no wasted energy.
Common mistakes:
+ Setting injection time to 10–15 s and cooking the motor.
+ Choosing the resistor only by “ohms,” skipping the energy calculation.
+ Boxed the resistor with no airflow, then wondering about the burnt smell.
+ Ignoring rising bus voltage and forcing parameters until the drive trips.
+ Enabling injection and dynamic braking together, randomly, on the same machine.
Correct application example:
A textile machine with a 5 kW motor stops/starts hundreds of times per shift. They tried DC injection but the motor overheated. Fix: a correctly sized brake resistor and proper parameters. Result: cool motor, controlled bus, clean operation. Read the load, then choose injection or resistor.
Wrong application example:
A workshop saw got a new drive. Operator set DC injection to 12 s and DC level to 100%. Days later the motor ran blazing hot; insulation swelled; motor and drive died with a burnt smell. A short injection or a small brake resistor would have done it. “More is better” is the biggest mistake here.
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