Electronics > Power/Renewable Energy/EV's

Brushed DC motor: Resistor in series with freewheeling diode

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Nominal Animal:
As I understand it, if one wanted to reduce the braking effect when power to an unidirectional brushed DC motor is cut off and a freewheeling diode is used, one could add a power resistor in series with the freewheeling diode, to bleed off the energy when the motor is acting as a generator.

Omitting the freewheeling diode means there is no braking effect at all, but then there is no clamping on the (negative) voltages generated either.

Similarly, on a stepper motor, one could have additional MOSFETs shorting each winding through a power resistor, as a sort of a "half-brake", when needed.  This might be useful when the motor is connected to a hand crank, as a "tactile" feedback for example.  (Short-circuiting the windings directly is usually a bit too strong, at least for the steppers I have.)

Have you ever seen this used in real life?  Aside from voltage and power dissipation requirements, are there any pitfalls here?

It's a silly question, I know; sorry.  :)

Siwastaja:
You have completely misunderstood.

Freewheeling diode does not brake. It creates forward torque for the small time during every PWM cycle, by allowing the current to flow in the same direction it was already flowing when the MOSFET was turned off, utilizing the energy stored in the inductance of the motor to drive the motor as intended instead of generating a massive voltage spike.

Braking happens when you actively drive the second switch you don't have (as you use a diode, which is automatic, and never turns on to enable braking), boosting the motor BEMF voltage, and have a load on the input side which can consume this generated current, like a battery.

Braking also happens when the motor is turning faster than the input voltage would be capable of turning it. Then BEMF > Vin,  and current flows through the freewheeling diode, rising the input voltage. In order to current to flow, you need a load (think about a zener diode or active equivalent) that turns on during input high voltage condition; or a battery which would get charged.

Because information about motor control online is crap and mostly bullshit, this motor circuit is best analysed as a buck converter, motor being both the inductor, and the (very large) output capacitor which voltage is ~ RPM. This way you get the correct understanding.

T3sl4co1l:
bEMF is in the same direction as applied voltage, it looks like a capacitor (namely, inertia), not an inductor (the winding inductance appears in series with that, however).  So yeah, no problem.  Probable "duh" moment, that's okay, I think I wondered the same thing years ago too. :)

Tim

Nominal Animal:
Dammit, I'm an idiot. :palm:

Why didn't I check this first?

Oh well, not the first time I expose my true stupidity to the world.... :-DD

langwadt:

--- Quote from: Siwastaja on August 21, 2021, 09:33:02 am ---Because information about motor control online is crap and mostly bullshit, this motor circuit is best analysed as a buck converter, motor being both the inductor, and the (very large) output capacitor which voltage is ~ RPM. This way you get the correct understanding.

--- End quote ---

and a (sync)buck converter in reverse is a boost converter, so depending on how it is switching the motor voltage can be boosted
and backfeed into the supply

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