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I have a 4-quadrant current controller which regulates the braking current of a brushed DC motor with a high-inertia load to a constant level, but the motor winding cannot handle the current required to halt the load in the time I want.

I thought I could dissipate the energy elsewhere by regulating the braking current and letting it drain back into the supply where a DC link regulator is, but I was wrong: the only thing I would be doing, would be to charge up the inductor, and discharge it into the DC link regulator.

I have the feeling now that I can't decrease the power the winding itself dissipates for a given mechanical braking power, since I can clearly see the mechanical breaking power is the back-EMF (constant at a given time) times the current (constant because regulated at the desired braking torque), regardless of what I do with the current.

Could anyone confirm or suggest how I could achieve what I want electrically (except brake over a longer time)?

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    \$\begingroup\$ If your motor can't do it then you're looking for a friction or magnetic brake, I suspect. \$\endgroup\$
    – Transistor
    Nov 19 '20 at 21:00
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    \$\begingroup\$ The shaft torque and the winding current are related. Just as you cannot accelerate an inertial load too quickly without overloading the windings, you cannot decelerate that load too quickly without overloading the windings. As you have noted, you can decrease the current by braking over a longer time. If your windings are overheating, there is not much else you can do. \$\endgroup\$ Nov 19 '20 at 21:04
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    \$\begingroup\$ You can't change the current, and you can't change the winding resistance, and the power is the current times the resistance squared, so... \$\endgroup\$
    – user253751
    Nov 19 '20 at 21:07
  • \$\begingroup\$ Perhaps you can blow a fan on it or something? \$\endgroup\$
    – user253751
    Nov 19 '20 at 21:07
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    \$\begingroup\$ You CAN reduce power in the winding by dissipating power in a braking resistor. However this inevitably reduces the braking current (compared to shorting the winding); braking torque is thus also reduced and braking time is increased. \$\endgroup\$ Nov 19 '20 at 21:08
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The motor heating goes as the winding current squared, which goes as the braking torque squared. So your motor heating will only allow a certain braking torque. If you only want to brake for a short time, you might allow the motor temperature to rise to absorb rather than dissipate the heat, subject to long enough being allowed between braking events to allow the motor to cool.

The motor voltage will depend on speed, so when braking from high speed you would be able to use an external load to dissipate some of the braking energy. At some lowish speed, the external load would need to be zero to allow enough braking current to flow. Below that speed you would have to actively drive a braking current into the motor.

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Obviously, the current of the electrical machine has to be in the acceptable range. No matter if this machine works in a motoring mode or in generator mode. Ideally the same time is needed for accelerating the mass of inertia and for stopping it. In a practical world it's much different, since there is also a friction and load, so the decelerating time is shorter than acceleration at the same current.

As for your question, you probably have some wrong interpretation. 4-quadrant means that electrical energy is converted into a mechanical energy and vice versa. Thus, when decelerating you get the electrical energy back that you have to store it somewhere or to dump it through a braking resistor and not into motor's winding, that's not a 4-Q.

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Clearly you have to dispose of a fixed amount of energy to slow the rotor by a certain amount. A direct short will give the highest current and therefore the best retardation, but if you use a resistor rather than a dead shirt then the resistor will dissipate some of the energy, and the part that the motor dissipates will be over a longer time, so the power will be reduced. It’s not clear whether a small extension of the braking time would be acceptable. Unavoidably, the braking force is proportional to the current in the windings, so if the maximum motor current doesn’t give you enough retardation then there’s not much you can do about it.

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