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I know that if you spin a DC motor by an external force, it can create electricity, hence it can be used to charge a battery or something. This is used in regenerative braking. But how can be the amount of braking or braking strength controlled/adjusted like it's done on many electric longboards or EVs?

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    \$\begingroup\$ The load of the generator is controlling the "amount" of braking. That is the current drawn from it. So by limiting the current to different values will change the braking strength. \$\endgroup\$
    – Eugene Sh.
    Jul 21, 2022 at 13:20

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The torque in a motor or generator is roughly proportional to the current flowing.

To make the motor apply a braking torque, you control a reverse current through it. This may be as simple as switching a resistor of a suitable size across its terminals, using the voltage the motor generates to drive the current, or switching between resistors for several degrees of braking. For regenerative braking, you would program your controller to draw the braking current from the motor, which it would then use to charge the battery.

For the most brutal uncontrolled braking, short the motor terminals. A current limited by the motor's internal resistance will flow. Don't try this on anything big.

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    \$\begingroup\$ "For regenerative braking, you would program your controller to draw the braking current from the motor, which it would then use to charge the battery." Might want to mention pulsing connections to the regen hardware. Pretty sure that's what the OP is after. \$\endgroup\$
    – DKNguyen
    Jul 21, 2022 at 13:36
  • \$\begingroup\$ @Neil_UK Right that makes sense. So if I understand it correctly there is current generated by DC generator and if I draw the current from it, it makes the DC motor or generator brake, or basically it's harder to spin. But what if I didn't draw the current from the motor? Where would it go? \$\endgroup\$
    – Fernando
    Jul 21, 2022 at 13:48
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    \$\begingroup\$ @Fernando "It" wouldn't go anywhere, because there is no current, and the only resistance would be from the mechanics. Same as unconnected battery. \$\endgroup\$
    – Eugene Sh.
    Jul 21, 2022 at 13:57
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    \$\begingroup\$ No, it is not "fed back". When there is no current, there is no back-emf which is resisting the rotation. \$\endgroup\$
    – Eugene Sh.
    Jul 21, 2022 at 15:44
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    \$\begingroup\$ @Fernando - A generator does not generate "current" it generates a voltage (also called back-EMF). The current depends upon the load. \$\endgroup\$ Jul 21, 2022 at 15:48
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For a typical MOSFET H-bridge DC motor controller, the body diodes will conduct and cause the BEMF of the rotating motor to be fed into the drive bus. If the motor is spinning fast enough, its BEMF will supply current into it. But that only works if it's greater than the battery voltage. If the battery is isolated from the H-bridge with a diode, the BEMF will appear on the drive bus, and a load may be applied there to receive the current and act as a brake. A resistive load can be varied to control the amount of braking, but it is just wasting the energy as heat. To achieve regenerative braking, you can use a DC-DC converter to adjust the current fed back into the battery over a range of speed.

This topic was also discussed here: Regenerative Braking circuit

Here are additional references, explaining how the inductance of the motor can be used to produce regeneration by using PWM or a chopper circuit:

https://www.pmdcorp.com/resources/type/articles/get/regenerative-braking-part-i-steady-state-analysis-article

https://www.eeeguide.com/regenerative-braking/

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With an H bridge the voltage can be applied in either polarity.

Shunt choppers can also be added to a motor:generator in either direction to brake the motor. This could be implemented with another FET or IGBT bridge.

Normally the high side controls the direction and the low side controls average voltage using PWM to open and close the diagonal switch. The direction may be changed after the vehicle stops.

The dynamic brake shorts to 0V the spinning motor's EMF with a duty factor (d.f.). which varies the average load resistance. The current integrates up during the pulse width and is limited by sensing with some threshold and maximum cycle width that determines the chopper frequency.

Regenerative dynamic braking simply adds steering diodes or FET switches to put the back EMF current into the battery instead of ground. The transition current is the same then decays until the next chopper cycle that shunts the motor.

How is amount of regenerative braking in DC motor controlled

The amount of braking is calibrated to the demand brake sensor with speed and current sensing. If a full stop is needed and there is insufficient back EMF, it is possible to drive the motor current reverse faster to stop, then toggle fwd/rev once stopped to lock the motor against external forces. But this latter process draws a lot of battery power and is better done with a mechanical brake assist when there is insufficient juice from the motor-generator to achieve the desired braking force. Calibration is needed to balance this shared braking.

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Increase the magnetization of non-permanent magnet DC motor so that the induced EMF is higher than the battery voltage. That causes regenerative braking. Control it by adjusting the magnetization current.

This approach isn't the top notch technique today, but it was possible before modern switching components. And its useless if the motor has permanent magnets.

Another old method is to accelerate a massive enough flywheel through a variator. It does not store the braking energy as electricity, but the directly reusable rotational energy is surely useful.

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