# bldc regenerative braking, braking torque

I'm trying to figure out the braking torque calculation for a BLDC simulation. Now if I'm looking at a datasheet I know the torque at a given rpm at acceleration. If I brake with regenerative breaking is that the torque at a given rpm as at acceleration? Or can I get any braking torque with the controller?

I'm no expert but I would add to John Birkinhead's answer that for a battery powered system you can sink the current back into the battery while the regen voltage is higher than that of the battery. This, of course, is only possible for rechargeable batteries and charge current limits will apply. As the motor slows down the voltage will fall and when it goes below the battery voltage then you'll have to burn it off in a resistor.

Figure 1. A typical VFD drive system with braking resistor controlled by PWM when the DC voltage rises above a certain level. Source: Invertek Drives (which is well worth a read).

For a mains powered device you (usually) won't be able to feed back into the mains so you'll need to monitor the DC bus voltage and, when it rises above a certain level, you'll have to dump it into a resistor.

• Does this apply to non-rechargable batteries as well?? – Huisman Apr 18 at 18:59
• Which part? By definition a non-rechargeable battery can't accept charge so reverse current protection would be required in that case. – Transistor Apr 18 at 19:01
• you can sink the current back into the battery ... that part ... i think you should be more specific here – Huisman Apr 18 at 19:02
• Thank you. Done. – Transistor Apr 18 at 19:29

The braking torque you can achieve is based upon the type of controller you have.

If you have a full four-quadrant controller with a dedicated speed and current loop then you have the ability to control the current while in forward generation and reverse generation since $$\T = K_t I \$$ (give or take).

If however you do not have means to control the stator current then your ability to control the braking torque is limited

When braking, the motor acts as a generator, and your braking voltage is the back-emf of the motor, with the current proportional to the braking torque. Both of these can be derived from the motor constant. You can brake with some controllers, but keep in mind that the power required for braking (voltage times current) that does not go back into regeneration must be absorbed by the controller and dissipated as heat in the circuitry.