Regenerative braking [tour 8]

Regenerative braking is a way of diverting the kinetic energy of a moving motor / vehicle back into the battery so that it re-charges the battery and provides a braking effect. Most of 4QDs controllers have regen braking as standard, on some models it can be varied or disabled.


What is regeneration?

It’s a way of using a motor controller to convert the kinetic energy of a vehicle into electricity. The electric motor is used as a generator and the current thus produced is fed back into the battery.


When does regeneration occur?

If enabled, the process of regeneration is entirely automatic and occurs when the actual motor speed exceeds the demand speed requested by the controller. When this happens the motors back EMF is greater than the output voltage from the controller, so current flows from the motor, through the controller and back into the battery.

It is actually the battery which is doing the braking, not the controller, since the braking energy gets dumped into the battery.

As the motor slows down, the regen current reduces, and hence the available braking force also reduces. For this reason regen braking should not be considered as a safety or emergency braking system. Any vehicle particularly those that carry passengers should also have an adequate mechanical brake for emergencies.

Regeneration is best thought of as a braking aid rather than a means of saving lots of energy. This is because motors are not designed to be generators and are not very efficient in the way they convert kinetic energy to electricity. Also there are often gear train losses to take account of, and lastly the process of charging a battery only stores around 85% of the energy going in.


When does regen braking not work

There is one unusual situation where you have to be wary with regen braking: for instance if you are at the top of a hill with a fully charged battery. The controller may overcharge the battery and it is possible that the over voltage trip fitted to some controllers may operate.

However – lead acid batteries do in practise stand significant overcharging, but it is not good for the battery, so such situations should generally be avoided.


I don’t understand!

The best way of thinking about 2 quadrant control is that, normally, the controller is supplying a voltage to the motor to drive it, and the motor is generating a back emf, proportional to its speed, which mostly cancels out the drive voltage.

If the motor now goes faster, its back emf rises and the current (caused by the difference between the controller’s output voltage and the motor’s back emf) falls. If the motor rotates fast enough, the motor current falls to zero as the back emf then equals the controller’s output. Now, if the motor rotates even faster, the current must go negative (feeding back into the controller) as the back emf is now greater than the controller’s output voltage. If the controller can accept this current being fed back into it, then braking starts to occur. The controller has to do something with this current. Crude designs simply dump it as resistive heating but it is more efficient (and not difficult) to feed the current back into the battery.

There is a more detailed technical description of how regen braking works in our circuits archive.


Can I vary the regen braking?

Regen braking can be controlled on all 4QD models by adjusting how fast you close the throttle. All 4QD controllers have adjustable deceleration ramps which vary the maximum deceleration rates and hence the amount of regen braking.

In addition our newer Pro-160 / 360 models have adjustable regen current limits.


Can I coast?

If you hold the speed so you are neither applying power to the motor nor braking it, you will indeed coast: the motor current will be zero and the controller will be doing no work.

We are working on new software for the Pr0-160 / 360 that will give a “coast mode”.


Can I disable regen braking?

Some controllers (see controllers list below) can be altered so regen braking does not occur.

Disabling regen braking ‘on the fly’.

We sometimes get asked for a controller where regen braking can be switched on and off. Some controllers can be supplied with the feature disabled, and on some it can be enabled / disabled either as an on-board option, or via software.

  • Switching regen OFF when running is ok.
  • Switching regen ON when running. This can be done BUT ONLY only when the motor is running at full speed or is at zero. At any intermediate point there would probably be a mismatch between the motor’s back emf and the controller’s output, which could result in a large jerk and current spike that could damage the controller.

Can I control the battery re-charging?

In general for the older 4QD products the answer is no. A controller could either control the braking level or the battery charging level. However – braking is (in most applications) essential and must be controlled. Charging is generally a secondary consideration and occurs because the controller has to dump the energy somewhere.

But we are working on software for our new Pro-160 / 360 that will give control over charging for re-charging under sail marine applications.


How much energy will regenerative braking recover?

As far as the controller is concerned, virtually all of the energy it can get has to be returned to the battery – the controller simply has no way of dissipating this energy so must dump it.

However many gear trains are not efficient when driven backwards, so they will waste a significant part of the energy the motor generates.

The motor, also, isn’t designed as a generator, so may not be so efficient. Then charging a battery is only around 85% efficient.

So sorry, we can’t really give an accurate answer to this question.


What happens if there is no battery connected?

In this case, the controller still attempts to feed braking energy back to the battery. It does this by using the motor’s inductance to step up the voltage at the battery terminals. If there is no battery connected this voltage can rise to quite a high level (typically 47v for lower voltage controllers and about 68v for 48v controllers) at which point the controller stops regenerating. So there is no braking.


Controllers

  • The following controllers have regen braking which cannot be disabled.
    • Pro-150
  • The following controllers have regen braking which can be can be disabled by an on-board link or modification.
    • 4QD series [link]
    • Pro-120 Mk 2 [link]
    • DNO series [modification]
    • Porter [modification]
    • Uni series
  • The following controllers have regen braking but could be supplied without.
    • 2QD series
  • The following controllers do not include regenerative braking.
    • 1QD series
    • SST-031

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