How does the regerative braking work?

How does the regerative braking work?

Dosa anyone have a back-of-a-napkin schematic explanation for how the regen works in the Tesla? It's fascinating to me how its able to kick in so instantly, I'm curious what is physically happening under there.

dortor | June 15, 2013

they stop "feeding" the motor power - but since it's still spinning inside of a magnetic field it becomes a generator "pushing" electrons back into the battery - it's a natural by product of an electric motor in that it's motion through a magnetic field induces a current…

this video might help

the tesla motor has magnets in the "rotor" and when that rotor is spinning it is moving electrons in the copper coils surrounding the motor…this is an electrical current.

when you apply power to the copper coils surrounding the rotor they create a magnetic field that "pushes" against the magnets which cause the rotor to move due to magnetic force…

dortor | June 15, 2013

this video is also very good…

Brian H | June 15, 2013

When driving the car, the magnetic fields in the motor slightly lead the rotors, and PUSH. When slowing, the magnetic fields slightly trail the rotors, and PULL.

Brian H | June 15, 2013

The PULL mode forces electricity to flow back into the battery, recharging it.

dortor | June 15, 2013

oh i guess to fully answer the question there is a "braking" effect because mother nature always balances her energy ledgers - power taken from some place has to show up somewhere else (and there is always a bit of loss in the transfer).

so when the car is "at" speed it has a certain amount of kinetic energy - once you take your foot off the accelerator and tesla stops feeding power to the copper coils in the stater there is resistance inside the motor as the magnets in the "rotor" push against the magnetic field in the copper coils - this is the car converting it's kinetic energy (it's motion) to electrical energy - as this energy is converted the kinetic energy is "lost/converter" and becomes "less" - the car slows down until it has exhausted all it's kinetic energy…

where does all the kinetic energy go?

a) conversion to electrical current
b) overcoming aero dymanic load (wind resistance)
c) rolling resistance of the tires to the road
d) changes in road surface and slope
e) conversion to heat energy via friction if you apply the brakes

so the braking effect is a natural outcome of the conversion of kinetic energy (the car's motion) to other forms of energy that work against the car when it's no longer receiving power to over come it's environment - all cars do this - just most cars don't have option 'a' from the above list - and the majority of kinetic energy is converted to "heat" in the brakes… - but all cars deal with items b-e when power is no longer applied, and now that I think about it they continue to have power consumed by the engine and there is probably some energy conversion/loss to think about there with the transmission and engine and gearing…

Brian H | June 15, 2013

Oops, crossed with dortor's explanation. Same story: making electricity rather than using it when regen braking.

dortor | June 15, 2013

no problem BrianH - it's actually very very cool and very very efficient - it's amazing ICE engines are dominant because at a minimum they are just sooo complex when compared to an electric motor…and they have many many inherent downsides - where as an electric motor just becomes more and more impressive the more you learn about it.

ian | June 16, 2013

Great explanations and video links! Thanks!

stevencoberly | June 16, 2013

Does anybody know how efficient the regen process is? Suppose I am cruising down the road at 40 miles per hour. With a mass of 2108 kg, that's about 350,000 J or just under a tenth of a kWh. I I take my foot off the go pedal and "coast" to a stop, how much of that kinetic energy makes it back into electrical energy in the battery?

Brian H | June 16, 2013

Some found that going up a steep hill and then down again returned as much as 80%. YMMV. ;)

stimeygee | June 16, 2013

Great, thanks for the explanations.

So what happens when you turn the regen to "low"? How is it varying the "pull" effect?

NKYTA | June 16, 2013

@ stimeygee

From what I've seen, depending on speed, it limits the maximum regen to 30kW, whereas you get up to 60kW with "Standard Regen". AFAIK, the brake lights go on starting at 30kW, so if you are on the freeway and only slightly lift your foot off the Go pedal, your brake lights won't go on. Good for freeway passing/merging. I usually leave it on Standard unless I'm on I-5 and it is really traffic-y.

MrB | June 16, 2013


Pretty sure there are no magnets in the rotor of the Model S motor since it's a 3 phase AC induction motor. The general explanation is still correct though.

olanmills | June 17, 2013

NKYTA, I think stimeygee was asking how the car physically modulates the regenerative effect. I am wondering if it does this by sending some power to the motor, which will "resist" the braking effect, but then I'm not sure about that because I thought that the main battery cannot be charged and discharged at the same time.

Also, the motor is not connected directly to the axle since there is a single speed gear box to convert the higher RPM of the motor to the RPM appropriate for the axle, correct?

Brian H | June 17, 2013

The wheels force the gears to spin the motor, which drives electricity into the battery. It's no longer a "motor", but a generator, like a hydro-electric dam has, at that point.

jnb | June 17, 2013

What is the second brake caliper looking thingy at 11 o'clock on the rear brake rotor? How does it fit into the process?

Theresa | June 17, 2013

There are two brakes on the Model S. One set is the parking brake and the second is the standard caliper brakes used for stopping the car. The parking brake is electrically operated and the standard brakes are hydraulically operated. Neither are involved with the regen of the car.

create | June 17, 2013

I believe the electric one is also the e-brake (when you hold down the park button at the end of the shifter).

dortor | June 17, 2013

@ MrB - from all the videos and explanations I've found on the web there has to be magnetized "rods" embedded in the rotor for the whole thing to work - but I will admit I don't know exactly how the Tesla motor works - so I'll defer to someone else on that topic - the other point however is the the Tesla is proof that Magnetism can be used to accomplish "work"

thanks for the correction MrB…

MarkV | June 17, 2013


As you have figured out, this all boils down to magnetic forces. Recall when you try to push the positive end of one magnet against the positive end of another magnet the two magnets will repel each other. Whereas if you line them up positive to negative they attract. Remember the old saying likes oppose, opposites attract? If you had two magnets with holes in them mounted on an axle and pushed them close to one another they would align themselves positive to negative and would be happy. If you held one steady and rotated the other and then let go of it, it would rotate back into its preferred alignment. So, with an electric motor we simply energize one set of magnets in the stator (the fixed magnet) which causes the magnets in the rotor (the rotating magnet) to realign themselves relative to the magnets in the stator. Then we turn off one row of magnets in the stator and turn on another one which causes the rotor to move again. We simply keep doing this at a very fast rate and that causes the rotor to rotate and a variable speed depending on the frequency at which we turn the stator magnets on and off. This is called a variable frequency drive.

Now, for the regenerative braking aspect we simply reverse the way we energize the magnets so that we basically create the situation whereby we are pushing two of the same poles toward one another and repelling action slows the rotor down while at the same time creating a flow of electricity in the opposite direction and putting some of that back into the battery. This switching from one mode to another is done in a matter of milliseconds so we cannot sense any change. Of course the conversion of electricity to mechanical movement is not 100% efficient and likewise the conversion of mechanical back to electrical is not either. So, you should not expect to be able to climb a hill and then get 100% of the energy back when you come back down. That said something is better than nothing and infinitely greater than what and ICE car generates.

July10Models | June 17, 2013

The MS motor does not have magnets but a magnetic field is created in the rotor with DC. The strength of that field determines how much the rotor couples with the rotating electric field in the stator. When you pick a lower regen brake setting, the DC current supplied to the rotor field is lower cause less coupling or more slippage. The field can be made strong enough to lock up the rear wheels on regen with a motor this powerful. The Tesla supercar is going to be awesome.

You can feel the DC field cut on when you stop on an incline hill. Field cuts in right before the wheels hook up in the forward direction. Some people have reported feeling the detent as a thud like cylinders in an ice.
The power to the motor stator positive or negative must be accompanied by field current or the motor will quickly overheat because it would offer no reflected impedance to the stator current.

satyrias | June 17, 2013


"So, with an electric motor we simply energize one set of magnets in the stator (the fixed magnet)

That is not correct. There is no permanent magnet in the Stator. That is an electromagnet; that means, the electromagnetic material produces magnetic field when the current flows. Once the flow is cut off, that is not a magnet anymore. So, the circuitry will turn on and off in sequence around in one set of field coils in the Stator to create magnetic field in cycles. That is why the armature rotates from the repulsion.

cmeyers | June 17, 2013

Read about induction motors here:

There isn't a DC current applied either, the field in the rotor is "induced" hence the name induction motor. The rotor consists of copper conductors in an iron core (steel laminations typically), can be seen in the cut-a-way motor in the show rooms or on the mega factories show.

An alternating field in generated in the stator windings and like a transformer, this induces current to flow in the rotor. The flow of current in a conductor creates a magnetic field.

If the motor controller aligns the magnetic poles of this alternating magnetic field in the stator with the predicted position of the magnetic poles in the rotor then no torque is created because all poles are aligned.

If the motor controller causes the magnetic field to lead the rotor's then torque is created in the motor as it tries to catch up.

Current in the motor is directly related to the torque created in relationship to the position of the magnetic pole phases.

As an electrical winding passes through a magnetic field a voltage is induced into it. The voltage has a relationship with the velocity that it moves. As the windings move faster through the field a higher voltage is created. This applies to moving the motor or as a generator, higher voltage equals higher RPMs in a motor.

The difference between the speed the magnetic field in the rotor is moving, remember that it has to be induced, and the outer windings will generate a specific voltage. This back EMF voltage from the motor, added to the voltage the motor controller is applying to the motor, done out of phase, can create a voltage higher than the voltage in the battery. This causes energy to be returned back to the system. The motor controller is in control of everything here, and it can regulate the maximum voltage generated and the maximum current.

A controller like this is called a 4 quadrant controller because it can apply positive voltage, positive current, negative voltage or negative current, and operate in any of the 4 quadrants if this was drawn on a 2D graph.

I doubt my description really helps but at least know this. The company is called "Tesla" because the induction motor was Tesla's most successful invention.

riceuguy | June 17, 2013

Quick clarification on the brake understanding is that they are not triggered by a specific regen level but rather using an accelerometer so that the car is approximating when brake lights *should* come on; that may in many cases translate to the deceleration provided in conjunction with 30kwh of regen, but I don't believe that's what causes the brake lights to illuminate.

murpcorp | September 2, 2015

Is regen 80% efficient then ?