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Regenerative brake 3

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Sajuuk

Sajuuk

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Jul 13, 2020
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Hello,

I know some basics about electric motors (sync, async, DC) but i know nothing about regenerative braking used in some cars (e.g. hybrid or fully electric).

I read on wikipedia that it is not (yet) possible to completely and safely stop a vehicle only by using it and a manufacturer still needs brake pads.

Is it really the case ? Where can I find some research on this ?

Could it be possible with the right electronical system ?


Thanks for your answers :)
 
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Aside from what Lionel said, the technical issue with regenerative braking not being able to fully stop you is one of the law of diminishing returns. Regenerative braking means transmuting the kinetic energy of a moving object into electrical energy by using the drive motors as generators, then moving that electrical energy into something else. In the case of a car, the electrical energy is put back into the battery bank. But the AMOUNT of braking energy that you have available is based on the amount of kinetic energy in the system. So as it works to slow you down, it is ALSO diminishing the braking energy at the same time. As you approach very slow speeds, there is almost no kinetic energy left, ergo no braking energy and you end up coasting at the end. So the mechanical brakes are necessary to finish the job at the very end of the deceleration cycle, because coasting into an intersection at a red light is considered a bad thing...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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But with electronics it is easily possible to get full breaking to a complete stop, although it would not be purely regenerative. This would constantly generate heat if parked on a hill, so mechanical brakes will always be needed.
 
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Regenerative braking relies on using the "back EMF" of the motor - the magnetism plus the rotation turning the motor into a generator, and the drive being one that is capable of feeding that back into the battery pack.

The back EMF depends on the RPM of the motor. The battery pack is a (relatively) fixed DC voltage.

When you are down to a crawl speed, the EMF generated by the motor becomes insignificant. So how do you come to a stop using regenerative braking only? Is the drive capable of rectifying the (let's say) 2 volts generated by the motor with the car puttering along at 5 km/h, rectify that to DC, and somehow step it up in voltage sufficiently to charge the battery? Worse, what happens if you are trying to stop on a hill?

If it is an induction motor, and you are trying to stop on a hill, the drive would need to actively supply a small AC power to the motor to actively try to turn it backwards (the opposite direction of whichever the hill is trying to drive it). Induction motors cannot act as generators without a small magnetising current being supplied. Without that, they just sit there, spinning, and doing nothing.

If it is a permanent-magnet motor, you can hold it stopped by supplying a small current to one phase, just enough to stop the motor cogging over to the next magnet. Still, you have to hold the motor stopped by using active current supplied to it.

The practical reality is that once the car has slowed down to some fraction of its design road speed (what that fraction is, is unknown to me), it becomes no longer viable to try to recover the kinetic energy via regenerative braking, so they don't bother ... and that means, the car needs conventional brakes.

There's a separate problem at high speed. If you are doing (let's say) 140 km/h and a moose runs across the motorway in front of you, you then need to do a panic stop. If you were to try to recover that energy, the generating capacity of the powertrain probably represents a battery-charging rate that nothing in the system can absorb. Thus ... you need conventional brakes.

And there's yet another problem if the car has been fully charged at a charging station on top of a big hill. Driver hops in, first thing to do is drive down the hill. What do you do with the regenerated power when the battery pack is already fully charged and unable to accept it? Again ... conventional brakes!
 
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How slow can you get with regenerative braking?
For a conventional induction motor rated at 1760 RPM the slip RPM is 40 RPM.
You can get regenerative braking down to about 40 RPM or about 2.3% of rated speed at rated torque and rated full load current.
If your regenerative braking is limited to rated full load current, then your deceleration will be limited to full load torque.
panic braking demands more braking than that.
To give some idea of the effectiveness of regenerative braking, if a motor with a slip frequency or slip RPM of 2.3% will give a rated speed of 100 MPH, regenerative braking at rated torque will be effective down to about 2.3 MPH.
These figures are based on an off the shelf, standard squirrel cage induction motor.
The numbers are probably different for a purpose built EV motor but this will give you some idea of what is possible.
By the way, in this example, comparing a stop from 100 MPH with no regenerative braking with a stop using the maximum possible regenerative braking,
The energy dissipated in the conventional brakes with regenerative braking will be about 0.052% or 0.00052 of the total braking energy.
With 99.95 percent of the braking done by regeneration, take what you can get. It's pretty good.
Don't begrudge the conventional brakes doing the little bit extra.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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A BLDC motor is a type of synchronous motor and theoretically it may be able to regenerate at slower speeds than an induction motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Still, the voltage generated is in proportion to the speed. At 10 km/h, it's not going to be generating much voltage to work with. It has to be stepped up to the DC voltage of the battery pack somehow.
 
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That can be done. Brian.
The torque depends on the current and the current depends on the slip frequency or slip RPM.
Are you near any transit lines driven by Linear Induction Motors?
The Sky Train in Greater Vancouver Canada is an example of a light rail transit system driven by Linear Induction Motors.
The vars would regenerate from about 40 MPH to a very slow walk.
You could hear the sound of the induction coils as the trains slowed into the stations.
You could hear the coils cut off and hear the mechanical brakes apply.
From the cutoff of the coils to full stop took about 5 or 6 feet as I remember.
Not very far as I recall.
The system was fully regenerative.
There were very large resistor banks to dissipate the energy when regeneration exceeded demand.
Is there anyone here who is located in Vancouver and can drop down to a Sky Train Station to verify my memory as to stopping distance?
The newer additions to the system may not be using Linear Induction Motors.
Check one of the older lines.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Right, but it's still got a mechanical braking system that takes it down to a stop. And it doesn't have to deal with steep hills.

Bottom line, the regenerative system can do a lot of the braking, but you still need the mechanical one. It may be able to be lightened or downsized, but it still has to be there.
 
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Is it really the case ? Yes - see the answers above

Where can I find some research on this ? Don't know - try some searching

Could it be possible with the right electronical system ? In theory yes, in practice highly unlikely.

Apart from the low speed stopping, the emergency stopping requires a higher level of retardation than is economic to deliver from the regeneration system including the motors etc. Also for individual mobile items like cars and trucks, the cost of a back up electrical heat dissipation system for when the battery is fully charged is too much.

There are known issues with some hybrids where long descents end up charging the battery to maximum and then the brakes would have difficulty slowing the vehicle down enough if they had been de-rated. So they still need to work.

Trains a bit different as you're sending electricity back to the grid to use somewhere else and their max braking is alot less due to the steel on steel friction factor of the wheels.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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At slow speeds, not worth doing and not enough braking would be the reasons to quit using regenerative braking.

The HP of every electrical motor/generator is directly proportional to rpm. So, at very low rpm there is hardly any power available to be regenerated into the batteries. Starting at highway speeds the motor system might have 20-40kW of regenerating power and this regeneration can last for quite a few seconds leading to a fair bit of energy going back into the battery bank. At 2mph, you might be looking at < 1kW of regenerating power and < 1s of braking so almost no energy is available to go back to the battery.

As for as fully stopping a vehicle with the electric motor. Yes, that is easily possible, but at some point as the speed reduces the energy flow will reverse and begin flowing into the electronics and motor again. Once again as speed reduces the regenerative power available from the motor drops and at some point it becomes less than the electronics is consuming. At zero speed there is no regenerative power, so the electronics have to use power/energy to hold the motor stopped.
 
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an interesting anecdote in regards to regenerative braking:
Years ago I worked with a fellow who had driven trolley buses for a few years before going back to school.
The subject of regeneration came up.
He said that while the buses had the same brake system as a similar diesel powered bus, they used a lot of regeneration when stopping normally.
He said that from time to time, the trolley pole would come off of the wires.
When that happened, it took a lot more brake pedal pressure to slow down.
Lionel, your braking torque is dependent on the current and will hold up as long as the speed is above the slip RPM or frequency.
The limit is not the motor but the ability of the electronics to inject the small regenerated voltage back into the batteries.
Yes, you still need service brakes.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Bill, I suggest you read what I posted again and try to understand it this time. Torque have nothing to do with the amount of energy available to be regenerated and the hard lower speed limit is reached when the electronics use more power to operate than the motor is producing.
 
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Got it.
Me said:
The limit is not the motor but the ability of the electronics to inject the small regenerated voltage back into the batteries.
Click to expand...

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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My field is positioning servo control. Most commonly used now are permanent magnet synchronous ("brushless DC") motors, but also brush DC and AC induction motors. In all cases, we decelerate all the way to a stop by regeneration alone. (Many precision systems use air bearings, so don't even have friction to help decelerate.)

The deceleration process causes current to flow "upwards" through the inverter stage's flyback diodes to the DC bus -- in our case to the bus capacitor banks. It works just as well going from 1 rpm to 0 rpm as it does from 3000 rpm to 2999 rpm. (Although due to the V^2 relationship to kinetic energy, the regenerated energy is a lot greater in the 3000-2999 rpm change than in the 1-0 rpm change.)

We used to do a fun demonstration when most people didn't believe you could control an AC induction motor as a positioning servo motor. We had a standard 5 HP induction motor with an inertia wheel attached sitting loose on the floor. We would run it at 1800 rpm in one direction, then command a very rapid deceleration/acceleration to 1800 rpm in the opposite direction. Because it was not bolted down, we could get the motor to do a complete flip as it did this.

Curt Wilson
Omron Delta Tau

 
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The limiting factor on the deceleration is almost always the rate at which the storage devices on the DC bus can accept current, not the motor or the inverter. Capacitors have a far greater charging rate than batteries, but a good-sized battery bank can absorb at a pretty good clip.

My hybrid SUV has over 200K miles on it, and I'm only on my second set of brake pads front and rear. Overwhelmingly, my decelerations must have been regenerative, not from friction braking.

Curt Wilson
Omron Delta Tau
 
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Curt, what percentage of short term torque can you develop in a panic braking situation relative to rated torque?

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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