How does regen braking actually stop the car?

Hi euro...,


A motor can be made into a generator simply by driving the shaft, and hooking up the motor to some sort of load. In the Prius its a battery will lower voltage than the generator output - so that current flows into the battery. And the braking energy is used to cause the chemical reaction in the battery run backwards, or charge the battery.

One way to illustrate this is to get a toy permanent magnet motor. Leave the terminals open and spin the shaft. It spins easily. Next short the terminals with a clip lead and spin the shaft. The shaft will take allot more torque to turn. Next hook a small resistor to the motor terminals and spin the shaft allot. The resistor will get warm just like friction brakes get warm.

Diesel Locomotives are electric drive as well, and do not short the motor terminals, but connect them with very larger resistors, which are under the fans on top of the locomotive. As the train comes to a stop, the fans kick on, and the heat is exhausted. Ever see heat coming off a locomotive just pulled into a station? That is the heat from the braking resistors.

 

Upon light to moderate brake application, regenerative braking is the only thing that is slowing the car down until you reach ~8MPH. At that point, the rear brakes engage first, followed by the front brakes. It's expected that the average Prius driver will get around 100K miles from a set of pads/shoes.

 

If you want a more fundamental explanation, electric motors work through magnetic fields that attract and repel each other as the rotor of the motor spins. The fields are switched back and forth quickly, so they are always attracting and repelling at the correct times, but for the purpose of our discussion we can think of a couple of permanent magnets that you hold in your hands. If you use bigger magnets, they push and pull harder. With an electric motor, they use electromagnets, so the strength of the pull and push can be controlled by how much electricity is running through the coils. With the Prius, when you take your foot off of the gas and you get a little drag with the blue arrows, the control system is running a small amount of electricity through the magnet coils of the motor. As you press the brake, the amount of electricity in the motor coils is increased. More electricity in the coils means a stronger push and pull from the magnets. The harder you push, the more electricity. What you have to remember is we are talking about really, really strong magnets that are working very close together. The forces are incredible.

That's really all there is to it on a conceptual level: magnets pushing and pulling on other magnets. Time the fields one way and it pushes the car using electricity. Time the fields the other way around and it slows the car but makes electricity.

Tom

 

Here: http://www.autoshop101.com/forms/Hybrid16.pdf is a complete mechanic's view of the braking system.

 

Perhaps I should read the information at the link posted by bbald123 before commenting, but I'm feeling kind of lazy at the moment.

What you've posted here qbee makes it sound like electricity is being used by the control system to generate electromagnetic fields? I thought it was the other way around when braking. I thought mechanical force caused the permanent magnets in the motor/generator to spin past the coils dragging the magnetic fields through them. I thought that a changing magnetic field in a conductor had a physical effect of creating an electric current which the control system then fed back into the battery.

What I've been confused about, and hopefully when I get around to reading that link I'll understand better, is how the amount of electricity generated in the coils can be adjusted. If I release the gas, a small amount of electricity is generated? If I shift to "B" more electricity is generated? If I press the brake even more electricity is generated? The amount of electricity generated is continuously and smoothly adjusted as determined by the needs comunicated through the brake pedal?

If the permanent magnets are of constant strength and distance from the coils, and if the number of coils is constant, and if the rotational speed of the motor shaft is constant, then how is a variable amount of electricity generated?

 

This
"Selecting B on the shift lever will maximise regenerative efficiency" is not true, avoid B mode unless the battery is already at full charge. Even then if the battery is at full charge the control system will use MG1 to spin the ICE to dump excess energy to avoid damaging the battery from being over charged.

 

Read the last line of my post again. What is happening in both cases involves an electrical current circulating through the coils. In the case of the MG being used as a motor, the electromotive force is being supplied by the inverter, so the motor consumes energy and supplies force. In the case of regenerative braking, the electromotive force is supplied by the spinning rotor so the MG works as a generator converting the rotor force to a circulating current. The more current allowed to circulate, the larger the force required to produce it. If you spin a magnet inside of an open coil, you will induce a voltage in the coil, but no current flows because there is not a complete circuit. The amount of energy required to spin the magnet is only that necessary to overcome friction. Say we attach a heater to the output of the coil and then spin the magnet again. Once again the moving magnetic flux induces a voltage in the coil, but this time a current flows through the heater. The power for this heater comes at the expense of magnetic drag for the spinning magnet. If we attach two heaters in parallel, then the magnetic drag doubles to supply power for both heaters.

In this fashion, the control system regulates the amount of power drawn from the MGs during regenerative braking. If only a little braking is required, the controller allows a small amount of current to flow. If a lot of braking is required, the controller opens up and allows a lot of current to flow from the MGs.

Either way, braking or motoring, the current in the coils controls the amount of power in the MGs. In one case the controller is supplying power, in the other it is consuming power and putting it back in the battery.

Tom

 

Generating electricity requires work. The more electricity you generate, the more work is required to do so. So, if you hooked a gasoline generator to you house and turned on one light bulb, a certain amount of work and effort is required by the gasoline engine to generate the electricity. As you turn on more and more light bulbs, more and more effort is required from the gasoline engine.

In regenerative braking, the "engine" that is doing the work is the momentum of your car. When your car is moving, it has a lot of kinetic(movement) energy stored up in it's momentum. As this kinetic energy is consumed the car slows down. The faster the kinetic energy is consumed, the faster it slows down, until is has stopped and has zero momentum and therefore zero kinetic energy.

So, we use the car's kinetic energy (from it's momentum) to turn the generator to generate electricity. As we consume the kinetic energy by turning the generator, the car slows down. The more electricity we generate, the faster the kinetic energy is consumed, and the car decelerates faster.

The amount of electricity you generate is measure by the "electrical current", which is the number of electrons that flow past a given point. Electrical current is like water in a pipe; open the faucet wider, or open more faucets, and water flows through the pipes faster. In your house, light switches are like faucets; the more light switches you turn on, the more electrical current flows into your house.

Another kind of switch is a transistor. It has three wires, two of which are used to carry the electical current, and one which is used to control the flow. By varying the voltage on the control wire, you can vary the amount of current that passes through the trasistor. It's very much like turning the handle on a water faucet; you can make very fine adjustments to the amount of current that passes though the transistor.

So, in regerative braking, the volatage applied to the control wire of the transistor is proportional to how hard you press the brake pedal. Therefore, as you press harder on the brake pedal, more current is allowed to flow through the transistor and into the battery. Therefore, more work is required by the generator and more kinetic energy is consumed, which slows down your car.

 

I think the OP and the rest of us know that, Tom (incidentally, the magnets in MG1 and MG2 are permanent, not electro-magnetic).

What has been puzzling me, and I think the OP, is exactly what mechanism translates the mechanical motion of the brake pedal into additional current flow (i.e. higher amperage) in the windings of MG2 (and I assume MG1), thus increasing the back EMF in the motor, which is now operating as a generator? This increased current, together with the PSD gearing, is what produces the variable braking effect. This current is obviously modulated via the brake pedal, but how?

rah

 

So the modulation is effected via a power transistor, which gets variability (permittivity?) through an applied voltage? Better an more efficient than a simple resistor I hope?
rah

 

The motor/generators are actually three phase AC motor/generators. The output is fed to a multi-transistor control unit (the one on top of the engine). It uses pulse width modulation to control how much power is fed back to the battery. The battery can't take the full power the MG can generate without shortening its' life however, so sometimes the power is "dumped" by spinning the engine via the other MG (mostly when in "B" mode) or simply not used, requiring the system to use friction brakes to slow the car. The electronics reads input from brake pedal travel and fluid pressure to "know" what to do. The brake master cylinder is a very complex device. Which is one reason bleeding the brakes is so complex.

 

I was referring to the windings in the MGs, not the permanent magnets. The windings act as electromagnets, but if you replace the permanent magnets with electromagnets, it's irrelevant for this discussion. It would change the complexity and efficiency of the MGs, but not the theory.

I think you misunderstood the OP's original question. It was on a much simpler level, that's why I used crayons for the explanation. If you want to go into technical details, I'll put my EE hat back on:

1) The brake pedal is attached to two transducers. One is a potentiometer that is used to measure the rate of pedal actuation. This is used to sense a panic stop situation. The other transducer is a hydraulic to voltage device that measures pedal pressure by means of hydraulic pressure. When you press the brake pedal it actuates a hydraulic piston which pressurizes a closed system consisting of the pedal piston, a spring loaded piston, and the pressure transducer. The spring loaded piston simulates the feel of a normal hydraulic braking system.

2) The output of the two brake transducers is routed via CAN bus to the motor ECU and the braking ECU. The two ECUs carry on a negotiation about how much braking effort is needed from each. In a panic stop, the braking ECU takes over and actuates the hydraulic friction brakes. Otherwise the motor ECU tries to do the braking through regeneration.

3) Several special cases exist which can limit or influence regenerative braking. Loss of traction will cause the friction brakes to be used. Insufficient regenerative braking will cause the friction brakes to assist. Too high an SOC will cause MG1 to spin the ICE to burn off extra energy. Use of B mode will cause MG1 to spin the ICE to burn off extra energy. Too low a speed will cause a switch to friction brakes. I may of missed some, but you should get the idea.

4) Regenerative braking is accomplished by PWM control of the three-phase AC MGs. Energy produced is routed to the battery, discarded by spinning the ICE, or a combination of both depending on the SOC and the transmission mode (B or D).

Tom

 

Tom,

Thanks for the explanatory details - I have a much better appreciation now for what is going on under the braking/re-generation scenario. It's pretty complex, to say the least. Just impresses me more on Toyotas ability to make it all work together so seamlessly, to simulate what people have been used to over the years.

rah.

 

The manual this comes from is written by Toyota. I'll take their word for it for the nonce. I will be checking for more rapid regen in B mode on one particular hill the next time I drive it.

<<<CORRECTION>>>

The manual I linked to is indeed incorrect. I just checked the Toyota site.

Regen is always used. It is emphasized in D mode and ICE braking is emphasized in B mode.
If the battery SOC is full regen'd energy is dumped as heat in the MG windings.

I apologize for the mis-information.

 

B mode discards energy by spinning the ICE with MG1. Instead of more rapid regen, you actually get less saved energy. Look at it this way: The friction brakes are only used in one of these three cases:

1) Under ~8 mph.

2) Panic stop or loss of traction.

3) When the SOC of the battery is too high.

Otherwise ALL of the braking is done though regeneration. You can't do better than that, or in other words, B mode can't regenerate more than what is there.

Essentially, the only difference with B mode and D is that B mode tells the controller to immediately start wasting power, and not to wait for the SOC to get too high. This is useful on a long downhill where you know the SOC will max out, as it will save wear on the friction brakes.

This explanation refers to the Gen 2 (U.S. numbers). The Gen 1 has a more primitive system.

Tom

 

Here's another way to look at regenerative braking:

A Prius in motion has kinetic energy equal to 1/2*mv*v or m*v-squared, where m is the mass of the car, and v is the velocity. Notice the energy is the square of the velocity.

Regenerative braking, in the simplest terms, converts kinetic energy into electrical energy that is stuffed into the hybrid battery pack. Converting kinetic energy in this way reduces the kinetic energy of the car, hence slowing it.

Kinetic energy - Wikipedia, the free encyclopedia

Harry

 

Rather than sending waste heat into the atmosphere from conventional brakes the energy is harvested and siloed for later use. This is energy that the engine has expended or sown in making the car move.
Proving that with Hybrid Synergy Drive As ye sew, so shall ye reap.

 

I'm going to throw another question into this thread because there is great info being given here! Thanks everyone who has contributed!

This has been bothering me for a while. At below ~8mph when the friction brakes are used, the MFD often shows power being the battery to the wheels, even under braking. Is this accurate, and if so WHY???!! I'm trying to stop, dont be sending power TO the wheels! Any info on this, such as why its doing it?

One of my few gripes about the car is related to this. I'm coming from a manual transmission, I hate that the prius sends power when you let off the gas to simulate an automatic. Id rather it sit there until I ASK for power by pressing the accelerator. Minor, but kind of annoying...