Toyota Prius Forum banner

1 - 20 of 29 Posts

·
Registered
Joined
·
27 Posts
Discussion Starter #1
I mean how mechanically? Is there some kind of generator that gets shoved against the rim?
 

·
Registered
Joined
·
1,215 Posts
Nope, it causes MG1 and/or MG2 to become generators, recharging the battery. Generators naturally resist the force, and in the case of the Prius, that resistance has a braking-like effect (slows down the car).
 

·
Registered
Joined
·
130 Posts
How is the braking effect varied within the generator? I'm not asking how the brake pedal works. I want to know what happens inside the generator when the ecu tells the generator to provide more or less braking effect.
Thanks
 

·
Registered
Joined
·
928 Posts
This is potentially a huge subject, so I'll give some quick highlights and you can ask for or search for the details you're interested in.

First, look at Graham's website and click on "The Motor / Generators" on the left hand side. This should bring up an animated picture of a motor/generator similar to that used in the Prius. Note that this picture shows the winding generated magnetic fields following the magetic poles on the rotor. This pushes the rotor in the same direction it's currently spinning, so the device is behaving as a motor. If the winding fields were ahead of the rotor poles they would push the rotor in the direction opposite it's spin. This tends to slow the rotor down. It also forces current through the windings, so the device acts as a generator. The windings are connected via a rectifier to the battery, so whenever the voltage on a winding exceeds the battery voltage, current flows to charge the battery. When the winding voltage is lower than the battery, the rectifier doesn't let current flow from the battery.

There is a separate circuit consisting of 6 power transistors in an "H-bridge" which is controlled by the ECU to allow current to flow from the battery through the windings in the correct amount and direction to set up the desired magnetic fields in the windings.

The H-bridge is an active circuit which draws power from the battery to set up the winding fields to create the desired torque in the same or opposite direction as the rotor spin. When it's set in the opposite direction, the passive rectifier circuit converts the AC winding current into DC current that charges the battery.

So during regenerative braking, the MG torque opposes the motion of the car, slowing the car and recharging the battery.

The axel of the MG's rotor is connected to the wheels via the final drive assembly consisting of a chain and several fixed gears, a differential and a pair of drive shafts (one to each front wheel).
 

·
Registered
Joined
·
130 Posts
Robert!

Thank you so much. This is what I've been looking for. I want to be able to understand the inner workings of the regen braking system in the same way I understand the power split device. I have been all through Graham's site but the info there is too limited and simplistic for the type of understanding that I'm trying to gain. I don't mean to take away from his fine efforts in any way. His is a great source of info.

I think that the inner workings of the regen system is just as interesting as the hybrid drive system. Now that my appetite is whetted, is there a source where I can learn more about this or can you explain this in a little more detail? I think the H-bridge is what I would like to know more about. I hate to ask, as I know this would be very involved, as you say. I look forward to your response.
Ray
 

·
Registered
Joined
·
928 Posts
Hi Ray,
There was a fantastic series of articles in Embedded Systems Programming magazine in the Sept. 2001 timeframe when I got my car. It explained much of the theory of how to control torque in a brushless motor like the ones used in the Prius. The H-Bridge circuit they described was very similar to the Prius inverter circuit described in the Prius New Car Features May, 2000 book from Toyota. If you don't have the book, you can get the picture from the Yahoo! Prius group. The main difference is that the Prius drawing also shows the rectifier diodes.

When I get in to work Tuesday, I'll post the exact issues of Embedded Systems. I hunted around their website but couldn't find a way to view back issues, so you may have to hunt at a library.
 

·
Registered
Joined
·
22 Posts
Don't forget some links to other good descriptions of how the Toyota system works:

http://auto.howstuffworks.com/hybrid-car14.htm

and

http://auto.howstuffworks.com/hybrid-car14.htm

The second one comes with annoying popups, but does have good info.

For the record, three phase AC Inverters driving "Brushless DC" motors, AC Synchronous motors, and regular old squirrl cage induction motors have been around for a long time.
In industry, we have been switching from DC motors over to AC motors in the past few years, as AC Inverters have become extremely efficient and easy to control in high-performance applications.
We use what are known industrially as "AC Vector Drives", and apply them daily in the 1 to 500 horsepower range. The real breakthroughs in AC Drive technology is two fold. One part, is using microprocessors to mathmatically model the motor, and seperate out the actual Torque-generating field component from the Magnetizing current. This isn't necessary on the Prius, as it has a synchronous (permanent magnet) motor. The second advance is the use of high power HIGH SPEED transistors to actually drive the motor windings.
[/url]
 

·
Registered
Joined
·
928 Posts
Ok, I have the Embedded Systems reference now. I completely blew the year though, It was August and September of 2000. I recalled reading about it right when I was getting my new Prius, which is a 2001, so I foolishly put that as the year. I actually bought Pikachu in Sept. 2000.

Anyway, Don Morgan devoted two months of his "Spectra" column to controlling motors (including the kind in the Prius) with digital signal processors (DSPs). The first column was in the August 2000 edition of Embedded Systems Programming, pp. 17-20 and the second column was in the September 2000 edition, pp. 179-186.

It appears from context, and my vague recollection, that Mr. Morgan also covered the subject in the October 2000 issue, but I can't find that issue. I think he got a bunch of letters asking for further detail, because the subject he said he would cover in October was really covered in November and that article says the previous column was "hijacked". I probably lent that copy to someone, so it will take additional tracking down. If I find it, I'll post article reference details.

I checked the http://www.cmpreprints.com website, and it appears that for mere mortals who just want one copy of a back issue/article, you have to call Neeta Acharya on 1-800-790-5970.
 

·
Registered
Joined
·
68 Posts
brakes

I saved this some time ago and I believe it was Wayne's. There was some dispute at the time w. Vitaliy and I believe the whole thing is very complex, but here is what I have.
The following is a description of the Prius brake operation. Lots of
things switch on different operating conditions. One of these
devices switching is probably what you hear. The only time the brake
pedal pressure is applied directly to the wheel cylinders in on a
hard application of the brakes, on a system malfunction, and maybe
when the car is not turned ON. The car has a separate hydraulic
system that normally applies the pressure to the wheel cylinders as
calulated by the computers.

"NORMAL BRAKE OPERATION (With Regenerative Brake Cooperation control)

During normal braking, the master cylinder cut solenoid valves are
closed and the fluid pressure circuits to the wheel cylinders remain
independent. Accordingly, the fluid pressure generated by the master
cylinder will not directly cause the wheel cylinders to acutate.

The skid control ECU calculates the regenerative brake force value
out of the required brake force and transmits the calculated value
to the HV ECU. Upon recieving the value, the HV ECU generates a
regenerative brake force. At the same time, the HV ECU transmits the
actual regenerative brake force value to the skid control ECU. The
skid control ECU controls the solenoid valves in order to cause the
hydraulic brake system to generate a brake force value (which is
obtained by subtracting the regenerative brake force from the brake
force value required by the driver)."
 

·
Registered
Joined
·
928 Posts
Alright, I finally figured out how this web thing works 8)

You can access back articles in Embedded Systems at their archive website.

Look at Don Morgan's spectra columns for August, September and October of 2000.

Once you know how to control the torque in an electric motor, just point it opposite the motion of the car and conservation of energy will transfer most of that energy of motion to chemical energy in the battery. All it needs is a path, and the rectifier bridge provides that path.
 

·
Registered
Joined
·
15 Posts
It seems to me thee are four states.

Electricity is generated by the movement of the car whenever you ease off the throttle. It is also generated when braking whilst in D shift and also when B is selected on the gear shift, with and without simultaneously braking.

What I would like to know is do you get more electricity by braking in the D position or by not braking in the B position? When going downhill or approaching a speed restriction I often avoid braking altogether by selecting B because I thought that some extra gearing got thrown in to produce greater engine braking and that this generated more electricity than you get by just lifting your foot off the throttle and applying the brake in D.

Cheers
Nick
 

·
Registered
Joined
·
928 Posts
In B mode, more electricity is generated than in D mode (foot off all pedals in either case). But in B mode, less electricity is stored in the battery than in D mode. This is because in B mode, the smaller motor (MG1) uses most of that electricity to spin the unfueled engine (against its will). If you push the brake pedal in D mode, it will generate more electricity. If you push it hard enough, it will generate more electricity than B mode does, and all of it will be sent to the battery (which stores around 80-85% of the electricity sent to it).

Technically inclined, please substitute "electric power" or "electric energy" as appropriate for the uses of "electricity" above.
 

·
Registered
Joined
·
15 Posts
Thank you Robert for that clear explanation. This is something that has been puzzling me since I bought the car.

So it looks like it is better to stay in D and use the brake in order to top up the battery and only use B when we want to avoid braking for a long period like on a long shallow incline.

Nick
 

·
Registered
Joined
·
928 Posts
Nick said:
So it looks like it is better to stay in D and use the brake in order to top up the battery and only use B when we want to avoid braking for a long period like on a long shallow incline.

Nick
Yep, use B when you're on a long downhill stretch to avoid (or reduce) riding the brakes for a long time. Stay in D for short term slowing/stopping.
 

·
Registered
Joined
·
130 Posts
Thank you so much for the link Robert. This clears up the mystery for me. Although the info is not Prius specific, I assume that it fits our system but I would be surprised if the Toyota engineers didn't come up with some super duper way to increase efficiency in their implementation of the concept.
 

·
Registered
Joined
·
299 Posts
Wow, lots of great info here!!

Here are a couple of questions which may be obviouse or not:

1) Is the electric motor also the generator?
2) If the answer to #1 is yes, how is it possible for the electric motor to provide power to the wheels AND charge the battery at the same time, like my monitor sometimes shows?
 

·
Registered
Joined
·
220 Posts
There are two motor/generators, MG1 (main motor) and MG2 (main generator). They can both be used as either a motor or a generator.

As I understand it, the smaller generator (MG2) charges the battery when the ICE has extra energy beyond driving the wheels, and when the car is coasting, and the larger (MG1) is used for charging the battery during braking. Being 50 KW, it can provide enough resistance to slow the car with real braking power.
 
1 - 20 of 29 Posts
Top