Regenerative braking question 1

[dmh13433]

We were all talking about the Toyota Prius at break the other day, and the question came up about how the 3 phase electric motor turns into a generator when you step on the brakes or flip the switch. All we know is from the manual, it says it is a 3 phase motor that powers the car.
From what I know about 3 phase motors(squirrel cage), you can't make a generator out of it unless you try to overspeed. Or maybe the rotor is different? Maybe one of you guys can answer this question for us.
Thanks

 

[itsmoked]

If you drive a three phase motor that's hooked to a power source it will generate automatically, nothing else is needed. On Prions(as I am want to call them) they obviously are not running on a power network but a Variable Frequency Drive. The VFD takes power from a DC bus capacitors and batteries in this case and creates the AC waveform the motor needs. Well when the motor gets driven instead, the power can be returned to the DC bus and hence the batteries.

Specifically to your question: the motor is "over-sped" by virtue of the VFD driving to a lower speed than the drive train is providing.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

To put some numerical examples to that:
Say you are going down hill, so it's an opportunity to regenerate. Speed sensors tell the VFD that the motor rotor speed, created by the kinetic energy of the moving car through the wheels and transmission to the motor is, let's say 1000RPM to make it simple. If you want to regenerate, i.e. turn the motor into a generator, the relative RPM of the stator rotating magnetic field created by the VFD needs to be slightly slower than the real RPM (said 1000RPM). So the VFD simply creates a 970RPM rotating field in the motor stator windings. The 3% "slip" difference between the two is what makes the motor into a generator, the VFD then takes that electricity and reconverts it back to DC and pumps it into the batteries.

Largely oversimplified, but that's the basic concept behind the technology.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Warpspeed]

That is correct.

What happens is a rotating magnetic field is set up within the motor, and is created by the three phase drive voltages (plus a small magnetizing current).

If the rotor is spun up to be at EXACTLY the same Rpm as the rotating magnetic field, no torque is generated, and only the small magnetizing current for the windings is consumed from the source.

If the rotor slows below synchronous speed, current will be drawn from the voltage source, and it acts like a motor.

Torque fed into the rotor (attempting to overspeed it), will also produce a current, but it will be exactly opposite in phase and flow back into the voltage source.

The three phase drive voltages do not change, but current flow mirrors torque flow, and it can flow in either direction.

This can easily be seen on a two channel oscilloscope connected with a current shunt or a current transformer to one channel, and the voltage waveform to the other channel. The current waveform will change in amplitude with torque, and be seen to drop to zero and abruptly change phase as torque reverses.

It is not quite that clean and simple because of frictional losses, copper and iron losses, and the fairly constant magnetizing current, but these effects are relatively minor and can be ignored for this discussion.

 

[jraef]

Forgot to mention. I own a Prius, and you're right, they really don't explain it well in their manual. I'm in the drives industry so I knew intuitively what they were doing (and liked it). Remember however, they are selling a highly engineered complex system not to engineers, but to soccer moms and bean counters. I know that for my wife "slip" has an entirely different meaning and she gets a glazed-over look whenever she overhears me say "rotating magnetic field". Not only dos she not know, she does not WANT to know. She just gets in the Prius feeling good about saving the world and stops less often for gas. For her, even the simplistic blurb in the manual is boring and I feel she is more in line with the majority than I am.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

I worked on an elevated transit system years ago. The propulsion was from a pair of 400 Hp. linear induction motors on each car. There were substations along the line to supply DC to the busbars. As described by itsmoked, jraef, and Warpspeed, the VFD frequency was lowered to below the equivalent linear speed and the units regenerated. It was very effective. You could see and hear the trains de-celerating as they came into the stations. There was a bit of a magnetic hum from the motors. When the train was almost stopped the drive cut off and the service brakes came on to stop the train. Up to that point it was all regenerative. You could hear the magnetic hum stop, and see the mechanical brakes apply.
The transition from regenerative braking to mechanical braking was very smooth. The effectiveness of the regenerative braking was such that the distance travelled from the cutoff of the regenerative braking to dead stop was about 3 feet or less.
Each substation had a large resistor bank to disipate regenerated power if the traffic was light and there weren't enough trains running in a sector to absorb the regenerated energy of the trains stopping.
respectfully

 

[jraef]

A side bonus of the Prius; it now has 87,000 miles, have not replaced the original brake pads yet, and they are only 1/2 worn according to the last inspection 2 months ago. Ever since the loss of asbestos in brake linings I have been lucky to get 40K from a set on my other cars. Any cost savings however will likely be eaten up (and then some) by the first battery replacement, but that's still a ways off.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

I take it jraef; You have one of the originals to have put that many miles on it. To bad they advertise that absurd mileage 60MPG?, provided by the EPA no less. My Mom went to a new owners meeting(Camry) and the 2 to 1 question was why don't I get anywhere near that mileage??

What MPG do you get jraef?

Waross; I had a buddy doing research on the Earth's magnetic field. There was a phenomenon that all the researchers in the field called the "Weekend Effect". It was a fixed shift in the Earth's field that only occurred on week days. After a few years of research they finally stumbled on the fact that it was BART and those long DC rails.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

BART? After some research, I think that it might be the Bay Area Rapid Transport. The rail system in the San Francisco Bay area?

The SL probably does the same. And the RER.

Gunnar Englund

http://www.gke.org

 

[dmh13433]

Thanks for the replies everyone, that was quick!!

 

[jraef]

Keith,
You're right, it's an "oldie" 2001, first batch available in the US. My wife drives it, she gets 45MPG consistantly, but she's a home visit therapist and does a lot of freeway driving at non-commute times. That is actually worse for mileage on the hybrids than city driving, contrary to ICE-only power trains. My sister in L.A. drives a new one (2005) and because she takes back roads to work instead of freeways, she gets 52MPG consistantly.

I agree, the 60MPG is almost pure hype, but to be honest, every MPG rating I have ever seen is pure hype too.

Gunnar, you rode on BART with us from San Francisco! Maybe you were asleep and I didn't notice...

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

Jeff,

Not the cable cars?

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

Yes! Remember now! Those modern cars. Sorry.

I do also remember the skunk - and Peter's Brass Rail and Car Wash. Or was it Brass Wash and Car Rail?

Gunnar Englund

http://www.gke.org

 

[itsmoked]

skogs how could you not remember the unsettling whining howl (or sleep through it)that came out from beneath your BART car occasionally. So loud you can't talk over it????!?!?

jraef; Thanks for the info. That is nice mileage! Better than my V8 T-Bird... Your better half is a PT? Same as mine..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Smoked,

I am used to worse. The Stockholm subway is a lot worse. At least some cars are. And, I wasn't sleeping. No, I wasn't.

Gunnar Englund

http://www.gke.org

 

[jraef]

That's good to klnow, because as I remember it, we were conversing. Had you admitted now to being asleep, I would be insulted!

Pete's Brass Rail and Car Wash, where there is no brass, no rail, no cars and the only washing is done on plates, utensils and beer glasses.

Keith,
We'll have to get together sometime. My daughter is about to choose her University and I think it's going to be UCSC, so we'll unboubtedly be in and out of your neck of the woods for the next 4 years (hopefully not 8!).

 

[UKpete]

The Prius uses PM synchronous machines by the way! Unless they've changed it.

 

[itsmoked]

Hey maybe jraef!

I couldn't count on my toes and fingers how many campus tours I've given for prospective students and relatives.

Let me know if you want one. (prolly past that by now?)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com