Quick Question re: Inverter

Prius MG's are 3 phase AC motors and 3 phase AC generators.
Also, the A/C compressor is driven by 3 phase AC motor.
Therefore, we need DC to AC inverters.
The 12V aux battery is charged by DC to DC converters.
Plese refer to page-13 of following article.
http://www.autoshop101.com/forms/Hybrid02.pdf

Ken@Japan

 

One point that may make it easier to understand is that the big inverters are converting the DC to a three phase AC at the exact frequency needed to match the engine speed and wheel rpm. This requires the computers in the car to figure out the exact frequency needed by each motor-generator and then program the inverter to generate this exact AC frequency.

 

Hi M8s,

You cannot step up or down DC without first converting it to AC, or undulating it in some fashion BTW. That is one of the synergies of the AC electrical power distributions invented by Tesla. By generating the power as high Voltage AC, one can transport it over great distances (because as AC it travels as an electromagnetic wave between the wires, more than a actual transportation of electrons as with DC). Then once at the destination the AC can be stepped down with a simple transformer to house-safe voltages.

The big advantage of an inverter (as was previously stated) and synchronous motors is that speed control is a simple matter of frequency control.

An Automotove Alternator typically includes a three-phase diode rectifier pack to convert the AC into DC besides the electrical machine which functions as a three-phase AC generator. Its is somewhat symantic to call the Prius electrical machines "motor/generators". But, as they are seperate from the inverter (which functions as the diode rectifier pack in generation mode) the electrical machine is a AC generator. Without the inverter, its not a full alternator. But this is only Automotive terminology.

The reason they use AC synchronous electrical machines is reliability. The winding can be the stator, and the comuntation is done by the Inverter. Not a sliding brush mechanism, that generates sparks, EMI/RFI, carbon dust and wears out. Why they have not done this with more consumer goods (electric power tools for example) I know not. Some asian washing machines have gone transmission-less and use the AC synchronous motors to get the speed/torque ranges neccassary.

 

AC is transported across wire much better than DC because it is very nearly lossless. Due to the nearly constantly changing polarity and levels in AC current, the losses almost cancel themselves out. There is electron migration in a wire carrying AC, but it is very slow.

 

The main reason that they have not is cost. Built-in obsolescence doesn't hurt either (from the manufacturer's viewpoint).

 

Here is another more intuitive way to look at this. All electric motors work by magnetic attraction and repulsion. An electromagnet in the motor pulls a magnet toward it, then as it passes, pushes the magnet away. Think of yourself spinning a merry-go-round at a playground as you stand next to it. You reach out and grab one of the bars, then pull it toward you. You keep pulling until the bar is right in front of you, then continue the motion by pushing the bar away as it passes. Now you reach back and grab the next bar, pull it toward you, and push as it passes. You have to synchronize your pulls and pushes with the speed of the merry-go-round.

This same thing is true with motors. The motor must switch its pulling and pushing to synchronize with the rotation of the motor. If it just pulled and kept pulling, the motor would wiggle a little and then lock into alignment and move no more.

Conventional DC motors synchronize the pulling and pushing of the electromagnet by using a mechanical commutator. The commutator is a ring of copper plates and carbon brushes. The copper plates rotate under the brushes and act as electrical switches, switching the current back and forth in concert with the rotation of the motor. This alternating current causes the electromagnet to pull and push as needed at exactly the right time, so the motor will spin.

Brushes and plates are fairly inefficient. They produce friction, sparking, and wear out with time. Modern DC motors are called "brushless" motors because they replace the brushes and commutator with electronics. Transistors are used to switch the current at the proper time.

When modern brushless motors get large, like those used in the Prius, the electronics are usually housed in a separate box. This separate box is called the inverter or controller. The inverter takes DC current and converts it into the back and forth current needed by the motor. It's just a fancy and much more efficient commutator.

Tom

 

Hi M8s,

Maybe, you thought a DC motor, such as used on low cost EV's or golf cart, is good for Prius.
Prius uses the IPM (Interior Permanent Magnet) motor which is far more efficient than a DC motor.
JMAG : Inductance Analysis of Interior Permanent Magnet Motor (DP)
TOYOTA: Company > Technology

Ken@Japan

 

Price. A PM (permanent magnet) or wound field DC brush type motor is much cheaper than a syncroness AC motor because you are required to have a frequency drive controler to run the syncroness AC motor. As the price for frequency drive come down you will see more of them being used.

 

Actually, that really isn't true. HVDC line transmission has become much more popular in the last decade, due to much improved solid-state IGBT's and sharp reductions in the price of such components

There are a few companies that specialize in HVDC systems, such as ABB

The ABB Group: Ultra High Voltage DC Systems

Typically, the break-even when you contrast a given power capacity HVDC to HVAC, is around 50 km for undersea installations, and 1000 km for land

HVAC grid interconnection is complicated due to the fact it's difficult to maintain synchronization over long distances. This frequency "drift" is well documented in the Transmission and Distribution industry. Failure to maintain tight intertie synchronization can lead to some pretty spectacular failures.

There are Rube Goldberg gadgets that allow auto-synchronization for very long distance HVAC power transmission. The one I'm familiar with is the GE Variable Frequency Transformer.

GE Energy - Variable Frequency

The test installation was Hydro Quebec, which not only has to deal with very long transmission distances, but also GIC from solar weather

HVAC not only has "skin effect" issues, you must have a separate conductor for each phase (Hence, the three wires you see on pylons are due to three phase power), and for a given power rating the conductor must be sized larger for HVAC.

The power is still generated with polyphase generators. The first step is to electronically convert AC to DC, which is rectification. At the other end, you must convert DC to AC, which is known as inversion.

In theory, bipole HVDC is more efficient at high power transmission, compared to polyphase HVAC, especially at higher power levels and longer distances

 

A little off topic but does arcing or coronal discharge result in x-ray emissions?

Arcing would be a short duration event but given the currents, I would expect a burst of x-rays that after changing one's underwear, might lead to caution about voluntary medical exposure. Just to keep the rad dosage low.

Coronal discharges both AC and DC are another matter and having had a Tesla coil as a teenager, something to be respected. I inadvertently caused ozone bronchitis in my family (Gosh what a fresh smell, ozone in the morning!) Still, I didn't have the means (or thought) to check for x-ray emissions.

Bob Wilson

 

The only experience I have in that area is a MURI/ARPA project around 12 years ago. The goal of the project was to determine how rapid flight through atmosphere and into exoatmospheric areas, resulting in corona discharge, would impact vehicle power systems.

We found out that most of the pulse peaks from that sort of corona occurred around 17 kHz

Corona discharge under carefully controlled conditions will return nuclei - charged condensation nuclei - that can be experimentally collected

An arc flash event - say a flashover in a three phase motor control center - will generate plasma. The plasma arc will have most of the radiated energy ranging from infrared to ultraviolet. Consider that most circuit interrupters have a dead short rating of around 20,000 amps.

Say a phase-to-phase arc flash event, you can expect the circuit protection to not drop out for several cycles, possibly ten cycles. The energy from such an event is devastating, over a megajoule. I've seen metal electrical panels twenty feet away with severe ablation and erosion from an arc flash event

THis is a pretty horrific security camera shot of three electrical utility workers involved in a switching arc flash event. WARNING: Not for the quesy or faint of heart!

Magna Electric - Video of Arc Flash Incident

Generating x-rays is a bit more involved than a simple arc. The frequency is typically from petahertz to exahertz range, or, put another way, from 0.001 to 10 nanometers. The energy can range from 120 eV to 120 keV

A-ha! This PROVES you're an Evil Genius! I had always suspected it

Corona can occur on HVDC and HVAC. But the emission of other energy is dramatically lower with HVDC. A primary reason for new and proposed HV transmission lines in Scandinavian countries is due to the perceived health danger from radiated EMF caused by HVAC power lines.

In contrast to HVAC, HVDC has negligible EMF emissions.

 

Aren't DC systems also much more prone to electrolytic effects
anywhere there are dissimilar metals?
.
_H*

 

Glad you commented on this Jayman, because I too thought the comment regarding A.C. high power distribution was not correct. The high power intertie bringing electrical power from Northern California to Southern California is one such HVDC line I believe, courtesy I believe of ABB group, the latter being the company formed from the merger in 1988 of ASEA in Sweden and Brown Boveri in Switzerland. The Swedes and the Swiss are no slouches when it comes to high-class engineering.

Related to Bob's comment on ozone, I well remember when I was a student in London I did a lot of traveling on the Tube, which uses a relatively low voltage/high current third rail to carry power. The pungent smell of ozone as trains transited the station was very noticeable at times, caused by the arcing between the rail and train pick-up shoe.

 

The simple answer is: so much more can be done with AC. Even more thanTesla ever imagined. Tesla and Edison had constant arguments over using DC ( Edison ) or AC ( Tesla ) If it had been up to Edison we would have batteries all over the place, with thier inherent dangers, poor Edison, he did not know Ohms law and didn't want to learn it. We can Thank Tesla for the design of the present day Motor/Generators, still relatively unchanged.

 

Yes. High voltage DC transmission lines produce a constant magnetic field, so electrolysis effects occur in one direction. AC lines tend to cancel out the effect since it goes back and forth. Obviously both can cause problems, but you are correct that DC offers more of a challenge.

Tom

 

In a marine environment with a suitable electrolyte - seawater - absolutely, yes! This is galvanic corrosion. HVAC systems can also suffer from strange corrosion effects, as the EMF will attract dust and chemicals out of the environment, resulting in rapid corrosion

Especially at an industrial site, there is very high risk of galvanic corrosion. There is a special glop you're supposed to use on the splices. The materials selected are carefully engineered to minimize corrosion effects