New 42-V Alternators for Automobiles

[MolaKule]

New 42-V Automotive Generator's based on Permanent Magnet Rotors.
By MolaKule

The following excerpts are from IEEE Transactions on Energy Conversion, Mihai Comanescu, Ali Keyhani, and Min Dai, Design and Analysis of 42-V Permanent-Magnet Generator for Automotive Applications , Vol. 18, No. 1, pages 107-112, March 2003. These excerpts are for educational purpose only.

"The actual trend in automotive design is to replace some of the mechanic and hydraulic systems with electric systems. As the cost of digital control technology keeps decreasing, the use of of new electric and electronic devices is expected to provide new features and improved performance as well as increased comfort and customer satisfaction."

"The actual power demand in an automobile [today] is in the range of 1.2 to 1.5 kW. The automotive electric system in use for the last 30 years is based on the Lundell machine. This is a three-phase synchronous generator equipped with a field winding and brushes. Its output voltage is applied to an ac/dc converter. The output voltage is controlled by regulating the field current [to the spinning rotor, which creates a changing magnetic flux]….Thus, no control is needed for the converter, a six-diode bridge rectifier. Despite the relatively low performance of the Lundell machine, this system was for many years the best compromise between efficiency and cost."

"With new loads being added to the automotive power system, it is estimated that by 2005 the power demand of a typical car will be in the range of 2.5 to 3 kW. Examples of such loads are electric air conditioners, electric steering systems, electric brakes, or high-energy discharge lamps. At this level of demand, the system based on the Lundell machine becomes inefficient."

"The losses in the alternator are significant and the increased currents require thicker wiring harnesses. The cost of the system increases while the performance drops. Considering the concerned for improved fuel economy and reduced emissions, the need for an electric system with improved efficiency can now be justified with energy savings. With the new [voltage] level of 42-V as standard [system] voltage, automotive companies have started to examine the design of electrical systems and loads to ensure the transition. Various solutions for the future power systems are examined in [references]. Recent advancements in permanent-magnet (PM) materials technology make the PM machine an attractive solution."

Summary by MolaKule:
The new alternator will generate 42-Volts at 75 Amps for a power rating of 3 kV, at 1,800 RPM. The Power Factor is 0.854. The field required by the Permanent Magnets is about 1.6 T. The overall efficiency is a remarkable 90%. There are three phases of power, with each phase displaced by 120 degrees with respect to the other. The alternator has 16 coils per phase (for a total of 48 coils) with two (2) turns per coil. Each conductor is AWG 17 guage wire. Each phase is rectified by a two-diode "full-wave" rectifier system.

The external diameter is 165 mm, the external length is 124 mm.

[ April 25, 2003, 02:09 PM: Message edited by: MolaKule ]

 

[Jay]

This is an interesting topic.

I've heard various voltages mentioned for the new automotive standard from 36V to 42V. I've often wondered why they settled on such an oddball voltage. 48V is a widely used telecom standard and there are just tons of switch-mode downconverters, etc. made expressly for 48 volts. Maybe someone can shed more light on why IEEE settled on 42V (or 36V or 38V or whatever).

The standard automotive field-coil alternator is a beautiful design from an electical standpoint. Regulation is very easy and efficient, and the output voltage has good ripple characteristics. It's a very ugly design from a mechanical standpoint, though. The field-coil windings will fly apart from centrifugal force at fairly low rpms, so it has to be geared down and mounted externally of the motor. This takes up a lot of space under the hood.

Motorcycles currently use permanent-magnet alternators which are beautiful designs mechanically. Since magnets rotate and not windings, the magnets can be mounted directly on the crankshaft and do dual duty as alternator and flywheel. This is a compact, lightweight arrangement that needs no bearings. Regulating the voltage is a bear though! The voltage output varies so widely from idle to redline that it's very difficult to regulate. The regulators that the industry has chosen are extremely crude, inefficient, and unreliable, and the DC output is very dirty compared to the field-coil design.

The new hybrid cars have the most beautiful alternator mechanical design of all. They use high-output permanent-magnet motor/alternators and they kill 4 birds with one stone. These function as alternator, flywheel, starter motor, and electrical supercharger.

[ April 25, 2003, 07:51 PM: Message edited by: Jay ]

 

[Rick in PA]

I think the voltage has to do with the standard lead acid battery. With about 2 volts per cell, current systems using 6 cells have a nominal voltage of 12 volts. Add a couple of volts for charging and get, about 14 volts when the car is running (I know these numbers aren't exact, just "ballparking"). The new systems under development would use 18 cell batteries for a nominal voltage of 36 volts, with a charging voltage of 42 volts.

I guess they could use 20 cell batteries for 40/48 volt systems. I'm curious why they settled on 36/42 volt systems. I thought the next step would be to a 24/28 volt system. (Sort of like the step from the 6 volt systems to the 12 volt systems, remember them?)

 

[MolaKule]

As with any introduction of new technology, there are always more questions to be answered.

From the references given in the paper, the 42-Volt system is an offshoot from the aerospace industry where 36 Volt batteries are used and the charging voltage is 42-Volts for purely DC systems.

Of course, for commercial and military transports that use 400 Hz AC, the voltage has always been 220-110 Volts AC.

Someone in aerospace did an optimization study and determined that a 42-Volt system represented the optimum Power-to-Weight ratio. As in aerospace, the highest power machine with the lowest weight wins.

Theoretically, with a 42-Volt system, the sizes of starters and system wiring could be reduced by almost 70%. And batteries would have smaller plates, but a larger number of plates.

Enter new battery technolgy, where the Power Density is going up, then electrical systems could represent a significant weight savings.

[ April 25, 2003, 11:26 PM: Message edited by: MolaKule ]

 

[K-Man]

Call me a cynic, but I suspect that 42 V was chosen precisely because it isn't compatible with anything else with common use outside (possibly) aerospace. This would prevent jerryrigging in the field and would require use of proprietary parts and systems.

I'm not convinced that the gizmos the new 42 V systems are designed to run are necessary. They will undoubtedly be expensive to replace, and what will their durability and longevity be in the field? Motor vehicles are becoming increasingly unaffordable, and this trend won't help.

 

[Jimbo]

Where in aerospace is 36/42 VDC used? I am familiar with the 24/28 VDC and the 400 Hz AC systems only.

One reason to go 42v is that it is considered the highest voltage that can be used without risk of injury due to electric shock.

 

[Steve S]

How much will this save me $$$ on my next car? Although brushless alts. are a good idea I cant under stand with all the engineering going on why can't there be less parts in our cars.

[ April 28, 2003, 05:04 PM: Message edited by: Steve S ]

 

[MolaKule]

I have often wondered why they just don't replace the harmonic balancer with a set of permanent magnets and put a coil around it with an external regulator?

Maybe it would see too much heat, although it would be in the airstream to obtain most of it's cooling, although radiator heat might cook it. And corrosive salts could splash it as well?

 

[Steve S]

Molakule,great idea no belts, less parts, as to the dirt problem. we'll have to figure it out.

 

[Jimbo]

The goal is to replace heavy and leak-prone hydraulic systems with electric ones. This will first eliminate the power steering pump and hoses as they go to magnetic power steering. Magnetic brakes would be next, elimiating cylinders, boosters, pipes and hoses. I'm sure people didn't trust hydraulic brakes when first used in the 1920's.

The aircraft industry has been looking at electric replacements for hydraulic systems for years. So far, no one is taking the leap.

 

[Jay]

quote:

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One reason to go 42v is that it is considered the highest voltage that can be used without risk of injury due to electric shock.

---

Possibly, but 48V (52V charging) is perfectly safe if you are dry. You might get a little tingle if you are wet. Household voltage is 120Vrms and nearly twice that in some countries.

quote:

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I have often wondered why they just don't replace the harmonic balancer with a set of permanent magnets and put a coil around it with an external regulator?

---

Regulation of a field-coil alternator is so much easier than a permanent-magnet alternator. But the cost and size of efficient PM alternator regulation is coming down. Integrated PM alternator/starter motors are coming.

I think the biggest benefit of the higher voltage standard, at least for the short term, is that it allows auto manufacturers to use less copper. 12-V hasn't been a good standard for some time now.

 

[Kestas]

Our company is involved in the 42v drive-by-wire FILO car. The press was even at our company when the concept car was making its rounds in the US. The reason they picked 42v was that it is an even multiple of the lead acid battery (14 x 3).