Brushless DC motor as AC Generator (Automotive Application)

[gordguide]

Greetings.
I am exploring the replacement of a DC Generator with an AC Generator (aka "Alternator").

Briefly this is an attempt to modernize a classic automotive electrical system. The 1950's era Generators have a few issues I would like to address. One, reliability ... although they are generally reliable with appropriate maintenance there are some common issues. Batteries can be destroyed by certain operating conditions (excess heat in battery); low RPM output is problematic and often does not meet load requirements.

The original 12V DC Generator is rated at 14~15A @ 72F and can be expected to output 10~13A at typical operating temperatures. Although this is broadly sufficient for basic operation, it has proved to be somewhat low with night lighting, etc. Modern versions of the vehicle upon switching to the 12V AC Generator system were rated at 32A.

WE cannot simply adopt the later OEM AC Generator because it's adoption requires significant physical modifications. Electrical modifications are not an issue.

Design Constraints:

-Although 6V and 12V Generators were used, we can limit our requirements to a 12V system.

-Physical Dimensions:
OD: 95.1mm 3.744"
Length: 233.6mm/9.197"

Typical Automotive alternators that would otherwise be suitable generally have a larger OD than required; length is usually not a problem. Brushless Permanent Magnet DC motors are more likely to resemble the original Generator physically, which is important.

-Generator RPM

Should have suitable output at 1000 RPM; OK if battery is needed to supplement output for full current demand but not ideal
Should be able to charge battery and operate systems at 3500 RPM; lower is better
Should be capable of operating at 10,000 RPM; if necessary we can shave this downward 1,000 RPM but not much more. Drive gear ratio cannot be easily changed.

Output: 20A would be nice; whatever we can get above that is a bonus; I see no need to exceed 30~40A.

Now, cutting to the chase:

Which criteria should I be looking at for specifying a suitable Brushless DC motor? I am not an EE and am a little confused by the typical specifications since they are assuming motor use.

 

[waross]

The reason for the large diameter of automotive alternators is to increase the peripheral speed of the rotor to improve low speed output.
A Brushless Permanent Magnet DC motor is actually a type of AC motor which will not run on straight DC, it needs a controller to invert the DC to AC in order to function.
The voltage output will be proportional to the speed.
If you require a charging voltage of 13.8 Volts at 1000 RPM you can expect 138 open circuit Volts at 10,000 RPM. You will have to design a voltage regulator circuit to accommodate this range of voltages.
I am not saying that you can't do it, just making you aware of some of the challenges you may face.
An option that you may not have considered is to modify the existing generator. You may be able to locate a grade of brushes that are suitable for higher currents. The armature may be rewound with state of the art wire insulation that will allow the armature to run hotter and safely output a greater current. Rewinding the field may permit charging at lower RPMs. This will work if the magnetic circuit of the generator is not close to saturation. If the rewinder is able to add a few turns to the armature coils this will also allow charging at lower RPMs. After the modifications to the original generator the original voltage regulator may still be useable if it can be adjusted to limit the current at the higher current level.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gordguide]

Thank you for the excellent reply.

Yes, I am aware that decent charging voltage at 1,000 RPM is unlikely ... even automotive alternators rarely are capable of that, unless perhaps very lightly loaded.

I think it would be sufficient if charging voltage were available at higher RPM ... I believe some V-8 automotive alternators generally can charge the battery at roughly 1800 engine RPM and it is quite likely that alternator RPM is higher still. It's my understanding that common 65A GM "1-wire" alternators will not attempt to charge until the engine RPM has "flashed" to 1800 RPM via a throttle blip.

I didn't mean to imply that I feel a need for 13.8+V at 1,000 generator RPM; I envision that the battery would be doing most of the work there.

However we do need the ability to charge the battery at some reasonable speed; thus my estimation that if a charge voltage were available at 3500 generator RPM (with a 1.4 ratio as found on the existing generator drive configuration that translates to 2500 engine RPM, which is within the expected RPM in daily driving). A bit lower would be nice but not absolutely necessary.

So, perhaps 14.4V @ 3500 generator RPM, falling to whatever below that, and capable of safely running to 10,000 generator RPM without damage. I understand the voltage regulator must deal with excess voltage above the safe charging limit, perhaps 14.8V.

The original DC generator tended to create higher voltages than that which leads to battery failure. It also requires a rather large battery to absorb over-voltage in general. I was hoping modern AC generator (alternator) regulators could do their part in eliminating that particular problem.

 

[gordguide]

Further to my above post although I haven't explored it fully, I was thinking of purchasing the high current GM "1-wire" alternator rebuild kits, which sell for perhaps $30 and then incorporating the components for the built-in regulator used on these models and which are always included in the kit.

There are 80A to 130A "high output" variants of this AC-Delco OEM alternator used in high load vehicles as well as the 65A versions commonly fitted as base alternators in GM vehicles up to perhaps the late 1980's, and the rebuild kits are available for all the variants.

These are brush-type alternators so if that creates issues regarding the voltage regulator as compared to a brushless AC motor that would be good info to know.

Also, assuming this is still a workable idea, I still need some guidance as to what specifications to look for when perusing motor catalog listings that would be useful for re-purposing the motor as an AC generator.

For example they will list voltage and watts to drive the motor. Is this useful information that can be used to predict voltage output and current (amperage) capability?

 

[waross]

The brushes in an automotive alternator carry the field current, not the load current.
If you can fit an automotive alternator into the space available that will be the best solution.
Full load current will be limited by the motors ability to reject the heat, whether that current is input into a motor or output from a generator or alternator.
If you want an alternator that will output 35 Amps, select a motor with a maximum current rating over 35 Amps.
Go somewhat greater to allow yourself a "fudge" factor. The relationship between input current and output current may not be as crisp as it is with conventional DC motors or synchronous or induction motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wayne440]

Consider a permanent magnet alternator, rectifier/regulator assembly as used on small Kubota tractors. They are commonly available in 12V, 14A and 12V, 19A rating and may be small enough to use along with a false rear housing to resemble the OEM generator.

Here is a page with examples, one of many sources.

http://www.iowamotorparts.com/kubota_alternators.htm

 

[potteryshard]

There are a variety of alternators constructed to resemble generators for the vintage car markets. I don't have any references at hand, but have stumbled across ads for such replacements on many occasions.

 

[gordguide]

" ... The brushes in an automotive alternator carry the field current, not the load current.
If you can fit an automotive alternator into the space available that will be the best solution.
Full load current will be limited by the motors ability to reject the heat, whether that current is input into a motor or output from a generator or alternator. ..."

So, If I understand you correctly, I will be dealing with the load current and must use a regulator that is appropriate for that, not a conventional brush-type alternator regulator. Also, the Brushless DC motor's ability to reject heat is a prime factor in assessing suitability for this purpose.

Yes, I understand that an automotive alternator is the best solution. More on that following.

" ... If you want an alternator that will output 35 Amps, select a motor with a maximum current rating over 35 Amps.
Go somewhat greater to allow yourself a "fudge" factor. ..."

Am I to assume by that you mean select a motor with a current rating a bit above 35A @ xV; ie 35w x 13.8v = 483 watts so I would be looking for a motor that is rated at or near 500 watts? Could I instead select a motor rated at 35A @ 24V = about 250 watts (these seem to be more common in the catalogs).

" ... Consider a permanent magnet alternator, rectifier/regulator assembly as used on small Kubota tractors. They are commonly available in 12V, 14A and 12V, 19A rating and may be small enough to use along with a false rear housing to resemble the OEM generator.

Here is a page with examples, one of many sources.

http://www.iowamotorparts.com/kubota_alternators.htm

..."

From your link and further links on that page:
I have looked at the Kubota (diesel) tractor alternators, they seem popular for those seeking what I am also seeking. I am also familiar with the Alton alternators (France mfg) for similar vehicles.

A problem I've found in considering the Kubota is very little information seems available regarding the physical dimensions of the alternator. This is a common issue when you are trying to re-purpose an OEM replacement part ... catalogs simply list it as a direct replacement for a given application without further specification. I may try to examine the alternator at a shop that sells/repairs alternators. It would also be nice to see a graph of output vs RPM with the recommended regulator to see where the "knee" of the output is ... ie at what alternator RPM it is outputting maximum amps and at what output the regulator kicks in to limit voltage, and what that limit voltage is.

Apparently Alton has explored this application about 10 years ago and decided against offering a product. Altons use a gear drive to increase the alternator RPM vs the engine RPM. This requires an offset drive which is not ideal; possibly not practical, in this application. A centre-driven shaft is ideal.


Thank you both for your helpful and informative replies.

 

[waross]

" ... The brushes in an automotive alternator carry the field current, not the load current.
If you can fit an automotive alternator into the space available that will be the best solution.
Full load current will be limited by the motors ability to reject the heat, whether that current is input into a motor or output from a generator or alternator. ..."

So, If I understand you correctly, I will be dealing with the load current and must use a regulator that is appropriate for that, not a conventional brush-type alternator regulator.

Conventional automotive alternators control the field strength which in turn affects the voltage. A PM type alternator has a fixed field strength and other means of regulation must be used.

Also, the Brushless DC motor's ability to reject heat is a prime factor in assessing suitability for this purpose.

" ... If you want an alternator that will output 35 Amps, select a motor with a maximum current rating over 35 Amps.
Go somewhat greater to allow yourself a "fudge" factor. ..."

Am I to assume by that you mean select a motor with a current rating a bit above 35A @ xV; ie 35w x 13.8v = 483 watts so I would be looking for a motor that is rated at or near 500 watts? Could I instead select a motor rated at 35A @ 24V = about 250 watts (these seem to be more common in the catalogs).

Forget Watts. There are electrical watts, mechanical Watts, the Watt rating may or may not include losses. Heat (BTUs) is determined by the resistance of the device and the square of the current. Double the current and you will quadruple the heat generated.

Consider a permanent magnet alternator, rectifier/regulator assembly as used on small Kubota tractors. They are commonly available in 12V, 14A and 12V, 19A rating and may be small enough to use along with a false rear housing to resemble the OEM generator.

Here is a page with examples, one of many sources.

http://www.iowamotorparts.com/kubota_alternators.htm

..."

Good advice.
From your link and further links on that page:
I have looked at the Kubota (diesel) tractor alternators, they seem popular for those seeking what I am also seeking. I am also familiar with the Alton alternators (France mfg) for similar vehicles.

A problem I've found in considering the Kubota is very little information seems available regarding the physical dimensions of the alternator. This is a common issue when you are trying to re-purpose an OEM replacement part ... catalogs simply list it as a direct replacement for a given application without further specification. I may try to examine the alternator at a shop that sells/repairs alternators.

It would also be nice to see a graph of output vs RPM with the recommended regulator to see where the "knee" of the output is ... ie at what alternator RPM it is outputting maximum amps and at what output the regulator kicks in to limit voltage, and what that limit voltage is.

With a fixed field the voltage is quite linear with speed.
Automotive regulators are voltage controlled. The current is determined by the difference between the alternator voltage and the battery voltage, and the resistance (and impedance) of the complete charging circuit.
Load dump events often result in transient voltages that may exceed 100 Volts. In years past there were kits available to modify the regulation circuit so that about 90 Volts or 100 Volts could be produced for the operation of small drill and grinders from a pickup engine. The downside was the speed that the engine must be run to turn the alternator fast enough to develop 100 Volts. eg: wide open. I haven't seen these kits since the original fuel price crisis.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wayne440]

I have a Kubota lawn mower with such an alternator, I can provide approximate dimensions if that will help. Off hand it appears to be somewhat smaller in diameter than a typical 1960's automotive generator. My thinking was that you might fabricate a "can" on to the rear to simulate the shape of the OEM unit, and perhaps even fit the rectifier/regulator inside said can.

 

[gordguide]

Thanks again guys.

The "Kubota" alternator is a Nippon Denso mfg Part# 100211-1670 and/or 100211-4520. This is a "one-wire" alternator with built-in voltage regulator. The (voltage regulator?) "hump" at the rear of the housing is a problem; the unit may need to be dismantled and if possible whatever components are sticking out there removed.

Assuming that went well, the mounting ears must be removed and then the housing itself turned down on a lathe to meet the maximum OD for my needs. If a thin-wall housing were used ... say Stainless Steel of perhaps 5mm ... you *might* be able to get it to fit.

I realize now I had looked at that option previously in my research, prior to contemplating the brushless motor idea.

I also found some very sketchy information on alternators used on Suzuki automobiles ... very small cars ... sold in China and India markets only. These use 3-cylinder engines of perhaps 60 HP. Those do look promising size wise in comparison to the Nippon Denso unit but again I met the limits of trying to find any data whatsoever on an OEM-replacement part, so that's still an unknown. I have a business associate in Shengen China I might be able to get to dig up the information; he has a Chinese speaking wife. So far I haven't explored that further.

The original idea of trying to find a brushless motor substitute came from research that led me to Magneti Marelli who have a product for F1 race motorcycles that is very similar save for the characteristics whereby it doesn't generate significant power until 5,000 rpm and is designed to run to 20,000 rpm. Product literature indicated it was created by modifying a stock brushless DC motor.

Similarly, McLaren makes a possibly suitable unit that is designed for " ... single-seat racing cars and motorcycles ..." that has very attractive specifications. They manufacture regulators specifically for this type of unit (one 25A and one 60A).

Naturally I don't necessarily want to be buying Formula1 parts.

However, interestingly, McLaren state that this type of unit:
" ... Permanent magnet alternators can tolerate high ambient temperatures and vibration allowing them to be directly mounted to racing engines. The separate regulator can be mounted in a location where the temperatures and vibration levels are lower. When used together with the DC2 “Buck” regulator, the alternator provides high current, at stable voltage, over a large range of rotation speeds from a very small and lightweight package. ..."

I will continue my explorations; again the responses here are excellent. Thanks again.

 

[ScottyUK]

Have you looked at motorcycle alternators? Might be a bit shy on current but definitely smaller than the typical automotive type.

Some bike alternators are definitely PM machines with an external regulator in the high current path (which you could hide somewhere else) rather than wound field types.


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If we learn from our mistakes I'm getting a great education!

 

[potteryshard]

This looks like a good product:

http://www.gener-nator.com/orders.htm