# Alternator as a BLDC



## pariah (Oct 10, 2008)

I know that this question will probably cause enough flame for me to be cremated but I'll ask just the same. I've tried to find the answer in all the places that i can think and since I don't know anyone with electronic knowledge I'll ask here. I'd like to know if there is a formula to calculate the power and torque for a 190 amp alternator being driven by 144 volt. There it is so flame on.


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## remy_martian (Feb 4, 2019)

I seriously doubt you could run the alternator continuously at nameplate power (no idea where you got the 144V from). Power is easy...P = i * V * e
You can swag the efficiency at 0.9.

Torque is horribly complex as it's related to machine geometry. Bench test it...


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## kennybobby (Aug 10, 2012)

Maybe that's a typo, did you mean 14.4V ? 

In either case, where and how are you applying the voltage to drive it like a motor--to the stator windings or to the rotor coil or both?

The rotor usually has a coil that is fed 12V for excitation of the magnetic poles using slip rings.

The stator is usually a 3-phase winding with diodes to rectify the AC output.


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## pariah (Oct 10, 2008)

kennybobby said:


> Maybe that's a typo, did you mean 14.4V ?
> 
> In either case, where and how are you applying the voltage to drive it like a motor--to the stator windings or to the rotor coil or both?
> 
> ...


Like I stated I expect to be flamed for this. The voltage is indeed 144. The plan/dream is to chain 3 of these alternators together to drive a vehicle about the size of a Gem. They are water cooled so there is the hope that they can survive. Of course I could use half the voltage but I hope for more. I know that an inverter controller is needed but as to what I haven't quite figured out that one. I thank all who answer.


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## brian_ (Feb 7, 2017)

Unless the alternators are being re-wound, the only point in using much higher than the normal rated voltage would be to maintain torque at higher speed.... but how fast can you spin these things without destroying them?

There is a thread for this: Alter-Motor Project Build Thread.



kennybobby said:


> In either case, where and how are you applying the voltage to drive it like a motor--to the stator windings or to the rotor coil or both?
> 
> The rotor usually has a coil that is fed 12V for excitation of the magnetic poles using slip rings.
> 
> The stator is usually a 3-phase winding with diodes to rectify the AC output.


Yes... unless these are PM alternators (which do exist but are not the most common), the rotor will need controlled DC excitation and the stator will need to be driven by a 3-phase inverter. The need for controlled rotor excitation (normally the only thing controlled in alternator operation) was one of the first complications the author of the above thread encountered when he tried to run his alternator as a motor.

There is one line of production EVs which use a wound-rotor 3-phase synchronous motor and thus have a controller-inverter to handle it: Renault Zoe motor synchronous motor

Due to the complexity of this separately-excited system, and the need to run three small alternator-motors instead of one properly-sized motor, the cost of controller-inverters seems likely to be far more than the system is worth... but it's still an interesting project.


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## olegil (Jan 7, 2012)

Well, I for one think this is an excellent question.
I'm not an expert, but from my understanding, you use the current regulation of the rotor to limit the voltage on the output from overcharging the battery when used as alternator.
When used as motor (as in, ripping out the diodes and hooking it up to a BLDC controller), providing a constant current on the rotor would make it a permanent magnet rotor, no? It's not like a permanent magnet would have any field weakening built in. There's a good booklet out there on alternate uses for alternators, and 115V three phase (though high frequency) is one of the things it explains how to get, by simply bypassing the diodes and setting a constant current (or even a constant voltage) on the rotor slip ring. The author claimed to push a fairly moderate (60amp? not sure) alternator up to I think 6kW or so.
If you absolutely must have field weakening, then a simple DC-DC regulator which adds a current proportional to the speed into the feedback loop (laymans terms, large-ish resistor from speed control leaking current into the feedback) will provide that. You can obviously start with a bench top PSU connected to the rotor and work out the optimal voltage for different speeds and torques, then make something more autonomous later.
Sorry if the electronics parts of this sound too advanced, but it really is very basic, schematics wise. The difficult part of DC-DC regulators is in the PCB layout (basically same problem as motor controllers in general, with large currents and high frequencies generating magnetic fields which can induce currents in the wrong plane and destroy its own components).


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## olegil (Jan 7, 2012)

Btw, which exact 190A water cooled alternator? I know there's one that has a double stator (and rotor?), with 6 windings for two independent outputs. It might be that these can be connected in series to effectively halve the voltage seen by the windings. Will take some extra modifications to get it out of whatever delta/wye connection it has so at least one of the two sets has 6 connection points instead of 3 or 4. But this should not normally be rocket science.


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## brian_ (Feb 7, 2017)

olegil said:


> I'm not an expert, but from my understanding, you use the current regulation of the rotor to limit the voltage on the output from overcharging the battery when used as alternator.


Yes, or more precisely, the rotor current is regulated to control the rotor's magnetic field strength, which then determines the output voltage for a given speed.



olegil said:


> When used as motor (as in, ripping out the diodes and hooking it up to a BLDC controller), providing a constant current on the rotor would make it a permanent magnet rotor, no? It's not like a permanent magnet would have any field weakening built in.


I agree, it would behave like a PM synchronous motor... but those normally shift the phase of the applied voltage at higher speeds, and call that "field weakening".



olegil said:


> If you absolutely must have field weakening, then a simple DC-DC regulator which adds a current proportional to the speed into the feedback loop (laymans terms, large-ish resistor from speed control leaking current into the feedback) will provide that. You can obviously start with a bench top PSU connected to the rotor and work out the optimal voltage for different speeds and torques, then make something more autonomous later.


That makes sense, as long as the intention is that rotor current varies inversely with speed - less rotor current at higher speed.


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## ChazFisher (Sep 30, 2017)

I'm doing some work now with a 2008 GM eAssist alternator, to use as the motor in an electric motorcycle. It looks and behaves as an AC Induction motor, from the controller's perspective. So look for online calculators for an AC induction motor. I _think_ the typical current rating is the DC current, so you'd need to convert that to individual phase current.


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## remy_martian (Feb 4, 2019)

The eAssist is good for about 20hp, isn't it?

Always had my eye on those, just too many projects with 400hp motors in them, lol. 

Please keep us posted on your findings/results/info and share any specs on those.


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## brian_ (Feb 7, 2017)

ChazFisher said:


> I'm doing some work now with a 2008 GM eAssist alternator, to use as the motor in an electric motorcycle. It looks and behaves as an AC Induction motor, from the controller's perspective. So look for online calculators for an AC induction motor...


Right... but a typical alternator is _not_ an induction machine.


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## remy_martian (Feb 4, 2019)

eAssist may not be a typical alternator...


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## brian_ (Feb 7, 2017)

remy_martian said:


> eAssist may not be a typical alternator...


Very true. It functions as the alternator, the starter motor, and a P0-position hybrid machine. It is an induction machine, not a powered-rotor machine.


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## remy_martian (Feb 4, 2019)

Seen any specs for it - if anyone has them, it's you


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## brian_ (Feb 7, 2017)

remy_martian said:


> Seen any specs for it - if anyone has them, it's you


I only remembered the basic system type and thought that it was induction (because GM likes induction rather than PM for machines that routinely spin in idle mode) so ChazFisher's comment looked right.

Here is more detail from GM found in a quick search:

*eAssist details*
The Sierra’s eAssist powertrain is a light electrification system leveraging General Motors’ latest electrification technologies. It builds on the advanced technologies of the 5.3L V8 engine, including direct injection, variable valve timing and Active Fuel Management (cylinder deactivation), complementing them with a compact lithium-ion battery pack to enhance efficiency through:



*Stop/Start capability: *Additional fuel savings are achieved by turning off the engine when stopped at a stop light or heavy traffic and restarting the engine when the driver lifts his or her foot from the brake pedal
*Electric power boost: *The on-board electric motor provides a power boost during acceleration and passing, while enabling the engine to operate in four-cylinder mode for longer periods, for additional fuel savings
*Regenerative braking: *By using the on-board electric motor as a generator, the energy recovered while braking is converted to electricity to recharge the onboard battery system
*Aero enhancements: *A 6 percent improvement in aerodynamics contributes to fuel economy due to a standard soft tonneau cover and automatic grille shutters, which can close at speeds over 30 mph in order to further reduce drag and fuel consumption.

The eAssist system includes a 24-cell, air-cooled 0.45 kWh lithium-ion battery pack, located under the center console (or front bench seat). It adds only about 100 pounds to the vehicle’s curb weight.

A compact induction motor, located on the accessory drive, provides 15 kW of peak regenerative capability, thanks to advanced software controls based on the Chevrolet Volt. It performs like an electric torque booster, providing up to 44 lb-ft (60 Nm) and up to 13 hp (9.7 kW) of additional boost in high load situations. There’s also a liquid-cooled power inverter module located under the hood.


The combination of specs which is a bit surprising is that regenerative power is greater than motor power - usually it's the other way around, but it makes sense in this case. The 24-cell specification, assuming a common lithium-ion chemistry and all cells in series, implies a nominal voltage around 90 V, versus the more typical 48 V for mild hybrids. Perhaps it's 12P2S, for a nominal 45 V and a match to typical practice.

The Ram eTorque system is similar, as are other BAS (belt-drive alternator starter) hybrids that have been offered over the years, but I don't know if all are induction designs.

Very different from a typical alternator.


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## lj516 (Mar 11, 2020)

brian_ said:


> Very different from a typical alternator.


Actually, internally they are nearly identical. They will still have the slip rings used to provide field weakening for a "stock" alternator rotor with magnets placed in the "teeth".


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## brian_ (Feb 7, 2017)

lj516 said:


> Actually, internally they are nearly identical. They will still have the slip rings used to provide field weakening for a "stock" alternator rotor with magnets placed in the "teeth".


No, that's completely wrong. My comment to which you are replying, lj516, was about GM's eAssist, which (as I explained in that post) is an induction machine (motor-generator). As an induction machine, it will have neither a powered rotor winding (and the accompanying slip rings and brushes) nor any permanent magnets at all.

Your description is close to a PM-assisted powered rotor synchronous machine. That would be more efficient than without the permanent magnets, and could be applied to a machine built as an alternator but being used as a motor, but is completely different from the induction machine used in the eAssist system.


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## MineAngerBR (Jan 5, 2020)

Hello, i saw that this thread is about alternators, so i'm here to talk and give my 2 cents on the idea.



pariah said:


> I know that this question will probably cause enough flame for me to be cremated but I'll ask just the same. I've tried to find the answer in all the places that i can think and since I don't know anyone with electronic knowledge I'll ask here. I'd like to know if there is a formula to calculate the power and torque for a 190 amp alternator being driven by 144 volt. There it is so flame on.


Sincerely, I personally never saw people getting anywhere closer to 144V for testing as a Traction Motor, the maximum being 82V max/72V nominal battery packs, and the "medium/standard" voltage range is always between 36-48V nominal.



remy_martian said:


> The eAssist is good for about 20hp, isn't it?
> 
> Always had my eye on those, just too many projects with 400hp motors in them, lol.
> 
> Please keep us posted on your findings/results/info and share any specs on those.


GM's eAssist is known in other forums, like EndlessSphere: Buick Lacrosse 2012 e-assist moto-alternator - Endless Sphere
And ElMoto: eAssist AC Induction Motor
It shows a nice powerful mid-power system for karts or motorcycles being fully responsible for powering it.



lj516 said:


> Actually, internally they are nearly identical. They will still have the slip rings used to provide field weakening for a "stock" alternator rotor with magnets placed in the "teeth".


You're talking about the GM BAS Alternator, also known as Gen1. That was indeed a 36V air-cooled PM-assisted wound-rotor alternator, and unfortunately i never saw any real tests on those, the closest that i got to see people using it was this thread on ES, but with watercooled rotors, as OP is preparing to work with them:


Water cooled car alternator - Endless Sphere


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## Cloter (Nov 11, 2021)

I've asked the same question and got to work to solve it. *Yes*, an alternator can be converted to a semi BLDC ('semi' because you still have the slip rings, that are equivalente to brushes, aren't they?). Yes, like many people said before, you do need to supply a voltage to the rotor. I used an old Chevy 30A alternator (made by Delco-Remy in Brasil) and the rotor measured 4,5 ohms, so driving it with 9 volts pulls 2 Amps. You can do this math to obtain a reasonable magnetic field and keep heating under control. I designed, build and programmed a controller based on an ESP32S micro controller and 100V/50A MOSFETs for the power stage. The whole thing works beautifully. It's the basis for an electric bike that I am building.
And here I have a *question*: the picture shows my attempt at the e-bike 'drive train'. All works well, but the three bolts get very hot. First I thought I was mistaken, then I ran several tests, and the bolts warm-up really quickly and before any other parts of the alternator. I replaced the original (regular) steel bolts with stainless steel, so they would not interact with the magnetic fields, and there was no significant change. I'm thinking 'Eddy currents', 'Foucault Currents'? Any of you have seen this effect? Should I create a new topic to ask this question?


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## remy_martian (Feb 4, 2019)

You asked this in another thread. Why are you splattering the problem everywhere on the site? Pick one...


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## Cloter (Nov 11, 2021)

Cloter said:


> I've asked the same question and got to work to solve it. *Yes*, an alternator can be converted to a semi BLDC ('semi' because you still have the slip rings, that are equivalente to brushes, aren't they?). Yes, like many people said before, you do need to supply a voltage to the rotor. I used an old Chevy 30A alternator (made by Delco-Remy in Brasil) and the rotor measured 4,5 ohms, so driving it with 9 volts pulls 2 Amps. You can do this math to obtain a reasonable magnetic field and keep heating under control. I designed, build and programmed a controller based on an ESP32S micro controller and 100V/50A MOSFETs for the power stage. The whole thing works beautifully. It's the basis for an electric bike that I am building.
> And here I have a *question*: the picture shows my attempt at the e-bike 'drive train'. All works well, but the three bolts get very hot. First I thought I was mistaken, then I ran several tests, and the bolts warm-up really quickly and before any other parts of the alternator. I replaced the original (regular) steel bolts with stainless steel, so they would not interact with the magnetic fields, and there was no significant change. I'm thinking 'Eddy currents', 'Foucault Currents'? Any of you have seen this effect? Should I create a new topic to ask this question?





remy_martian said:


> You asked this in another thread. Why are you splattering the problem everywhere on the site? Pick one...


Ok, you're right. But in my defense: my rationale was to put the question here to call the attention of the people who posted to this thread, and were obviously interested in the topic. Then I realized I could post to a boarder public, creating a new thread - and I did that too. If this goes against the site rules, or shows that I have no web etiquete, I'd prefer to delete this one. Just let me know.


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## brian_ (Feb 7, 2017)

Cloter said:


> *Yes*, an alternator can be converted to a semi BLDC ('semi' because you still have the slip rings, that are equivalente to brushes, aren't they?)


Not a "semi BLDC" because BLDC means "brushless DC", but it's not brushless at all and not DC at all. The typical alternator has brushes running on slip rings to power the rotor, while a traditional brushed DC motor has brushes running on a commutator to power the rotor and commutate (switch windings so it can run on DC). The slip rings are like the commutator (but in solid rings instead of segments), not "equivalent to brushes".


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## Cloter (Nov 11, 2021)

brian_ said:


> Not a "semi BLDC" because BLDC means "brushless DC", but it's not brushless at all and not DC at all. The typical alternator has brushes running on slip rings to power the rotor, while a traditional brushed DC motor has brushes running on a commutator to power the rotor and commutate (switch windings so it can run on DC). The slip rings are like the commutator (but in solid rings instead of segments), not "equivalent to brushes".


Thank you for your very precise explanation on the differences of BLDCs, traditional brushed DCs and typical alternators. We all known that a typical BLDC will never receive DC directly either (but some sort of PWM or sinusoidal current) despite its name. As for my point about the heating of the bolts, any insights?


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## Filekidb86 (7 mo ago)

pariah said:


> I know that this question will probably cause enough flame for me to be cremated but I'll ask just the same. I've tried to find the answer in all the places that i can think and since I don't know anyone with electronic knowledge I'll ask here. I'd like to know if there is a formula to calculate the power and torque for a 190 amp alternator being driven by 144 volt. There it is so flame on.


I use two alternators on my evbike with a 9kw motor controller. They are 6 phase, typical 3 phase starts smoking at the Copper windings lol. It's 72v rn and wouldn't have any issue putting 144 on it. Last time I had 4k rpm but there's plenty of videos and articles replacing the rotor with neodymium magnets to make it pm motor.


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## MineAngerBR (Jan 5, 2020)

Filekidb86 said:


> I use two alternators on my evbike with a 9kw motor controller. They are 6 phase, typical 3 phase starts smoking at the Copper windings lol. It's 72v rn and wouldn't have any issue putting 144 on it. Last time I had 4k rpm but there's plenty of videos and articles replacing the rotor with neodymium magnets to make it pm motor.


Hey, would you be able to share pictures and/or videos about your build?
And about the videos where people use neodymium magnets for a PM rotor, usually they just glue them with CA glue and epoxy them out, but for faster spinning units makes me worried about grenading and damages/harm to driver/people around the unit, a proper built rotor could be safer


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