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Automotive alternator to Brush-less motor conversion

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Carbon_Rod

Sun Sept 04 2011, 02:21AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

The alternator has delta windings rated at 100 A @ 12v , and around 10 A for the field current.

I have seen projects that have approached over 1 HP output for similar BLDC conversions.

Does anyone know the maximum RPM these things can run at continuously without burning off the slip rings?
The 8000 RPM continuous is stated on similar alternator specifications...
But I am looking for a maximum rpm limit someone has run their driver at for a few moments.

Cheers,

Patrick

Sun Sept 04 2011, 02:59AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im not sure i can give you a useful answer based on what youve said.
Are you planning to put power in through the brushes?

Carbon_Rod

Sun Sept 04 2011, 05:36AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

If you are interested in re-building one, there are various projects explaining details of the conversion process:

etc...

Patrick

Sun Sept 04 2011, 05:42AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ive already got my 2-cycle 15Hp, two alternator generator assembled, i was merely trying to answer your question based on my alternator experience. But i dont get your question.

Carbon_Rod

Sun Sept 04 2011, 06:39AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Note, there is no permanent magnet as the ECU usually regulates the output by driving the rotor.

The 240A 3 phase driver in use can in theory go over 200kHz, but I do not expect the motor will get anywhere near that speed.

The common "redline" for gas engines is roughly around 9500 rpm, and thus a default alternator will unlikely ever exceed 38000 rpm (with a common 4:1 belt drive).

I'm trying to find the maximum "practical" drive limits for a stock delta configuration.

i.e. something similar to this item:

Dr. Slack

Sun Sept 04 2011, 07:51AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

While your brushless motor will function perfectly well, being converted from an auto alternator means it is not going to be very efficient, especially as you push the power up.

Don't forget that an alternator has to produce 1kW or so to recharge the battery after a start, and a few 100 watts continuous to run headlights, but it's being powered by a 100kW power source for which the owner wouldn't notice whether the odd kW or two was going missing as heat in the alternator, or the radiator, on in eddies behind the car, or friction in the tyres. This means that the manufacturer has used the thinnest wire he can get away with, that will withstand the temperatures from self-heating on peak loads.

Use of a winding to provide the armature field rather than a permanent magnet is also an efficiency hit. Consider what excitation to use for the armature field. As you increase the field, you "waste" power there, but you increase the output voltage, which reduces the output current and so output I2R losses for the same output power. At any given output power, there will be an armature excitation which gives you lowest total losses.

Ash Small

Sun Sept 04 2011, 09:48AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I looked into this some time ago and 20,000 RPM is the usual design limit. (three times engine speed @ 6,000 RPM = 18,000 RPM)

I've never driven one to destruction though.

Carbon_Rod

Mon Sept 05 2011, 07:19AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

@Dr. Slack
Overall efficiency for this project is not dependent on just the conversion losses. I anticipate the #8 AWG wire heating up etc. will further reduce the power conversion efficiency well below 60% (indeed, this test unit is not "continuous rated" like the 200 A industrial version).

I have seen projects that convert to a permanent magnet rotor, and Neodymium magnets work great below 300'C. Perhaps if the test is too ridiculously out of spec with the defaults... I will take some time to try further upgrades.

@Ash Small
18000 rpm sounds like a conservative estimate, but I will likely still mark the parts with color changing welders crayon to give an early warning of thermal problems.

Cheers,

radiotech

Tue Sept 06 2011, 05:48PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The self exited ecu's in alternators start up by drawing residual magnetism induced power at a certain speed,
and ramp up the the field and regulate it referencing the connected battery voltage as needed. Once the alternator
is stopped they unlatch. This is how the always-connected alternator will not drain the car battery.

I'm not sure about what you are asking about sliprings, but the current in the sliprings determines how close to
saturation the iron in the rotor is brought to. beyond this the magnet wire just heats up.

By ohms law, an alternator externally excited without any battery will develop about 150 volts DC output
from the bridge rectifier., which if then switched to excite the field through the rings would destroy the brushes.

There was a gadget in the days of external regulators, that allowed vehicle alternators to power tools that could
run on 120 volts DC. (drills, grinders etc). The gadget was just a swtichbox intercepting the alternator wiring.

Ash Small

Tue Sept 06 2011, 07:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

radiotech wrote ...

The self exited ecu's in alternators start up by drawing residual magnetism induced power at a certain speed,
and ramp up the the field and regulate it referencing the connected battery voltage as needed. Once the alternator
is stopped they unlatch. This is how the always-connected alternator will not drain the car battery.

.

I'm not sure this is completely correct, at least for all alternators.

I went through a phase some years ago of stripping alternators down and fitting the rotors onto motorcycle crankshafts, and machining up carriers to hold the stators in place, with various brackets, etc to hold the brush carriers in place (needless to say I was running a 'dry' primary back then) so I have 'a bit' of experience here.

Most vehicles have a 'battery warning light' on the dashboard, which comes on when the ignition is on, but the engine isn't running (it also comes on if your alternator belt breaks, comes on at low revs if you blow a diode or the brushes need changing, etc, i.e. when the alternator isn't charging the battery at a greater rate than it is discharging.

During the research I did back then (long before the internet) I discovered that the wattage of that 'idiot light' is critical to the 'correct' functioning of the alternator. It's what 'we' would call a 'current limiting resistor', and provides initial current to excite the windings. I also learned that, for 'correct' operation (ie for 'guarantee purposes') the 'correct' regulator and 'correct' wattage bulb were essential.

It's the diodes in the rectifier that prevent the battery from draining when the ignition is off. If one of the diodes blows 'short circuit' the battery will drain.

There are various articles on the internet about de-regulating alternators, to get 120V or to use them for arc welding.

I'm afraid I'm not able to offer any advice on brushless motor conversions, though.

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