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Removing the diode bridge in alternators

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olafsson

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Good day

I was wondering if there was a way to calculate the AC voltage that alternators generate.

Also if someone here knows a little bit about alternators then any additional information on problems or things to look out for when removing this bridge is very welcome.

thanks
 
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and also the amps....

if it helps, then let's say that we have a 150 amp alternator that generates 130 amps at 3500rpms (with bridge)
 
Hi.
Seems you are talking about an automotive alternator; you did not say. Removal of the rectifier bridge will leave the stator windings with no connection to anywhere, unable to obtain any power from the alternator, and the voltage regulator would be driving the field at full blast as sees nothing coming out.

Replacing the rectifier bridge with wires, will create a lot of smoke as the three phases will be short-circuited.

With no rectifiers or not using the rectifier output, tapping the 3 phases for using the AC generation, should produce about the same 1800 Watts, or 600 W each phase. The AC voltage and frequency will be proportional to the field exitation and speed rate.

Changing stator windings to more turns would increase AC voltage.
 
Hi.
Seems you are talking about an automotive alternator; you did not say. Removal of the rectifier bridge will leave the stator windings with no connection to anywhere, unable to obtain any power from the alternator, and the voltage regulator would be driving the field at full blast as sees nothing coming out.

Replacing the rectifier bridge with wires, will create a lot of smoke as the three phases will be short-circuited.

With no rectifiers or not using the rectifier output, tapping the 3 phases for using the AC generation, should produce about the same 1800 Watts, or 600 W each phase. The AC voltage and frequency will be proportional to the field exitation and speed rate.

Changing stator windings to more turns would increase AC voltage.

Yeah, I was thinking about automotive alternators. Knew that I would have to replace the bridge with something else. Also the voltage regulator would have to be replaced with a desired ac voltage regulator.

Is there any way for me to figure out approx voltage that I could expect from a converted alternator? Or can you just throw at me numbers that you find likely if I'd spin the alternator at 3500rpms?
 
For a standard automotive alternator, think of it as a constant-current source where the output current is ≈ k*If, where k is like 25 for a 50A rated alternator. In other words, the output current is proportional to field current.

If you bypass the regulator (i.e. the field current is limited only by the field's resistance) with 12V applied, the field current is ~2A (ie the field resistance is about 6Ω). This causes the alternator to output its full rated output (typically 50 to 60A for most automotive alternators).

Normally, the output of the six diode stack is connected directly across the battery, which holds the output voltage down. If you open-circuit a fully-excited, spinning alternator, the open-circuit output voltage soars to ~150V, hence my description of the alternator being a constant-current source.

If you get rid of the diodes, nothing changes, except that you will have three-phase AC out of the stator windings, instead of full-wave rectified three-phase, which has mostly DC, and only a bit of ripple. The field excitation would still be DC, and could be chopped just like it is in a car, which would cause the average AC output current to be modulated proportional to average field current.
 
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You can get AC out of an alternator while it is generating DC. If not much output current is taken from the DC output, the AC will be a fairly good 3 phase sinewave at about 16 V peak to peak, from about -1 V to + 15 V. You may need a power source to get the alternator field to start.

The more DC current you take, the more the sinewave will be clipped, but the amplitude will stay the same, as it will be controlled by the voltage regulator.

Some cars use the AC output for heaters or to use the frequency to run tachometers.
 
Actually alternators are more of a constant voltage source with a limited upper current level just like any other common generator is. If they are working right the vehicles electrical system voltage will still stay around 14 volts if the battery is disconnected while the engine is running.

What type of alternator do you have? most can have the 3 phase AC tapped off before the diodes so that they can supply constant voltage AC output along with their normal output.
 
Actually alternators are more of a constant voltage source with a limited upper current level just like any other common generator is. If they are working right the vehicles electrical system voltage will still stay around 14 volts if the battery is disconnected while the engine is running.

It depends on whether the voltage regulator is working. It takes time, about 1/4 - 1/2 second, for the voltage regulator to respond on a car alternator, because the field winding inductance is so large.

If you disconnect the battery when the alternator is putting a lot of current into the battery, there will be a huge voltage surge. That is because the alternator, as Mike says, appears more like a constant current source than a constant voltage source.

If the system survives the surge, the voltage regulator will reduce the field current and the alternator will carry on working at 14 V or so. So in the short term, it is constant current source. In the long term, it is a constant voltage source.

If you want to run a car with no battery connected, make sure that there is little current into the battery before disconnecting the battery. It's also a good idea to have some load, like some lights or a heater, turned on before disconnecting. With the battery disconnected, the voltage is very sensitive to changes of load. Turning on the headlights can cause the voltage to collapse, because the alternator can't increase its output current fast enough, especially as the filaments are cold and low resistance. Turning on the headlights will certainly cause other lights to dim for a moment.
 
Actually alternators are more of a constant voltage source with a limited upper current level just like any other common generator is. If they are working right the vehicles electrical system voltage will still stay around 14 volts if the battery is disconnected while the engine is running. ...

Only because of the voltage regulator. If the regulator is bypassed, as I said in my earlier post, the open-circuit output voltage will indeed soar to ~150V. Do you remember the little box you could buy out of the back pages of Popular Mechanics which would let you use the alternator in your car to power a 120V drill or SkillSaw? Have you ever heard of a "Load-dump"?
 
Yes I know what your talking about I have been rebuilding and customizing alternators for most of my life now.
Without further info its impossible to know exactly what the OP wants to do and for what reason.

The Delco 10Si and similar models of alternators have and internal diode set that the regulator uses just to power itself. In those alternators is possible to remove the main rectifiers and still get a regulated AC output from them.
 
Yes I know what your talking about I have been rebuilding and customizing alternators for most of my life now.
Without further info its impossible to know exactly what the OP wants to do and for what reason.

The Delco 10Si and similar models of alternators have and internal diode set that the regulator uses just to power itself. In those alternators is possible to remove the main rectifiers and still get a regulated AC output from them.

Only wanted a rough estimate on how much voltage and amp could be achived with this conversion. No worries on exact numbers.

Would it be a fair estimate that at 150V with each phase creating 600W that the output amps would be ~12?
 
There are two things that limit what the alternator will produce: thermal limits, and core saturation. When an alternator is rated at say 100A @ 14V, that is ~1400W, so no matter what you do with or without diodes, you will still be limited to 1400W. Also, if you unload the output side of the alternator and let the voltage rise, the thermal limit will still apply, ie, less current will be available at the higher voltage to avoid overheating it. Core saturation comes into play at the other end, i.e. when you load it more heavily by asking it do deliver current into a low impedance...
 
There are two things that limit what the alternator will produce: thermal limits, and core saturation. When an alternator is rated at say 100A @ 14V, that is ~1400W, so no matter what you do with or without diodes, you will still be limited to 1400W. Also, if you unload the output side of the alternator and let the voltage rise, the thermal limit will still apply, ie, less current will be available at the higher voltage to avoid overheating it. Core saturation comes into play at the other end, i.e. when you load it more heavily by asking it do deliver current into a low impedance...

Yeah, ok thank you for clearing this for me. Knew(or more fairly certain) that there would be a good reason on why alternators aren't used in this manner, just needed to know why;)
 
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