Shorting an Alternator 1

[roydm]

Most motorcycles and I'm assuming lots of other small motors have a permenant magnet 3 phase alternator feeding a 3 phase bridge and the battery.
When the battery reaches full voltage the most common method of regulation seems to be by shorting the alternator coils rather than interupting the circuit.
My question is with the coils shorted you obviously get Amps but no Volts, so where is the power going? Is it just reflected back somehow?

 

[dpc]

shorting the alternator coils

There were old contact type voltage regulators that may have shunted the field circuit briefly, but not the alternator output.

Anything made recently should use a solid-state voltage regulator.



David Castor

http://www.cvoes.com

 

[Skogsgurra]

The impedance is mainly inductive so the shorting produces reactive current, which doesn't heat much, nor does it load the prime mover.
Shorting with triacs is still used. At least in trial bikes. That's where I have seen it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

The output is not shorted so much as limited.
A common circuit used on small bikes was a Zener diode across the alternator. The diode voltage would be chosen to be the maximum charging voltage, typically 13.8V for 12V nominal systems.
The zener would pass enough current to limit the voltage to 13.8 Volts. The alternator would be designed to be capable of self protecting itself. Remember, the Zener is only conducting at higher voltages when the RPMs are high and there is a lot of self cooling by the air movement.
The energy would be expended in both the alternator windings and in the Zener's heat sink. The losses in the alternator winding will be based on I²R and R is small as Gunnar pointed out. The losses in the heat sink will be based on IE where I is the total current minus the load current and E is the open circuit voltage minus the Zener voltage.
Some small bikes ran the headlight at all times as a design feature to reduce the load on the Zener. A burned out head light could result in a blown Zener diede followed by all the other lamps failing. Then when you replaced the headlight and the other lamps wothout realizing that the Zener was also shot, you could expect to burn out the headlamp and the other lamps the next time that the motor was run at high RPMs. Larger systems used more complex shunt regulation as mentioned by Gunnar. Some systems still required full time head light illumination to avoid regulator failure followed by lamp failure at high RPMs.

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

 

[roydm]

Thanks Skogs, that's the explanation I was seeking.
I have tried to attach the circuit from a 2005 Suzuki, I know Yamaha use similar and I think Harley do too.
Regards

Roy

 

[roydm]

The reason I asked, I now have a Motto Guzzi with a single phase alternator. It's regulator operates by interupting the bridge (2 diodes 2 SCRs).
I am wondering if there would be an improvement (more output) if it were posssible to re-wind the stator as a 3 phase and use a shorting type regulator as per Suzuki.

Waross,
I remember those Zenner diodes on British bikes 40 years ago, I haven't seen a bike with that scheme for a while. Actually it was about the time SCRs came on the market I think.

Thanks
Roy

 

[Sparweb]

Roy,

I'm surprised to read the answers you are getting because that's not how I do it at all on my generator systems. Perhaps the resistance in each winding of a motorbike alteranator is pretty large, so shorting doesn't cause much over-heating. But on my generators, it's really low, so shorting the 3-phase leads causes a major dynamic braking effect.

I don't know the winding resistance of your particular alternator, nor that of the Suzuki example. But it would seem to me that braking an alternator would just cause it to get very hot and stress its shaft unnecessarily.

Must be something else going on...

Maybe that SCR isn't ON all the time, just switched with a Pulse-with modulated input to the SCR so that the alternator output sine-wave peaks are "clipped" by being shorted momentarily.


Steven Fahey, CET

 

[roydm]

SparWeb,
No, the resistance would be < 1 Ohm. Remember also it's a permanent magnet rotor, perhaps the current is limited by that as well.
No the SCRs turn on and stay on for remainder of cycle, the circuits are very crude.
I think you will find that most motorcycles are 3 phase and work like that. The Moto Guzzi is quite unusual with a single phase. I measured the short circuit current with a clipon and from memory it was about 27 Amps.
What Gunnar said makes sense to me.
The regulator would only short out coils on intermittent cycles, most of the time it's trying to keep the battery and load satisfied.

 

[Sparweb]

Well, it just goes to show you that what works for one type of machine won't for another.
I've built a couple of wind turbines, which have permanent-magnet 3-phase alternators, and when I short the power leads, it makes the turbine stop. And I mean woah! The currents surge so high in the low-resistance wires that the current spike causes a very large I-squared-R power load. It's shed as heat for just a moment but there isn't enough power in the wind to keep things turning. It's called dynamic braking, and I do it deliberately to shut-down the turbine if I am disconnecting the battery, or if I'm lowering the tower.

I think I just guessed why the alternator doesn't go open-circuit, though. If the alternator's 12V cut-in speed is around 1000 RPM, then at 10,000 RPM the voltage will be up around 120V. Dangerous!



Steven Fahey, CET

 

[Skogsgurra]

Steven,

It is not about dead-shorting the alternator. It is about 'cutting' excessive power and that can be done very effectively by using triacs to short out part of the sinewave.

The winding's impedance is mostly reactive and increases with speed - as does voltage - so, when you need to reduce output voltage, the frequency is high and so is the impedance of the winding. No probs, actually.

You mention resistance, but that is not very relevant in this case. All alternators or generators worthy of the name MUST have low resistance to keep losses low.

Google to see how it is done.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[LionelHutz]

Steven, it's historically just been simpler to design them so the over-voltage is shunted down to acceptable levels or it's just shorted. It's likely easier to do a short with a semiconductor and deal with any alternator heating then to try and do a linear regulator and deal with the semiconductor heating. I wouldn't be surprised if new designs use a permanent magnet generator with a switching supply though.

 

[Sparweb]

...the frequency is high... slap to the forehead.

of course, thanks Gunnar.

My wild AC stuff works at around 15-30Hz, so I usually find little or no reactance effects. I have to bear in mind that it's a peculiar condition, and I can't generalize too much.


Steven Fahey, CET

 

[roydm]

My alternator has 14 poles (7 pair) so at 1000 RPM the frequency is 116 Hz. The regulator doesn't even start to cut back until around 2000, 230 Hz
Getting back to the question, would a 3 phase arrangement work better than a single phase. I'm not concerned about the efficiency just weather it would give better charging.
I assume with 14 coils I have 14 slots, would it be possible to split this up into 3, all be it unballanced?
Thanks
Roy

 

[waross]

The shunt regulator doesn't "Short" the alternator. It acts as a load and increases the current to limit the voltage to the charging voltage. That is 13.8 Volts in many designs.
The effect on the alternator is similar to connecting a very large battery rather than shorting the output completely.

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

 

[roydm]

The shunt regulator doesn't "Short" the alternator.
Waross
The regulators we are refering to have SCRs wired directly across each coil before the 3 phase bridge. I posted a schematic further back.
Roy

 

[waross]

What is the firing angle of the IC?

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

 

[roydm]

My guess for thye firing angle is zero°, If the voltage is above setpoint from the previous 1/2 cycle the gate is turned on before the voltage starts to rise. Looking at it with a scope you see complete half cycles missing, mot partial.
Here is the complete circuit, note the waveforms I drew to the right hand side, this is what you see on red wire to chassis, nice clean half wave pulses.
Roy

 

[waross]

Please excuse my confusion. This thread started with questions about regulators that shorted or, perhaps stated more accurately, shunted the alternator output to control the voltage.
The description of the circuit you have just posted explains that this regulator works by interrupting the current rather than shunting it. I seem to be stuck in a time warp back in shunting regulators. I must have missed something important somewhere.

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

 

[roydm]

I'm sorry I got a bit off track.

This originated on a motorcycle forum where I posted instructions on overhauling a single phase regulator. We were discussing the relative advantage of shorting out the coils of a 3 phase winding over open circuiting a single phase type.
I was trying to figure out what happens to the power when the coils are shorted out, now I know

I still don't know if there is an advantage one over the other as far as the output goes, I suspect a 3 phase type would be more efficient (put out more Watts) than the same alternator wired as a single phase assuming the same rotating magnet and equivalent slot space.

Looking at the waveform I can see that the alternator is only charging for about 50% of the cycle. I suspect a 3 phase would have overlapping current pulses but would it be more efficient?
I'm not sure that I can wind it as a 3 phase given the number of slots available but I should be able to go 2 phase at least.

I'n not even sure that I want to go to the trouble of re-winding the coils, I can probably live with a weak alternator but it has been an interesting topic and I learnt a lot.
I thank you all sincerely for your input.
Regards
Roy