Preventing Voltage Spike from Alternator Cut-Off

[john61ct]

I did not know an external monitor could tell a Balmar VR when 100% SoC is reached?

And I did not imagine you thought a BMS is acting as that controller,

Originally Posted by Dockhead

But I haven't seen a BMS which provides this functionality.

Goes double for me, delays neither.

figured something custom-coded e.g. Arduino or a PLC?

In any case my feedback was about the logic of it, not so much implementation.

Look forward to more. . .

[Dockhead]

Originally Posted by john61ct

I did not know an external monitor could tell a Balmar VR when 100% SoC is reached?

And I did not imagine you thought a BMS is acting as that controller,

Goes double for me, delays neither.

figured something custom-coded e.g. Arduino or a PLC?

In any case my feedback was about the logic of it, not so much implementation.

Look forward to more. . .

You need to do some more reading. In particular, read the Batrium manuals. The Batrium BMS will connect to and control Victron charger/inverters via Canbus, and have control outputs for SOC and high voltage. And low voltage. And other stuff.



That's just one example of a BMS who can do these control functions.

[john61ct]

But not to Balmar VRs or the other 99.999% of charge sources out there.

I personally would not want a design that depended on a particular piece of proprietary hardware.

Too many BMS companies come and go. . .

[nilespf]

Originally Posted by Wotname

PN, you have posted reasoned response and I completely agree with the emphasised sections yet IMO there is more to understand. The issue I have with the claim the regulator can mitigate the level of the spike is simply that the regulator can only create a +ve potential to the B+ terminal. Yes, a working regulator can ensure the B+ terminal returns to ~+14V but the voltage spike (created by the collapse of the flux field around the stator) will be -ve at the B+ terminal - at least it will try to be -ve until the diode pack fails.

I cannot see how the regulator can compensate for the -ve going spike.

Unfortunately I don't have any generalised numbers for the stator windings inductance and I think the bigger problem would be determining (guessing?) the delta time for calculating the back EMF.

If I still had access to alternator test bed and a 'scope, it would be nice to run up an alternator with a reasonable load (say 30 A), pull the load and observe the spike on the 'scope. Probably best done with the diode pack removed and a fixed field current and although the output would AC, the spike would be similar. Unfortunately I'm no longer in a position to carry out such an exercise .

Other than the reverse emf spike I think we agree.

Load dumping is a well understood phenomena and is actively addressed by the auto industry.



There are some old salts that will swear you cannot remove a battery from the car while it's running and I would guess some cars may still be on the road that will sustain damage if the battery is removed while the engine is running. From the postings on this thread some have experienced it first hand.



I get the feeling you know your away around alternators so back to the reverse emf issue.

Take a look at the two files I have attached. They put some numbers to the spikes. Some spikes can be modeled with as little as 1 msec edges most are around 3 to 5 msec.

If we use my setup:
Idle RPM is 900 and the engine to alternator pulley ratio is 1.7, alternator has 12 poles.

At idle the stator output frequency is (N*P* 1.7)/120=153 Hz=6.5 msec period.

The rise time/fall time of each phase will be about 3 msec give or take.
Given medium framed alternators there is not much room for many stator windings. My searches show documentation using (40 to 45) * 3 loops per phase. I used an on line tool to get an idea of what the per phase loops inductance calculates out to be. Using 135 loops, 17 ga wire

the inductance calculates to 3.27 mh. The back emf is V=L*d(i)/d(t).
The dominating factor here is the how fast do you want the flux to drop. If we use 30A, 3.27mh and have the d(t) at 1 msec then V= -98V Note that is a negative spike at time of drop out. This voltage will subtract from the what the voltage will rise to from the same drop out due the the output becoming unloaded. Use an order of magnitude less time (.1 msec) and the voltage spikes to -981 V. What the unloaded phase voltage goes to due to the loss of load is another set of calculations. It involves factors that can be assigned typical values and as such it can be shown the unloaded voltage of the alternator is trying to rise upwards to about 2 kV. If the regulator is still connected at time of disconnect it cannot help with peak values. Documentation I have read suggests load dumps can last for 400 msec. If that is the case the regulator needs to have only a 2 Hz bandwidth 500 msec). Also while the terminal voltage is trying to rise the flux has begun to fall. The system needs only a 2 Hz regulator bandwidth to restore base line voltage. You can play with the numbers and see that if charging at a low value the back emf as well as the rise in unloaded terminal voltage will be subdued. I'm not an alternator engineer but I am an EE designer. If reading between the lines of the included documents the assumptions of regulator bandwidth are reasonable.


Bottom line is I hope the power plant suppliers in our industry catch up to the auto industry. Myself, I cannot see putting in a bunch of Transient Voltage Suppression devices all over the boat (hence my original post about system knowledge) but rather have the alternator wired up so the regulator stays on line.



PN

[expozen]

Originally Posted by nilespf

Bottom line is I hope the power plant suppliers in our industry catch up to the auto industry. Myself, I cannot see putting in a bunch of Transient Voltage Suppression devices all over the boat (hence my original post about system knowledge) but rather have the alternator wired up so the regulator stays on line.

I do feel more and less confused.

To be clear: are you saying that if I keep power to the regulator, I can cut the alternator output without consequence?

[Wotname]

Originally Posted by nilespf

Other than the reverse emf spike I think we agree.

Load dumping is a well understood phenomena and is actively addressed by the auto industry.
......

I had a quick look at the links and they look interesting (to me) and deserve a closer look. Thanks for posting them.

It might be a few days before I have time to digest them - the weather has starting to become good ATM and that's cuts into the keyboard time - which is a good thing

Standby

[nilespf]

Originally Posted by expozen

I do feel more and less confused.

To be clear: are you saying that if I keep power to the regulator, I can cut the alternator output without consequence?

Yes providing your regulators battery voltage sense is not compromised.



If cutting alternator output means:
1) disconnecting the cable that carries the charge current to the battery at the alternator when the alternator is providing charge current would not be condoned by anybody. If the charge current is low and the engine is at idle you may get away with it. Nobody advocates a load dump. In my system because the new Balmar MC-614 charge controller receives non switched power from the battery not the battery buss AND the Balmar alternator has a separate terminal lug for the output, I can disconnect (prior to engine start) the alternator . Battery sense voltage originates at the battery terminal.



2) If cutting alternator output means dropping the field current to zero then I would be OK because the battery is still connected but not being charged. Still the battery sense voltage originates at the battery terminal. As mentioned previously, the field coil drive point in the regulators have a protection diode for back emf that WOTNAME and myself have been discussing.



3) The original Mitsubishi alternator used the oil pressure switch to bootstrap the battery voltage (at the battery not the battery buss) directly to the R terminal and indirectly to the L terminal via the charge lamp. The bootstrap connection (from the oil pressure sw) was connected to the same point of the output of the ig switch. This allowed continuous power to the regulator regardless of ig sw position, regardless of battery disconnect switch position. One can argue the battery was no longer 100% isolated when the disconnect switch is made but I can assure you it saved my ass any number of times before I figured out how it was wired.


pn

[nilespf]

EXPOSEN


Take a look at this link it drives home the point. Note how they mention a "discharged" battery read...alternator is providing heavy current!!!!


https://www.electronicdesign.com/pow...tion-headaches



Note I mention the regulators battery voltage sense. In automobiles there is no way that can happen and they don't even worry about it. They choose to address "accidental" load disconnect from "discharged" batteries. On our boats who know how they are wired or for that matter what modifications have been made over the years.



pn

[expozen]

Originally Posted by nilespf

https://www.electronicdesign.com/pow...tion-headaches

This is a very interesting article. I think I'm going to start doing more research about this topic by way of the auto industry. It's seems like they've put much more thought into it.

Originally Posted by nilespf

disconnecting the cable that carries the charge current to the battery at the alternator when the alternator is providing charge current would not be condoned by anybody.

Yes, but this is what I'm trying to plan for. Not because it's likely, but because it's a possibility in many of the systems I've seen (which to be honest isn't many because I have very little experience), either by way of a BMS opening a charge or main relay, or by a user disconnecting the battery by turning a manual isolation switch. So I'm trying to understand what legitimate (in real life) options are available to manage the scenario of load dumps caused by an alternator or possibly other charge sources.

Originally Posted by nilespf

In my system because the new Balmar MC-614 charge controller receives non switched power from the battery not the battery buss AND the Balmar alternator has a separate terminal lug for the output, I can disconnect (prior to engine start) the alternator . Battery sense voltage originates at the battery terminal.

I appreciate your advise about considering where the power for the regulator comes from, and where the voltage sense is fed from.

I've drawn up many different configurations to try to figure the best way to come about this. As of now, the one I feel the most hopeful for is what I originally posted in the beginning of this thread (using a battery isolator to shunt the output of the alternator in the event of a load disconnect). But I still don't know enough to confirm its merit. I've incorporated the regulator power feed and voltage sense into the schematic and attached below.

What I question is... if the BMS opens the charge relay while the alternator is outputting its full rated current:

1) Does the current actually shunt to the starter battery? What if the start battery is already full and it can't accept that amount of current?

2) Could the spike in voltage generated by the cessation of current between the battery isolator and the charge relay be enough to possibly damage the shore charger and/or solar charger?

3) Could the regulator still think that there is a battery connected to the voltage sensing input if the solar was also charging at the same time? Balmar says that if the voltage sense of the MC-614 senses < 10 volts it will shut down the alternator. But what if the solar is simultaneously charging in float and outputting ~13.2 volts, how does the regulator distinguish between voltage coming from a battery or voltage from another charge source?

4) Am I just in way over my head?

[nilespf]

Originally Posted by expozen

This is a very interesting article. I think I'm going to start doing more research about this topic by way of the auto industry. It's seems like they've put much more thought into it.



Yes, but this is what I'm trying to plan for. Not because it's likely, but because it's a possibility in many of the systems I've seen (which to be honest isn't many because I have very little experience), either by way of a BMS opening a charge or main relay, or by a user disconnecting the battery by turning a manual isolation switch. So I'm trying to understand what legitimate (in real life) options are available to manage the scenario of load dumps caused by an alternator or possibly other charge sources.



I appreciate your advise about considering where the power for the regulator comes from, and where the voltage sense is fed from.

I've drawn up many different configurations to try to figure the best way to come about this. As of now, the one I feel the most hopeful for is what I originally posted in the beginning of this thread (using a battery isolator to shunt the output of the alternator in the event of a load disconnect). But I still don't know enough to confirm its merit. I've incorporated the regulator power feed and voltage sense into the schematic and attached below.

What I question is... if the BMS opens the charge relay while the alternator is outputting its full rated current:

1) Does the current actually shunt to the starter battery? What if the start battery is already full and it can't accept that amount of current?

2) Could the spike in voltage generated by the cessation of current between the battery isolator and the charge relay be enough to possibly damage the shore charger and/or solar charger?

3) Could the regulator still think that there is a battery connected to the voltage sensing input if the solar was also charging at the same time? Balmar says that if the voltage sense of the MC-614 senses < 10 volts it will shut down the alternator. But what if the solar is simultaneously charging in float and outputting ~13.2 volts, how does the regulator distinguish between voltage coming from a battery or voltage from another charge source?

4) Am I just in way over my head?

Can you list the model #'s of switches. I found a blue sea product that fits the diagram for AFD function but no pin-out on the diagram.


Also is it fair to state that no connection exist if any wire crosses another. The wires must terminate into each other before a connection is made. If so where does the starter get voltage?



If house is used for engine start due to start battery disconnect switch activated (meaning open), will the field have voltage? If voltage is blocked, would you consider using a regular disconnect sw in place of the AFD sw and re-wire the ignition power?


regarding 1&2), It will perform as a sea anchor, (if the battery were connected). If battery is connected the alternators output will see one load being switched off and a resistance load (battery) being added. From what we have seen the data suggests the spike is modeled in the auto industry as lasting about 200 msec. Most energy in the spike will be dropped across the start batteries internal resistance and absorbed there. Your correct, if the start battery is reasonably charged, the internal resistance will be higher and the voltage bump will be greater. If not connected (as shown), the voltage will indeed rise and high enough and long enough to pop the alternator and possibly anything else connected on the alternator side of the disconnect. It would be considered a full on load dump. My guess is there is one place on your schematic in which a crossed line does actually have a connection LOL.


3) It doesn't. You can pick any point on the schematic, put your voltage sense there and the alternator will output until the 614 says that sense line is at the chosen voltage. As drawn, if the engine is running, and the solar goes into equalization mode, the alternator regulator will back the field down in an attempt drop the buss back down to whatever voltage you have programmed it to. The Alternator cannot sink current so it will literally let go of the line but the solar controller is in it's own routine and will happily keep the buss at ~15+V. Because the alternator cannot pull the line down the stator voltage will drop and in some boats the stator output is the tickle for the RPM gage. Now the gage appears to become intermittent.


I would like to see the BMS your looking at.
Hope this helps

pn

[NPCampbell]

I guess I am naive. I have always assumed that it was common practice for every inductive load to suppress it's own back EMF. Simple relays routinely embed flyback diodes to suppress RF, protect downstream electronics and extend contact life. Why don't alternators? If the back EMF generates voltage greater than the PIV of the suppression diode then pair it with a load resistor, etc? Why should you have to use an external protection circuit?

[nilespf]

Originally Posted by NPCampbell

I guess I am naive. I have always assumed that it was common practice for every inductive load to suppress it's own back EMF. Simple relays routinely embed flyback diodes to suppress RF, protect downstream electronics and extend contact life. Why don't alternators? If the back EMF generates voltage greater than the PIV of the suppression diode then pair it with a load resistor, etc? Why should you have to use an external protection circuit?

Good point. If the alternator generates the spike, let them deal with it. My bet is some manufacturers of high end alternators have schemes to mitigate effects of load spikes. Some even have built-in bootstrap circuitry that once the alternator is spinning, a portion of the output is fed back to the regulator as a voltage sense to at least keep the alternator output under control. It doesn't help load dumping but it will maintain a closed loop operation status. It seems like the stuff we have to work with is glam'd in and not purpose built.


pn

[transmitterdan]

You guys are on a wild goose chase about the EMF of the field coil. That’s not what breaks the diodes.

The diodes that “break” are the rectifier diodes that take the multiphase AC main output and rectify it to produce DC. An alternator is basically a current controlled current source with an inherent voltage limit. The field current controls the flux rotating through the stator. So the stator makes a current that is proportional to the flux rotating through it. When the battery is disconnected the stator keeps trying to force current into infinite resistance and the voltage rises until some maximum limit based on turns ratios is reached. But this limit is way above the avalanche breakdown voltage of the multi-phase rectifiers and thus the ones connected to ground begin to die and short out the stator.

All alternator regulators I know about prevent back EMF of the field winding by placing a diode from ground to the field circuit (anode to ground) so when the field is turned off the maximum field voltage is about -1V. So no damage happens by turning off the 12V power to the regulator. Balmar recommends that method to cut the field. They don’t recommend opening the field circuit with a relay because that would defeat the protection. Open the 12V supply to the regulator and the excitation field
quietly goes to zero alter a few milliseconds.

[nilespf]

[QUOTE=expozen;2974836]This is a very interesting article. I think I'm going to start doing more research about this topic by way of the auto industry. It's seems like they've put much more thought into it.



Yes, but this is what I'm trying to plan for. Not because it's likely, but because it's a possibility in many of the systems I've seen (which to be honest isn't many because I have very little experience), either by way of a BMS opening a charge or main relay, or by a user disconnecting the battery by turning a manual isolation switch. So I'm trying to understand what legitimate (in real life) options are available to manage the scenario of load dumps caused by an alternator or possibly other charge sources.




Does your boat allow this to be implemented?
It is only for load dump control and if the disconnects are put up against the batteries it may adapt to the rest of the boat. The logic input can burn a warning LED and or buzzer. The topology perhaps adapts to any methodology you choose to implement the BMS.


pn

[john61ct]

Originally Posted by expozen

I'm trying to understand what legitimate (in real life) options are available to manage the scenario of load dumps caused by an alternator or possibly other charge sources.

…

I've drawn up many different configurations to try to figure the best way to come about this

Note I am not claiming this is "best".

I am asking what is "wrong" with just

hard-wiring a small and cheap SLA battery into the alt / VR output

?

Maybe even a properly rated capacitor if that has a longer worry-free lifetime?