Myth of alternator damage cause by BMS disconnect?

[Wotname]

Originally Posted by tanglewood

When the alternator output gets disconnected, doesn't it create a reverse EMF? I.e. it's a reverse voltage spike? No?

Inside the alternator, the stator winding are producing AC; when a external disconnect occurs the voltage spike created in the stator winding is the reverse polarity of whatever the polarity was at the moment of disconnect. The forward biased rectifying diodes inside the alternator will rectify this as +ve on the alternator output post (if they aren't fried). The reverse biased diodes will likely to be fried!

However if the alternator diodes do survive, then the spike on the output will always be +ve, due to the rectifying function of the diode pack.

[RaymondR]

Originally Posted by team karst

I think of it this way:

Alternator is chugging along, delivering 50 Amps. Suddenly, the output is disconnected. Since the regulator has a time constant, plus the field current can’t collapse instantly, plus stator windings, being coils, therefore inductive, they hold energy, there becomes a high positive voltage at the B+ port to keep the current flowing for a time. This transient energy may manifest itself in switch arcs, and other undesirable voltage excursions.

Also my understanding of the problem.

early high current diodes had a limited reverse bias or "zener" capability and would fail under the high reverse voltage.

[team karst]

Originally Posted by RaymondR

Also my understanding of the problem.



early high current diodes had a limited reverse bias or "zener" capability and would fail under the high reverse voltage.

Yeah; there is a compromise with Reverse breakdown and low forward V drop, desirable for low waste heat. Ie: shottkey tends to have low reverse breakdown.

[RaymondR]

Also just realized that leaving the stator coils in circuit provides a number of low impedance inductors which are more likely to damp out voltage excursions than put them in.

[Dieseldude]

The diodes act as check valves would in fluid system. They let current flow in only one direction. If the reverse voltage exceeds the reverse polarity capability of the diode, it will be destroyed. This can be compared to excessive reverse pressure on a check valve blowing the valve. Automotive type alternators produce 3 phase AC, as this is more efficient than a DC generator, allowing for a much more compact unit. Each phase has two diodes. One diode on each phase allows the positive half of the AC wave to pass through the to the positive terminal, and the other diode which is connected reverse polarity to the positively polarized one allows the negative half to pass through the negative or ground terminal of the alternator. Minor inverse transient voltages will be stopped by the diodes. Any inverse voltage that exceeds the inverse voltage capability of the diodes will destroy them. If you are lucky, the diodes will quickly go open circuit before damaging anything in the system. If unlucky, they will go short circuit and allow the over voltage to pass to all loads in the system and reek damage to sensitive equipment. Previous posts discussed using varistors connected to the alternator positive terminal to limit over voltages. But these can overheat and burn if an over voltage is sustained beyond their rating, so they need to have a fuse in series.

[RaymondR]

I don't know about more efficient but certainly more compact and lighter, I can remember trying to hold some of the monsters up one handed whilst trying to feed bolts into the mountings and it was not a job for 100 pound weaklings.

One of the reasons they went to alternators was that you could spin them a lot faster which was better for city driving where cars sit at traffic lights idling a lot. With an alternator you can have a high ratio drive so that they produced a decent amount of current whilst the engine was idling and then they will still stay together when the engine revs to it's highest revs.

[Dieseldude]

Originally Posted by RaymondR

I don't know about more efficient but certainly more compact and lighter, I can remember trying to hold some of the monsters up one handed whilst trying to feed bolts into the mountings and it was not a job for 100 pound weaklings.

One of the reasons they went to alternators was that you could spin them a lot faster which was better for city driving where cars sit at traffic lights idling a lot. With an alternator you can have a high ratio drive so that they produced a decent amount of current whilst the engine was idling and then they will still stay together when the engine revs to it's highest revs.

Increased efficiency includes the fact that alternators can produce more power per unit mass and more power per revolution then would a DC generator. So this agrees with your points about weight and low RPM operation. Alternators are better on these considerations, but they are are electrically more fragile, as the diodes can be a weak point. But of course DC generators have no diodes. The power output of a DC generator is cleaner, in that diode switching ripple are not present. But DC generators can suffer lower output at low engine RPM. It should be noted that the battery in the system offers some stability. It smooths out diode ripple. If the battery is weak, fails, or is disconnected, the alternator alone will tend to produce "noisy" power with more diode ripple and spikes.


Modern electronically controlled engines and electronic accessories can exhibit strange behavior due to a weak or disconnected battery because of a lack of stability that the battery normally provides. Also, a bad alternator diode can case similar problems, as the power will be electrically noisier. So the battery and charging system need to be well maintained.



In addition to increased efficiency, 3 phase power is steadier than single phase. The 3 AC wave forms are staggered 120 degrees over a cycle. This can be compared to a single cylinder engine verses a 3 cylinder. The single cylinder produces only one power stroke per revolution but the 3 cylinder produces 3 power strokes, so its power is more continuous in that the intervals between power strokes are shorter.

[RaymondR]

I'm not sure about the smoother power. DC generators were excited with an electromechanical regulator which, if you took the cover off, looked like having a thunder storm under the hood. Your AM radio would go crazy with static when a car fitted with some of them drove down the street.

[Dieseldude]

In that reguard, yes, mechanical regulators can be noisy. Not sure how much regulator noise appeared in the vehicle's electrical system and how much dissipated through the air as electromagnetic radiation and appeared as radio noise. But a modern electronic regulator would not be so noisy. It is difficult to say how much AM radio noise was caused by the regulator and how much was caused by the ignition system. Ignition noise is a reason that ignition wires started being made with resistive conductors instead of copper. The resistive wires tend to transmit less radio noise.

[Wotname]

Originally Posted by team karst

I think of it this way:

Alternator is chugging along, delivering 50 Amps. Suddenly, the output is disconnected. Since the regulator has a time constant, plus the field current can’t collapse instantly, plus stator windings, being coils, therefore inductive, they hold energy, there becomes a high positive voltage at the B+ port to keep the current flowing for a time. This transient energy may manifest itself in switch arcs, and other undesirable voltage excursions.

The field current has little effect on the voltage spike, after all the field current (and thus the field magnetic field) can and is varied constantly by the regulator.

The spike is primarily generated by the collapsing of the stator windings magnetic field.

Any time current flows through a conductor, a magnetic field is created around the conductor. Any time any conductor is moved through a magnetic field, a voltage (EMF) is produced in the conductor likewise any time a magnetic field is moved through a conductor, an EMF is produced in the conductor. Electrical theory 101.

The alternator is delivering say 50 amps. This is (in essence) the RMS stator current of all stator windings combined. This current creates a magnetic field around the stator windings and while the output stays constant, the magnetic field stays constant.

If the output is disconnected (open circuit), the stator current ceases immediately. This causes the stator magnetic field to collapse immediately. The rapidly collapsing magnetic field moves through the stator conductors thus creating a EMF in the conductor. The size of the EMF is proportional to the size of strength of the field (thus proportional to the current flowing), the rate of change and the number of conductors.

The said 50 amps has created a pretty strong magnetic field, the rate of change of the magnetic field is huge (i.e. goes from max to zero instantaneously) and the number of conductor is large (i.e. all the turns of the stator) thus the EMF created will be large. While the polarity of the EMF spike is opposite to that of the EMF before the disconnect, the alternator diodes rectify it so it appears as a +ve spike on the output post.

[LeighC]

In addition to increased efficiency, 3 phase power is steadier than single phase. The 3 AC wave forms are staggered 120 degrees over a cycle. This can be compared to a single cylinder engine verses a 3 cylinder. The single cylinder produces only one power stroke per revolution but the 3 cylinder produces 3 power strokes, so its power is more continuous in that the intervals between power strokes are shorter.[/QUOTE]

Your engine must be a 2 stroke

[SOLAR SUPPORT]

Most factory made boats have battery banks isolated from each other by a diode type isolator. Those batteries are charged from a common source at the same time by the alternator when the engine runs. Imagine a factory boat. It's service batteries are upgraded from AGM to two plug&play type Lifepo4 batteries connected in parallel. In this system, the battery isolator is diode type, the engine battery is AGM type. While the engine and service batteries are on and charged with the alternator, the system voltage somehow rises to 14.6 volts and the charging circuit of the Lifepo4 service batteries is canceled by its own internal bms system at the same time. In the meantime, can the engine battery absorb the high voltage spike originating from the alternator?

[goboatingnow]

Remember the numbers are easy to calculate the inductance of most car alternators ( 40-70 A) is around 350 -400 uH , and the formula V= L*(di/dt) hence there is a massive difference in the voltage spike from a 5 uS relay disconnect and a 10mS mosfet disconnect.

TVS can be used but to survive multiple triggers need to be massive to handle the peak power in the order of 4Kw needs to handled. Several big D2PAK type TVS will be needed.

Varistors are not a solution for this sort of protection they are too slow.

The other thing with is that the TVS knee really needs to above the avalanche rating of the alternator diodes. If it’s below it then it will have to handle the full alternator load dump energy.

The next thing is this will still allow voltage excursions upto the TVS break over point. This is typically 28V in a 12v system. So all your down stream systems need to handle that.

Taking all that in a couple of big ass TVS diodes ( ST Micro make some nice ones ) can provide low cost alternator protection. Certainly it will protect the alternator if not everything down stream

For good down stream protection an “ hot swap “ controller IC makes a good protector. These can implement current foldback. In essence they try and prevent the voltage rising above a precise point ( say 15v) with full disconnection if this is not realised ( typically in 20mS)

These use mosfets in series.

The alternative is a precise TVS comprising voltage sensing and a big ass mosfet shunting the alternator. This is a good compromise between the TVS only diode and the more complicated hot swap controllers.

I’ve a couple of these to build and test over the next few months.

Note that voltage spikes are quite common in alternator systems. Any abrupt change of current causes a spike.

The ISO test is now very stringent and quite hard to met in its entirety. The good news is the actual real life load dump waveform is rarely anything as bad as the ISO test waveform.

[Dieseldude]

Originally Posted by SOLAR SUPPORT

Most factory made boats have battery banks isolated from each other by a diode type isolator. Those batteries are charged from a common source at the same time by the alternator when the engine runs. Imagine a factory boat. It's service batteries are upgraded from AGM to two plug&play type Lifepo4 batteries connected in parallel. In this system, the battery isolator is diode type, the engine battery is AGM type. While the engine and service batteries are on and charged with the alternator, the system voltage somehow rises to 14.6 volts and the charging circuit of the Lifepo4 service batteries is canceled by its own internal bms system at the same time. In the meantime, can the engine battery absorb the high voltage spike originating from the alternator?

Battery pates act like capacitor plates, so will absorb transient voltages to some degree. Is their capacitance sufficient? I do not know. It can get very technical to calculate theoretical values of voltage spikes that are impractical to measure. And can the capacitance value of the battery be obtained from the manufacturer ? Perhaps someone with lots of battery knowledge would know a ball, park figure. Battery electrolyte is a poor dielectric, so I do not expect the capacitive value would be much.

[goboatingnow]

Batteries have about 1F ( farad ) per 150Ah of capacity so they act as very substantial capacitors ( at one level )
Hence they have enormous ability to absorb spike energy.