PMDC generator (as 2nd alt) to rapid charge house battery bank size and setup. Help!

[John Franklin]

I am repowering my sailboat and would like to add a secondary alternator to charge the house bank.
Repower will be 68HP@3000rpm.

Ideally I would like to charge the bank as efficiently and rapidly as possible to reduce engine run time while charging.

I do not have solar or wind generation as of yet.

Since PMDC generators are the most efficient of the 3 options can I use one of these with a serpentine belt and external smart regulator that monitors alternator temp and battery temp? Is this safe?

How would I choose a suitable size PMDC generator/alternator? Not too light and not overkill.

Additions will be made to the boats systems over the years but not much more than what I have already. Perhaps autopilot and watermaker. Upgrade to lithium once my SLA batteries are shot. Currently 4 golf cart wired in series-parallel plus one starter battery. Magnum charger unit.

Any help or advice would be great!

[lmxr]

PMDC machines provide an interesting option for DC power generation. However, they are not the easiest to use. A first limitation is that their EMF depends only on speed. It therefore will work well if you can guarantee that the PMDC will operate within a small speed range, but that is not how boat engines are usually operated.

The traditional automotive-style alternators are far easier to regulate because their EMF is proportional to the product of the speed and the field coil current. The field coil current of a few A is very easy to control using a (smart) regulator. With a PMDC such a regulator cannot be used because a PMDC has no field coil.

A second difficulty lies in heat. Permanent magnets are very sensitive to overheating. Luckily, on a boat there's ample cooling available in the form of seawater, though this will add to the overall system cost.

While somewhat difficult, it is not impossible to use a PMDC on a boat. In regards to regulation, a first option is a DC-DC step-down converter. The PMDC should be designed such that its EMF is always higher than the battery voltage and the DC-DC converter will lower the voltage (and increase the current). A second option is a DC-DC step-up converter, in which case the PMDC should be designed such that its EMF is always lower than that of the batteries. This latter option is particularly interesting as the DC-DC converter would consist of only a few FETs, as for the coil it can re-use those in the PMDC, decreasing the cost somewhat. In regards to cost, the higher the minimum voltage the better, as the cost of power equipment is roughly proportional with the current.

In practice, a firetruck-style alternator - far larger than traditional car alternators, and more efficient too - might provide a more achievable option.

Hope this helps somewhat, let know if you have any questions!

[John Franklin]

Originally Posted by lmxr

PMDC machines provide an interesting option for DC power generation. However, they are not the easiest to use. A first limitation is that their EMF depends only on speed. It therefore will work well if you can guarantee that the PMDC will operate within a small speed range, but that is not how boat engines are usually operated.

The traditional automotive-style alternators are far easier to regulate because their EMF is proportional to the product of the speed and the field coil current. The field coil current of a few A is very easy to control using a (smart) regulator. With a PMDC such a regulator cannot be used because a PMDC has no field coil.

A second difficulty lies in heat. Permanent magnets are very sensitive to overheating. Luckily, on a boat there's ample cooling available in the form of seawater, though this will add to the overall system cost.

While somewhat difficult, it is not impossible to use a PMDC on a boat. In regards to regulation, a first option is a DC-DC step-down converter. The PMDC should be designed such that its EMF is always higher than the battery voltage and the DC-DC converter will lower the voltage (and increase the current). A second option is a DC-DC step-up converter, in which case the PMDC should be designed such that its EMF is always lower than that of the batteries. This latter option is particularly interesting as the DC-DC converter would consist of only a few FETs, as for the coil it can re-use those in the PMDC, decreasing the cost somewhat. In regards to cost, the higher the minimum voltage the better, as the cost of power equipment is roughly proportional with the current.

In practice, a firetruck-style alternator - far larger than traditional car alternators, and more efficient too - might provide a more achievable option.

Hope this helps somewhat, let know if you have any questions!

@lmxr
Thank you for the information! I will look into the brushless style heavy duty alternators.

[ItDepends]

Adding to that thought...

If you are running you main engine to charge your batteries, the mechanical efficiency of the alternator is not in any real sense an important variable. The complexities of regulating a PMDC system pretty much negate that.

Do not over think this. Standard alternators are the go to system on drive engines for very good reasons.

Not that PM alternators don't have their place. My genset is actually a PM alternator that I use to charge my battery bank, and it works great. But that is a different use case.