Menu Sign In Contact FAQ
Banner
Welcome to our forums

Standby / backup alternator for the TB20 (and other types)

Engine is back

so we are refitting the fuel pipework and are arranging it to take the backup alternator, and it does fit, as reported before, by rotating the assembly at the fuel pump inlet appropriately

With the tapered thread, it is a matter of luck where that thread becomes tight but we use PR1422 anyway. If this angle cannot be achieved you will need an additional adaptor there which is adjustable with a standard stop-nut (or whatever it is called).

The fuel pressure switch clears the alternator by 5mm and clears the engine mounting frame (the white tube) by 10mm

I now have to get the field approval done in the USA. The alternator I have is just a mockup.

Administrator
Shoreham EGKA, United Kingdom

Impressive scenery…
Is the situation under control here ?

Everything is under control

Administrator
Shoreham EGKA, United Kingdom

A little update on the previous debate about how much current these produce.

You do have to be very specific on the RPM. From the B&C website:

The small difference between 1.3 and 1.5 makes a large difference to the generated power.

The TB20 is of course a Lyco engine.

Administrator
Shoreham EGKA, United Kingdom

I took the back off the B&C “mockup” alternator I got a year or two ago.

The first thing I noticed is that it contains 8 diodes, not the usual 6. So it looks like a 4 phase alternator. Interesting!

The second thing I noticed is that the field connection has two terminals but they are shorted; the field is actually between either of them and the alternator ground.

The third thing is that the brushes aren’t apparently serviceable, but that corresponds to the B&C ICA (instructions for continued airworthiness)



I have no idea what the bit under the aluminium heatsink is…

I am going to spin this one up and see if it works… My plan is to borrow a 1kW variable speed drive and do a proper test on the one I buy.

Administrator
Shoreham EGKA, United Kingdom

I have just measured the field resistance as 3 ohms which is a bit unreal, even for a 12V system. Obviously with a switch-mode regulator it will be workable (with an awesome peak current especially in a 28V system, and resultant muck on the main bus) but if the regulator fails shorted, the power dissipation in the armature will be 200W which will destroy it pretty fast, unless the CB trips. As well as draining 8A from the battery which by then is all you have… B&C are not responding to emails anymore until one phones up reception and asks the lady if they have gone bust…

The full size 60-70A alternator is 11 ohms.

This thread started off as a general “backup alternator” thread, so I may as well mention this unit

It looks very similar to the B&C unit, and is probably made by the same company, and is probably the same as the GAMI Supplenator, but is not quite the same as the B&C especially as it contains the voltage regulator internally. Different castings too.

In the absence of an STC this would b a field approval, same as the B&C one which is a lot more expensive…

The Plane Power page is here and the biggest issue seems to be that there appears to be no 28V version.

Administrator
Shoreham EGKA, United Kingdom

I’ve been doing more research on this alternator, of which I have one to play with.

As mentioned above, the field resistance is just 3 ohms. I managed to make contact with the designer of their voltage regulator (a different company, not B&C) and this turned out to be extremely interesting.

To enable the same alternator to be used in a 12V or 24V system, the voltage regulator internally limits the voltage applied to the field to 14V. For maximum output power the field current is 3.5A but this is not the current drawn from the aircraft bus, because the regulator is switch-mode. 3.5A into 3 ohms is 10.5V so in a 14V system and assuming 90% regulator efficiency, the maximum bus current for the field power would be 2.9A or 1.45A for a 14V or 28V bus voltage, respectively.

However there is another twist. B&C is probably alone is having a voltage regulator which applies regulated DC to the field. The others (e.g. Lamar) use a PWM (pulse width modulated) method where they just connect the field directly to the bus via a MOSFET and switch that on/off at some 200-600Hz, with the duty cycle adjusted as required. With a flywheel diode present, the result is that the field current is a sawtooth waveform whose average value is above zero and corresponds to the field current which a regulated-DC regulator would be feeding in. Either method works equally, and an equivalent PWM solution for the B&C alternator would simply limit the duty cycle at 50%… But the B&C regulator will generate a cleaner bus voltage, without any regulator ripple. There will still be the ripple due to the 3 phase (or 4-phase?) rectifier but you won’t have say the 200Hz ripple which you would get with e.g. a Lamar regulator.

This leads to the question of control loop stability. Apparently, most vehicle alternator systems have a gain of around 50 i.e. the field current goes from zero to maximum as the bus voltage changes by 1/50. That happens to correspond well with what I found here where it goes from zero to max over a bus voltage delta of 0.6V which is a gain of 47. It’s always satisfying to see experimental evidence coming together

My next job is to borrow a 1kW variable speed drive and test the alternator with it, to see what current it really does.

Administrator
Shoreham EGKA, United Kingdom

A further update: the alternator is 3 phase, not the 4 phase it appeared previously. And the aluminium heatsink is just a dummy cover for a missing voltage regulator – other versions of the alternator (I still don’t know who makes it but it looks pretty generic) have a voltage regulator built-in.

Also this post claiming the alternator does only 6-7A is nonsense. I spoke to one of the original designers of the B&C product line and the only way to get just 6-7A out of it would require it to be so hot it would melt the copper. The context is clear since my preceeding posts show the alternator model.

Administrator
Shoreham EGKA, United Kingdom

Apparently all these alternators (B&C and GAMI) are made by DENSO.

Administrator
Shoreham EGKA, United Kingdom

I took this pic today, of an installation of the B&C alternator on a turbo SR22

Can anyone advise the maximum RPM it gets at that mounting point?

Administrator
Shoreham EGKA, United Kingdom
Sign in to add your message

Back to Top