Greedy Motors + "Picky" ESC = Big Headache!

[John Candy]

This sorry tale relates to a GRS kit for the GWR"Flying Banana" AEC railcar.
The kit has languished (part built) for many years and if you go to http://g3forum.org.uk/index.php/topic,1661.msg14304/topicseen.html#msg14304  you will see the progress and problems encountered.
However, just when I thought I had it all sorted, a fresh problem reared its head.

The Aristocraft bogies are fitted with motors which had performed satisfactorily on short tests (a few seconds) of the electrical circuits but had never been properly run on a length of track.

How I overcame the problems is described in the railcar build article (follow link above) but what transpired was to cause a whole day of testing and head scratching as described below.

The motors (wired in parallel) ran for a short period then ground to a halt. The battery (a nominal 21.6V 2100 mAh "AA" cell pack) was fully charged and registering a tad over 24V.
The light on the Futaba R/C receiver went from green to red and then extinguished and the ESC (an Electronize FR8T-AM) then cut-out.
First thought was a battery fault but it still registered a full charge.

I then tested the set-up with a spare Buehler motor and recharged battery but had same response but when connected directly to the battery the Buehler motor ran normally. I then tested the Mabuchi motors removed from the bogies by connecting directly to the battery but they turned and stopped after a few seconds.
I swapped the Electronize for a Mtroniks Viper 10 HV (24V / 10A) unit but that produced just a "stutter" in the test motor and promptly "died"....presumably toasted the internals!

The principal problem was the Mabuchi motors...... "Googling" for problems with Aristocraft bogies revealed a widespread problem with some bogies fitted with faulty (Chinese) motors each drawing 5+Amps!
To get to the bottom of the problem with the Buehler motor, I swapped out the battery pack for a lower voltage (nominal 16.8V charged to 18V) and the Electronize ESC worked fine with the Buehler motor. Swapped for the higher voltage battery pack and back to "square one"...... a whimper from the Buehler motor and the ESC shut down.

So the greedy motors were only part of the problem (battery could not cope with current draw) but the ESCs were being "picky" about voltage......although rated at 24V, they were actually shutting down at anything over 21+V (in the case of the Viper it completely died at 24V).

Hope this may help someone with a similar problem (the first batch of ten GRS "Flying Banana" kits were supplied with the Aristocraft bogies...later batches had USA Trains bogies).

John.

[cabbage]

John,
try running the motors in SERIES not PARALLEL. Thus the same amount of current flows through both motors at half the voltage. I do know that the Mtroniks Viper series do have a minimum voltage of 10V before they cut out, if it was me I would also have designed in a high voltage cut out.

regards

ralph

[cabbage]

Having asked a few friends the problem may be with your battery pack. Some AA cells are fitted with a resister in order to prevent short circuits. My "black 5" will run on a race pack capable of drawing 45Amperes @9.6 Volts. It uses sub "C" cells to do this. Have a look at the entry on the Wiki to refresh your memory.

regards

ralph

[John Candy]

Ralph,
Thanks for those suggestions, I will experiment further when I feel less tired .... spent several days demolishing a brick wall in the garden and carting rubble to the tip, so now knackered!
The cell packs I normally use are standard Strikalite "AA" packs but may now look at using sub "C" type for a new pack for this model.
Regards,
John.

[John Candy]

In the course of sorting out the railcar motor bogie issues, I have learnt a bit about detecting and isolating  faulty cells within a battery pack.

I use the Axtronics Smart Chargers (as supplied by Strikalite) in conjunction with the same suppliers NiMH cell packs.

I have deduced that when you plug into the charger a "good" cell pack, the RED charge light illuminates immediately and stays on until the pack is fully charged, whereupon the GREEN light comes on. If the RED light comes on and goes out after a couple of seconds, then the GREEN light comes on and goes out after a couple of seconds, followed by the RED coming on and staying on, then you have a "problem" cell pack.
The light will eventually go GREEN (indicating a full charge) BUT, although the voltage will indicate a "full charge", you may find that the pack is NOT charged to capacity in terms of mAh and will run down very quickly. Another indication is when the voltage "decays" very quickly under load. The pack may indicate 20V when unconnected but attach a motor and (monitoring the voltage with the motor running) the voltage will quickly reduce to only a few volts ( a "good" pack will maintain a steady voltage under load). When the load is removed, the voltage will spring back to the 20V (or whatever the nominal voltage of the pack) so checking voltage of a disconnected pack will not give a reliable diagnosis of battery health.

I had two packs which gave a "dodgy" signal with the Axtronics charger, so I stripped away the shrink wrap and tested the individual cells. In one pack (which had the older style green/opaque shrink wrap) I found a single cell which had leaked and corroded and was giving a reading of only 1.06V, whereas all the other cells in the pack were reading 1.38V. I removed the "dud" cell and reconnected the remaining cells and the pack is now charging as it should to the rated capacity of 2100mAh. The second pack had the more recent transparent shrink wrap and there was no sign of leakage or corrosion but one cell was reading just 860mV, whereas all the others were reading 1.4V. As with the first pack, I removed the "dud" and the pack is restored to normal health.

So, before you throw away a seemingly dud pack, it is worth checking the voltage of the individual cells and removing/replacing any which are more than 150mV below the remainder. 
Photo shows the leaking/corroded cell.

John

[cabbage]

John,
Hopefully you replaced the ruined cells with tagged cells of  same capacity!

Regards

Ralph

[John Candy]

Ralph,

I haven't replaced the faulty cells, since I don't require the additional voltage..... the 18 cell pack was charging to 24.85V ...... that one "fried" the 24V-rated Viper 10 HV ESC ...... and now charges to 23.5V with 17 cells, while the pack which was formerly 16 cells but is now just 15 is charging to 21.04V. These two are higher voltages than fitted in my other locos..
I don't have any shrink wrap large enough to encapsulate the packs but all exposed connections and bare metal surfaces have been given three coats of "Performix Liquid Tape" (a paint-on rubber insulation sold for automotive use).

Regards,
John.