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[Damien]

I have a speed 400 motor running a 30mm 3 blade prop on 5 x 1800mAh sub-c nicads, this config lasts about 2 minutes on the water before the battery Voltage drops to where the rudder servo goes berserk, i assume because the ESC ceases to work correctly.
I put an Amp meter in line and with the power of my sub chaser and with it held captive in the test tank the current drawn is 8.75 amps and the batteries last about 40 seconds.
3 questions
Is the motor over propped causing the high current ?
or over motored the boat is 34" x 6" and weighs 1.5kg with batteries ?
I noted after 3 x 40sec runs the motor was hot is water cooling needed for these motors?
 
I have 2 x 7.2v 3600mAh nimh packs waiting until i can afford a charger.

Any suggestions greatly appreciated.
Damien.

[FullLeatherJacket]

Where does one start? I would suggest the following:

1. Sounds like your NiCad batteries are well memoried-up. Discharge each pack across a 6v bulb until the voltage drops to 6v, then SLOW charge the pack at no more than 180mA. Repeat.
2. What sort of boat is this? If it's a displacement type (e.g. warship) then it's probably over-powered with a Speed 400, which is a very fast little beast. If so, drop down to a 385 and use 12v. If it's a planing model then it's certainly underpowered; a 30mm prop is pushing the envelope for this motor. Upgrade to a 500-sized motor for the same prop.
3. If the motor gets too hot to touch then it's being abused. Better get the combination of motor/prop right than torture the poor thing further!

Suit yourself - works for me.
FLJ

[malcolmfrary]

Silly question - does the battery charger work?
Follow FLJ's advice, but if its a displacement model, consider a 385 rated for 12-15 volts and run on your 5 cells, but make sure thet are fully charged first.
If you can check the voltage of each cell, see if one is a bad one. Charging that one and cycling it a few times might bring it back to life, but it might be worth considering some nice new NiMH cells.

[stallspeed]

 If a smaller prop increases the thrust there is your proof that are indeed overpropped (see red curve on graph ) http://www.mabuchi-motor.co.jp/en_US/technic/t_0201.html
..... or if rpm has fallen to < 50% or current drain > 50% motor stall.

Five cells are not enough to run with BEC as you need 6 volt to regulate.You will see that the output voltage at the end of the cell discharge cycle is 1 volt from which the nominal capacity comes according to the red curve in this graph http://www.mpoweruk.com/performance.htm

It is normal for small brushed motors to reach maximum rated temp very quickly and 400 size will do that in < 1 minute.
Changing the motor to a 385 would only help if the 385 had a greater (torque/current ratio)-who knows?

[DickyD]

Understood not a word.

[Damien]

Thanks for your thoughts guys.
The Nicads are near new i bought 20 cells 12 months ago and made up 4 packs they were charged once and left as i've been without transport since. they were charged at 4A quick charge prior to use lake side. After the first mishap where my out of control sub chase T-boned a 5' schooner, fortunately as the batt's were about to expire it was a slo-mo crash. I tested the cells in the pack used 4 were 1.12v the other was zero. i sacrificed a pack to replace the dud, i guess i'll use the rest to add a cell to the others.

Ohh yes the charger does work.

I hate that radiation treatment i've had for a pituitary tumor has all but wiped out 35 years experience as an Electronics Technician.
At least there's lots of knowledge at Mayhem.
Damien

[Damien]

As it turned out i had some 12v 5 pole motors on hand. along with specs.
12v free load speed 20,800 rpm
Current 0.44 at max efficiency speed 17,100rpm Torque 9.1
Current 2.07 at max power speed 10,400 rpm torque 25.5
Current 5.00 at stall torque 51.0
Same diameter and shaft size as the Speed 400 so fitted it to the Sub Chaser fitted Ampmeter and dialed up 12v on my power supply same 30mm 3 blade prop, about half the force required to hold her steady in the test tank and only draws 1.6A 12v 10 cell pack of sub-C's drop nose 1/2" so i'll put a pack of AA 2800mAh nimh cells togetherthese should give a good time on the water.
Note these motors cam fitted with a bridge rectifier across +/- so i assume they're a hair dryer or some such appliance, a chap i spoke to at the Surrey park club yesterday uses 2 x 385' in a 3' sub to great effect.

Thanks for the input guys very much appreciated.
Damien   

[stallspeed]

Damian,you are right at the limit of rechargeable AA cells and they can get very hot above 5 amps.The internal resistance is ten times that of racing sub-C cells.

[Damien]

After changing motors to a 5 pole Johnson 385 the current is now a comfortable 1.7A ( no doubt this will be lower when the ship is running free) on 12v = 10 subC 1800mA nicads, but I still had the rudder servo chattering constantly, so using a servo extension lead i cut the red wire to disable the BEC and fitted a Rx battery, ruder chatter still the same.
Feeling frustrated and ready to throw the lot against the wall, I changed the Tx & Rx, still using the Rx batt. no rudder chatter whooo hoo ran it for 5 mins not a glitch so i changed it back to BEC  again no rudder twitch.
I can only conclude there is a problem with the Tx Rx combo (probably why it was cheap on ebay).

Rest of day getting ready for lake trip tomorrow.
Damien.

[stallspeed]

If you switched between 27AM and a pcm set it would have been no surprise.
Tell us what sets you used so more can benefit.

That setup would be ideal for the surplus radio in that you could have put a 3 amp 6uH chokes on the motor cables.

[Damien]

Thanks stallspeed,
The first set was 27mHz AM 2 ch trigger and wheel, the second is 27mHz FM 3ch again trigger and wheel.
I'll certainly keep the chokes in mind for the future.
Damien.