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Old August 13th, 2013, 18:31   #97
mcguyver's Avatar
Join Date: Aug 2005
Location: Northern Alberta
Originally Posted by ThunderCactus View Post
Motor failures are generally due to heat cycling, and it seems to me heat cycling is worse on LiPo batteries. I'm guessing because of the higher voltages there's just more wattage going to the motor creating more heat.
AFAIK the NiCd's supply comparable amperage to the LiPos and that's why systema always used them
The issue is really what happens to the motor when the battery becomes depleted. I am lazy, so I will ask you to Google "discharge curve for" whichever battery chemistry you want to compare. In lipo, the discharge is rather linear up until it is depleted, then it drops like a rock. In nicad, its curve is very shallow in comparison.

When users notice a slowdown in motor soeed when shooting, it is a cue that it is time to change the battery. With nicad, it recovers a bit and you can get a few more shots out, and a dead battery always happens at the worst time, doesn't it? With lipo, when you hear a slowdown, it may already be too late.

Without getting heavy into the science/engineering aspect of motors and all the math, think of a motor like any other inductor or transformer. Because it is. So now you can apply all the laws and formulas regarding inductance, as well as a helping of motor theory. When the voltage drops to an inductor, inductance drops, the magnetic field strength the armature is capable of is reduced, and the motor slows down. Nicads will deliver current but at reduced voltage. Inductors by their nature resist changes in current (like capacitors resist changes in voltage) and they will transform their charge current (when they are getting current from the battery) into discharge current (when they geting no or reduced current from the battery). And, being inductors, this discharge current is a function of time, the shorter the time, the higher the current. This is why we as electricians must make provisions for and be aware of inductive spikes when you open inductive loads like motors, and it is the reason there are motor ratings on switches, breakers, contactors, etc.

Now, with lipo, they are a constant voltage battery, their current delivery does not follow a linear curve in relationship to voltage like nicad does. The battery may still be reading 11 volts and not be able to generate much current. This means the inductance of the armature stays constant, and it is dumping current as much as retaining it. In a motor that spins at 30,000 rpm, with 2 pole reversals per turn, this means that the armature is charging and discharging 60,000 times per minute. It is a veritable inductive jackhammer, and the windings are paying the price.

Perhaps lipo alarms would prevent some of it, quality batteries might help, but I think it is something that users will just have to live with.

And I understand the FCC motor is not the end all be all motor, but we want it to be! and that's why we're so critical of it lol
How much work is it really to redesign the head properly? At the very least I'd like to see the original 4 axially mounted screws at the head of the motor where the most force is going to be applied.
There is lots of design changes that could be made. Windings could be thicker, but that means more turns, more armature mass, and the weight penalty would mean slower RPM. The brushes can be improved. The trueness of the comm could be better. Spring strength could be better. Height adjustment to accomodate the variety of aftermarket lowers and gearboxes would be nice. The better attention paid to smooth layering of windings, a soldered comm joint, solid lashing and dipping to prevent winding resonance are good places to go.

Perpendicular screws are not a good idea mechanically, but there is alot of more critical areas to address to performance and longevity.

I could write an essay on why side mounted screws are a REALLY bad idea, but you just need to look at any industrial motor out there
Yep. Good luck finding a motor out there done like that. PTW motors are by far the cheapest motors I deal with.
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