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Rapid Strike and Stryfe VOltage Increase

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#1 Hammy



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Posted 05 September 2013 - 01:20 AM

I hope this is the correct sub-forum, apologies if not.

Last week, I made the following measurements on a Rapid Strike with 2x RM2 motors fitted (all the thermistors, capacitors, inductors and diodes removed), and

Voltage / Power Consumed
3V / 2.75W
4V / 4.45 W
5V / 6.8 W
6V / 9 W
7V / 13 W
8V / Insane, too scared to make a measurement.

Today I repeated the same measurements on a Stryfe, also equipped with RM2’s and all internalk components removed

Voltage / Power Consumed
3V / 2.5 W
4V / 5W
5V / 8W
6V / 12W

The measurements for both the RS and Stryfe were made with an R&S HAMEG Bench supply, and this can deliver 10A of current.

Making measurement with just the trustfires and a Fluke Multimeter (the yukki orange one)

The RS, with 4 trustfires when open, voltage measured 16.8V, but dropped to 6.5 V as soon as the flywheel motors engaged. (The pusher motor was disconnected at this time).

The Stryfe had 2 trusties fitted, and when open circuit the voltage measured 7.8V, and dropped to 6.25 V when the flywheels are running. 6.25 V means that more than 2A of current is been supplied by the batteries.

And both setups smell like an Ozone factory with the flywheels running.

I could obtain better measurements if I had more Stryfes and RS’s and more time, but I hope the above figures give an idea.

OK conclusions for now.
If you have Solarbotic RM2’s fitted, it is advisable not to use more than 2 trusties, even in a RS, otherwise expect burnt out motors and maybe the batteries to catch fire. Even 2 batteries is risky because of the high current been delivered.

When trusties are delivering large currents, they get hot, expect your battery compartment to melt if you are running on 4 trusties.

For a hand-held device, I think above 1-2 Amps, is well above the safety margin.

It is best to use a Lipo with a regulated supply to the RM2’s. And I will think of a solution for this in the coming weeks, unless someone beats me.
I am thinking to limit the operational voltage to 4.5V, but it is 6 V for the 1st few seconds of flywheel operation to get the motors up to speed, then the 4V switches in.

Edited by Hammy, 05 September 2013 - 01:22 AM.

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#2 azrael



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Posted 05 September 2013 - 12:17 PM

If you search, it's been documented that people have brush failures at higher than 2s for RM2 motors.

You should not be using trustfires with these motors, anyway, as they cannot supply the current that the motors ask for at high voltages. This makes for slower recovery/start up time.
Trustfires can barely supply 2A at all.
Also, their voltage when under load is poor.

Remember, voltage supply is not everything, you must also consider the current discharge rate of batteries.
Check out the article on my under either the Rayven guide or tech articles, infamous goes over it quite a bit.

There is no safety issue, as long as you are using batteries correctly and making sure they never drop below their nominal level and overdischarge. I have never had any problems with high current setups, and I recently helped build a robot where each motor consumed 100A at stall. No issues whatsoever.

Edited by azrael, 05 September 2013 - 12:18 PM.

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