[FTC]: AndyMark Motor Stall Current

[ItsTebowTime]

Greetings: my team is currently having a problem in autonomous with our robot power levels. Directly after we raise and lower our lift, the bot drops power significantly and drives incredibly slow for a few seconds before regaining speed several seconds later.

We think we've narrowed it down to one of two things:
1. AndyMark motors on the lift or
2. Thin wire for our battery circuit.

Based on this useful document from Cougar Robotics http://www.cougarrobot.com/attachmen...C_Motor_V2.pdf, the TETRIX V2 motors have a stall current of 1.37 amps, whereas the AndyMark motors that we're using for the lift have a whopping 11.5 amps http://www.cougarrobot.com/attachmen...NeveRest40.PDF. Give that we're using 2 AndyMark motors for the lift and 4 on the drive train, does this seem like it could be the issue/should we switch the lift to TETRIX motors?

The other possible problem we noticed was our wire gauge that gives the motor boxes power is 20 gauge (same as DC motor wire) instead of something thicker like 16 or 18 gauge.

We're currently leaning towards the AndyMark motors as the source of our problem, but we'd just like some input on what might be the problem/how we could fix it before we make any major changes to our bot. Thanks so much for any suggestions!

[cgmv123]

If you have thicker wire, try that first. A drop in voltage is caused by resistance (Ohm's Law), so lower resistance will lessen the voltage drop.

[MattRain]

I'm sure its your wiring. Make sure that you are in specs of the Game Manual 1. R10.d

[aklego]

11.5 A is the published stall current but I'm wondering if that is a typo. The motor controller has a max current of 4A/channel and an 11.5A stall current is likely to trigger the overload protection in the controller. Also, a stalled drivetrain at 23A would cause that battery fuse to blow....

[GeeTwo]

11.5A of current on a manipulator (elevator) should definitely not cause:

the bot drops power significantly and drives incredibly slow for a few seconds

if you're using anything close to recent FRC standard PDB and wiring, and have a battery that's anywhere close to competition-ready. My first suggestion is to load-test the battery or try a new battery. If the battery is good, figure out just how much current you are drawing, using a hefty current meter or (if you don't have one) a small resistor (much less than 1 ohm; probably a length of wire with a known resistance per unit length) in parallel with a voltmeter: I = V/R.

[Bryce Paputa]

Originally Posted by GeeTwo

11.5A of current on a manipulator (elevator) should definitely not cause:

if you're using anything close to recent FRC standard PDB and wiring, and have a battery that's anywhere close to competition-ready. My first suggestion is to load-test the battery or try a new battery. If the battery is good, figure out just how much current you are drawing, using a hefty current meter or (if you don't have one) a small resistor (much less than 1 ohm; probably a length of wire with a known resistance per unit length) in parallel with a voltmeter: I = V/R.

This is all FTC stuff, not FRC. Their battery is much smaller, small enough to use a 20A automotive fuse (same one in spikes) instead of a 120A breaker.

[Mr V]

Originally Posted by ItsTebowTime

Greetings: my team is currently having a problem in autonomous with our robot power levels. Directly after we raise and lower our lift, the bot drops power significantly and drives incredibly slow for a few seconds before regaining speed several seconds later.

We think we've narrowed it down to one of two things:
1. AndyMark motors on the lift or
2. Thin wire for our battery circuit.

Based on this useful document from Cougar Robotics http://www.cougarrobot.com/attachmen...C_Motor_V2.pdf, the TETRIX V2 motors have a stall current of 1.37 amps, whereas the AndyMark motors that we're using for the lift have a whopping 11.5 amps http://www.cougarrobot.com/attachmen...NeveRest40.PDF. Give that we're using 2 AndyMark motors for the lift and 4 on the drive train, does this seem like it could be the issue/should we switch the lift to TETRIX motors?

The other possible problem we noticed was our wire gauge that gives the motor boxes power is 20 gauge (same as DC motor wire) instead of something thicker like 16 or 18 gauge.

We're currently leaning towards the AndyMark motors as the source of our problem, but we'd just like some input on what might be the problem/how we could fix it before we make any major changes to our bot. Thanks so much for any suggestions!

What I see listed on the Tetrix site shows 1.37a as the max load current, not the stall current. The graph shows the stall current to apparently be 7.5a. So yes the NeverRest motor does draw significantly more current so it can cause more voltage sag in the battery. However since Tetrix specifies a 20a fuse the load that the NeverRest draws is well within the design range of a good Tetrix battery. So what is the age and condition of the battery? Has it been properly cared for, not overcharged and not stored for long periods of time w/o charging?

Originally Posted by cgmv123

If you have thicker wire, try that first. A drop in voltage is caused by resistance (Ohm's Law), so lower resistance will lessen the voltage drop.

No thicker wire will make the problem worse. The problem described indicates that the problem is battery voltage sag caused by a large load since it recovers after a short period of time. So decreasing the resistance in the circuit by using larger wire will result in greater battery voltage sag due to the increased draw and a longer recovery time.

Originally Posted by aklego

11.5 A is the published stall current but I'm wondering if that is a typo. The motor controller has a max current of 4A/channel and an 11.5A stall current is likely to trigger the overload protection in the controller. Also, a stalled drivetrain at 23A would cause that battery fuse to blow....

There is no indication that there is any stalling occurring so that 11.5a draw is only for a few milliseconds.

A fuse's rating is not an absolute it can handle over currents for a period of time. From the littlefuse specifications page a 110% current will have a minimum opening time of 360,000 seconds. 135% of rated current has a minimum opening time of .75 seconds and a max opening time of 600 seconds.

So at 115% of the rated current (23a) you can expect that it would take at least several seconds up to several minutes before it will blow.