# Understaning current draw in drive trains

Hello,

I was trying to calculate a gear ratio to my drivetrain using the jvn calculator, I used 2 cim motors in the vex pro 2-cim ball shifter, i used a 1:6 gear ratio and got a free speed of around 15 fps and a real life speed of 12 fps which seems like a healthy speed, however the pushing current draw was 194 amps which seems wrong so I’m guessing that either I didn’t configure something correctly or I don’t understand what pushing current draw means, can anyone help me understand how current in drive trains work.

In addition now that the new falcon is out I’m hearing a lot of talk from my mentors about current limit, I don’t understand why that is neccessery since from my understanding (which is probably wrong) you have ratio so that the nominal current draw will be healthy and my breakers won’t jump.

Pushing current is the current when the motors are stalled (aka pushing another robot). 194 does seem high. In general aim for 50-80. I do wonder if you did not configure the low gear as a shifting gearbox has two speeds.

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That tab lists the pushing current draw per gearbox instead of per motor… so between 2 cims it’s 95 amps/motor. I wonder if that’s why it seems high.

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Our team has been using this tool to select motor / gearbox combinations for a couple of years. This has been updated to include the new Venom and Falcon options.
https://www.chiefdelphi.com/t/frc-1018-robot-speed-simulation-2019/354716/4
This may help you to understand the current draw as the robot is operated.
Make sure you pull the 191106 version.

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So, there are two things “current limit” brings to mind for me:

First - Circuit breaker limits: Each motor must, in steady-state, draw no more than 40A, otherwise it will trip its breaker. Additionally, the total robot current draw must be no more than 120A, or the main breaker will trip. These are all thermal breakers, which means you can exceed the rating for some period of time (depends on how much you exceed the rating).

Any time you see a motor stall current or max power rating (where it draws more than 40A), you have to keep in mind that you’ll only be able to achieve that operating point for a short period of time.

Second - Current Limits in motor controllers: Most FRC motor controllers have some ability to actively limit the current draw, by cutting back on the amount of voltage they apply to the motor. Ramp rates or other algorithms can be used to attempt to prevent the high-current draw situations in the first place. If you want to ensure your motors don’t exceed some limit, this is one way to do it.

As you also mentioned, mechanical design goes a long way in keeping the motors operating in a “happy” state. It’s important to remember that you don’t need to always keep your motors below 40A, just don’t let them stay above 40A for long enough to trip the breaker.

As you may have noticed, the motor curves on Vex often show more than a 40A draw. These show optimal, transient performance of the motor. The curves may look different if you enforce a lower current limit, either by design or software (or mistake).

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