fun with motors...

hello again everybody…

a little over a year ago, I posted a question on the forums, asking about the “safe” number of 393 motors (6), and I thank everybody for the info i received on the subject. So, as I understand it, If I used a power expander, I could put 4 393’s on the cortex (two between 1-5 and two between 6-10) and two 393’s on the power expander. BUT I heard of teams using 8-10 of them during the Sack Attack season.

The question is this:

How many 393’s do you plan to use during the Toss Up season?

(poll is above)

My team plans to use all 10 393 Motors. It worked fine last year for us.

If you have a power expander, there’s almost no reason not to be using all 10 393 motors; if you have leftover motors when designing your robot, put them on a crucial system as backup/extra power; drive power or hang power will come in handy.

Most of the time you will not be stalling out all sub-systems at the same time, making it possible to have 10. The free current is .37 apiece making it low enough that if there is no load on the motors, they could all be spinning. Also, if you go over the 4 amp limit it will not immediately trip. The breaker has to heat up and then trips.
http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/0100-400%20A%20Frame%20FA-LA/FA-FC-FH/0600DB0105.pdf
Page 2 shows a EXAMPLE graph of how a breaker trips. If this graph is same as the breaker on the cortex, it would take about 10 seconds to trip with a load of 5 times the max load of 4 amps or 5 393 motors stalled out.

Last year we had a claw that used 2 393 motors on a power expander. We had only these 2 motors on the expander so when the breaker did trip, the it would only take out our claw, and not the rest of our robot.

We’re going to use 8 or 10 this year, depends on if the intake needs them or not. Last year we used 10 with no issues and had a power expander.

This year to save weight, we plan on going without the power expander and will have to see how the cortex holds up, but any issues will likely be end user error, for example stalling out the drive in a pushing match.

One of the tricks we employ is putting half of the motors from each subsystem (drive/lift) onto each circuit breaker so you split your pull of current.

We used all 10 393s, 6 on the cortex and 4 on the power expander. Our drive was 6 motors, so we put the arm on the power expander. We didn’t have any problems, except when we got into pushing matches against heavier robots, which is likely not a good idea under any motor setup.

Cylinders EVERYWHERE.

-Nick

Last year we used eight 393s without a power expander. No clue if that’s the reason why we had connection issues, but when it was tethered, the robot worked fine. This year we would like to use 10, however it looks like we’re going for either 7 or 9

You may want to have your Vexnet keys checked because we have had the same problem and it was because of bad keys. If you have spares test them in different pairings to find out which one(s) as bad.

There is no option for 12, so I am going to say 12.

Like most things in Engineering, use of 393 vs 269 vs Pneumatics etc is a design tradeoff with advantages and disadvantages of each.

In our case, our current designs allow for 12 393’s without significant risk of stalling. In past, motor load has been an issue so we haven’t used 12.

So yeah, designers, make it work.

Ok, I have a quick question about the stall current of 393’s…

In this thread

https://vexforum.com/t/fun-with-high-strength-motors/21599/1

I asked about why the “safe” number was 6, and afterwards I operated under the belief that the stall current of a 393 motor was 3.6 Amps, given the calculations in that thread.

On the Vex website, however, it lists the stall current as 4.8 Amps… [http://www.vexrobotics.com/vex/products/accessories/motion/276-2177.html

Is there a reason for the difference? Is one or the other incorrect?](http://www.vexrobotics.com/vex/products/accessories/motion/276-2177.html)

I asked about why the “safe” number was 6, and afterwards I operated under the belief that the stall current of a 393 motor was 3.6 Amps, given the calculations in that thread.

On the Vex website, however, it lists the stall current as 4.8 Amps… http://www.vexrobotics.com/vex/products/accessories/motion/276-2177.html

Is there a reason for the difference? Is one or the other incorrect?

4.8A is the correct number, it was revised based on some of last years testing. See this post

https://vexforum.com/showpost.php?p=309781&postcount=40

and this thread

[https://vexforum.com/showthread.php?p=310291

The 393 motor was designed to be a 3.93W motor, turns out it is in fact closer to a 4.3W motor.](“https://vexforum.com/showthread.php?p=310291”)

Ah, thank you very much for the explanation :slight_smile: