Neo braking mode information

I didn’t see anyone else had posted this data…

I just checked the winding resistance on a room temperature NEO motor: 4.99 Amps produced 0.75 Volts. 0.15 Ohms from X to Y. This means that each coil is 0.225 Ohms. Just FYI, when I plug X into Y, I get quite a bit of drag torque and some cogging. With 5 Amps going from X to Y the motor holds position quite strongly; more torque than I was able to casually put on the shaft with my fingers.

Running the model, it looks like you might get 1.4 Nm of braking torque at full speed (5800 RPM) and 880 Watts of braking power going into the motor windings. There’s a very good chance that I’m calculating this wrong, though. I’m not 100% sure how to treat the shorted three phase windings. You REALLY don’t want to do that for very long! This braking torque tapers off as the motor speed falls, of course.

Since I can’t leave well enough alone… I took some actual measurements! Sorry, but we are getting close to the limits of my scale, and the hanging scale is waaay below its measurement range.

Torque arm: 200mm
Speed: 575 RPM according to my lathe chart
12.7 grams or 0.12N of load open circuit
204 grams or 1.99N two leads plugged into each other
243 grams or 3.35N all three leads connected via alligator clips
103.6 grams or 1.01N two leads plus a 0.005 Ohm shunt
Reads 16.6 Amps AC
66 Hertz I don’t know if this is right or interference…
Also I don’t know the pole count on a NEO

Open circuit: 0.024 Nm at 575 RPM
Two leads shorted: 0.398 Nm at 575 RPM
Three leads shorted: 0.67 Nm at 575 RPM
Two leads + shunt: 0.2 Nm at 575 RPM

1 Like

And yet more measurements up to the full speed on my lathe… which isn’t all that fast :frowning: The way I’m doing it, its not easy to get more than three consistent speeds.
Here’s the drag with no electrical connections at all. Very small torque due to the bearings.
Only shorting two wires together (using the Andersons) delivers less drag than all three shorted (even though its done via test leads). The Spark Max brake mode is all three shorted, so it should be fairly close to this, at least for a cold/slightly warm motor.
And, finally, the exotic testing. I am injecting 0 to 5 Amps between two terminals. I am using a diode and a 3.6 Ohm resistor so that the current doesn’t jump all over the place due to the motor’s generated voltages. Please be aware that this data is of limited accuracy. My smaller scale only measures to 0.1 gram. The largest torque on here varied from 14.7 to 17.3 grams! One takeaway from this is that 5 Amps is really not enough current to get any real braking on this motor. At zero speed there is a noticeable cogging torque if you are turning the shaft by hand, but with any lever arm at all its minor.

1 Like

Have you considered running the NEO in an active braking configuration? That is, put it in closed loop control and either set the target speed to zero, or the target position to a constant. (These may give different results.)

1 Like

I would LOVE to do that, but I don’t have the hardware to make it happen! There’s four braking scenarios I’d like to test and have actual quantitative data on:

  1. Coasting
  2. Spark Max brake mode
  3. Active braking to zero speed
  4. Spark Max brake mode until RPM falls to a setpoint, then active braking to zero speed

Stopping doesn’t seem to be treated right on ILITE, and I’m trying to build my own sheet so I fully understand it before I start tinkering under the hood :wink:

Here’s an ILITE simulation of the chassis. Note that the reverse drive section has constant acceleration, where the starting doesn’t. I’m 98% sure that the stopping torque varies with speed the same way that starting does. The distance shown is or 25.2 feet or 7.6 meters.

This isn’t totally apples to apples, but here’s my simulation of the same chassis. I’m not using ABS(accel) and its metric units. I’m seeing the reverse drive acceleration increasing and then flatlining once its current limited. An interesting bit is that (according to my model) the “brake mode” braking torque is actually higher than you can get from reversing the motor! You can see they cross; the green acceleration curve has an automatic switch to whichever is braking harder. The tuned one is me tinkering to see if there is a lower energy stop that’s still pretty quick. The brake mode setting doesn’t use battery power for stopping, which is a pretty significant perk! But, that comes at the cost of much longer braking time/distance because the braking is related to speed.

I would LOVE some of the motor gurus to weigh in here! Even better would be for Rev and Vex to set up and run these cases with their dynamometers and give us a real answer!

I realized that I hadn’t plotted my measured brake torques vs known motor performance and my predicted motor braking! Um, that’s a lot higher than my prediction! Edit: added the two leads shorted data… When I change the resistance that the generated voltage sees from my measured 0.15 Ohm to 0.065 Ohm I can hit the highest data point. My base model of this does not factor in the 3-phase aspect of the whole problem, so its not surprising its off…

1 Like

Even more nuttiness here… In order to get faster I chucked the NEO shaft in my milling machine. :exploding_head: The #1 takeaway was “bigger scale needed”. The two new data points are blue for the SparkMax brake mode and yellow for my crimped 10 AWG shorting plug. Clearly, the resistance in the test leads I was using was seriously skewing my braking results! Need more data…
And then I hooked the Spark-in-a-box tester to my test motor.

OK, first ever real-live two quadrant drive mode data! The NEO was set for 10 Amp current limit. I started the NEO running against the drag of my mill’s spindle, belt, and motor. Keep in mind that this is NOT full load. I don’t have a good way to load this case up accurately and controllably. Using the spindle brake didn’t do it… Once I got that data point, I turned on my spindle VFD and slowly ramped up until the NEO was barely turning forward. Data. Then barely turning backward. Data. THEN, I ramped up till it was running backwards at about the same speed.
I can’t get the REV client to run the motor, so no current data. I need to troubleshoot that!


@GeeTwo Yes, I finally tried it and the SparkMax active braking is quite vigorous! Post 7 has a data point with brake mode.

Today’s quick test came back with a kind of scary answer: the current limit doesn’t seem to be active when the load is overhauling! The first phase of this graph you can see the RPM spools up in the positive direction. I have this set for a 1 Amp current limit, but you can see lots of peaks above it. After it reaches full speed I turn up the VFD on my mill and force the motor to stop (you can see the RPM cross zero) and then run in reverse. You can clearly see the reverse RPM phase has a LOT more than 10 Amp draw; looks like up to 40 Amps! After I hit full reverse I let the motor slow and spool up the right way again, then brake mode stop.
The graph really explains what’s going on in the torque graph in post 17, where the reverse quadrant torque is WELL above the forward quadrant torque. That’s because there’s a lot more current!

And, just for fun factor, here’s braking events with and without “brake” mode. I think you can tell which one is which :wink: Yes, there’s a lot of drag on my spindle…