Driving a high KV Motor

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DubCar
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Driving a high KV Motor

Postby DubCar » 18 Feb 2017, 21:36

Hello!

I am attempting to use a VESC to drive a Traxxas Velineon 3500 Motor (part number 3351R from Traxxas). We attempted to use the config files for both BLDC and FOC mode that a research group at MIT has used. Unfortunately after one successful run we started getting the DRV8302 fault, and it appears we burned up the board. We sent it back to Ollin for repairs, but after speaking with Ollin it appears that the config file was at fault. Here were his words:

"Whoa! That motor has an extremely high kv. You are exceeding the make erpm by over 100k! Instant death for a vesc. The vesc operates best around 60k erpm max but can go as high as 100k. You need a lower kv motor, you could try asking on Vedders forum if switching the shunts to the .005mOhm versions will help but you really need to find something else with the same bolt pattern and can size. Our specialty is high powered skateboards so you will get more appropriate info from vedders forum. A lot of roboticists there."

I'm not very familiar with motor control (Comp. Eng. background), and would love some advice on how to get this motor up and running! I can't attach any files for some reason so I've linked the config files!

https://raw.githubusercontent.com/mit-r ... 00_foc.xml
https://github.com/mit-racecar/hardware ... n_3500.xml

Best,
Colin

rew
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Re: Driving a high KV Motor

Postby rew » 19 Feb 2017, 09:08

To select a motor we need to know the target RPM and power levels that you intend to run at. If you are running that 3500 KV motor at 50V, you would be running at 175k RPM.

However in fact, the traxxas documentation hints at a maximum RPM of around 44k RPM. So... A more appropriate motor would have a kV of around 875. On the other hand, this is assuming you need to hit that 44k RPM.....

Traxxas documentation is not good as I've been unable to find the required specs based on the motor model number. I've only found: this motor is meant for the VXL-3S ESC and that specifies 3S maximum voltage.....

DubCar
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Re: Driving a high KV Motor

Postby DubCar » 19 Feb 2017, 22:32

We really don't need to run the motor beyond 20-30 erpm. I believe you are right that the stock ESC takes a maximum of 3S voltage as well. This is what we are driving the VESC with. So it seems like the <l_max_erpm>100000</l_max_erpm> could be set far lower then? I'm still getting acquainted with the configuration spec.

rew
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Re: Driving a high KV Motor

Postby rew » 20 Feb 2017, 08:10

20-30 ERPM means around 3-5 RPM on most motors. (most motors have 6, 7 to 10 sometimes even 20 ERPM/RPM).

If I round the 20-30 off to 35, you are running the motor at "max 0.011 V". You would be much better off with a motor that has a lower kV so that you get a more realistic voltage. So for instance 1000 times less kV, or 3.5 would mean that your motor would run at 35 RPM at around 10V.

There is another motor constant, it is expressed as "Nm / A": How much torque do you get out of the motor for each amp that you put through it. As the KV goes down, this number goes up. You might say: I want a motor that does 100Nm at 1 Amp. Sure, that can be arranged. But then... If you want to run it at 1000 RPM, you'll need thousands of volts. So... what are you going to do with that motor if you're going to run it so very slow?

If you are going to run a motor really, really slow, consider getting a "gimball motor". Those have really low KV numbers.

For example: https://hobbyking.com/en_us/turnigy-hd- ... -bldc.html

DubCar
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Re: Driving a high KV Motor

Postby DubCar » 22 Feb 2017, 01:46

Could someone also help me understand a bit more what Ollin meant by "see if switching the shunts to the .005mOhm versions will help"? Thanks!

rew
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Re: Driving a high KV Motor

Postby rew » 22 Feb 2017, 09:28

There are a bunch (two) of shunts. On the stock VESC those are 0.001 Ohm resistors. You can get those resistors also in the value 0.005. This will reduce the maximum current. If the power in the resistors is the limiting factor by a factor of 25, but if you use the same model of resistors as on the VESC, that is not an issue. At least a factor of five though. So you'd get max 10A out of your VESC.

But still... With the numbers you are mentioning, something does not add up. At 10A, you'd be able to get a maximum of 0.1W out of your motor. Then why are you using an expensive 100+W motor?

What is your application? What torque do you need? Are you really going only 30RPM?

DubCar
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Re: Driving a high KV Motor

Postby DubCar » 22 Feb 2017, 21:28

So if I understand correctly I can swap out the physical resistors on the VESC for the 0.005Ohm, which will reduce the maximum current? Could you explain (or maybe link some material that would explain) how this might help?

And 30 rpm was a typo, we want to be able to run from 3k-30k. We are attempting to build a 1/10 scale autonomous vehicle using the Traxxas Slash 4x4 as our platform.

rew
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Re: Driving a high KV Motor

Postby rew » 22 Feb 2017, 23:33

How many pole pairs does your motor have? 7 like my DT750? 10 like my sparta? 11 like the gazelle? 18 like the honda?

DubCar
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Re: Driving a high KV Motor

Postby DubCar » 22 Feb 2017, 23:39

It's this motor here: https://traxxas.com/sites/default/files ... nual_0.pdf

Most the internet seems to agree that it's a 4-pole. They used to use a 2-pole design, but switched to 4-pole a couple years ago.

rew
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Re: Driving a high KV Motor

Postby rew » 23 Feb 2017, 08:43

So it has two pole pairs? Then it should stay below the 100k ERPM limit for the VESC.

Similarly, with the motor capable of 65A continuous, and that this is above the VESCs capability, it does not seem like a good idea to limit the current by increasing the resistor values. Possibly this is one of those "very low inductance" motors that Benjamin talks about when he says: "... that would kill a VESC-4 instantly". Watch the two most recent videos from Benjamin to find the quote.


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