Max motor amps

General topics and discussions about the VESC and its development.
Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Max motor amps

Postby Hummie » 02 Apr 2017, 19:35

ive been riding at 200 motor amp limi and 48 battery amp limit for a while and the performance is a huge improvement with low speed power and much less cogging. Do I only have to worry about heat, just as with the battery amp limit, or are other dangers possible with the high motor amp limit? I heard there's a sensor that can't register beyond 160, is it in the motor amps or battery....and should I have already blown up?

rew
Posts: 914
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Max motor amps

Postby rew » 03 Apr 2017, 06:15

It's the motor amps that are measured. This happens while the lowside mosfets are on.

The current sense resistor is 1mOhm, so at 200A the voltage across it will be 200mV. This is amplified 10x by the DRV chip: 2V.
Then this is offset by 1.65V to allow 0-3.3V measuring of the voltage by the CPU (with currents being allowed to be both positive and negative!). This 1.65+2V exceeds the measuring range.

Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Re: Max motor amps

Postby Hummie » 03 Apr 2017, 07:07

Maybe I'm not loading the motor enough to hit the 200 motor amps? Is it measuring current going to or coming from the motor?

rew
Posts: 914
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Max motor amps

Postby rew » 04 Apr 2017, 06:15

With an ideal motor and an ideal motor-driver, at standstill you can circulate 200A through the motor without any battery current as long as the motor isn't running.

If you're running at 5% of max speed, you need 5% of the motor current from the battery.

In practise, with say 20mOhm of winding resistance, you'll need 4V to push the 200A through the motor. At 200A, that means 800W of losses inside the motor. That's going to heat up quite quickly.

But yeah, you might not be hitting 200A motor current. If you can't view or log the motor current while you're testing, then you can see if setting the limit to 100A makes a difference. If there is no difference you're not hitting 100A.

Hummie
Posts: 105
Joined: 10 May 2016, 04:05

Re: Max motor amps

Postby Hummie » 04 Apr 2017, 15:24

Thank you I'll get logging today!

Begs another question: how can you have 200 amps in the motor and no inductance and motion.

rew
Posts: 914
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Max motor amps

Postby rew » 05 Apr 2017, 03:16

The motor is an inductor electrically.


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