Current limit of motor

Discuss hardware related to the VESC such as the NRF nunchuk.
tobawo
Posts: 5
Joined: 03 May 2016, 17:29

Current limit of motor

Postby tobawo » 10 May 2016, 08:23

Hi!

Just a quick easy question regarding the max. motor current:

The max. motor current value in the BLDC_TOOL referes to the current that is flowing through the coils while PMW is High.
So Motor current * duty cycle = battery current, right?

So I assume if I have a motor and in the datasheet it says max. motor current 20A, then this is a thermal issue and therefore continuous current over 20A is not allowed and therefore this is the value I have to put into max. battery current (motor current averaged over time) and the motor current value (peaks) in the BLDC_TOOL can get a lot higher?

Thanks!

rew
Posts: 940
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Current limit of motor

Postby rew » 11 May 2016, 19:22

In principle, that is the "max motor current' that you should enter.

Suppose you have a 20A motor and enter 20A as the Max battery current. At 10% PWM, the VESC will easily transform 5A battery current into 50A motor current....

tobawo
Posts: 5
Joined: 03 May 2016, 17:29

Re: Current limit of motor

Postby tobawo » 12 May 2016, 14:32

Yes but the 50A motor current only flow for 1/10 of the time so what is the problem?
I thought the max amp of a motor is because of heat dissipation, so the avg current that the motor "sees" should matter:
0,1*50A + 0,9*0A = 5A = battery current?

Whats my error in reasoning here?

jaroslavnovotny
Posts: 49
Joined: 16 Feb 2016, 13:10

Re: Current limit of motor

Postby jaroslavnovotny » 12 May 2016, 15:27

Heat takes time to travel and dissipate. The current spike and the heat generation is immediate.

pf26
Posts: 296
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Current limit of motor

Postby pf26 » 13 May 2016, 12:12

tobawo wrote:Yes but the 50A motor current only flow for 1/10 of the time

No, the 50A may flow continuously, even with 5A battery current, (PWM increases the current and reduces the voltage seen by the motor)

rew
Posts: 940
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Current limit of motor

Postby rew » 16 May 2016, 08:13

Just to provide some contrast to the "no" above: Yes! :-) So the battery only sees 5A on average.... :-)

(but as pfj26 already says, the motor will see 50A continuous!).

Laurens
Posts: 1
Joined: 12 Oct 2016, 16:27
Location: Gent

Re: Current limit of motor

Postby Laurens » 12 Oct 2016, 16:29

Is it possible to run this motor with the same firmware on the blog ? :
A2212 motor:
KV: 1000 Max
Efficiency: 80%
Max Efficiency Current: 4-10A (>75%)
Current Capacity: 12A/ 60s

Thanks ! :

rew
Posts: 940
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Current limit of motor

Postby rew » 13 Oct 2016, 12:00

This is a very "small" motor compared to what most people use their VESC for. It lists 12A max, for 60 seconds. The "continuous" max is even less (and not listed in what you quoted). The VESC can run about 50A continuous.

I do recommend that you run your VESC at min 12V. This is for the FET drivers to be properly powered.... You would get enormous RPMs if you apply the full voltage to the motor, but that is not a problem. You can ask VESC to limit RPM or duty cycle to stay away from the very high RPMs.

lizardmech
Posts: 171
Joined: 19 Jan 2016, 10:54
Location: Australia

Re: Current limit of motor

Postby lizardmech » 15 Oct 2016, 13:39

rew wrote:Just to provide some contrast to the "no" above: Yes! :-) So the battery only sees 5A on average.... :-)

(but as pfj26 already says, the motor will see 50A continuous!).

I have a question, lets say I had two setups, a 60V 20A max current battery and a 120V 10A battery but they both have the same 150V mosfets. At low speeds the controller is converting it to 20V 60A, the 120V one runs cooler due to only 10A traveling through the high side mosfets? Also which mosfets get exposed to the full motor current is it only low side ones get exposed to that?

pf26
Posts: 296
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Current limit of motor

Postby pf26 » 17 Oct 2016, 10:04

All Mosfets are exposed to the full motor current, but only during fractions of the time. The higher the battery voltage, the lower this fraction of time.


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