Motor current limit

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tdejong
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Joined: 09 Feb 2016, 15:33
Location: the netherlands
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Motor current limit

Postby tdejong » 09 Feb 2016, 15:57

I am using the VESC and I have a question regarding the motor current limit.

First of all I must admit that I really like this controller, no other ESC was able to control my rewinded c80100 motors, but the VESC is amazing. Very smooth and has no problem to start the motor from 0 under load.

I use the VESC in a prototype electric outboard, which consists of a rewinded c80100 (50KV) submerged in oil and directly drives a boat propellor.

However I noticed that this controller does not deliver the same maximum power as another ESC. The motor initially draws about 25A max with 24V of batteries. The power was limited by the controller which I set to 50A battery current and 60A motor current.

I experimented with higher current limits and noticed that much more power is available. The motor draw max 45A with the battery and motor current limit set to 100A.

I connected my laptop (linux+bldc tool on a USB thumbdrive) to plot the current diagrams (great fetaure !) and I noticed that there are steep spikes, especially on phase C which are spiking to 100A and therefore I assume the VESC still limits the power capability of the motor. I am aiming for 50A battery current for 20-50V

Maybe somebody can enlight me on the motor currents in the diagram, and perhaps help me to find a reason for the spikes
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benjamin
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Re: Motor current limit

Postby benjamin » 09 Feb 2016, 16:40

Are you able to load the motor enough to reach the maximum current? If the duty cycle goes to max the current can't rise unless you load the motor more.

tdejong
Posts: 3
Joined: 09 Feb 2016, 15:33
Location: the netherlands
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Re: Motor current limit

Postby tdejong » 09 Feb 2016, 20:11

Thanks for your reply, the motor has a high load (12x10 boat propellor, static) and should max out at around 1200W (23V/52A) when used static.

I studied the current plot and I noticed that phase 1 (purple) does not reach the upper current, like the other phases. It looks like the current from phase 2 (red) is flowing through phase 3 and 1 instead of phase 1 only.

Could it be that the top switch for phase 1 is not conducting?

I have another VESC to experiment, but unfortunately I resin casted the mosfet's and shunt resistor to an aluminium conductor on this VESC so a bit hard to check :)
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benjamin
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Re: Motor current limit

Postby benjamin » 10 Feb 2016, 14:42


tdejong
Posts: 3
Joined: 09 Feb 2016, 15:33
Location: the netherlands
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Re: Motor current limit

Postby tdejong » 10 Feb 2016, 15:38

Thanks for explaining the phase currents, it is clear to me that the plotted phase current is normal behaviour.

However, the motor current limit really limits the total power of the motor, before even the limit is reached at 95% duty cycle indeed.

When I set the motor current limit to 60A and battery current to 50A, my motor will max out at 25A battery current (measured at the battery) and 28A motor current (measured via the bldc tool)

What I noticed is that when examining the current plot in the milleseconds range is that the phase 1 current spikes into the motor current limit and with that reduces the total motor current/power by about 50%.

I can get the motor to run with 50A battery current using a 24V battery and the big 12x10 inch boat propellor, but therefore I have to set the motor current limit to 130A, so the spike in phase 1 does not limit the current.

I do notice that the controller limits the motor current based on the current spike in phase 1 at the beginning of the commutation phase for which the motor current is not correctly sampled (because of the 2 shunts). The spike is exactly 100A when I zoom in on it.


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