basic question motor controllers -- max current

timmy66 · Sep 18, 2017

Sorry for basic question:

I understand how the motor controllers, when they're rated for say 20A, will PWM the current coming from the battery pack to make an "average" current rating not to exceed the 20A.

But if you're looking at the more instantaneous current demand : how is that being limited?

Because in general, for a short period of time while a motor is in a specific position, there is a short circuit over the windings -- is that amount findable in the specifications somewhere?

And how do you ensure that these sudden bursts are never too bad for the battery pack? eg, for the BMS that may be watching the pack's output current closely? Is it because their (BMS) response time is mostly averaged over a second's window, too, that it doesn't kill the peak demands?

And deeper -- where to see the capabilities of batteries to see how they respond to those very rapid pulses?

Thanks

billvon · Sep 18, 2017

>But if you're looking at the more instantaneous current demand : how is that being limited?

Often by a current mode controller. A common method of controlling PWM for both motors and power supplies is called current mode control. An oscillator turns the FET on; once current reaches a limit, the FET turns off.

>Because in general, for a short period of time while a motor is in a specific position, there is a short circuit over the windings

Well, there is never an AC short. The motor always looks like an inductor, and in an inductor, current won't ramp up instantaneously. The controller uses this delay to decide when to stop current flow.

>And deeper -- where to see the capabilities of batteries to see how they respond to those very rapid pulses?

You have to look at its AC impedance; most batteries do not provide that info.

casainho · Sep 19, 2017

timmy66 said:
Sorry for basic question:

I understand how the motor controllers, when they're rated for say 20A, will PWM the current coming from the battery pack to make an "average" current rating not to exceed the 20A.

But if you're looking at the more instantaneous current demand : how is that being limited?

Because in general, for a short period of time while a motor is in a specific position, there is a short circuit over the windings -- is that amount findable in the specifications somewhere?

And how do you ensure that these sudden bursts are never too bad for the battery pack? eg, for the BMS that may be watching the pack's output current closely? Is it because their (BMS) response time is mostly averaged over a second's window, too, that it doesn't kill the peak demands?

And deeper -- where to see the capabilities of batteries to see how they respond to those very rapid pulses?

Thanks

This is something I want to implement on our OpenSource firmware for Kunteng/BMSBAttery S controllers. I was able to limit the max current but only the average value but I would like also to limit the instantaneous current... if is possible and if makes sense, and if the hardware permits.

Just post 2 videos showing motor running with 1 and 2 amps current limit: https://endless-sphere.com/forums/viewtopic.php?f=30&t=87870&start=425#p1320654

Alan B · Sep 19, 2017

If you want to limit the real peak current you'll have to measure it, and quickly. The response time of the current sensors and the ADC need to be short. Shunts aren't all that fast. Many controllers use shunts on the battery side to measure current, but that's just the average current feeding the capacitor bank. That's not really where you want to measure current, the peak current is on the motor side, through the FETs. And it takes fast sensors to see it. Ideally on all 3 legs of the motor side, in case there is a short and the return path is not through the other phases.

The real limitations on peak current are from system resistance and inductance. Every connector, piece of wire, the FETs, the battery resistance, the motor resistance contributes to limiting the current flow. Low powered systems have enough resistance to limit peak currents inherently. High powered systems, not so much.

timmy66 · Sep 19, 2017

Thanks Alan/billvon;

so this is the method (current measuring at FETs) used by even the cheap USD$20 chinese controllers that are hooked up to say 9c / Crystalite wheels?

What are the sensors they're using to do that??

Alan B · Sep 19, 2017

No.

Most controllers use a shunt between the battery and the capacitor bank. They are slow and don't see the real peaks.

casainho · Sep 19, 2017

Alan B said:
No.

Most controllers use a shunt between the battery and the capacitor bank. They are slow and don't see the real peaks.

The new generation controllers, that implement FOC like Scrooser/Citycoco scooters, or the electric unicycle controllers, don't have shunt but only phase sensors, be them resistors, hall current sensors or reading the mosfets voltages.

Punx0r · Sep 20, 2017

I think the battery/bms will not see severe spikes. Cable inductance and the capacitors on the controller's input should smooth out most of it. So the battery/bms should see an average with a bit of ripple.

timmy66 · Sep 20, 2017

@Al - so it sounds like the slow (average) current methods all depend on the peak instantaneous needs not getting chopped by the typical battery BMS, which also has a specification that's probably measuring in the same way that the cheap controller is.

Any idea what typical motor instantaneous currents are? For example, on a 48V 20A (continuous) spec, would you be seeing peaks in excess of 100A? Battery data sheets I've seen don't seem to give peak power capabilities.

Why is measuring with the shunt so slow? Or is it that the reaction time on an inductive load would have no real effect, even if trying to limit it?

atarijedi · Sep 20, 2017

To prevent damage to batteries, just make sure they can output continuous current at what the motor uses at peak. That way you'll never damage the batteries. BLDC motors, for all intents and purposes, will use as much current as you can throw at them, until they burn themselves up or Vbemf counteracts the applied Voltage.

basic question motor controllers -- max current

timmy66

100 mW

billvon

1 MW

casainho

10 GW

Alan B

100 GW

timmy66

100 mW

Alan B

100 GW

casainho

10 GW

Punx0r

1 GW

timmy66

100 mW

atarijedi

100 W

Similar threads