Motor Current

[venividivici]

Hi, i'm planning to build my own electronic speed controller for my astro flight cobalt 035 motor.

the motor spec sheet says that it it's max current is 35A.

does this mean that i can put in my equation that the maximum current that the motor can draw is at 35A?

has anyone tried checking how much current their motor can draw when it is just starting to move?

also do motors used in rc flight reach their stall current?

from what i have read stall current is the current drawn by the motor when it is forced not to rotate. so making the motor stall isnt used in rc flight since we want the motor to rotate freely.

i have only checked the motor's current at full power and it draws 27A.

thanks!

[Andy W]

The max current means the point at which you will probably start to damage the motor in some fashion. The motor doesn't somehow limit the current it draws to some fixed nuymber. You can easily push more power through the motor, however, just don't expect it to last very long..
..a

[Frank B]

If you make a crash landing and forget to cut the throttle before the plane hits the ground, the motor will be stalled because the prop can't rotate. I have tried this a couple of times, and typically I forget to throttle back because all of my "brain power" is trying to keep the plane from crashing. I have destroyed one motor this way, but never a speed controller.

[venividivici]

thanks! we custom designed our airframe so that the props could still rotate even if the craft crashes to the ground.

im just worried because the stall current of the motor is 210A and i would be designing the speed controller to handle around 50A max.

other than stalling the motor, is there any other situation in which the motor would be drawing more current (or something near its stall current) than it is rated to do?

[vintage1]

I have destroyed an ESC that way, but not the motor :)

A stalled astro 035 will probably do 150A plus. And smoke something...its a toss up as to whether the motor, pack or ESC will pop first. That's why people fuse motors! Pop a 50A fuse in there, and chances are it will never blow unless its a prop strike at full power, and then rate your ESC for more.. You can make a nice fuse by simply having a section of PCB that blows at 50A as well. To replace, solder some 50A fuse wire across the blown section :D


In general when desining power electronics for this sort of load, you tackle it from two different angles.

First off, you want to make sure the instantaneous current won't blow the FETS outright They have quite lightweight wires in the packages, and these tend to be what blows first, so if - say - you might expect stall or start current to be e.g. 90A. that might dictate use of three parallel 30A FETS.

Then you want to look at sustained maximum current. Let's say you decide on 40A, and the RDS (on) of your FETs is 50 milliohms, so at 13A a piece (three in parallel), each one will be doing I^2R= 8.45 watts apiece. That's a fairly heavy load for a single FET in free air. The more FETS you use, the lower the paralleled total resistance is, and the less heat both in each FET and in fact overall, you generate. In short you can trade weight and cost for efficiency and coolness.

Whether ypu need more fets to meet the instataneos current, or the peak sustained power rating, is somethig you will have to do the maths for.


Of course those FETS need to be driven - and driven quite hard - when switching at part throttle, or they will end up in the worst possible heat state for a semiconductor - part on with high voltage AND high current across them. That means you need to also think about how to drive them, and have relatively substantial transistors to do that. Peak cuurenst into the gates will be quite high as they represent a significant capacitance - probably several hudred pF from memory - and when switching at - say - 3Khz you want to aim for rise times at least as good as 2-3us. Say you are switching 12v, and ou want to do that in 1.2uS, that is about a volt per microsecond on the gate, which equates to an amp per microfarad, or a millimp per nanofarad parasitic. And remember also that if you are using FETS in drain output mode, the Miller effect multiplies the effective drain to gate parasitic capacitance by a significant amount. Maybe 100 times greater than that. This is where you need a fast scope to see that you are getting clean fast edges on the drain and source terminals. I'd expect to see a 100mA driver for a big power stage to really hammer it fast.

I hope that helps outline the tricky bits.

Good luck!

[venividivici]

I have in here some IRF3803 logic level fets rated at 0.009 ohms and am thinking of using 3 of them for the speed controller.

The max continuous current of the Astro 035 motor is 35A. So i was planning to give my esc around 45A capability to be sure. then the rdson of the fet becomes 0.0135 at 125 degrees. So i guess the power dissipation of each fet would be 15*15*0.0135 = 3 watts

As for the instantaneous current, this is the part where im not sure of yet (help :) ) Based on the fet data sheet, its Id at 125 degrees is 80A (140 at 25 degrees). The stall current of the motor (voltage/armature resistance) = 8.4V/0.04 ohms = 210A. Does this mean that if by using the 3 fets, it will also be able to handle the high amount of current during startup?

As for the mosfet driver, i have in here a couple of max627 mosfet drivers rated up to 1.5A.

thanks!!!

[venividivici]

also, is there an equation which can compute or estimate how much startup current there is in a motor when starting to rotate?

[GregoryJ]

3W is a lot to dissipate, even for a TO-220 package (about the best for heat dissipation). I'd go for a sub 5mOhm MOSFET in a D or D2-PACK. Sub 3mOhms are available but are a bit more expensive. Consider driving the gate at more than logic levels, the Rsd(on) is then reduced further - most mosfet drivers drive the gate with the supply voltage. You shouldn't have a problem with startup currents - mosfets can handle a heck of lot - see specs. For continuous operation, you may consider a shunt resistor (a few mOhm) to measure output current and then switch off above a threshold. One more thing... make sure the flyback diode can handle the current at 50% PWM!!

[vintage1]

Some drivers even use bootstrapping to raise gate voltage ABOVE supply rails.

I don't think peak instantanous currents will be a problem, but dissipation really is. Totally agree with the above on that. Its been a long time since I did any serious design , but 1.5W for T0-220 rings a bell with me as being reasonably safe, or a couple of watts bolted to something metallic.

You get a double whammy by increasing FET count. Less power per fet, and less power overall because the combined RDS on is lower.

This thread really belongs in the new DIY electronics forum. More transistor heads in there :D

[venividivici]

thanks for the replies!

Ok, by driving the gate at a higher voltage (10V probably), my rdson would be at .006 ohms (.009 ohms at 120 degrees).
this would make the Pd = 2W.

I based my design from design example 1 which i got from the net http://homepages.which.net/~paul.hills/SpeedControl/Mosfets.html

The design example derated his power requirement to 20%. im not sure about this, but if i derate my computation also by 20%, it will give me 0.4W which i think is pretty okay and would also mean that 3 mosfets would do. Please enlighten me on this one if i should have or should have not derated my values.

* to the mods, if you can transfer this to the DIY electronics forum, thanks. if it can't be transferred, just notify if me if you have to lock this one and i need to continue my thread in that forum.

[GregoryJ]

Sure, you've got to derate them 'cos its unlikely that they'll be operating at 25ºC. If they use 20% as a ballpark figure, I'd go with it, it sounds reasonable. Still, if your 3 mosfets in parallel have a combined resistence of 2.4mOhm (derated by 20%) and you want to sink 45A, P=45*45*0.0024 = 4.86W. This is of course shared between 3 mosfets but its still going to get hot - look at the specs but its probably in the order of 50ºC/watt so your mosfets will heat up by 80ºC above ambient. I'd still go for sub 5mOhm ones - there're tons of them around.

[vintage1]

Excellent tutorial on that URL you posted. I haven't time to go over the calcs in detail, but I will have a crcck at it maybe this evening.

On teh 'unlikley to be opearting at 25VC thing, there are two points.

The maximum allowable junction temepertuire is teh thing that causes failure. You should exepect that to be way up over 150C, so derate your RS on to take account of that sort of temperature.

Powr dissipation that allows the temp to NOT rise that high is a function of the CASE temperature and the internal thermal condictivity, ort may also be quoted as AMBIENT temperature of the device in a box full of still air...read the bit on hetsinks and digest it.


You can make a nice overcurrent detection circuit by monitoring the on voltage of the FET. If it rises too far, either its drawing too much current, or its too hot, or both.

[venividivici]

yup, i have considered the temperature. the IRF3803 has an Rdson of 0.006 at 25 deg. the data sheet says that it becomes 0.009 ohms at 125 deg.

isnt it that parallel mosfets distribute the current? so if i have 4 mosfets sharing 45A, each would take 11.25A.

then to compute the power dissipation of a mosfet, it should be 11.25*11.25*0.009 = 1.13W

right?

so this means that a mosfet would have to deal with 1.13W of heat which it can take w/o being destroyed.