Limiting kV
 

Hello.
Im wondering, is there anyway to limit kV on a motor. Well not necessarily on the motor. Maybe on the ESC. I've gotten my self a 3800kV motor and thinking maybe thats a bit much. I also gotten the Castle MMP. Is it possible to do with Castle link?

I think the simplest way would be to limit the throttle EPA on your TX.

Just knock down the pinion size, or go with fewer cells. Also adjust the ESC to provide less timing advance.

Don't use EPA to limit motor performance; the ESC and motor are most efficient running at maximum throttle (at anything less than full throttle, the current ripple cause by the PWMing will cause losses - damn I^2R gets ya every time!).

I have no idea what that means but sounds like he knows what he is talking about so go with that haha.

Hmm I was thinking of stepping down in gears, I though that would lower the torque significantly.

On the ESC you can adjust the throttle curve. Would that help my situation? or should I just save my self a headache and buy a second motor?

Eric: But if you limit your throttle epa on your radio to 90 to 99% from my exprience on my 1/8 e truggy its still pretty efficent and fast . But going less than that it will defietly effect the efficently and power ive tired it before.

100% epa on a dry track at least heck no lol.

Im going to get a castle link field tuner to limit the timing or punch so i can run 100% epa.

What do you guys mean when you guys talk about the efficiency being effected with lower EPA?

Basicly if you lower the throttle epa to much the motor and the esc will have to work harder although with plently of holes in your body for cooling it whouldnt really make it hot . With lower turn kv motors i can see the issue but with a 2200kv motor i run its pretty efficent.

Just use "torque limit" setting in the MMP (using Castlelink)

Effectively lowers the kv

Cheers

You mean the punch control right? Doesn't that only hold down huge bursts in AMP so it prevents fast acceleration but he would still be able to reach top speed?

Timing and gearing is probably his best option (and controlling his trigger finger too)

No I mean the "torque limit" that has been available on Castle ESC's for a while

Punch control does not change the topend at all, simply changes how throttle is applied

Timing will not help him with a 4 pole motor, all it does is add heat

Agree gearing may be an option if he wants to lower his speed

Cheers

Isnt that the samething as turning down the expo on your radio or no ?

Nothing like playing with expo or throttle endpoints

Expo simply changes the the throttle response relative to trigger position

Throttle endpoints (travel) changes the ESC's duty cycle

ESC's are more efficient when WOT (wide open throttle) equals 100% duty cycle


The "Torque Limit" setting limits the kv of the motor therefore limits torque

Cheers

Ok thanks yea there was times when i lowered the expo on my radio to calm down the trigger response especially on a dry/dusty track but now i will mess with the torque limiter.

Them 2200kv motors are torquey man i think its mostly because its sensorless and the kv of the motor also.

As long as it's not the first thing you do, there is nothing wrong with using the epa to manage the total power of the system. With brushless motors the fets already have to switch on and off several times every rotation, so they are never at peak efficiency like a brushed esc would be when holding full throttle.

The kv constant of a motor is a physical parameter that is established by the flux linkage between the rotor and stator, and the number of turns per phase. This is not something that can be altered by the ESC.

The ESC delivers a voltage waveform to the motor winding and controls the phasing ("timing") relative to the rotor position. Those two parameters are effectively the only thing that the ESC can adjust to affect the performance of a permanent-magnet brushless motor. And since there is one "best" timing for a given speed and load, this really leaves the voltage as the only adjustment. For any delivered winding voltage that is less than the battery voltage, the ESC is running at a duty cycle (PWM rate) less than 100%.

Running at a reduced ESC duty cycle isn't the worst thing in the world, but it does cause more heat to be generated in the ESC, motor, and pack. Because of this, it is best to select a motor kv, battery voltage, and pinion size that maximizes the amount of time spent at full output.

The switching behavior at 100% output is determined by the rotational frequency of the motor. For example, with a 4-pole motor at 30,000 RPM, the rotational frequency is 1 kHz. Compare this to the typical PWM carrier frequency of 8 kHz that the FETs are switching at during anything less than full throttle.

I have no idea how Castle implement their "Torque Limit" feature mate

I cant imagine it is the same as simply winding back the EPA

Agree that gearing and motor selection is the best option

Cheers

8 khz only at half throttle. Its not a step function by any means and since the switching frequency it never is close to 0 the resulting difference in impedance from half throttle to full throttle doesn't waver much. Current plays more in creating heat at that point.

The carrier (switching) frequency is maintained up until the point where the "modulation index" (another term for the commanded duty cycle) goes to 100%. This happens regardless of the motor rotational frequency. Put an oscilloscope on one and take a look sometime...

I^2R losses (caused by current flowing across the effective drain-source resistance) are indeed the main source of heat at high motor loads. But if you're just putzing around at part throttle, the switching losses (caused by charging and discharging the gate capacitance) can be quite considerable.

The command duty cycle is maintained, but that doesn't mean the fets are cycling at that rate.

The switching losses are only considerable relative to the current losses. They are still very small.

??? Explain this statement further, please. As written, it does not make sense.

Have you ever calculated them for a BLDC controller? They are not trivial.

Lets assume a 10khz duty cycle and linear on/off cycle to throttle % command.

At 99.99% throttle every cycle except 1 would be "on"
At 99% throttle every 100th cycle would be off.
etc.
at 50% throttle every other cycle would be off.

The change from on to off is the main loss, so the fewer times the fets actually go from on to off the lower the losses are regardless of the duty cycle.

Yes, I've calculated and measured all sorts of losses. I purposefully italicized "relatively" because while the switching losses are a good chunk, they are not the root cause of heat in an esc by any means.

My head is spinning trying to understand electric terms lol.

In a MOSFET, there is the same amount of power required to turn from "off" to "on" as there is the other way around, since you're either charging or discharging the gate capacitance. IGBTs are indeed asymmetrical, but not used in hobby controllers.

Regardless, your statement simply confirms the point of this thread - run the controller as close as possible to 100% in order to maximize controller efficiency.

If you have indeed done the calcs, then you should also realize that there are significant losses in the FETs during the "off" portion of their switching cycle, as the intrinsic diode forward-conducts as it provides a path for the winding current to "freewheel". Yet another reason to minimize running the ESC below 100%!

Don't mind the technical talk - all you need to know is that you should properly select the motor kv, pack voltage, and gearing in order to achieve the desired performance. Don't just simply turn down the travel volume, especially if the ESC is marginal for the application.

Its not just about the power used to turn the gates on and off. I'm talking about the impedance from stopping and starting the current going to the motor. Also, I never said you shouldn't avoid running your system as efficiently as possible. My very fist post explicitly said "as long as it's not the first thing you do" when referencing cutting back on the epa. Of course you should pick the most power you need for your track rather than limit an overly powerful motor. My point has been that the losses of backing off the epa is nominal in regard to the overall efficiency of a system that typically only sees full throttle for a few seconds a lap.

What are you talking about the winding current "freewheeling"?

Yep, and my apologies go out to you (and any others reading the thread) for beating this issue into the ground. I'm going to try to walk away after this post - we'll see how successful I am ;)

The motor winding has inductance. The current in an inductor cannot be changed instantaneously, and as such it will attempt to find another path (if it can't find another path, the voltage will rise in the circuit until a path is created; this is the "flyback" principle that is used in things like automotive ignition systems).

So, the FETs in the ESC turn on and current starts to flow through the winding - that part is pretty intuitive. Then the ESC commands the FETs (at least the low-side ones) to the off state in order to limit the current in the winding to some desired level. At this point, the motor winding current can no longer follow the same path (which is the whole point of turning off the FETs), but it cannot instantly stop flowing. It ends up flowing through the intrinsic diode that is a by-product of the FET design (look at the FET symbol in a datasheet or schematic and you'll see a diode connected across the drain and source; this is the one I'm talking about). Since it takes several tenths of a voltage to get this diode to conduct, the VI losses can be quite significant.

This thread CA NOT DIE! You guys are hilarious! ....eherrmmm...but the conversation seems far too simplistic and linear...pray tell...how did you factor frequency variance into your calculations...?:smile::smile:
JK pleeeaaaaase don't answer that!

Easy answer; No you can't change the kv of your motor without changing the motor itself. Get a smaller pinion or a different motor.:smile:

Not sure the following question has been answered

Does Castle's "Torque Limit" function work differently than simply lowering the throttle EPA ?

Guess we can ask them ..............

Cheers

Crusey, could you contact Castle?
On the rc-monster forum Patrick has said that torque control is not a direct current limiter, but controls torque directly, and in another post that they're working towards traction control, so I guess it is based on measuring phase delay of the motor.

I dont use it but will ask the question on rc monster

Guess the better question is do all of the following have the same effect

Limiting throttle travel (EPA)
Torque Limiting
Limiting Forward Power

Cheers

Yes - the means of accomplishing these is the same (delivering less voltage - and thus less current - to the motor winding).

That statement makes absolutely no sense. In an "ideal" electric motor, torque is directly proportional to current, and so controlling output torque is accomplished by controlling the winding current. There are a lot of other variables that factor into this, but that's the general concept.

Uh, what? Please explain what you mean by "measuring phase delay", because that is not a term I've heard in several years of designing BLDC motors and controllers.

The most likely way to implement traction control would be to limit the rate at which the motor is allowed to accelerate. This would be rather simple to implement in software, but the calibration of it for a given setup (motor/voltage/gearing/tire size) would be pretty tricky for the average hobbyist.

Look up Patrick's comments in this thread, especially post #38.

I mean measuring how far off the motor is from its ideal no-load position the more the difference, the higher torque it is forced on. Since the motor is sensorless, I think it can be determined by comparing phase of the the control signal and the back emf. I accept that phase delay is not the correct technical term for it, but it was my "best estimate". What would be the right phrase?

I agree that it takes a lot of parameters to set up right, especially without an accelerometer.

Had a quick scan of that thread, makes my head hurt even more

Torque Limit will not effect top speed ?

So it appears it does not have the same effect as limiting EPA or forward power

Cheers

Didn't think this would be so controversial.
I ordered some spur gears and some pinion gears and I'm going to play around with the ratios.
Thank you so much to all of you for so much information that my brain is still comprehending.

http://www.beeranyone.com/beer-forum...eer-smiley.gif

Thanks - that makes a lot of sense. Basically, what is being discussed here is a "feed-forward" control scheme, where a model of the motor is used to determine the required modulation index (PWM duty cycle) to achieve the desired current (which then in turn controls the amount of shaft torque). This is often much easier to implement than a feedback system, where to the winding current is measured by the controller (seems simple in concept, but is rather difficult in practice).

Note that the end result is the same - the winding current is limited in order to limit torque.

The problem with this is that any proper control algorithm (sensored or sensorless) should not allow the rotor to "lag" as more torque is applied.