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Measuring the Power Consumed by a Brushless Motor

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HyperSpeed

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I find the following to be an interesting topic--as for all of the technology we have available--I have yet to see anyone actually confirm the power being used in cordless electric-powered devices or R/C vehicles which use brushless motors.

When it comes to electric power systems consisting of lithium battery packs feeding power through an ESC and into a brushless motor powerplant; sure, we have ratings on lithium packs that make claims they can output X number of amps. Then we have ratings on the ESCs too, stating they are rated for X amount of continuous amperage and can handle X amount of burst-current for so many seconds. Yet how can we actually verify what is really happening when the devices are being pushed? How do we know we really need the 300A ESC and not the 200A ESC? Sure, the batteries make claims of C-ratings they can deliver, but what do we need to do if we want to see the cold hard data in graphical form from conditions during use, to actually know how many kW a motor is sucking up?

On the other hand, let's say that we have somewhat of a blind system; for example, a power tool like a brushless chainsaw. We may know how many volts the pack is capable of supplying--we may even have looked inside the pack to see what kind of cells it is wired together with internally, and based on manufacturer specs we can also know what those cells are capable of outputting as far as current, but finally we arrive at an enigma of what kind of limiting logic the unknown ESC in the system is programmed with to feed an also unknown specification of brushless motor in such a system. In the end, the best most of us can do is to know that there is a given voltage of battery available, yet we really have no clue as to the kW power the system is actually working with, and as such we really have no idea what device is using that voltage to make any form of comparable power level between them. It seems to be a very crude form of making worthwhile comparisons between products, other than stating the obvious: "The system in competitor A is a 40V system, the other system in competitor B is a 56V system, and competitor C uses an 80V system". That's pretty much it--there's no HP/kW breakdown between them to make a more relative comparison with.

With all of that being said, I would like to take the required steps to figure out how each system is using the available energy to actually produce a real-world power figure. I would like to know what I could potentially gain by using a different ESC in the system, for example.

So let's hear it; what does one need to do to know how they can potentially modify a power system to unlock more available energy from the cell pack?

I had an idea that is rather crude, but straightforward in the general sense, I believe. Use a hi-amp meter with a shunt rather than a fuse, and install short, heavy gauge power wire between the pack and the ESC input side, then run the device under maximum expected load in use, and record peak amperage levels witnessed. Then repeat the test while monitoring the voltage levels from the pack as they sag under the same device loads. Take the standard calculation for wattage, V*I, and the wattage levels before efficiency loses will then be known. The only problem here is this doesn't tell us the power made by the system, but rather the power used by the system. I don't know how efficient most brushless ESCs are in delivering this power to the motor, but I would assume it would at least tell something about the system power for a start, which is a lot better than knowing nothing at all.

I would like to upgrade the ESC and motor in one of my power tools, as from a first glance, it is probably heavily limiting the potential power being fed to the brushless motor--probably for the sake of run-time and user safety. I have made this assumption by first noticing the small-gauge power wires between pack and ESC, as well as between ESC and motor. If this were an R/C car, there is no doubt that those wires would be much heavier to minimize as much resistance as possible. This is why I feel there is probably much power to be gained from the tool I seek to gain more power from. Let's face it, chainsaws are probably the number one tool out there that users seek more power from. 😁
 
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KilroySmith

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It's a bit of a difficult question, because the current going into a brushless DC motor is kind of a discontinuous AC signal, rather than DC. The current is also highly dependent on load - at no load, the current is low, and at max load (stalled), the current is maximum. So you're not gonna be able to measure anything on your bench - unless you plan on running your chainsaw at full tilt hacking through some tough old oak on your bench. Sounds like a marriage-stressing act.

Anyway, the practical way to determine if the wiring/ESC is throttling your chainsaw involves a couple of analog voltmeters. Digital voltmeters, unless you have some pretty specialized ones, will have a great deal of difficulty dealing with the current waveforms. Put one analog voltmeter across the battery terminals, and one across the motor terminals. Then hack into that old oak at full throttle. The difference in readings between the two meters is the voltage drop in the wiring/ESC. Ideally, you'd want this to be zero. Any voltage drop across the wiring/ESC represents power that isn't getting to the motor, and that you could possibly improve. If you have a 56V chainsaw, and you're seeing a 2V difference between the battery and the motor, that's less than a 4% drop, and it's unlikely going to be worth your effort to improve it.

Note that you can use this to isolate individual pieces. Use an analog voltmeter with one probe where the wiring connects to the battery, and the other probe where the wiring connects to the ESC, and you can measure the voltage drop across the wire to see if it needs to be upsized. Measure voltage across the ESC, and see if it's dissipating too much power. Etc.
 

OverTheTop

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To measure the power of the motor you just need to measure the current in each of the three windings, and also the voltage across the winding.

It is a fairly straight-forward measurement if you have a current clamp and a relatively modern oscilloscope. Actually even a decent clamp miltimeter that is rms capable would work. Assuming the currents are balanced (quite likely very near) then things get even easier. The current clamp is put around one motor wire (it measures the current flowing by magnetic coupling) and a voltmeter put across the windings, measuring RMS voltage. Spin the motor up and load it. Measure current and voltage. Scale voltage by scaling factor (1/1.73) to account for the fact that the winding voltage is applied across two phase-shifted windings. Multiply calculated voltage and current to get power. Multiply by 3 to get total power from the three phases. Simples :).

A decent four-channel scope with a current probe and maths functions will pull the numbers out quite quickly, but will cost a bit more than a true-rms clamp meter.
 

David Schwantz

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This is so simple, If your Castle Creations 200A ESC cuts out on take off, you need the bigger one :)
 

HyperSpeed

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OK, so it sounds like the easiest way is for me to get an RMS capable clamp meter, then. Is the RMS-capability something that's typically quickly visible on the meter markings itself, or is it a separate mode of actual operation, etc.? When I speak of a separate mode of operation, I mean I.E. "AC/DC/RMS voltage"--would it look like that, or more deeply hidden in the meter specs?

I've needed a good clamp meter for some time now that's capable of high-amperage DC operation for other needs, so this seems like the perfect opportunity to modify my needs of a meter model and purchase one.

I'll be purchasing a digital model, I can't see myself ever buying another analog model, so I'll just eat the additional cost it brings to get it right rather than guesstimate with an analog model. Though I appreciate the thought input in this regard, KilroySmith.

Thanks again for the help guys.
 

HyperSpeed

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This is so simple, If your Castle Creations 200A ESC cuts out on take off, you need the bigger one :)
Pardon my lacking ability to censor out the sarcasm here if there is any, but is this what Castle ESCs actually do if some sort of amperage value is met? I am curious--though I hadn't planned on actually using a Castle ESC. The ESC must support 14S lithium, and I've narrowed my options down to about 3–4 ESCs initially at this point, with more research to do. My research is basically first completing the task I've posted about here--determining the kind of kW value the factory motor is actually using with one pack, then I'll switch battery packs and see how much extra power is gained. The other part of my research will be determining if an air or land based ESC will be some kind of limiting factor based on the features supplied; I know an air based ESC would be limiting if it requires a high amount of cooling and minimal heatsinking is supplied on the ESC.

I can say at this point that there doesn't appear to be any sort of throttling actually going on, the only kind of throttling I can detect is basically built into the battery pack side, not the tool side, and that's really just over-temperature protection of the cells, and of course minimal voltage cut-off. The ESCs in this company's tools seem to operate as a typical R/C ESC would, in that the more amperage the cells can provide, the more power which is produced. This is immediately evident with the wind-up speed of the motor as the pack aH increases. My biggest gains will likely come from a different brushless motor, and different cell chemistry, followed by a sprocket gear size change that's driving the chain. I most likely will try to shove the largest LiPo packs I can inside of their largest battery housing, and remove the 18650 cells, then charge the packs with my PowerLab 8 battery workstation I use to charge all of my LiPo packs.
 
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OverTheTop

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Is the RMS-capability something that's typically quickly visible on the meter markings itself, or is it a separate mode of actual operation, etc.?
You will need to look at the meter for the specifications. Look for "true RMS" or something like that. There is also likely to be an upper frequency limit on the ability of the meter. If it is an RMS meter it will always give you RMS meausrements as there is no point in doing otherwise. They are usually a little more expensive than just the averaging meters, but well worth it. It basically takes lots of very quick voltage or current readings that it uses to calculate the root mean square value of the waveform. The RMS voltage value, when applied to a resistor, will generate the same amount of power as a DC voltage of the same value. Same for the RMS current in the resistor. It doesn't matter what shape the waveform is, you can always work out the power dissipated in the load by using the RMS values.

As an example, if you put 110Vac (RMS implied) across a resistor (say a radiator heater or something) it will produce the same amount of power as putting 110Vdc across that heater. The actual peak voltage of the 110Vac is sqrt(2) times bigger (156V peak, or 312V peak-peak, sine wave) but the power it will drive into the load is the same as 110Vdc. There are mathematical relationships for known waveshapes (hence the cheaper meters that assume this) but the true RMS meter will give you the proper measurement based on the entire waveform, not just the peak value.

Any non-linearities in the load will distort the sine wave which means that for anything other than basic measurements an RMS meter makes the difference between a real number and a very rough approximation.
 

FredA

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Why not measure in front of the ESC.
They are a system - and you care about system efficiency.
And you can't use one without the other.

And if you can still isolate individual effects by swapping motors and keeping the ESC constant and vice-versa.
 

David Schwantz

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Yup, the CC ESC will give you seris of beeps to tell you error message. If it cuts out a full throttle, usually is to many amps flowing. They also makeee a link you can usse on your computer. Good lucck.
 
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