How to measure ESC AC electrical power output (2023)

Aug 18, 2022, 01:55 AM

• #1

Bromium

Thread OP

Question

Hi, I ended up with a few no-name ESCs and I would like to see how efficient they are. That would depend on what motor they are driving. Given a particular motor which I have no idea of its efficiency, I would like to measure the ESC power outputs at multiple RPMs and torques. DC input is about 400W but can be up to 1 kW at 14S = 60 V.

I have a basic idea how to filter and measure the phase voltages, and plan to measure the phase currents with the INA169. A microcontroller will do the ADC summation and multiplication.

But how do I calibrate this thing? Is there an affordable test instrument that can measure AC ESC electrical power output?

I need to measure the absolute efficiency of an ESC so if I purchase a new ESC, which is pretty expensive at high voltages, I can determine if it meets the advertised spec.

Thanks to all.
Bromium

Last edited by Bromium; Aug 18, 2022 at 04:58 PM.Reason: more info

Aug 18, 2022, 07:41 AM

• #2

Fourdan

Hi Bromium
On a bench (static WOT)
a) no load testings (Rm, V, Io, rpmo ...)
b) tests with several props (V, I, rpm)
From a) and b) it is possible to estimate/compute the losses (Iron, ohmic, ESC) via EXCEL several algorithms
Win = Wout - several losses
There is the concept of a dynamic resistance Rd (including the ESC and the windings)

see my post
Another possibility is to use a Tytorobotics bench (with a measurement of torque at the shaft)

Iron losses (vs rpm) can also be measured with a bench 2 motors back-to-back (driver-driven)

Louis

Aug 18, 2022, 12:14 PM

• #3

Ron van Sommeren

That would be a LOT of work. Controller losses are negligable compared to total system power.
But I guess you already know that

Unless you have have something special in mind.
The more info you can give, the better the answers can be.

Vriendelijke groeten Ron

• Without a watt-meter you're in the dark ... until something starts to glow •
• E-flight calculators • watt-meters • • Cumulus MFC •

Aug 18, 2022, 12:30 PM

• #4

eflightray

Most model fliers will use a model wattmeter, for the amps/volts, and a tachometer for the prop RPM,.
The rpm is also an important input, as it's the prop that is actually doing all the work when flying.

.

Aug 18, 2022, 01:09 PM

• #5

rocketsled666

Aug 18, 2022, 03:30 PM

• #6

Bromium

Thread OP

Hi Fourdan -- thank you for your reply. Yes, I have built a test stand, so I can measure the torque of the motor, and therefor the Motor Pout = Torque * RPM.

But how will that help me measure ESC Pout = Motor Pin? I have no ideas what the efficiencies of the motor are.

I will study your links. Maybe it is explained there?

Bromium

Aug 18, 2022, 03:48 PM

• #7

Fourdan

Hi Bromium
It is difficult to measure Pout of the ESC.
Rolf Zimmermann (SLS) knows perfectly his ESC , the MOSFETs and the algorithms (real time) to evaluate intermediate Pout ESC (= Pin motor 3 phases)
But the signal shapes are not perfectly sinusoidal ... not easy.
Better to have other approaches to evaluate
* iron losses
* motor ohmic losses
* power at the shaft
* ESC low loading losses, voltage drops,
* ESC high loading losses
Anyway you have to make some guesses for some parameters
A calorimetric testing of the ESC is also possible (but complex)
Generally we forget the estimation of ESC losses , considering the system ESC+Motor not separable
Louis

Aug 18, 2022, 03:58 PM

• #8

Bromium

Thread OP

Hi Ron. Thank you for your reply. I have built a test stand that gives me motor torque and RPM. And I have the DC ESC Pin = Battery Pout. But am missing the AC ESC Pout = Motor Pin.

So I can currently measure the combined efficiencies of the ESC and Motor at different Torques and RPMs. But not the individual efficiencies.

The reason I need to figure this is if I detect that the combined ESC+Motor efficiency drops when using a particular ESC and a particular motor, then depending on whichever is the culprit, I would write that ESC or motor off and find an ESC or Motor better suited for the UAV, at the RPM/Torque that I intend to hover (which is about 400 W ~= 4 kgf thrust, depending on the thrust to power ratio).

When going to heavier UAVs, efficiency of each component becomes paramount. If you can only hover for 30 mins and you are going long range, and you need 10 mins to return home and 5 mins for reserve, then a 10% efficiency gain translates into 20% more range and hover time. Destination time increases from 15 mins to 18 mins, which can make a huge difference.

So this application is not really so special. A long range application runs into this problem. I need to measure ESC Pout and then later Prop Pout. What other information would you like?

Regards,
Bromium

Aug 18, 2022, 04:15 PM

• #9

Bromium

Thread OP

Thank you for your reply Louis. I tried unsuccessfully performing a calorimetric measurement. It depends so critically on the heat loss and is very slow. But I may have to try again. The good thing about it is that it makes caliberation simple.

Since the shunt resistance must be very low, e.g. 10 mOhm, the shunt output on each phase (14S = 60 V * 6.6 A) / 3 = 0.44 / 3 = 0.15 W which is hard to measure. I suppose there are two things I can do. Increase the shunt to 0.1 R, and also measure all three phases together, which will increase the power output to 4.4 W. On the other hand the higher shunt resistance will impact the behaviour of the ESC and/or motor.

I will study your links and get back to you.

Regards,
Bromium

Last edited by Bromium; Aug 18, 2022 at 04:26 PM.

Aug 18, 2022, 04:25 PM

• #10

Bromium

Thread OP

Hi eflightray -- thank you for your reply. I believe a model wattmeter can only work for DC. So it will work for the ESC DC power input but not the ESC AC output, which is what I need to measure. I already have an input DC voltage and current sensor for the test stand.

Regards,
Bromium

Aug 18, 2022, 04:49 PM

• #11

Bromium

Thread OP

Hi rocketsled - Thank you for your reply. Relative efficiencies may solve half the problem, but unfortunately not all the problems. My application is a heavy UAV (4 kg or more, 400 W). If I find the best ESC (at hover), but don't know if it is 90% efficient or better, I won't be able to replace it with a better ESC if it is below 90%.

At high voltages, ESCs can become rather expensive. A 14S = 60 V, 2 kW ESC can cost $1,000 which after shipping and taxes becomes $1,400. The manufacturer claims a 95% efficiency. If I can't measure the absolute efficiency, I would not be able to return it if it does not meet the spec or if it is defective.

Regards,
Bromium

Aug 18, 2022, 06:31 PM

• #12

Bromium

Thread OP

Louis - come to think about it, to do a calorimetric measurement, I would need two calorimeters, one for the current and also one for the voltage. After all, the RMS voltage is not constant or equal to input voltage.

Assume 60 V, 600 W DC input. The current calorimeter would be three 0.03 R shunt resistors, physically in a triangle with the thermistor situated in the center. Each shunt resistor is inline with each phase. At 100% power out, each shunt will see 10 A, 2/3 of the time. So that is 2 W each resistor at max power out, for a total of 6 W out.

The voltage calorimeter for AC 60V in and 6 W out would be three 300 R, 2 W resistors in a Y configuration where the center is NC. It can also be a delta configuration of three 600 R, 2 W resistors. Again, the sense resistors would mechanically be in a triangle with the thermistor in the center,

What do you think, and are the calculations correct? The 6W out of the calorimeters can probably be reduced to 2 or 3W?

How does one calibrate the Y configuration voltage calorimeter? Connect the three points together to V+ and the center to ground?

Thanks,
Bromium

Last edited by Bromium; Aug 18, 2022 at 07:17 PM.

Aug 18, 2022, 07:11 PM

• #13

KiwiDavid

It is indeed an interesting challenge and I have sometimes wondered if it's possible to make a dummy motor that you could dial up a load value and simulate the back EMF signals to make the ESC believe it was connected to the real thing.
I think the losses in the ESC will be primarily made up of RdsON of the FETs and switching losses. Switching losses will be dependent on PWM frequency, Gate drive power, Gate capacitance and voltage swing on the drain. I think even if you do find a repeatable measurement system with sufficient resolution you're looking at quite small losses compared to the total system power so reducing the loss by 5% on an ESC that's only dissipating say 5W is insignificant surely? Much better to increase the prop diameter by 5% since the total power for a multicopter is (I think) inversely proportional to the square root of the area. Perhaps Louis can confirm that.
I understand you're chasing optimum performance ( better than throwing horsepower at it) but I think you will get more return on optimizing motors and props.
Who uses 14S on a 400W drone anyway? You don't get low kV motors for that sort of power. Maybe the drone is 4Kg but has a 8-10Kg payload? Sounds interesting anyway and I wish you good luck.

Cheers,
David

Aug 18, 2022, 07:17 PM

• #14

KiwiDavid

Ooops. "UAV" - not necessarily a multicopter as I assumed. Ignore my last post.

David

Aug 18, 2022, 07:59 PM

• #15

Bromium

Thread OP

Good points David. The dummy motor will give you the relative performance of the ESC, but not the absolute power out and the efficiency. You would still need to measure and calibrate the swings you get from the ESC to get absolute efficiency.

Yes, increasing the diameter D of the prop is generally beneficial. Makes the prop more efficient, and also reduces the RPM for the same thrust.

It turns out that theoretically thrust is proportional to D^(2/3) and P^(2/3). So doubling the diameter gives you about 60% more thrust at the same power.

On the other hand, power is directly proportional to D, and doubling D reduces power by 50% for the same thrust.

Yes, you are right about the 14S. That is for a future 12 kg design that needs at least 1.5 kW. For the current 4 kg design it would be 8 or 9S. And you are right that there is more room to optimize on motors and props than on ESC. On the other hand, how can I optimize the motor without knowing what the power input to the motor is? Let's say I put a 18 pole motor on the ESC, and find out that motor output has dropped by 10%. Is this due to an inefficient motor or the ESC behaving badly with 18 poles? Or let's say I am getting 75% efficiency from battery out to motor out, which is what my thrust stand is telling me. Who is to blame? The ESC or the motor or a mismatch?

Optimizing props is a lot easier. The test stands gives the torque, and one can compare power in to the prop and its thrust and select the best prop. But I still need to know what the efficiency of the prop is. Is it 60% or the 90% that is advertised. How sensitive is this efficiency to the design of the prop and not just its diameter. There must be a way to calculate power from the thrust and diameter. I will also be installing an airspeed meter in the near future. I understand that will give me a better idea of the prop's power out. But the airspeed is not the same for everywhere on the propeller disc, and it is unclear how turbulence affects the result.

Regards,
Bromium

Last edited by Bromium; Aug 18, 2022 at 08:05 PM.