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Unread 08-10-2018, 12:27 PM
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Re: Stall testing 775 Pro, surprisingly poor result

Just wanted to clear up/reiterate our testing methodology to confirm our tests were valid.

We stalled a good 775 Pro in a gearbox with a 15 amp current limit set on the Talon SRX.

With a DC clamp ammeter placed before Talon could confirm that the Talon was being only drawing 15A and that the motor was being correctly limited. We confirmed this using PDB and Talon Current data.

We also measured current at the motor side using the DC ammeter. It appeared to draw about 40 amps. We were concerned about how the controller's modulation might affect our ammeter hall-effect measurement making it inaccurate so we decided to stop recording from the motor side of the talon. Though on second thought the 40 amp measurement was actually pretty accurate.

Through either measurement we can see that the motor is dissipating roughly 180W. While this is a lot, from reading the motor.vex.com data we expected to be able to maintain this for at least 30 seconds without significant damage. We decided to discontinue our test after seeing significant smoke 15-20 seconds in.

We wanted to try to measure the performance degradation so we used our 4 775pro gearbox to preform a "reverse dyno test" by electrically disconnecting a known good motor and the smoked motor (open circuit). We used the remaining two good motors to preform a velocity control loop that would try to maintain a set RPM. We then shorted only the smoked motor and measured it's current using the clamp ammeter. We then preformed the same test this time only shorting the good motor.

We don't expect this test to be entirely accurate but we did record significant degradation. The smoked motor was producing about 50% of the current as the good motor. This was far past what we'd consider acceptable and would need immediate replacement.

The intention of this post is to share and discuss the data we've collected with the community. Previously our dataset was only the one provided by Motors.Vex.com and these new findings have significantly changed our understanding of the 775 Pro. We don't intend to disparage the 775 Pro in anyway. We used them very successfully in our robot this year and we hope to continue to do so even more effectively in the future.
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Last edited by Marcus Q : 08-10-2018 at 03:27 PM.
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Unread 08-10-2018, 12:36 PM
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Re: Stall testing 775 Pro, surprisingly poor result

If I understand correctly, the only load on the powered motors during your test was the shunted motor under test. What (roughly) were the magnitudes of the currents you were measuring in either case, and what was the input speed?

One possibility is that you are effectively measuring current very close to the "no load" point on the motor curve. Current draw at this point can vary tremendously from motor to motor (or throughout the life of one motor) in my experience, since you are really only measuring the amount of friction present.
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Unread 08-10-2018, 12:42 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by Jared Russell View Post
If I understand correctly, the only load on the powered motors during your test was the shunted motor under test. What (roughly) were the magnitudes of the currents you were measuring in either case, and what was the input speed?

One possibility is that you are effectively measuring current very close to the "no load" point on the motor curve. Current draw at this point can vary tremendously from motor to motor (or throughout the life of one motor) in my experience, since you are really only measuring the amount of friction present.
Target input speed was 7500 rpm (at the motor), but it only reached ~6200 rpm with a good motor shunted, and ~6800 with the bad motor shunted (on account of the current limiting maybe). I'm forgetting the exact values for the output, but I just checked with Marcus and we both remember ~40A for the good motor, and ~20A for the bad motor.
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Unread 08-10-2018, 12:52 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by nuclearnerd View Post
Target input speed was 7500 rpm (at the motor), but it only reached ~6200 rpm with a good motor shunted, and ~6800 with the bad motor shunted (on account of the current limiting maybe). I'm forgetting the exact values for the output, but I just checked with Marcus and we both remember ~40A for the good motor, and ~20A for the bad motor.
Ok, then my theory is definitely not what is going on.
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Unread 08-10-2018, 01:47 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by mman1506 View Post
Just wanted to clear up/reiterate our testing methodology to confirm our test were valid.

We stalled a good 775 Pro in a gearbox with a 15 amp current limit set on the Talon SRX.

With a DC clamp ammeter placed before Talon could confirm that the Talon was being only drawing 15A and that the motor was being correctly limited. We confirmed this using PDB and Talon Current data.

We also measured current at the motor side using the DC ammeter. It appeared to draw about 40 amps. We were concerned about how the controller's modulation might affect our ammeter hall-effect measurement making it inaccurate so we decided to stop recording from the motor side of the talon. Though on second thought the 40 amp measurement was actually pretty accurate.

Through either measurement we can see that the motor is dissipating roughly 180W. While this is a lot, from reading the motor.vex.com data we expected to be able to maintain this for at least 30 seconds without significant damage. We decided to discontinue our test after seeing significant smoke 15-20 seconds in.

We wanted to try to measure the performance degradation so we used our 4 775pro gearbox to preform a "reverse dyno test" by electrically disconnecting a known good motor and the smoked motor (open circuit). We used the remaining two good motors to preform a velocity control loop that would try to maintain a set RPM. We then shorted only the smoked motor and measured it's current using the clamp ammeter. We then preformed the same test this time only shorting the good motor.

We don't expect this test to be entirely accurate but we did record significant degradation. The smoked motor was producing about 50% of the current as the good motor. This was far past what we'd consider acceptable and would need immediate replacement.

The intention of this post is to share and discuss the data we've collected with the community. Previously our dataset was only the one provided by Motors.Vex.com and these new findings have significantly changed our understanding of the 775 Pro. We don't intend to disparage the 775 Pro in anyway. We used them very successfully in our robot this year and we hope to continue to do so even more effectively in the future.
If I understand correctly what you are doing, I would say that your measurement methodology is correct. It would be interesting to see if you get the same or different results if you test multiple motors that had been purchased at different times.

I suspect you should get the same current reading regardless of whether you have your current clamp on the wires running to the motor or on the wires between the controller and the PDP. The Hall-Effect sensors generally have bandwiths greater than 100 kHz. These sensors were able to detect ringing in the motor currents on the 3-phase motor controllers I worked on at previous day-jobs.

I believe all the motor controllers used in FRC only have an H-Bridge between the input terminals and the output terminals with no energy storage devices (capacitors in parallel with the input or inductors in series with the input) so the input current should have the same modulaton waveform as the output current other than a small difference due to the current for the internal circuitry. If you are worried about the modulation messing up the reading in the DVM (a form of aliasing), see if you can get access to an oscilloscope with a long Acquire buffer (at least 1000 points) and a current probe. Most recent (digital) oscilloscopes have the ability to measure the RMS value of the waveform on the screen.
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Unread 08-10-2018, 02:18 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by philso View Post
I suspect you should get the same current reading regardless of whether you have your current clamp on the wires running to the motor or on the wires between the controller and the PDP.
This is not correct. Power supply current is not the same as motor current. To be more specific, the 2 current readings will be the same in 3 instances:

1. The controller's PWM is at 100%.
2. The controler's PWM is at 0%.
3. The load is 100% resistive (no inductance).

Otherwise all bets are off.
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Unread 08-10-2018, 02:29 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by Mark Wasserman View Post
This is not correct. Power supply current is not the same as motor current. To be more specific, the 2 current readings will be the same in 3 instances:

1. The controller's PWM is at 100%.
2. The controler's PWM is at 0%.
3. The load is 100% resistive (no inductance).

Otherwise all bets are off.

Are these motor controllers not H-bridges, i.e. two half-bridges? I haven't had the chance to get access to an internal schematic for an FRC motor controller.
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Unread 08-10-2018, 03:45 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by philso View Post
Are these motor controllers not H-bridges, i.e. two half-bridges? I haven't had the chance to get access to an internal schematic for an FRC motor controller.
I think you know this but for those reading this who are having trouble understanding it can help to think of it as conservation on energy problem. If 12v 15A (180W) is going into the controller then the controller must also output 180w besides for some negligible power being consumed by the circuitry and switching losses. If the controller is limiting the average voltage to the motor to say 4v then the output current is 180W/4v = 45 amps. If this were not the case and the current in and the current out was equal:

Input: 12v*15 = 180
Output: 4V*15A = 60W
180W-60W= 120W of power that would have to be dissipated by the controller.

This would lead to a very very hot motor controller.
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Unread 08-10-2018, 04:09 PM
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Re: Stall testing 775 Pro, surprisingly poor result

The mistake is this really isn't DC in the classic sense. The power calculation on the battery side is V*A*duty cycle. The motor controller is turning on and off the voltage really fast. The voltage on the down stream side just appears to be 4 volts. Think of it another way. Draw a circuit. The electrons are flowing from the battery minus around the motor and out the positive side of the battery. There are physical reactions going on in the battery that need those electrons. (Sorry Ben Franklin). Current is a measure of electron flow. What goes in gotta come out at the same rate. True AC components can have different currents because they are different circuits that are magnetically coupled.
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Unread 08-10-2018, 04:17 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by philso View Post
Are these motor controllers not H-bridges, i.e. two half-bridges? I haven't had the chance to get access to an internal schematic for an FRC motor controller.
I also haven't seen a schematic but based on what we have seen and measured this is what I'm assuming:

Safe assumption 1: The Talon is a textbook H bridge using 4, N channel FETs with typical gate drivers.

Less safe assumption 2: The Talon is using a sense resistor either on the top or the bottom of the bridge.

Super unsafe assumption 3: The Talon reports a super accurate current measurement.

Safe assumption 4: The Talon has a PWM frequency of 15kHz.


If the above is true, lets say that the PWM is 33%, 775 is locked rotor (electrically the motor is a .1 ohm resistor in series with 50uH inductor). When the FETs are on the current passes from the battery through the top FET, the motor, the bottom FET, the sense resistor and back to the battery. The current waveform starts at 40 amps and ramps up to 44 amps (due to the inductance) all of this current shows up across the sense resistor since it's in the current path. Next for 66% of the time, the FETs (probably the top FET) turns off and the inductance in the motor causes that motor line to fly negative into the diode of the bottom FET. If the inductance is high enough (and it is) the current from the inductor will circulate through the 2 bottom FETs and through the motor and NOT through the sense resistor. The current will start at 44 amps and decrease to 40 then the whole thing starts over again. So during this event the sense resistor saw 42 amps for 30% of the time (14ish amps average) and the motor saw 42 amps all the time.

If the sense resistor in not in series with the motor, you can miss a large chunk of motor current.

When you do math to get to watts in, watts out

PS current = 12V * 14A = 168 watts
Motor current = 4V * 42A =168 watts

All these numbers I spouted are from a PSPICE simulation however if I were to set up 2 current probes viewed on an o'scope I'm pretty confident the PS current would look like a saw tooth, 33% wide with 66% gap between the teeth and the motor current would be a triangle wave. This is why power supply current is not equal to motor current.
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Unread 08-10-2018, 04:24 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by nickbrickmaster View Post
Tangent about controlling torque... let's say you're holding a wrist. You lose some torque per volt because your motor heats up. This causes your controller to compensate by increasing voltage. I'd hazard a guess that this effect grows exponentially, so unless your stall power (torque * voltage) is small enough that passive convection/conduction/radiation can cool down the 775 faster than you can heat it up, eventually the motor will fail even at the smallest of voltages. I also guess that any current limit liberal enough to allow for any motion at all is probably too high to prevent this effect. So the only way to prevent this are (two sides of the same coin) a) make sure you don't stall your 775s for very long even at a "safe" voltage, or b) choose a "safe" voltage low enough that this effect doesn't run away in the time that you need to stall the motors.

Alternately c) use active air cooling or d) use a CIM

Option b seems popular, but knowing how long 775s can be stalled before they heat up too much may be helpful in being able to choose more aggressive gear ratios and stall voltages. Probably the other big thing is how much this "runaway" effect of torque control compares to the actual power draw due to naive I*V, because I have no idea, and there's a chance it's marginal enough not to matter.
Two questions:

1. Has anyone attempted to characterize the failure (partial or complete) in terms of temperature measured somewhere internal (like brushes or stators) or external (case) for various loading durations, currents, voltages, etc? Could a critical temperature be used as a measure of how close you are getting to failure? If so would it be a viable path to offer an instrumented version of the motor that could provide a temperature reading that could be used by either the drivers or the motor controller itself to safely stay away from damaging conditions?

2. Relative to option c), is there a way to know how effective the active cooling system is to be able to predict how much more time you can operate your motor at stall at a given voltage before it will fail? Would you just have to test it to failure with and without the cooling?

3. Since motor failure is caused by temperature and motors obviously get hot when they run, would the time to failure when stalled be reduced after the motors have run for a while (such as during a match) compared to when they start off cold? In other words is the normal running temperature a significant enough fraction of the failure temperature that the time you can stall the motor is measurably shorter when the motor is warm?

OK, that is way more that two questions.....
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Unread 08-10-2018, 06:21 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by Mark Wasserman View Post
I also haven't seen a schematic but based on what we have seen and measured this is what I'm assuming:

Safe assumption 1: The Talon is a textbook H bridge using 4, N channel FETs with typical gate drivers.

Less safe assumption 2: The Talon is using a sense resistor either on the top or the bottom of the bridge.

Super unsafe assumption 3: The Talon reports a super accurate current measurement.

Safe assumption 4: The Talon has a PWM frequency of 15kHz.


If the above is true, lets say that the PWM is 33%, 775 is locked rotor (electrically the motor is a .1 ohm resistor in series with 50uH inductor). When the FETs are on the current passes from the battery through the top FET, the motor, the bottom FET, the sense resistor and back to the battery. The current waveform starts at 40 amps and ramps up to 44 amps (due to the inductance) all of this current shows up across the sense resistor since it's in the current path. Next for 66% of the time, the FETs (probably the top FET) turns off and the inductance in the motor causes that motor line to fly negative into the diode of the bottom FET. If the inductance is high enough (and it is) the current from the inductor will circulate through the 2 bottom FETs and through the motor and NOT through the sense resistor. The current will start at 44 amps and decrease to 40 then the whole thing starts over again. So during this event the sense resistor saw 42 amps for 30% of the time (14ish amps average) and the motor saw 42 amps all the time.

If the sense resistor in not in series with the motor, you can miss a large chunk of motor current.

When you do math to get to watts in, watts out

PS current = 12V * 14A = 168 watts
Motor current = 4V * 42A =168 watts

All these numbers I spouted are from a PSPICE simulation however if I were to set up 2 current probes viewed on an o'scope I'm pretty confident the PS current would look like a saw tooth, 33% wide with 66% gap between the teeth and the motor current would be a triangle wave. This is why power supply current is not equal to motor current.
I can agree with your four assumptions. I was referring only to installing a current clamp on the motor wire and on the input wire as Brendan did in his tests and have not referred to the current measured by the motor controller or the PDP. I am not sure about the current flowing through the anti-parallel diodes of the two lower FET's after they turn off.

Referring to the schematics in this article, I am assuming a current clamp is installed on one of the motors wires, say the one going to Q3/Q4, and a second one on the supply wire at the top that goes to Q1/Q3.
  • With Q2 and Q3 on, I am assuming the current clamps are oriented so that they both measure a positive current. The two current clamps should measure the same wave shape.
  • After Q2 and Q3 turn off, the inductance of the motor windings keep the motor current flowing in the same direction as when the FET's were on. This current would have to flow through the anti-parallel diodes D1 and D4. The motor current would be positive as it decays but the supply current would be negative but the same wave shape and magnitude.
  • With Q1 and Q4 on, the supply current and the motor currents will be the same wave shape and magnitude but the supply current would be positive and the motor current would be negative.
  • After Q1 and Q4 turn off, the inductance of the motor will cause current to flow through D2 and D3. Both the motor current and supply current would be negative and with the same wave shape and magnitude.
Since the magnitude and wave shape of the current sensors is always the same, the RMS value at the two current sensors should be the same even though the polarity at the two sensors is different some of the time.

If I am not mistaken, PSPICE or LTSPICE, has a way to calculate the instantaneous power based on the instantaneous currents and voltages.
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Unread 08-10-2018, 06:32 PM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by wgorgen View Post
Two questions:

1. Has anyone attempted to characterize the failure (partial or complete) in terms of temperature measured somewhere internal (like brushes or stators) or external (case) for various loading durations, currents, voltages, etc? Could a critical temperature be used as a measure of how close you are getting to failure? If so would it be a viable path to offer an instrumented version of the motor that could provide a temperature reading that could be used by either the drivers or the motor controller itself to safely stay away from damaging conditions?

2. Relative to option c), is there a way to know how effective the active cooling system is to be able to predict how much more time you can operate your motor at stall at a given voltage before it will fail? Would you just have to test it to failure with and without the cooling?

3. Since motor failure is caused by temperature and motors obviously get hot when they run, would the time to failure when stalled be reduced after the motors have run for a while (such as during a match) compared to when they start off cold? In other words is the normal running temperature a significant enough fraction of the failure temperature that the time you can stall the motor is measurably shorter when the motor is warm?

OK, that is way more that two questions.....
It would be difficult to measure the temperature of the rotor in a motor that is spinning.

For the locked rotor case, it should be possible to attach a few thermocouples to the rotor, install the motor in the test rig and power it up to get some temperature measurements as long as the rotor is never allowed to spin.

One would want to use thin thermocouples to avoid affecting the air flow through the motor and to avoid adding thermal mass. Since the chart Brendan linked showed that the motors can die within 10 seconds at high currents, one would also want to use and automated datalogger set to a fast sample rate, say several samples per second.
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Unread 08-11-2018, 10:55 AM
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Re: Stall testing 775 Pro, surprisingly poor result

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Originally Posted by philso View Post
It would be difficult to measure the temperature of the rotor in a motor that is spinning.
I don't think there is a practical way to measure the temperature of the rotor. But is that the critical part that we need to measure? I've seen several failed motors that cough up their brushes. It also seems like the field windings would be susceptible to over heating. It seems like it would be fairly easy to measure the temperature of these components.

I've been involved in testing of DC generators for aircraft systems to validate that adequate cooling was being provided. Those generators are much larger than these motors, of course, but the basic internal construction is very similar. We are not worried about stall related issues since these generators are attached to the jet engines and therefore are always spinning at a fairly high speed when they are generating power. But we also use these generators to start the engines (as a starter motor), so they do start off at stall condition - they just don't spend any significant time there. For those cooling tests, we get generators that have been instrumented with thermocouples on the brushes and field windings. We don't feed those temperatures into the control system, we only measure it during our testing with a data acquisition system to validate that the cooling airflow is keeping the temperatures within limits. Generally, our temperature limits are set by wanting these generators to operate for thousands of hours without maintenance, but it seems like the same principles would apply if we were worried about short term damage caused by stalling the motor.

It seems like it might be possible to measure the temperature of some of these non-rotating components and develop a correlation between temperature and the point where initial damage occurs and where final failure occurs. With that data, you could come up with some better guidelines for how long you can subject the motor to power at stall. It would also give you a way to assess the effectiveness of external cooling fans.

I would be worried about using these temperature measurements as part of a control system feedback (to reduce the power if the temperatures start to rise) because thermocouples are generally not as reliable as you would want them to be for that type of application. But, hey, maybe it would work.

I was mostly just wondering if anyone had already gone down this rabbit hole.
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Re: Stall testing 775 Pro, surprisingly poor result

Quote:
Originally Posted by philso View Post
I can agree with your four assumptions. I was referring only to installing a current clamp on the motor wire and on the input wire as Brendan did in his tests and have not referred to the current measured by the motor controller or the PDP. I am not sure about the current flowing through the anti-parallel diodes of the two lower FET's after they turn off.

Referring to the schematics in this article, I am assuming a current clamp is installed on one of the motors wires, say the one going to Q3/Q4, and a second one on the supply wire at the top that goes to Q1/Q3.
  • With Q2 and Q3 on, I am assuming the current clamps are oriented so that they both measure a positive current. The two current clamps should measure the same wave shape.
  • After Q2 and Q3 turn off, the inductance of the motor windings keep the motor current flowing in the same direction as when the FET's were on. This current would have to flow through the anti-parallel diodes D1 and D4. The motor current would be positive as it decays but the supply current would be negative but the same wave shape and magnitude.
  • With Q1 and Q4 on, the supply current and the motor currents will be the same wave shape and magnitude but the supply current would be positive and the motor current would be negative.
  • After Q1 and Q4 turn off, the inductance of the motor will cause current to flow through D2 and D3. Both the motor current and supply current would be negative and with the same wave shape and magnitude.
Since the magnitude and wave shape of the current sensors is always the same, the RMS value at the two current sensors should be the same even though the polarity at the two sensors is different some of the time.

If I am not mistaken, PSPICE or LTSPICE, has a way to calculate the instantaneous power based on the instantaneous currents and voltages.
This is a nice link with a nice explanation of the workings of a bridge.

Safe assumption 5: The Talon only PWMs the high side FET and leaves the low side FET on.

The current flow in a Talon is represented on page 2 in the link, second illustration under SIGN/MAGNITUDE DRIVE section. Circulating current is missed in this method and in the examples I used in this thread, 66% of the current isn't measured. I'm going to ask our motor guru's why not use the other types of drive modes where everything goes through the sense resistor. There is a performance reason I just can't remember. If I get a good answer I'll throw it in here. If any of the assumptions are wrong I'll fix them too. Until then, power supply current is different then motor current.

In your example, when D1 and D4 are conducting, as indicated, the motor current is positive and the supply current is negative, not the same. The 2 current clamps will show the same wave form. They will have the same shape but one will be all positive and the other won't be thus the RMS value is different.
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Last edited by Mark Wasserman : 08-13-2018 at 10:53 AM. Reason: added info
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