Stall testing 775 Pro, surprisingly poor result

“The test results are also attached. With this setup we were able to increase the stall current limit to 29A (into the Talon, ~55A into the motor) without observing any damage.”

Please explain how the current roughly doubles through the motor. Please keep in mind, that when the switches are conducting, the load (motor) and source (battery) currents are identical.

Blowing air is fine too, as long as you don’t create a pressure vessel with it. See the fairly common inflatable hangers / blockers / etc of other years like 2016, 2013, etc.

Yep. It was easier to attach to the motor vents that way.

Considering the traditional legalities of pneumatic vs. vacuum systems that could impact implementation of these findings, I’m curious how much the numbers vary if you pull a vacuum instead.

I believe there was a Q&A this year that said a fan blowing into a vented system was not considered a “pneumatic system” for the purposes of the rules. However I can’t vouch for future rules / interpretations.

(Or what Chris said)

That explanation is above my pay grade (maybe Marcus or Al can try), but the Talon is not a simple on-off switch. It’s switching on and off thousands of times a second which, against an inductive load, reduces the voltage from 12 (input) to something (around 5.5V) on the output. The Talon wastes very little power in this conversion, so the output current must be higher (at low voltage) than the input current (at higher voltage). 29A*~11V in = 320 Watts in. 55A * 5.5V out = 303 Watts out. (All numbers listed from memory, subject to handwaviness)

If you’re a fluids guy, it’s very similar to a compressed air nozzle - the volumetric flow rate (current) on the high pressure (voltage) side of the nozzle is slow, but the flow rate on the low pressure side is really fast. But the power (pressure * flow) is roughly the same on both sides of the nozzle (less what the nozzle loses to heat and noise)

This discussion has taken place a couple of times throughout this thread. I suggest reviewing those responses or doing research on chopper drive circuits, inductors and DC-DC convertors.

Short answer is at the high 15.63 Khz switching frequency of a Talon SRX (measured) the motor is a large enough inductor that the motor acts as a low pass filter. This effect is the same reason a DC-DC convertor can produce more current at the output than the input. The assumption that “that when the switches are conducting, the load (motor) and source (battery) currents are identical.” is incorrect as a permanent magnet DC motor is not a purely resistive load.

The assumption that “that when the switches are conducting, the load (motor) and source (battery) currents are identical.” is incorrect as a permanent magnet DC motor is not a purely resistive load.

The drive H bridge is comprised of 4 switches. Suppose we number these Clockwise from the top left 1, 2, 3, 4. When driving, either 1 and 3 are on or 2 and 4 are on.

The electrical motor circuit is modeled as:
Va = Ra * Ia + La * d (Ia)/dt + Ke * omega

Do you agree that Va from this equation is the midpoint Voltage on either side of the H bridge? Assuming that you do, then there is no other current carrying path in the circuit.

When switches 1 and 3 turn on, battery current flows through switch 1 through the motor, through switch 3, and returns to the battery.
Thus, while the switches are on, the source and load currents are identical.

This is not true while synchronous rectification is active. While synchronous rectification is active - let’s assume switches 1 and 2 - current circulates from the motor to switch 2, to switch 1, and returns to the motor.

The switches always carry the same current as the motor. During the On time of the PWM waveform, the battery currents and the motor currents are the same. During the Off time, the battery current is zero, and the motor currents continue to circulate through the switches.

We had an interesting revelation tonight. Our single motor stall test at 9A dropped the vbus to 11V. Our comp bot got away with a current limit of 15 A without blowing motors. But at that current, 8 motors drops the vbus to 7ish volts**, which gives the same power through each motor (because the Talon current limit isn’t voltage compensated). So no need to panic (unless you only stall one gearbox :slight_smile: )

So I have another CTRE feature request: a voltage compensated current limit (aka a power limit).

** Which is a bit weird, because with a system resistance of ~.03 ohms, the voltage drop should have been only 15A8.03ohms = 3.6V.

Marcus,
Your descriptions are misleading. The output of all FRC motor controllers are pulsed between the battery voltage and zero volts. The PWM output will cause motor current to vary with pulse width. The peak output voltage of the controller will vary dependent on motor inductance and the width of the pulse. **However, the controller is not varying the output voltage. **The voltage when measured at the motor may be less than battery input because the combination of the motor inductance and series impedance of the controller and wiring at specific pulse widths will cause a voltage drop in the controller. Please refer to Fig. 8 in the Microchip doc you reference. Please note that the devices are connected to Vsupply and GND. The active devices are never biased in a linear mode as that would cause damage to the devices used. Q1-Q4 are operating in the switch mode where significant device dissipation occurs only during the transition from “ON” to “OFF”.
The discussion in this thread relates only to the 775 motor and not to all legal FRC motors and for a chop frequency of 15kHz. Motor controllers that may not be FRC legal can have chop frequencies as low as 100Hz where the effects of the motor inductance are much less pronounced.
The article you linked to train controls is still the reason motor controllers use PWM switching to this day. (The inductance of HO train motors is much less that of the 775 motor in this discussion) The full supply pulse still allows the motor to move during even short pulse durations because the full motor stall current (less other losses) flows the moment the pulse is applied. Please remember that “stall current” is defined as the current that flows when specified voltage is applied and the motor is not turning.

Brendan, You are correct in that the Talon is low loss, it is not a transformer however. When the voltage across the motor terminals is modified by the inductance of the motor and the series resistance, then the current is also modified. Full specified stall current will flow only when the voltage across the motor is 12 volts. When the voltage across the motor is 5 volts, approx. 55 amps will flow when stalled.

I do want to stress that the voltage measured at the motor is AC at 15kHz and most meters will not accurately measure that voltage/current (ammeters in particular).

The wire in the motor has a specific resistance that can be used for most of these calculations. At 12 volts, stall current of 134 amps will show that resistance to be about 0.09 ohms. This is the minimum resistance of the motor and may be (by motor design) the parallel resistance of two windings because the brush assembly will often bridge two windings (or more). Inductance is also static (when specified) as the magnetic structure of the armature is changing during rotation. If two windings are bridged then the total inductance is the result of the two inductors in parallel. The magnetic structure of the inductor includes the magnets and frame of the motor in addition to the core of the armature.

Phargo,
In analyzing the motor currents, it is assumed the series resistance of the motor is the resistance of the wire in series with a perfect inductor (one that has no resistance). The synchronous rectifier action will carry current only when the motor is turning and converting rotational energy into electrical energy. However, this current is also modified by the series inductance. Your calculations then have to be modified as the circuit resistance had now changed as it only includes the motor wire, motor leads and the “ON” resistance of the MOSFETs in the controller. During the conversion, the effective voltage is also modified by the efficiency of the motor. Power is lost in both cases. It is given up as heat during when voltage is applied or during the time when current is shunted in the controller.

I think the initial confusion in this thread is that the post that spawned this said the controller was directly varying the output current. This sort of implies that the controller as behaving by going “okay, let’s allow 20 amps through”, and that’s not how speed controllers work either. The controller is rapidly opening and closing the electrical connection. This results in an increase or decrease in average current, given a fixed load on the motor. But this is not the only variable controlling the current - the current is highly dependent on the load. This is why I think it’s misleading to say “motor controllers control current” - yes they literally allow or deny current flow, but they are not regulating the amount of current consumed in amps. And the

The switching mechanism of a speed controller also results in, effectively, an increase or decrease in average voltage, and the average voltage is less dependent on other factors than the average current draw is (voltage drop excluded for a minute). Under negligible load, an FRC motor connected to an FRC motor controller allowing a 100% duty cycle from a 6V power source will perform essentially identically to a motor connected to a motor controller allowing a 50% duty cycle from a 12V power source. This is why, as shorthand, we talk about motors as being commanded to particular voltages - effectively, they are - and it’s a lot easier to talk about “a motor running at 6 volts” than saying “a motor connected to a speed controller connected to a 12V power source with the speed controller running at 50% duty cycle”

Chris,
For the most part you are correct. Motor controllers do not vary voltage or current directly. However, it is current that turns the motor as that is what produces the magnetic field in a coil of wire. Since the resistance of the coil remains essentially the same, then changing average voltage or average current produces the same result. However, when looking at the motor as a load, there is a difference between 50% duty cycle and 6 volts steady state input voltage due to the switching frequency of the controller, synchronous rectification and the inductance of the motor as a load. Oz at CTRE may be able to give a better handle on the duty cycle that would produce the same result as 6 volts steady state. Even connected to a six volt battery there will be losses not included in manufacturer test specifications.

How did you achieve this current limit? Is it mechanical or programmed?

Programmed in the Talon SRX motor controller.