Current Draw of motor on the Cortex vs. Power Expander

We are trying to test the Current draw on motors in various states of loading. We hooked up an Ammeter on the input of the cortex with a single motor (motor port 3) attached to a wheel and slowly added drag to the wheel and watched the current increase as the load increased. Under a heavy load a single motor can shows an increase on current draw of up to 2 amps. But then we configured this same situation, but powered the motor using a power expander an measured the current in the exact same way. But the current flow to the same motor with the same load seemed to be about half of the increase of current to the power expander in this situation. We are using a bench power supply to provide power in both instances to remove the variation that would be caused by battery fluctuations. We have the voltage set for the power supply at about 8.15 Volts, to replicate a nearly fully charged battery. When on the power expander, under the load, the motor seems to stall in about the same way that it does with the cortex, the only difference is that the increase in the indicated flow of current through the meter is different. Does anyone have a definitive answer as to why we would be seeing this? I was kind of speculating that the Cortex kind of isolates the input from the PWM that is being generated directly from the Cortex. But because there is very little electronics between the battery and the motor, we are seeing a lower voltage due to the PWM oscillation. Am I off base here? Thoughts?

Dear jcgrif,

Thanks for your inquiry into getting the most out of the VEX EDR system. Using a bench power supply is an excellent way to provide a consistent voltage for a fair test. We do not observe differences in 393 Motor stall current between the 276-2194 VEX ARM® Cortex®-based Microcontroller and the 276-2271 VEX Power Expander. Let the motor cool completely between tests. Since the motor has thermal protection it is normal for the second test to have a lower stall current.

The Power Expander passes the PWM directly through to the 276-2193 Motor Controller 29. Motor Controller 29 performance is identical when powered directly from the Cortex or via the Power Expander.

The last bit of information to get the most out of the VEX EDR system is the thermal protection inside the Cortex and Power Expander. The Cortex has one positive temperature coefficient (PTC) device for motor ports 1 through 5 and another for ports 6 through 10. The Power Expander has one PTC for its four ports. Distribute your load equally between the three PTCs to minimize the chance of tripping.