Riddle me this: BLDC Supply Current vs Instantaneous Phase Currents

Hi All,
I'm working with a 3phase BLDC motor (High torque/current capable) and custom driver board. It has a current shunt on the phase returns(for measuring total current). A PWM High side/Low side On scheme is being used as well as halls for commutation, pwm frequency around 20KHz. A lab supply is being used to power the BLDC motor and controller.

When looking at the instantaneous shunt voltage and converting to current at a particular duty cycle(say 30%), the current is considerably higher than my supply voltage.

That is, if the motor is running steady, my supply is set to 12V, lab supply shows 2.4A, then for one pwm cycle, the current is 8A for the 30% period, then 0 for the remainder. If I multiply the instantaneous phase current by the duty cycle percent, it is the same as the supply voltage ( 8A * 30% ~ 2.4A).

So, the question is why is the instantaneous phase currents SO MUCH HIGHER than the supply current? Is this an effect of the motor, back emf, inductance, etc?

Any explanation, theory or links appreciated!

There will be a freewheel path, probably a diode across the motors windings.

The motor windings have a large inductance. The motor current is probably fairly steady at 8 A, but it is only supplied from the 12V supply for 30% of the time. The rest of the time the current is freewheeling, as it takes time for the current to reduce.

As an analogy, imagine riding a bike and only pedalling part of the time. While you are pedalling the bike speeds up, but when you stop pedalling the bike carries on. Over an hour, the bike has gone much further than the number of turns of the pedals would suggest. That only works where the bike has inertia and carries on for a bit when you stop pedalling. That is like the effect of inductance on the current in a circuit.

If you are riding on soft ground, there is a lot of drag and you can't freewheel. That is like a resistive load where the current stops dead when the voltage is removed.

Alternatively, the ammeter on the lab supply is too slow to react so it shows the average current.

Your rather answered your own question. The short answer, as Diver300 noted, is that the ammeter reads the average current, which is what you calculated by multiplying the instantaneous phase current by the duty cycle.

Thanks for the feedback!
I'm new to BLDC motors, so I was confused by the effect of the inductance charging and inrush during commutation. It was interesting to see the LR time constant on the scope, and then see how the duty cycle of each period spiked the current way above what I see on the supply (via scope). I ended up using an RMS calculation for the current, which fit the need.

Thanks again!