Glossary of Terms

[professor_shartsis]

illustration of 50% duty cycle during a single BLDC commutation:

(the actual frequency would be on the order of 20-40 thousand cycles per second)

@professor_shartsis

^you can see half the time (50% duty cycle), 60a current @ 50v is flowing from the battery, averaging to 30a @ 50v

the other half the time, the current flows inductively, and the motor “sees” 60a @ 25v the whole time. but in reality, during the “on times” the current in the motor might be increasing from 59.9a to 60.1a, and during the off times, the current is dropping from 60.1a back to 59.9a, which averages to 60a.

and some explanations for the illustration:

@professor_shartsis

@Krindor increasing the inductance in a circuit means increasing the time it takes for current to increase or decrease with applied or removed voltage.

so for example at 50% duty cycle, the battery could be supplying pulses of 60a, half the time, which averages to 30a. the battery side of the circuit has low inductance and so the current can ramp up to 60a and down to 0a very quickly.

on the motor side, the inductance is very high, so when the current from the battery shuts off, the circuit is rewired/switched so that the leads coming from the motor forms a loop, which allows the current to flow inductively (without applied voltage) for a short time.

the time scale of the on-off pulses from the battery are so short in duration that during the off times when there is no battery current, the current continues to flow in the motor entirely from the inductance.

so 60a from the battery half the time (30a average) can result in 60a in the motor the whole time, because the current still flows in the motor inductively while it is not flowing from the battery, but we say the 60a the motor sees the whole time is half the voltage of the battery if the duty cycle is 50%.

hope this helps.

@professor_shartsis

if you happen to be wondering what “powers” the current that continues to flow in the motor when you remove the battery current… it’s the collapsing magnetic field of the stator which stores energy in the magnetic field itself.

the inductance is directly proportional to the magnetic field produced by a given amount of current, and since the motor produces a large magnetic field for a given current, it has high inductance, and the current still flows for a while when you remove the battery current if you quickly short the motor to itself.

@professor_shartsis

when you send current through the motor, because of the high inductance (created by the coil shape of the winding combined with the iron in the stator, which forms an elecromagnet) it takes extra time to reach a given amount of current, compared to the time it would take if you used a straight wire with no iron stator.

during this “extra time” you are building a magnetic field or “compressing a spring” or “storing energy”… when you remove the battery voltage, the spring releases and powers the current that continues to flow for a short time without any battery voltage.