Is 2.5A of pk-to-pk current ripple too much for a BLDC motor coil?

[treez]

We have to set up a three phase, 650W BLDC pump motor. It must spin at 8000RPM constant speed, and pump fuel.

Each of the three motor coils has an inductance of 56uH, and resistance of 16mR.

I calculate that in between the commutations from coil to coil, each motor coil should he high frequency PWM’d at around 7.5KHz. This gives a 16 Amp current in the coil, with a current ripple (peak to peak) of 2.5 Amps -as the equivalent simulation shows (admittedly the simulation shows a buck converter but it’s the same concept and inductor current waveform)

Do you believe that the 2.5 Amps of ripple current is acceptable?…or will it cause too much core loss in the motor iron due to the changing flux at high frequency?

(the simulation is in the free LTspice)

 

[BradtheRad]

three motor coils

I'm not an expert on motors, but I've gathered that performance is better when you have several windings and several magnets, pushing and pulling.

Torque is generated in a motor when poles are attracting or repelling, at a close distance. The more percentage of the time this is happening, the better the performance. Emphasis on close distance. Hence a 7-turn motor is better than a 5-turn motor.

Perhaps you are doing this, by switching several coils simultaneously in series? For a total of 6 or 9 or 12 coils?
I guess this is okay, since you report that your windings are small Henry value, allowing several Amperes through them.

Nevertheless it is practical to avoid letting one lost component knock out 1/3 of the system. It should still allow the motor to limp along. Example, if you lose a coil or a switching device, will things become unbalanced and result in bearings breaking, pipes detaching, etc?

Of course, my free opinion is worth about the same as it cost.

 

[FvM]

15 % current ripple sounds fair. Most motors will accept it without relevant temperature rise. If the motor is operated near it's maximum ratings, a temperature measurement could give you more certainty.