6 Pole operation on the MMM - Long term effects?

[lincpimp]

Originally Posted by himalaya

No, FET life has nothing to do with the number of poles.

The FETs keeps switching on/off in a certain frequency(eg. 8kHz or 16kHz) no matter how hard or soft you pull your throttle, no matter how fast the ESC commutates(changing motor current directions ). The only difference is the FET on-state time width---so callled Pulse Width Modulation. There are 3 phase coils, so each FET works(switch on/off) only during 1/3 active time, in the other 2/3 dead time it just keeps shut off, no matter how many poles the rotor has.

And, the FET switching in the ESC is just NOTHING. Your computer's CPU power supply converter is also based on FET components, they keep swithing about 100 times faster than your ESC, year after year.

So, as I understand it the esc has a certain rpm limit on 2 pole, why would it have half that limit on 4poles?

The phases much switch more often on a higher pole motor per rotation than on a lower pole count motor. How else does the esc do it?

[sikeston34m]

Originally Posted by lincpimp

So, as I understand it the esc has a certain rpm limit on 2 pole, why would it have half that limit on 4poles?

The phases much switch more often on a higher pole motor per rotation than on a lower pole count motor. How else does the esc do it?

I've often thought about this.

I think you have to consider the winds you are dealing with and the physics of what is going on inside the motor.

If you use the same wind in two different motors, only one is a 2 pole motor and the other is a 4 pole motor. The 4 pole motor is going to be half the kv rating of the 2 pole.

As pole count increases, kv decreases for the same wind.

Take a look at some of the available winds and their kv's for various motors.

You can buy a 9000kv 2 pole motor, but you will never see even a 4000kv 12 pole outrunner.

It's a trade off though. Watts is Watts of power.

Lower kv means less rotational speed but more torque.

There is a limit on how many times a minute a ESC can phase shift for a 2 pole motor, but I believe the actual limit is the same with higher pole counts. There is just alot more phase shifts per revolution.

This goes along with what you said, "The phases much switch more often on a higher pole motor per rotation than on a lower pole count motor."

[himalaya]

Originally Posted by lincpimp

So, as I understand it the esc has a certain rpm limit on 2 pole, why would it have half that limit on 4poles?

The phases much switch more often on a higher pole motor per rotation than on a lower pole count motor. How else does the esc do it?

Yes, you're right, a high pole count motor requres the ESC to commutate faster than a low Pole motor, but that adds no more stress to the MOSFETs. Most ESC has fixed working frequency, MOSFETs are switching on/off according to this frequency. Faster phase changing results fewer switching times in a single commutation cycle. View from a longer period, say one second, the MOSFETs turn on/off the same times as slow phase changing situation.

An ESC's RPM limit comes from the phase detect circuitry, and how fast the MCU can safely process the detected phase information, not the FETs.