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BLDC Speed Computation

Question asked by bell.mark on Aug 15, 2013
I am working on a BLDC motor implementation.  The motor has 4 Pole Pairs with 3 hall sensors.

I am storing the time between any successive hall sensor interrupt.

The time between hall sensor interrupts is

(TIMER * LSB) where my LSB is microseconds

The total time for one mechanical revolution of the motor is

T = (TIMER * LSB) * 6 * NUMBER_OF_POLE_PAIRS

The rotation rate, in degrees per second, is therefore

(360*1000000)/T which is

60,000,000 / ( (TIMER * LSB) * NUMBER_OF_POLE_PAIRS)

Notice that the last equation is missing the factor of 6 in the denominator.

What I’m finding is that the rotation rate is correct if I still have the 6 in the denominator, which doesn’t make sense to me.

Help!

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