MCF8315A: unit clarification of the back emf constant used in the formula

Hi AK,

For MCF device BEMF constant is mV/Hz which is per phase (Phase to ct)

Measure BEMF  (Peak to Peak /2)/(SQRT(3)*Frequency(Hz)

What is the pole pair of the motor?

Thanks and Best Regards

Venkatadri S

Venkatadri Shantaram84961 said:

What is the pole pair of the motor?

This is a 4 pole motor , 2 pole pair. Does the mentioned formula hold true for a PMSM motor, i am trying to figure out the maximum speed for the motor from the back emf.

Venkatadri Shantaram84961 said:

Measure BEMF  (Peak to Peak /2)/(SQRT(3)*Frequency(Hz)

I dont follow, could you perhaps take a numarical example and explain this..

The motor manufacturer has given the back emf in RMS (Measureed from phase v terminal with respect to phase u terminal of the motor) as 12 Vrms/1KRPM

Correct me if i am wrong, i have derieved this formula to converts the back emfFrom Vrms/KRPM(phase to phase)  to V/Hz(Phase to neutral):

V/Hz = (((Vrms*sqrt(2))/kRPM) / ((1/60)*1000))/2

So for 12Vrms/KRPM would be 0.509v.

Asuming a DC bus voltage of 270v,

Calculating the Electrical frequency by the given formula , comes to 306 Hz., and the max speed which would be 120*306/4 = 9180 RPM?

Hi Ak,

I will check this and reply in a day.

Thanks and Best Regards

Venkatadri S

Hi Venkatadri Shantaram84961

Any update on the above clarification ?

Hi AK,

Consider steady state condition , then V (termial) =  I*R + BEMF    -->(BEMF = w * BEMF_CONST) where W is the speed. 

About the BEMF_CONST from the data 12Vrms at 1000 rpm speed, convert to per phase (assuming star) as per the equation

1000 RPM for 2 pole pair is 33.33Hz  --(1)

Measured voltage 12 * SQRT(2) / 2*SQRT(3)    --(2)

Divide (1) / (2)

Thanks and Best Regards

Venkatadri S

Thanks for the detailed calculations

Venkatadri Shantaram84961 said:

1000 RPM for 2 pole pair is 33.33Hz  --(1)

Measured voltage 12 * SQRT(2) / 2*SQRT(3)    --(2)

Divide (1) / (2)

So doing the above 33.33 / (12*SQRT(2)/2*SQRT(3)) , we get 2.26782 , This is the Back EMF in V/Hz ?

For a 270v DC bus, the maximum electical frequency will be (270/SQRT(3)) / 2.26782 = 68.737 Hz.

Converting the electrical frequency back to Mechanical speed in RPM (N) = (60*68.737)/2 = 2062.13 RPM --> This value seems to be very low? Could you kindly confirm if our calculations are correct..