Phase currents of Trapezoidal switching

AI Thread Summary
In trapezoidal switching, the RMS value of phase voltage is calculated as Vp = 0.4714Vs, leading to a current of approximately 16.52 amps per phase with a supply voltage of 20V and a phase resistance of 0.57 Ohms. The discussion highlights that current calculations must also consider inductance and back EMF, noting that equal applied voltage and back EMF result in zero current flow. Participants emphasize the importance of including various factors such as RdsOn and complex load impedance for accurate RMS current calculations. There are also queries regarding harmonic cancellation in systems with multiple phases and the implications of using only a fraction of the supply voltage. The conversation reflects the complexities of motor performance and theoretical versus practical outcomes in trapezoidal switching.
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Calculate the phase currents for trapezoidal switching
For trapezoidal switching or 180 degree switching these are the waveforms when i referred to the website
180 switching

1684308510462.png


when i calculated the RMS value of the phase voltage it is Vp = 0.4714Vs
so assuming the
Vs = supply voltage = 20V
and each phase - phase resistance as per the data sheet is 0.57 Ohms. So, the current is 16.52 amps in each phase? Are my calculations correct? Please advise.
 
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Show your assumptions
 
Now i understand that the currents also depend on the inductance and the back emf of the motor connected. But one doubt is if the applied voltage and the back emf are equal then 0 correct flow. How does it happen?
TonyStewart said:
Show your assumptions
Sorry I don't understand this. Assumptions with regard to motor?
 
Yes all assumptions include RdsOn of source and complex load impedance for RMS current, backEMF @ RPM but a quick simulation on my part proves you are close to my answer for a pure 570m load of 16.46 Arms ... in theory, in practise is always different.

for +/- 13.33V with steps normalized to 1,2,1,-1,-2,-1.

There are harmonic differences with 8 phases and others, which do you think is best for cancellation? Considering magnetics are strong odd harmonics. Ask your prof.
 
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If T is the shortest cycle at max RPM, and ##\tau=L/DCR## what would you expect for a good practical efficient motor for max Tau/T? and why? 1% , 10%, 100% ?
Also why would one ever want to use only 2/3 of Vs? Or just hypothetical Academic?
 
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