# Phase currents of Trapezoidal switching

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In summary, the conversation discusses calculations for the RMS value of phase voltage in a motor with a supply voltage of 20V and a phase-phase resistance of 0.57 Ohms. The conversation also touches on the influence of inductance and back emf on current flow. The participants also discuss the use of different waveforms and the best phase configuration for cancellation of harmonics. The conversation concludes with a question about the practical and efficient design of a motor and the potential reasons for using only 2/3 of the supply voltage.

#### PhysicsTest

TL;DR Summary
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

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.

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:
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.

PhysicsTest
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?