Evaluating energy usage in an H-bridge motor control system

Discussion in 'General Electronics Chat' started by stube40, May 30, 2010.

  1. stube40

    Thread Starter Member

    Feb 3, 2010
    I use an H-bridge to control a hybrid electric motor and I'm trying to calculate the efficiency by comparing energy used to energy delivered (measured by a torque transducer).

    I am capturing the voltage and current output of the H-bridge as it flows into to the motor. As you might expect, current and voltage goes negative as the H-Bridge reverses the polarity but also power goes negative at the point of reversal since our hybrid design uses a large inductor that has a high amount of energy stored in it and this energy gets released upon the reversal (see attached "Power", "Current" and "Voltage" PDFs).

    I just wanted to know if anyone feels I am evaluating the total energy used in the correct way. Here is how I do it:

    1) I capture a 6 second plot of the Voltage, Current and Torque Transducer data using an oscilloscope.
    2) I pull the scope data to a PC
    3) For every sample, I compute instantaneous input power as V * I (Note, this power goes negative at the point of polarity reversal).
    4) I compute instantaneous input energy by multiplying each sample by the time difference between samples (eg 1 / samp freq)
    5) I evaluate the integral by summing all the instanenous energy cells in the spreasheet. This number is my final "Total Energy Used" figure in J