Maker Pro
Maker Pro

Evaluating energy usage in an H-bridge motor control system

stube40

Feb 9, 2010
85
Joined
Feb 9, 2010
Messages
85
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
 

Attachments

  • Current.pdf
    53 KB · Views: 276
  • Power.pdf
    42.5 KB · Views: 275
  • Voltage.pdf
    34.5 KB · Views: 295

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
It's a bit further toward the high power end of the spectrum for me :)

It looks lke PWM is being used to control the current. Essentially that means current continues to flow through the motor (and the flyback diode) when there is no drive. Each pulse serves to pump more current into the circuit.

As the H-Bridge changes direction, the current has to be reversed and so the duty cycle increases dramatically to do this.

There are 2 possibilities.

The first is to track a sine wave rather than a voltage level through the H bridge. This would only work if the H Bridge is switching at a constant rate (say for a synchronous motor).

The second is to allow sufficient dead time when switching direction to allow the current to drop significantly before being started in the reverse direction. This may delay the change in direction too much for your application.

As I say, this is outside my comfort zone.
 

Resqueline

Jul 31, 2009
2,848
Joined
Jul 31, 2009
Messages
2,848
It's 20 years since last class but as far as I'm able to tell you're going about it in a correct way, as long as you know that each U & I sample is done simultaneously.
Of course if the waveform changes more between each sample than it's able to reflect then you'll get some error, so beware of spikes etc. upsetting the results.
 

stube40

Feb 9, 2010
85
Joined
Feb 9, 2010
Messages
85
Steve - yes your right, PWM is being used and that makes the whole calculation harder

Requeline - I hear what your saying and agree with you that the spikes are the biggest issue. If there is any jitter between channels at all on the scope then this is where the biggest errors will creep in.

I wonder if any averaging/smoothing could help here?
 

Resqueline

Jul 31, 2009
2,848
Joined
Jul 31, 2009
Messages
2,848
I'm on thin ice here but I figure only a high sampling frequency can assure accuracy by not giving any aliasing products. But at which point any errors become insignificant?...
Try to sample different lengths and see if you can discern any difference at any point. Say 6, 12, & 18 seconds, if the scope allows? If all are the same then all should be ok.
 

stube40

Feb 9, 2010
85
Joined
Feb 9, 2010
Messages
85
I'm on thin ice here but I figure only a high sampling frequency can assure accuracy by not giving any aliasing products. But at which point any errors become insignificant?...
Try to sample different lengths and see if you can discern any difference at any point. Say 6, 12, & 18 seconds, if the scope allows? If all are the same then all should be ok.

Good plan - thanks, will give it a go
 
Top