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Bipolar chopper drive circuit II: Calculating current

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We’re continuing our discussion about bipolar chopper drives and I’ll repeat the basic schematic of it here for ease of discussion. We also left off with +V being powered with only a one volt power supply. That’s not very practical, since most control logic requires something like 5 or 10 volts or higher to operate properly.  But hey, we’re trying to explain the drive operation, not the control logic.

Figure 1: Bipolar chopper drive circuit

Let’s set +V to 48 volts, the same value we used a couple of blogs ago with the unipolar drive. Forty-eight volts applied across the one ohm winding will cause the current to rise to 48 amps in five time constants or five milliseconds. But the DAC and the comparator shut the power transistors off when the current reaches one amp, just like it did with the one volt supply.

Figure 2: Exponential Current Rise and Instantaneous Current Equation

The exponential current rise is to 48 amps in 5 msec as shown above with the circle showing where the 1 amp level is. The question I have for you now is how long does it take to reach one amp?

Using the equation in Figure 2, all  you have to solve for is t when i(t) = 1 and where τ is L/R.

I always wanted to do this:

For all the text books that I had to pour over getting my degrees in Electrical Engineering where the author says “it’s easy to see” or “solving the equation is left to the student as a simple exercise” and I sat there saying “easy, simple, for who?” I’ll let you calculate the time rather than showing you how.

Ok, ok the answer I got was 21 microseconds (μsec.) Lots of time the answers to the odd home work problems were in an appendix at the end of the book. And we can agree that some folks might think this question is very odd.

I give you the answer because I want to compare it to the time it took to reach 1 amp with the L/R drive. Looking back a couple of blogs ago we had calculated the rise time to reach one amp in the winding to be 104  μsec. Here with the bipolar chopper drive we reduced the time to 21 μsec which is almost 5 times faster. And we did it without using two large 47 watt resistors, which probably need to be rated at 100 watts. This is a significant improvement in performance at a minimal increase in component count. (The L/R drive uses a total of 4 power devices and the bipolar chopper drive uses 8.) Plus we’re using the full winding.

The next improvement that can be gained by using a bipolar chopper drive is the fact that we can vary the current comparison point by digitizing the “current level control” signals.

Using a digitized sine wave on the phase A winding and a digitized cosine wave on the phase B winding allows the drive to micro step the motor.

The “voltage waveform at Sense Resistor” in our schematic shows what the voltage levels would look like as the motor is micro stepped. There are discrete changes in the signal, a staircase up and down, as the DAC converts the digitized signals from the “current level control.”

Next up will be a discussion about the effective magnitude of the current setting.

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