Gents,
Hope I haven't started a forum war here, but I still have a bit of a problem understanding why a good quality Digital multimeter connected in line with the PS output to the drivers fails to read an accurate current flow compared to a hall effect clamp meter. When i look at the chopwave graph Ian posted at post 44, the current trace has fairly minimal peaks and troughs associated with the switch activity. I read this as the current flow through the coil and not necessarily a waveform of the driver input supply. I would have expected the circuitry within the driver would have provided some smoothing to the input current. Additionally with 4 drivers all connected together I would have expected some further smoothing of this wave form as one goes high and the other goes low.
Is my thinking correct here??
Stuart
Stuart,
Unfortunately, it isn't as simple as that. Measuring PWM power is actually a substantial problem. One major problem is the phase angle, which is not taken into account at all by a DMM, no matter how good it is. The second problem is the harmonics generated by chopping the DC signal. You can have measurable power up to the 100th harmonic in some cases, although at the power levels we're talking about here it won't be that extreme, and obviously the power going up the harmonics will drop exponentially. Nonetheless, unless these factors are taken into account you will not get an accurate reading. The best DMM on the market is therefore pretty much useless to obtain an actual measurement.
Power output from a PWM can be expressed as Power = V * I * cos(e). Without taking this phase angle into account, the only real use of a DMM would be to compare motors to motors, or drivers to drivers, but the actual value you get is quite erroneous.
In fact, an analog meter will give results closer to reality than either an averaging DMM or true-rms DMM, because it will respond to low frequency harmonics in much the same way as the motors. It will not, however, take the phase angle or higher harmonic power that is also being used/lost into account. Higher harmonic power, from what I understand, will not be converted to usable torque, but it is still being drawn nonetheless.
After a little more research into the topic, I'm starting to also question the effectiveness of a DC clamp meter for the same reasons as above. While I'm fairly certain it would be more accurate, I'm not sure it would be close enough to actually be considered accurate. A power meter, however, will take the phase angle into account and measures both voltage and current simultaneously. With this method you will be within 1% of real life power draw. This, however, doesn't really help with real time current calculations as voltage is not constant, but could still be useful with the right methods.
A digital storage oscilloscope will do a pretty good job, but you will have to know what you're doing to get useful measurements. Also USB oscilloscopes with decent software would also do a good job, and many cases better because of higher resolution sampling. Standard oscilloscopes are 8-bit, better ones are 12-bit. This is relevant to measuring the harmonics.
This is a complex topic because of, in short, the phase angle and harmonics, and is made worse by the very fast rise and fall times created by modern PWMs. In addition you may also have to contend with reflected power depending on cabling lengths and such, but not likely significant in this application. Unfortunately I'm not well versed enough to say for sure one way or the other, I just know the potential is there.
There is obviously much more to this topic, but hopefully this will be enough to provide some insight into why standard meters are so inaccurate at measuring PWM power.
Guest
