I have been trying to find a better way to measure current on a 20khz PWM signal. This amplifier is 3 phase and is connected to a brushless servo motor with a leadscrew and about a 300 to 400 lb mass. On the 10 khz amps I have been able to use a DC clamp but the 20khz is out of the clamps range. I have a couple of emails out for a Wavetek Meterman AC68 clamp but no replies to date. I am trying to analyze the quality of the attached leadscrew's preload and have been monitoring it from the analog +-10vdc drive signals oscillation. The problem with this method is the mass and speed varies from situation to situation so it is only of marginal use. Any and all replies are appreciated, even if it's just a guess it may put me on the right track. Googled out on this one! Thanks in advance, Ross
Measuring PWM Current
Jan 03, 2004
7 Replies
Look at
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for good transformers which you can use to meassure PWM currents.
Thanks Jose, looks like one of these CT clamps will work just fine. I just sent out for product pricing...Thanks again, Ross
Whew!....I just got the pricing...$1200 !!
If current is reasonable (i.e., less than 20 amps or so,) you could insert a resistor can insert 0.1 ohm resistor in the current path. Then a 20 amp current will be coverted into 2 volts across the resistor (resistor will have to be large wattage - i.e., 50 to 100 watt rated) and you can use voltage probe. But since both sides of the resistor can be hot, you have to use a totally isolated o-scope like one of the battery operated, all plastic, hand-held scope (maximum operating voltage is around 600 V for these scopes).
If you already have one of these scopes (they are around $2000 each), then cost of even 100 watt resistors will be a lot lower than $1200 current transformer (or current probe plus current probe amplifiers which are not cheap, either).
Your advice is good and appreciated I have considered this approach but there are alot of issues with access too. I had hoped for a standalone amp clamp solution and may still go with that approach. As far as scopes go, I have three of them and two are plastic. Usually if I have to float the scope I use the differential method. That's probably why I am still around today! I will give this approach some more thought and once again, thanks for your informed input.....Ross
Ross Some time ago at Kennedy they had a problem with measuring a glitch in valve current and they flew me in to come up with a way to measure and explain the glitch.
First the voltage was about 28 volts so no danger of electrocution. The first attempt at measuring it was with a differential input scope, a tek
502 I believe. But the offset was to great for an accurate measurement What we settled on was a battery operated scope which we could connect the probes and then fire the valve. The Glitch was a bump in the wave shape when the valve popped open and the magnetic field overcame the spring pressure and the valve opened and one going the other way.
My time base will be quite a bit longer that your glitch. After giving this more though I think I might do best to use my Fluke 8062A (I think that's correct?) to monitor the voltage on the .1 ohm resister as the mechanism travels the full 38". I will try what Naim suggests, once I build up a safe test jig, and see how it works out for me. And I will also keep your suggestion in mind! I appreciate your time and Thank You much, Ross
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