Measuring PWM Current

[Ross Mac]

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

 

[Ross Mac]

"Ross Mac" <[email protected]> wrote in message

Look at http://www.pearsonelectronics.com/ 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

 

[Ross Mac]

"Ross Mac" <[email protected]> wrote in message

Look at http://www.pearsonelectronics.com/ for good transformers which
you can use to meassure PWM currents.

Whew!....I just got the pricing...$1200 !!

 

[Ross Mac]

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

 

[bushbadee]

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.

 

[Ross Mac]

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