Need attenuated leads for a scope

I need to see what is happening to the power at a large cnc machine. It is runnning off of a generator connected to a motor running off a phase converter. This could be a subject of it's own, in which I haven't formed an exact opinion on yet.

The drives are alarming out and shutting down and the generator growls a bit under quick burst of high loads. I need to see if the voltage, or frequency, or both is dropping.

I'm looking at buying a bitscope so I can capture what ever is happening. I know I need some 100x probes to drop the levels and not blow up the scope. But now I'm thinking that since it's low frequency and need bigger clips anyway. Can I just use a resistor? Or a voltage divider between the clips and connect to the scope?

Thanks
Cory

PS nice to meet everyone.

Oscilloscope probes are generally manufactured with a 1:10 attenuator - switchable 1:1 and 1:10.

If you have to use a voltage divider take care not to put more than 200V across each resistor. (better reduce to 100V to avoid self heating and loosing tolerance).

Calculate your voltage divider according to those criteria.

Boncuk

any progress?

... Just interested to see if you have discovered any problems so far.

If you have a wattmeter, you can monitor the power transients as you exercise the CNC machine. If you don't have a wattmeter, I think there are companies which rent instruments of this type.
...

At which point in the power train do you suspect the malfunction?

... Checked all your electrical connections, breakers, fuses?

I haven't do anything yet as I'm still looking into what scope to buy. I've needed something more portable than my 422 for years, and I hate to carry it around much,

We have changed almost everything at this point. First let me describe the situation. Let me again deny responsibility for the setup

This is a small machine shop with 5 cncs, and a 200A 1phase supply. First one 15hp phase converter is started. Then a second 30hp phase converter is started in parallel using the made up 3rd leg from the 1st phase converter. There is a 60A breaker between for safety.

Being that the cnc drives don't work the best with phase converters, a 30hp electric motor running off the phase converters is connected to a 40kva 3phase generator that runs 2 of the cncs. The spindles will come up to rpm and load okay, but when decelerating the spindles the drives error out when the rpm drops to the commanded speed.

Here is where I come in.

The generator has a low freq cutout with an indicator led. I noticed it flickered when that happened, and the generator should be more than enough to power 2 lathes let alone one. So we decided that the 30hp motor driving the generator wasn't enough and replaced it with a 50hp motor. We still get the same results.

So I'm wondering if the rpm is not stable enough under load and the voltage regulator is shutting down, or if the regulator isn't fast enough for the large quick current the drives pull. There is a good chance that starting the spindle is done in a different mode than changing the spindle speed. The spindle start has a ramp up function. Where the speed change is probably done in a closed loop mode for cutting and threading functions.

I've never used such a nice watt meter, I'll look into one.

Thanks
Cory

Boncuk said:

Oscilloscope probes are generally manufactured with a 1:10 attenuator - switchable 1:1 and 1:10.

Click to expand...

You can buy 1:100 probes as well - but 1:10 should be fine for just mains work, on any halfway decent scope.

Nigel Goodwin said:

You can buy 1:100 probes as well - but 1:10 should be fine for just mains work, on any halfway decent scope.

Click to expand...

I was planing to go with a pc based scope. All of them seem to have lower voltage inputs than other scopes. Also not needing the high frequency responce was my main thinking for the diy approach. Plus larger clips are needed.

Cory

... too much inductive load for the generator?

... Have you measured the voltage at the output of the generator with the load, and without the load? This would be phase to phase voltage, or possibly phase to neutral voltage. ... Just a measurement of the RMS voltage with a standard meter would be adequate.

The concern ... looking at simple things first, is that there is a large inductive reactance load on the generator, due to the lathes. It may be that this reactive load is too much for the voltage regulator on the generator to compensate.

A drop in the load voltage of the generator, as the lathes are brought online, might indicate that a set of load capacitors would improve the reactance situation ... bringing the load into the control range of the regulator.

... Another clue that the load reactance is too inductive would be a large current measurement, going from the generator to the lathes.

I don't think the inductive load is the issue though a VFD controller. Any thoughts?

clytle374 said:

I was planing to go with a pc based scope. All of them seem to have lower voltage inputs than other scopes. Also not needing the high frequency responce was my main thinking for the diy approach. Plus larger clips are needed.

Click to expand...

In which case you need to compensate you attenator just as with a normal probe, or the displayed waveforms will bear no resemblance to the original.

Nigel Goodwin said:

In which case you need to compensate you attenator just as with a normal probe, or the displayed waveforms will bear no resemblance to the original.

Click to expand...

Wow, thanks for the warning. I didn't think it would be an issue as 60hz.

Cory

clytle374 said:

Wow, thanks for the warning. I didn't think it would be an issue as 60hz.

Cory

Click to expand...

It's not with a 60Hz 'sinewave' - but presumably the idea is to see exactly what shape the waveform is?.

Assuming your scope has the standard 1MOhm input, you just need a 99M resistor in series with it's input to give 100:1. The capacitor across thar resistor needs to be about 1/100th 0f the input capacitance, and should be adjustable (tweak to give a perfect squarewave).

Great, thanks. Now to decide on a scope. It needs to be easy to carry and must have storage. Any thoughts on the bitscopes? I haven't been into electronic much(other than machinery rapair)in years, but I'm getting back into it now. Also I like the logic inputs of the Bitscope since with some circuits to handle the levels I could use it to log control wires in machinery.

Thanks again
Cory