how to choose a osciloscope to check GTO trigger wave form.

[cloonypan]

Hello everyone,

We have a inverter to drive a variable speed motor. The inverter was composed by GTOs. The vendor suggested us measuring both the gate-cathod voltage and gate trigger current for periodically maintainence.To do this work, we need a osciloscope to catch voltage and current sigals. Therefore I have to find out the adequate specification for this job.So my question would be what's the oscilloscope frequency band width commonly used for power electronic circuits?Is 200Mhz wide enough? Thank you.

 

[ScottyUK]

200 MHz will be good enough for anything on the power circuit of a GTO-based drive. GTOs are relatively low speed devices.

Your big problem may be finding suitable transducers or probes to connect the scope to the drive. GTO drives tend to be at the large end of the power range, with high voltages and high currents beyond the reach of most common probes. Tektronix do some nice current probes up to about 500A with a good bandwidth, although they don't tolerate overloads caused by through-faults on the power circuit very well. A Rogowski Coil based current transducer is probably a better bet if you don't need to measure a DC component. The Hall Effect devices from (e.g.) LEM are reasonably good but limited in bandwidth and sometimes susceptible to false readings caused by high dv/dt. Tek also make some nice high voltage probes although they are pretty expensive. You should use connecting leads incorporating an HRC fuse if you are connecting a probe to the power circuit.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Agree. But, there's a new kid on the block: The i-prober 520.

Have a look here:

http://www.tti-test.com/go/iprober/index.htm

I just bought one. It is dirt cheap (well, almost). It is contactless and it can take lots of abuse, it seems. It is based on a flux gate with a very high driving frequency and has something like 5 MHz BW.

Impressive device that measures DC levels also. In fact, if you use it without the "clamp", you can measure the Earth's magnetic field with quite good accuracy. You van also trace fault currents on PCBs and do lots of other useful things. How could I live without it?

Use without the clamp to measure GTO gate currents. Or use the clamp if you don't mind the low extra inductance.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

GTO's aren't fast devices so a 200MHz scope is more than enough.

You will need line rated isolation probes to measure the gate voltage.

 

[ScottyUK]

Interesting little device! A bit limited in the range for some of the stuff I work with and the sensitivity is dependent on the conductor width, but the technology could (presumably) be scaled up for higher current applications. Nice to see something new on the market.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

Scotty

It is a Vs/m2 (or tesla) device I have used it to look at 2+ kA peak waveforms by holding it at a distance from a 10x50 mm bar. In such cases, it is more interesting to look for waveforms than measure the absolute magnitude. Advice: use duct tape and a piece of wood to keep it in place.

The accuracy when using the clamp accessory is "ordinary".

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[sreid]

A question I've got to ask. Is looking at Gate Voltages and Currents a comon maintenance proceedure? It seems to me that this would be a dangerous place to be probing unless an expert.

 

[cloonypan]

Hi Sreid,

When this inverter stop to operation and cut off main power source, we use its own internal test function. This function will gererate GTO trigger signals then we measure them.

 

[Skogsgurra]

The reason one wants to check gate drive signals is that there are usually several components that may degrade during operation. If isolation is via optocoulers or fibre, the IR emitters and receivers need to be checked. There may even be dust in the fiber terminations. Another component that may need to be checked is the electrolytic capacitor used to keep turn-off energy.

GTO gate current is quite easy to check if you use a fast current clamp. It doesn't even need to be DC if the CT core doesn't saturate. But a DC probe is better. And the i-prober is, as I see it, ideal.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[cloonypan]

Thanks for Mr. Skogsgurra's.
I think our inverter GTO driver is a typical design. The GTO triggering signals were isolated by optical fiber, optocoupler and IR emitters. Since that, not only GTO signals had to be checked by oscilloscope but also the optical emitting peak power had to be checked by a optical power meter. About the electrolytic capacitor,they were not mentioned in vendor's book. Our inverter already 10 years old, your opinion also worth us pay more attentions on this degrade issue.

Cosider the suggestd new probe, I was interested it, too. But I need a probe which capable measuring 30 Amperes. I am going to ask if any probe type fits my need.

Vendor's instruction book did not tell us why we should do such maintaince work. I have more confidences via this discussion tread.

 

[ScottyUK]

Cloonypan,

Look at the Tektronix probes that I mentioned earlier.

http://www.tek.com/products/accessories/current.html

----------------------------------

If we learn from our mistakes I'm getting a great education!