Motor winding oscilloscope single and 3 phase?

[stelth21]

Hi, i'm a motor rewinder..i wind anything from 1/3hp 1 phase upto about 250hp 3 phase 2 pole winding..the main ones are s/delta..part wind..d/volt...When i test them i flash test them at 2,000 volts, then take the resistance readings and check the rotation of the motor using a ball bearing. I also check the amps on each phase so that their even.

Although i do all these checks the tests would be much better using an oscilloscope to check all 3 phase band widths, this is where i get lost!..years ago we used an analogue oscilloscope to check all 3 phases..if there was a problem with the winding one of the 3 waves? On the oscilloscope would be uneven..we could narrow the fault to a particuar phase..or when retesting an original winding it would tell us that one of the phases had a problem etc..all i remember doing was attatching three leads to each phase and checking that the bandwidths were even, but thats all i remember!...my question is what type of oscilloscope should i use for single and 3 phase windings? and if possible, how to use it..i also remember that a certain bandwidth would show a fault of bad braizing on one of the joins etc or a twisted coil..I'm not really a thecnically minded person so if possible could you let me know the model number or website i could look at and the cheapest way possible..Thanks in advance for your help..any advice required on windings let me know and i will try to help!

[stelth21]

Theres loads of oscilloscopes on ebay, not sure which one to go for.

[johansen]

what you want is a three phase power analyzer.. the ones with 3 voltage probes, neutral, and four clamp on amp meters, one for each phase and for neutral. some use rogowski coils instead of current transformers.

in anycase they are expensive and i'm supprised someone hasn't programmed their own yet.

basically an oscope isn't a good choice for this task, the reason being that the 8 bit adc front end isn't really enough, a 16 bit sound card would get you more information (0-400hz is pretty slow, oscopes are designed for 0-400Mhz)

if you want to cheap out buy a 300$ oscope that is known for a simple, easy to use, and bug free FFT math function.. but that won't tell you much.. it will tell you if one coil isn't the same as the other, but that's about it.

also most cheap oscopes can't give you information on the math output which is unfortunate.
so you won't be able to use an oscope as a watt meter.--i think most scopes over 800$ will do this.. i'm sure others will point out that rigol's latest 400$ scope can be hacked.. and i think it can be used as a watt meter.

As to making an amp meter, you can use any cheap clamp on amp meter as a current probe for motor rewind work..
You can literally buy a 8$ ac amp meter on ebay, open the case up and solder two wires from the sense coil into a bnc cable.. make sure you have a load resistor.
hopefully its something like an even 1000 turns or 2000 turns, not all of them are.

anyhow if you can get voltage in one channel and current into the other you can use the math function to multiply the two together, but again most cheap oscopes won't be able to tell you the average, rms, of that resulting math waveform. it would also be nice to be able to perform an FFT of that resulting math waveform.


so basically, those dedicated 3 phase power meters are designed to do all that and tell you what you want to know.. which is power on all three phases, amps, current and their harmonics.. and you want to know the backwards rotating harmonics, which have to be calculated not measured afaik..

you could perform these measurements with an oscope but you will literally be taking screen shots and counting squares under the math waveform.
also the 8 bit adc on most oscopes will quickly grind your gears because the best you'll get is 1 part in 100 for any rms voltage measurement.

[SteveH]

Hi, the company I work for does motor rewinds and tests. We use a Baker test set for doing the tests you describe.

[stelth21]

Thanks very much for your help guys!

[jboard146]

I recently used one of these for checking ~12 very large 3 phase UPS units.

Fluke 435 Series II Power Quality and Energy Analyzer
http://www.fluke.com/fluke/inen/Power-Quality-Tools/Logging-Power-Meters/Fluke-435-Series-II.htm?PID=73939

They are $$$, we rented it. If this is the work you do all of the time id is defiantly worth it. There are some older model with out the color screen.

You can get the reports/graphs off it as a QA to your customer.