oscilloscope waveforms - Page 1
Forum section: electronics / beginners

author: Pbl57lsu
author_rank: Newbie
timestamp: August 25, 2024, 12:46:43 pm
content: Does anyone have experiences viewing waveforms on a short between phases on a motor? We have a 3 phase motor that failed its initial inspection. megging between phase, the motor windings are supposed to show above or greater than .8mohm. the motor does not short between phases to ground, we believe it is a series of bad connections to the pump within the well at 160ft. The pump performs as needed but we suspect to fail because of said tests. This is a 3 phase motor running of a vfd from single phase 230v 60hz service to the pump itself at 3 phase 200v 50hz. There is substantial noise on the waveforms i presume from the vfd. I have a battery powered 2 channel oscilloscope and plan to capture between two phases at a time to atleast make sure they are 90 degrees out of phase as desired. Any pointers into this would be extremely helpful. I work directly for the owner and this is all a result of a incorrect motor replacement and the vfd purchase was the result of the error. the pump has a 3 year warranty but we do not run it often and there's a possibility it will not fail until after.

author: BennoG
author_rank: Frequent Contributor
timestamp: August 25, 2024, 01:13:44 pm
content: i would stick to measurements with the multimeter. Putting a scope on the output of an inverter is dangerous for you and for the scope.Without anything connected what is the resistance between the 3 phases on the motor ?U-V or L1 - L2V-W L2 - L3W-U L3 - L1Benno

author: Circlotron
author_rank: Super Contributor
timestamp: August 25, 2024, 01:17:52 pm
content: The fast rise time of the PWM voltage will blow up your probes even if the voltage is less than the probe maximum. I have first hand experience at doing this. A 1uS to 400V is a killer. Probe voltage rating is frequency dependent.

author: CaptDon
author_rank: Super Contributor
timestamp: August 25, 2024, 02:35:22 pm
content: If you're looking for phases 90 degrees apart you'll be real disappointed as 3 phase is 120 degrees apart. Are you suspecting a bad VFD? I assume you mean phase to phase above .8 milliohm??? A megger is used to measure leakage resistance to ground, a milliohm meter is used to detect lower than specified winding resistance which could indicate a shorted turn or shorted winding. Also, assuming the scope, the probes and you survive an unrecommended hookup the waveform from a VFD will be such a garbled mess it will be hard to make any meaningful diagnosis. You would need a 4 channel scope looking at three channels, one per phase and would also need 3 high voltage differential probes. From your statements I don't think you are ready for the task at hand. Too many mistaken ideas and statements. You typed .8mohm which would appear to be a winding minimum acceptable resistance. A megger would measure something like "Greater than 8 Megaohms (8 MOhms) leakage resistance to ground or pump case in this application.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 25, 2024, 02:56:25 pm
content: Quote from: CaptDon on August 25, 2024, 02:35:22 pmx 10! The OP clearly doesn't know what he's talking about or has a very odd setup. Yo the OP:Is this a standard 3 phase motor that you're trying to run from a VFD?200 volts and 50 Hz ishighlyunusual (pretty much NEVER used) in the US. And a motor designed for use with a VFD should have many more phases.Question #2 and 3. Has this setup EVER worked (and for how long?) or is this something new?Stop trying to use an oscilloscope for this! The induction in the motor will generate very high voltages when running and that is a very serious hazard to you and the oscilloscope. Not joking, I had a friend in high school that was shocked and was knocked unconscious while doing what you're trying to do and he drowned while he was unconscious.It really sounds like this is a much more complicated job than you're prepared for. In fact, more complicated than MOST repair people can understand. That's why most of them would simply pull the pump and motor and replace them.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 25, 2024, 04:24:39 pm
content: OK.People like to help, but that OP doesn't make sense.Are you saying that the motor was incorrect and a VFD was purchased to run the motor on the available supply?First as has been said DO NOT USE AN OSCILLOSCOPE.Test the motor windings resistance between phases and they should show equal low resistance on each test as per BennoG suggestion.Then test between phases and ground with an insulation tester.That should be above 500kΩ.Edit: don't flash test!If the readings are correct and the pump is working, stop worrying.If there is something else that is relevant. Please post an explanationclearlyRegards.

author: CaptDon
author_rank: Super Contributor
timestamp: August 25, 2024, 11:20:37 pm
content: I am baffled why at a mere 160 feet you have a 3 phase pump? Is it insane horsepower like 5 or more? You can easily run a 1.5 HP submersible pump from 240VAC single phase and eliminate all the other crap and generally they are so well insulated they only use the red and black wire and NO ground or earth return but you MUST have a double pole pressure switch! Sometimes a megger can be your worst nightmare since there must be a sensible test voltage limit. If you meg that pump at 5KV you will induce insulation punch through which will bite you in the ass about 6 months later. IF that pump is 3/4 HP or more I hope it has a torque snubber on it! What do you mean the pump failed 'initial test'? Where, out on the ground, on a workbench? What test did it fail since you say it is currently working?

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 26, 2024, 12:35:43 am
content: You want to check both Voltage and Current motor on and off.Voltage should remain stable, Current should not exceed X dependant on motor size.Jeff

author: jmelson
author_rank: Super Contributor
timestamp: August 26, 2024, 03:42:24 pm
content: A strong possibility is water has leaked into the connecting wires down to the pump motor. Measure current with a clamp-on meter and compare all 3. If one wire has MUCH less current than other 2, then it MAY be a short in the motor. (to Capt Don:) almost ALL deep well pumps are 3-phase. The reason is that you would otherwise require a starting switch and capacitor down in the well bore. The "3-phase converter" or VFD is kept above ground and it means there are no open contact points in the wet environment of the pump motor. Yes, the VFD output is 120 degrees/phase. But, it is all 400 V square waves of varying duty cycles.Your initial question does not make sense, however. If there was ANY sort of phase-phase short or even ONE winding turn that was shorted, the motor would draw a HUGE amount of current and the VFD would go to fault status. You talk about above ".8mohm" do you mean above 8 MEGOhm? As wrote the question, that is 0.8 MILLIOhm. You would NEVER use a megger between phases, only from each phase to ground or the delivery pipe. With a round trip of 320 feet, your phase-phase resistance would likely be several Ohms.Did an experienced well installer make these tests for you, or are you doing this yourself with very limited experience at this?Jon

author: Xena E
author_rank: Frequent Contributor
timestamp: August 26, 2024, 07:59:16 pm
content: Quote from: jmelson on August 26, 2024, 03:42:24 pmNormally:Single phase narrow form factor pump units have their starter capacitors mounted in the dry at the bore head... either capacitor start/run or with thermal starters...I don't see the point of a VFD except for matching non standard motor to the local supply...Regards.E

author: CaptDon
author_rank: Super Contributor
timestamp: August 27, 2024, 01:25:58 am
content: Not seeing 3 phase pumps in my area! Give example, my well is 300 ft and the 3/4HP pump is at 280ft. The only thing going down to the pump is a red and a black wire and the pipe. Power is 240VAC 60Hz. I forget how many stages the pump has, probably at least 6 or 8. Because of the foot valve above the pump and the fact that it is multiple centrifugal discs it starts up with no load. I always assumed it was a 3450RPM induction motor with no capacitor needed. The old pump motor got blown by lightning, I was in the shower when it happened. The motor developed a partial short, it would still run but drew too much current. Eventually the heaters in the contactor would trip and the pump only would make about 30PSI. The new pump (I replaced motor and pump) was guaranteed against lightning burnout!!! The pump and motor unit set me back about $450USD around 1985. I sold the farm to a friend and the pump is still running. It was a royal bitch pulling 280 feet of pipe out of a well with a hellish heavy pump on the end!!! All of my neighbors had similar setups. We don't have 3 phase in our rural homes and no one had a VFD or rotary converter. One neighbor had a rotary converter for his milling machines and lathes but that is a different story.

author: CaptDon
author_rank: Super Contributor
timestamp: August 27, 2024, 12:34:43 pm
content: I wonder since there is no evidence of an external capacitor if there is a capacitor in the upper end of the motor where the connections are made or if it is simply a centrifugal necklace start winding?

author: armandine2
author_rank: Frequent Contributor
timestamp: August 27, 2024, 02:22:18 pm
content: Delta connected motor?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2024, 10:51:28 am
content: Quote from: Circlotron on August 25, 2024, 01:17:52 pmIt's not; but explaining why, takes considerably more space than is deserved in this thread. Further reading:https://www.eevblog.com/forum/beginners/fast-rise-time-effect-oscilloscope-probes/msg5621055/#msg5621055Tim