Measuring large 3 phased motores - Page 1
Forum section: electronics / projects

author: audi80
author_rank: Newbie
timestamp: July 01, 2013, 02:15:49 pm
content: Hi PeopleI'm trying to figure out how i can measure voltage and current of various start methods of 3 phased motors, for educational purposes. I'm planning to use my DSO, but how to do it?I know the correct method is to use a Cat3/4 rated device, but is it possible with a normal bench DSO?Is it possible to measure voltage and current through a safety transformer? Well, i know it's possible but is it safe?Or should i float the motor (isolate it from mains-ground) and measure directly on the motor?Kind regards

author: JackOfVA
author_rank: Unknown
timestamp: July 01, 2013, 03:34:43 pm
content: Current transformer is the normal method for current data. Either wired in place dedicated current transformers or suitable clip-on current probes. Have to watch the current ratings, of course, as well as isolation voltage ratings.For voltage measurements, sometimes a potential transformer is used to obtain a stepped down isolated voltage, sometimes you might use a differential probe with an oscilloscope.All depends on whether you are looking at a 1/2 HP 3 phase motor running on lower voltage or some 1000 HP monster.Also depends on whether you are looking for highly detailed time information as might be collected with an oscilloscope or slower interval data as from digital multi-meters, as the various probes have different frequency response characteristics.If you can be more specific, you may receive higher quality answers.

author: audi80
author_rank: Newbie
timestamp: July 01, 2013, 04:22:23 pm
content: Thank you.I'm using motors up to 1kW, comparing direct start, star-delta start, soft starters and more... under different loads. To demonstrate diffrent senaries, it's not precision engeneering just illustartion of concepts.I do have some current clamps, rated up to 300A, i could try them...

author: JackOfVA
author_rank: Unknown
timestamp: July 01, 2013, 07:33:11 pm
content: Check the frequency response specifications of the clamp probes. Assuming they are good out to a few KHz it should show what you are interested in, start-up transients with resolution to a few degrees of the mains power cycle to some tens of seconds, with your digital oscilloscope.Spent a couple summers working at an oil refinery while studying electrical engineering - a "large" motor there would be several hundred HP and up. 480V 3 phase and up in voltage.

author: audi80
author_rank: Newbie
timestamp: July 01, 2013, 07:55:40 pm
content: Thanks exactly. I'll try that.Hehe, i ment large in comparisons with motors used in electronicsI'm teaching apprentice electricians in electromechanics and I would like to show them different motor starts in real life

author: notsob
author_rank: Frequent Contributor
timestamp: July 01, 2013, 10:41:42 pm
content: Make sure you let them experience what happens to a shunt motor when you lose the field.

author: audi80
author_rank: Newbie
timestamp: July 02, 2013, 06:50:01 am
content: Hehe i do

author: Kremmen
author_rank: Super Contributor
timestamp: July 02, 2013, 06:51:11 am
content: Quote from: notsob on July 01, 2013, 10:41:42 pmMemory lane stuff coming - skip if you don't care for such.Way back when my then employer Stromberg delivered no-break supplies to bank computer rooms to feed the big IBM dinosaurs thinking their unfathomable Cobol thoughts. The GTO or the IGBT were still far in the future so instead of inverters, these babies were electromechanical systems where a big DC motor drove a 3 phase generator that supplied the computers. The DC motor in turn was fed either from the grid or when that failed, from a righteously huge stack of lead acid batteries providing the 440 V DC voltage. The switchover was done with contactors controlled by relay logic, and the unavoidable moment of no torque on the generator shaft was handled by an enormous flywheel placed between motor and generator. The system worked perfectly and over the years saved many a day around Finland when summer thunderstorms caused a power break here and there.Yet this was a mechanical system and needed overhauling occasionally. This was one of those times in one of the bigger Finnish banks. The generator system was standing still and the maintenance man was preparing to do something, perhaps to replace the commutator brushes of the DC motor, when by mistake he inserted a jumper wire wrong in the relay cabinet. Due to a small but critical oversight in the original circuit design this particular mistake bypassed all safety interlocks and closed the battery main contactor _without the separately fed field supply_ to the armature of the standing machine. The DC resistance of a 150 kW DC motor is minimal thus the resulting armature current fed by the battery array was really, really high. The main contactor welded shut right away so anyhing the technician did made no difference any more.Now, the torque generated by a separately excited DC motor is 0 when there is no current in the field coil. But that is so only in theory. In practice the remanent magnetic field in the stator iron is enough to provide a small torque, in this case easily overcoming the very low friction from the ball bearings. The AC generator was cold so it offered no opposing load torque. Accordingly, the assembly started to accelerate slowly while the feeder cables were smoking. The DC motor was rated at 1500 RPM but unfortunately in the equation of shaft speed for a DC motor, the value for the field magnitude is below the division line. Some say that there is no division by zero, but in any case division using an inifinitesimally small denominator produces an astronomical result. Accordingly this motor was aiming for low earth orbit and something needed to be done.Messing with the controls did nothing due to the welded contactor that was beyond help. The technician did have access to a fire axe but didn't have the guts to hack the cables. This is understandable because 440 VDC can arc something vicious and molten copper is nor really Coppertone, so you don't want to try tanning with it. Finally when he couldn't come up with a solution and time ran out, the technician wrote in his report something like this: "When it was obvious that no practical way of fixing the problem was apparent and the situation turned alarming, i considered it prudent to vacate the premises". So he ran and moments later all hell broke loose.Probably it was the DC motor commutator that failed - we'll never know because this was before surveillance cameras were all over the place. Anyway finally the shaft seized due to some reason and the inertia of the 1½ ton flywheel tore the motor assembly to pieces. By this time the flywheel must have been revolving at several thousand RPM and now it went berserk in the generator room. When it got quiet again, there was nothing recognizable remaining in the room. Nothing. I saw the photos before and afterward because the system was designed in my dept so i knew the guys who did it. The 3 electrical cabinets 60x60x220 cm were flattened against the solid concrete wall and were literally pancaked. The battery stack, the size of a small locomotive, was in billion tiny fragments all over everything. Even the 4 by 1 by 1 meter solid concrete pedestal of the motor/generator assembly was beaten to a nub. To finish the destruction the battery acid had eaten and corroded all the concrete so the devastation was total. It was almost cartoon-like in its completeness.So yes, it is a b a d idea to try running a shunt motor or separately excited motor with the field unconnected.

author: BillyD
author_rank: Regular Contributor
timestamp: July 02, 2013, 07:17:11 am
content: Great story Kremmen, thanks for sharing! Would love to have seen those pictures!

author: audi80
author_rank: Newbie
timestamp: July 02, 2013, 07:17:45 am
content: That's a great story Kremmen, there must have been some people ashamed about the safety breach

author: Kremmen
author_rank: Super Contributor
timestamp: July 02, 2013, 10:45:36 am
content: Thanks.Yes obviously there were red ears but at the same time i recall that the problem was real subtle so that you practically needed to see that this could happen before anyone was able to really spot the shortcoming. During normal operation the control logic didn't fail a single time.

author: SeanB
author_rank: Super Contributor
timestamp: July 02, 2013, 07:13:42 pm
content: Worst I saw was an electrical fire on a towing tractor. Never seen a guy remove the lightweight battery cover ( a 600 by 600 by 10mm thick plate, it was the lightest panel on the unit though, with 4 M8 bolts holding it on in the corners) so fast with a spanner, while everybody else was pouring dry powder into the burning interior and underside of the engine cover to keep the flames down. then the cover came off and the battery was disconnected. Only reason this was done and it was not left to burn out was that it was attached to a fully fuelled and just serviced fighter plane and was being towed out of the hanger to go for it's first test flight.Next week was a a busy one for my friend at GSU, adding battery disconnect switches to all of the other vehicles. a month later he asked me if I could help him ( motor mechanic and no electrician on base ( post probably still open now)) rewire it, as the fault was wiring looms that chafed through.I did it, though the holes in the chassis plates benefited from a quick pass from a very large cutting torch to both remove the remains of the old looms and make the original 50mm flame cut hole a 200 by 100 panel hole you could get your hand through. Was not going to worry the 20mm plate it was made through, this tractor was the size of a small car, but came in empty at 16 tons. Used a 5 cylinder Deutz engine, a 3 speed Borg Warner truck box ( with a pin to lock it from engaging any other gear than 1, N and R) with a 100mm long drive shaft coupling it to a MB truck axle. Favourite fault there was the propshaft being pretzeled and tearing itself and the rear of the box apart. Most accidents with these involved either stupidity on the operators part ( normally fatal as the driver was the highest part) or involved a brick or concrete wall, which came off second best in most cases. Brakes could be best described as an optional feature, as the original designers did not use a power assist master cylinder, and a later mod was the addition of front BREMO Golf GTI dual calipers, still with the original master cylinder.One was brought in one day with a complaint of rough idle, and it turned out the one pot had seized, and they did a jump start with 24V ( it was a 12V system) and drove it down to charge the battery thinking it was flat. Needed a new pot on that engine complete, as the only way to remove it was to cut a hole and burn through the conrod in the crankcase.

author: superUnknown
author_rank: Regular Contributor
timestamp: July 03, 2013, 07:44:30 am
content: Great story!I miss working for Finns, but my liver sure doesn't!

author: SeanB
author_rank: Super Contributor
timestamp: July 03, 2013, 05:54:29 pm
content: Dunno, a few guys there made a pretty good moonshine. Witblits ( white lightning) which was about 180 proof or so, with a flavour of Marula ( the fruit used, it grew all over wild) and had it's uses as something you could use to start fires, clean glass or strip paint. Apparently you could drink it as well, but I was by then teetotal, having seen way too much of the effects of alcohol and having built up a very high tolerance to neat spirits. Like Dave I do not like beer, aside from cooking with it. Funny thing is my father worked in a brewery......