Allowable voltages on Scope

On the day of 25 Mar 2006 18:42:49 -0800... " snipped-for-privacy@aol.com" typed these letters:

You can get oscilloscope probes that will allow you to hook your scope to thousands of volts. Just multiply the reading on the scope by the multiplier on the probe.

The grounding part. I don't know. But if it has a 3 prong power plug the chassis should already be grounded.

Devonshire

Do you have a probe ? It doesn't sound like you have a x10 probe.

If the probe Inputs 10:1 - means what the probe says - or docs on the probe - and you have one - you are in good shape to use it.

Try not connecting a ground. The Earth ground might be common. Second - ground the ground clip to the ground - earth ground on the box.

The RPC is likely 3 wires and maybe a gnd for safety. Then you are looking leg to leg and need to use 2 probes - subtracting the signal of one from the other SO A is on one probe, B on another. The GNDs of the scope - to safety GND or open.

That should work - but you should have something like that in the instructions.

Martin

Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member

snipped-for-privacy@aol.com wrote:

According to snipped-for-privacy@aol.com :

*What* watts? It only needs to be a reasonably high *resistance value. A typical 10X scope probe should be all that you need on each channel. The input impedance of a typical scope is 1M(egohm), and a 10X probe is 10M. The 240V (RMS) is about 340V, and applied to a 10M load will pump all of 0.0116 Watts into that.

So -- a 10X probe would allow you to look at things up to 2.5KV (actually, probably a lot lower, with full scale being 8 divisions times

20 V/div, or about 160V Peak (assuming ground being set to the bottom or top grid line.

Of course, a probe typically has a lower voltage limit than that worst case calculation.

There you are -- with one of *their* 10;1 probes, you can handle a swing of 1200 volts (-600 to +600), so your voltage from the RPC will be within that.

There is where the problem is. Normally, you want to look at the difference between two of the three phases, assuming a delta wiring. You probably can't get to the joined center of the windings in the motor, if it is wired as a Wye format (which is pretty likely for US made dual voltage motors. Unfortunately, the ground side of *all* of the probes are connected to a single point, so you can't do this without six probes feeding into three differential input pairs, which your scope does not have.

So -- what I would suggest is that you get three filament transformers which will take the 240 V on the input side, and produce something like 12V on the output side. Hook each primary between two of the three wires, and join one side of each secondary to the same point on the other two. Connect that joined point to the scope's ground, and hook the probes to the other side of each transformer. (A quick check for whether you have one of the transformer secondaries reversed relative the other two is to hook an AC meter between 1 & 2, 2 & 3, and

3 & 1. If all three measure fairly close to the same, things are fine. If one of them is way higher or lower than the other two, then one of the transformers is reversed, and you need to swap it around until all three are fairly close.

Once you have this done, you can measure the phase relationship between the three outputs.

Good Luck DoN.

wrote: (clip) They advised to get a large resistor and drop the voltage across that and then use the 5v and look at the waveforms. That would be a large resistor and I would not know where to find one that large that would handle the watts. (clip) ^^^^^^^^^^^^^^^^ Couple people suggested a probe. If you don't have one, you can accomplish the same thing with resistors. But you don't want to *drop* the voltage with a resistor--you want to *divide* the voltage with a pair of resistors. They don't have to be high wattage resistors, but they should be fairly high values of resistance to keep the wattage down. A scope has very high input impedance, so that is okay. A resistance of about 250k will draw about a quarter watt. Most resistors are rated for that power.

you aren't trying to measure voltage, just phase relationships, right? get yourself two 10 meg resisitors. connect the two resistors in series. connect one end to ground. connect the probe to the middle. connect the other end to the circuit under test.

this will divide the voltage in half and limit any current to a safe value. 10 meg resistors will cost you 5 cents each. power dissipation in 20 megs (the series value) with 400V applied is e squared/R = 160000/20000000 watts = 8/100 watts = or .008 watts, so a tenth watt resistor will be more than adequate, and you won't be albe to buy smaller wattage than that.

So, I dont' know what idiot advised you that you would need a huge resistor - clearly not someone who knew anything at all about electronics

Bill

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Steve

Tektronix has probes for this measurement. They also used to have a divider that was permanently connected to the high voltage so all you had to have is a 5V probe. And there are also viewing transformers for viewing things like this.

Bob AZ

Steve

Tektronix has probes for this measurement. They also used to have a divider that was permanently connected to the high voltage so all you had to have is a 5V probe. And there are also viewing transformers for viewing things like this.

Bob AZ

Resistors also have voltage ratings. If you are exceeding the rating, connect multiple resistors in series, ensuring that no resistors voltage rating is exceeded.

Joe Gwinn

Using earth-grounded sensitive instruments to investigate utility-connected power sources can involve special equipment for many measuring procedures.

Isolating accessories are often required for operator safety. Opto-isolated interfaces are available for insertion between the probes and the ocilloscope, to insure that the scope's chassis remains at earth ground potential.

The great feature of oscilloscopes is that they're very sensitive, and can display very low signal levels, even without direct connections. Placing a probe tip near a transformer or inductor will often provide adequate signals for the scope to display. The added length of a screw-on hook tip can act as a pickup, sort-of like a small antenna.

Typically, an isolation transformer is utilized to troubleshoot/test line operated equipment, but in the case of an RPC (or a motor VFD), a fairly huge isolation transformer would needed, to be able to sustain the load current demands. Many high voltage industrial electrical installations can't be measured without special equipment.

Using an inductive method would be a safe approach of isolation when accurate voltage measurements aren't required. Scope probe hook tips can provide a small inductive and/or a capacitance-coupled pick-up method. Placing the closed hook tip near a magnetic field will often be sensitive enough to produce useable waveforms. Laying the plastic housing of the hook tip directly on an insulated cable will have the same effect. A small plastic cap (something like those automotive vacuum tap caps) could be placed on the end of the hook tip probe to insure that it's fully insulated.

A snoop loop makes a more sensitive inductive pickup. The loop doesn't need to be wrapped around a magnetic field, just close enough that some inductive coupling takes place. Such a loop can be fabricated on the terminal side of a panel-mount type of BNC connector. Many scope probes include a probe tip-to-BNC adapter (to enable the probe tip to be mated to a BNC connector), which will allow the snoop loop to be easily fitted to a scope probe. I've read articles about sensitive probes that were built using heads from floppy disk drives, but haven't tried the idea.

A simpler technique would be taping a length of insulated wire along the side of the individual power leads. Connecting the scope probe hook to the conductor in the isolated pickup wire should be sufficient for phase angle displays. (connect to only one end of the pickup wire).

If you're wanting to check the phase relationship of the manufactured/generated 3rd phase of an RPC, you wouldn't need to connect directly to any of the electrically hot terminals.

I think you'll want to check the RPCs phase relationships with a running motor connected to it, but maybe not.

Many RCMers and HSMs seem to be getting more involved in machine electrical and electronic circuits. Hopefully, all or most, will know enough about the equipment they use to avoid damage and injury.

Although it's not used often, my Sencore 3100 Auto Tracker has full auto-ranging capabilities to 100MHz and two isolated 2kV inputs for the x10 probes. Additionally, the LCD display provides AC, DC, P-P, frequency, time, and even ohms (separate test leads for ohms). Almost any measurement is as simple as connect the probe, then read any parameter.

WB ..............

snipped-for-privacy@aol.com wrote:

I suggest powering the oscilloscope from an isolation transformer; with appropriate safety caveats of course. Private email to OP outlining same.

According to Robert Swinney :

That still leaves you with at least one scope probe whose ground side would be high relative to the other two -- aside from the scope chassis floating at some awkward (unsafe) voltage above ground. Most controls are properly insulated, but the BNC input connectors (plus any sync or other inputs which are ununsed) so contact with any of those could be dangerous.

[ ... ]

With the problem that the voltages would be rather different on the three phases, as two are running center tapped either side of neutral for the input power (thus providing voltages which are 180 degrees out of phase relative to neutral), plus the wild leg which will be a notably higher voltage.

Better would be to float the scope on an isolation transformer (with adequate precautions against operator shock) and to connect the ground of the scope to the three-phase neutral formed by the junction of the three windings of the parallel set (assuming a 240/480V motor wired in 240V mode, since the center of the other parallel Wye is buried inside the motor housing and not accessible.

But I still prefer three identical transformers of 240 V primary and something like 12V or 24V secondary, which would allow producing an isolated neutral for the scope ground and low enough voltages to eliminate the need for the 10X scope probes (though if you have three of them, it still might make it easier to connect it all.

The transformer does not need to handle any particularly high power level, as long as the isolation between primary and secondary is good, and the output voltage is a convenient value for measurement. I've seen tiny potted transformers to run small control circuits which would be ideal for the purpose, and probably the most affordable of the suitable transformers available new.

Obviously, if the output of the RPC were a nice grounded center Wye connection, all you would need would be 10X scope probes of sufficient voltage rating, and access to that grounded neutral.

Enjoy, DoN.

The fluke scopemeter provides an excellent solution to this problem.

The common on the probes can be brought out and connected to any voltage, as the unit is insulated and battery operated.

One can clip the common to one incoming hot leg, one probe to the other incoming hot leg, and the second probe goes directly to the manufactured leg. In this way the phase relationships between the incoming power and the manufactured leg can be directly displayed, phase angles measured, and the power factor computed.

Jim

(...) "In this way the phase relationships

Can one compute power factor without current measurements, Jim?

--Winston

I did this by putting a small (one ohm) resistor in series with the incoming line. Then I could simultaneously display current and voltage that was feeding my converter. However, doing this means the common on the scope *still* has to go to a hot leg, and the scopemeter is great for that.

Jim

Now I see. Thanks!

--Winston