Three phase Measurament

[kennycaldas]

Hi everyone,

I'm trying to measure the phase and the power factor for a three-phase motor. The problemn is that the motor is running with 480V and the SCXI module that I have can only handles until 300Vrms. I'm thinking about using some voltage transformers to step down the voltage and then connect it to the SCXI module. If I do that, will it affect my phase angle and power factor?

Assuming that this configuration works, how can I find the phase in each wire? Is it possible to run all samples at the same time and then make some comparisson between their plots to measure that?

Thank you!

Kenny Caldas

[Hooovahh]

Lots of questions.  So the scale operation won't be a problem.  If you scale your voltage from +/-300V down to +/-10V, just scale it back up in your program after taking the reading and you won't have a problem.

But what you will have a problem with is sampling multiple voltages at the same time.  And I mean the real same time, the same clock pulse.  This means you need a card that supports simultanious sampling.  You can then take N samples on N channels on the same device and you will be able to plot all of them independently, or on the same graph.  There are then some tools that can calculate things like Power Factor and phase.  There are free tools as well but this works nicely assuming you took the data properly.

[johnsold]

Any potential transformer or current transformer will have some phase shift. The manufacturer should specify the phase shift just as they specify the voltage ratio. If you are only concerned about the fundamental power line frequency, you will probably not have much problem. On the other hand a variable frequency drive for the motor may require much more care to get good measurements.

If you only want the fundamental frequency and have modest phase resolution requirements, you may be able to tolerate the errors produced by non-simultaneous sampling. You will need to calculate the errors to see if they are within your requirements.

Please consider the safety aspects of the meaurements you will be making. Voltage and current transformers along with suitabel protection devices can prevent damage to expensive equipment and reduce the risk of injury or death.  Even though the nominal voltage is 480 V faults due to lightning or a car hitting a distribution pole can result in transients which are much higher.

Lynn

[kennycaldas]

Thank you for your answers!

Unfortunately I can't afford now the Electrical Power Suite add-on, so I'll have to go with the free tools that are already installed in LabVIEW. I have the SCXI-1140, which can handle up to 8-channel simultaneous-sampling. Do you think using that module will the enough for capturing the phase shift and determining the power factor? I'll stay have to step down the voltage though, since it only works with 10mV to 10V. 

I'll need good measures for this project, to see the variations on the efficiency of the motor in different conditions, so I'm trying to get the most accurate I can.

Thank you again!

[johnsold · Accepted solution]

That device should be fine. Put the rest of your budget into good transformers.

Lynn

[SteveD123]

Just want to emphasise Lynn's point about 480 nominal volts and what you might actually have.

Get your design and wiring checked by someone certified to do so before connecting.

Some further points:

How is the motor being fed?

a. If fed by a Direct On-Line starter or a Star-Delta starter, there will be a big spike when the contacts open to turn the motor off, or during the star-delta transition.

b. If fed by a PWM inverter, then the inverter synthesises the AC 50 or 60 Hz wave form by chopping DC very fast, and relying on the motor windings to act as the low pass filter. Good luck trying to instrument that.

Fault Level: You must have fuses as close to the instrumentation pick-off point as possible. That way, if there is a fault in your wiring the instrumentation fuse pops, and you do not have the full fault current flowing through your instrumentation wiring. Fault current will probably be about 4 times motor start current if the motor is fused correctly, and if it is an induction motor.

I would use isolation amplifiers on the voltage inputs. That way you can use a simple potential divider on the input to scale to the voltage you want, but make sure that the isolation amplifier has a suitable frequency range.

So it would go something like:

Line voltage->Fuse(or other protection device)->Potential divider->Isolation Amplifier->SCXI module.

IF you sample each channel fast, e.g. 10,000 samples per sec., then the time difference between samples and hence sampling induced phase error will be small.

Also look at the safety issues with Current Transformers here:

http://en.wikipedia.org/wiki/Current_transformer

[johnsold]

Good points, SteveD

Lynn

[kennycaldas]

Hi SteveD,

The motor will be fed by a Direct On-line starter. I'll have an Electrical Engineering to help me with the wiring, I'm still trying to come up with some ideas on the best way to go before buying anything.

I'll take a look at your sugestions.

Thank you very much for you help!