Author Topic: how to perform differential voltage measurement with an oscilloscope?  (Read 4735 times)

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Online rakesh

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hi everyone

I bought an induction motor controller which is actually a VFD drive http://kellycontroller.com/kim481024v-48v100aac-induction-motor-controller-p-1229.html . It is used to produce 3 phase AC at a maximum of about 48V and it can deliver a maximum current of about 100Amps. I use this controller to run a 0.5HP motor which is obviously a 3 phase. The motor spec is rated voltage :40V full load current: 12Amps connection :delta rpm at full load:2810. The controller gets a 72V DC from a 150Ah battery bank and produces the required voltage according to the throttle response. This controller needs a speed feedback from the encoder for its running. I want to do a load test on the motor to measure the change in power output at various loads. To find the motor efficiency i should measure the input power to the motor which is coming from the VFD drive controller. Since our normal voltmeter and ammeter couldn't show the accurate readings since it is a switched pwm waveform at a frequency of 16.6KHZ. Hence i planned to connect it to a oscilloscope to get the accurate value of the voltage and frequency of the 3 phase AC that is given to the motor.

When i connected my oscilloscope to the two terminals between phase to phase to measure the voltage. The power from the controller seems to flow through the ground probe of the oscilloscope and eventually the controller shuts down immediately for over current protection. One of my friend suggested me to measure the voltage by means of DIFFERENTIAL VOLTAGE MEASUREMENT by means of two probes of an oscilloscope. Am using a TDS1012B tektronix oscilloscope which has a sampling rate of 1Gs/second.

Since i have a very little idea of differential voltage measurement could some one tell me how to do the measurement with an above mentioned oscilloscope.And also kindly suggest me if there is any other ways to do the measurement of the voltage and current coming from the vfd drive controller.

Online David_AVD

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Offline IanB

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #2 on: September 04, 2012, 07:07:20 PM »
The ground probe of an oscilloscope is connected to mains protective earth. So if you attach this probe to any part of a device that has a voltage referenced to the mains ground you will create a short circuit. However, if your device being tested is completely battery powered and is not connected to the mains in any way you should not have a problem. Unless of course you connect two ground clips to two different parts of the circuit. Then you will make a short circuit between the ground clips.

To your particular question, the simple way to make a differential voltage measurement is to measure two different voltages with two probes and then use the math function on the 'scope to subtract one measurement from the other. When you do this you must either leave the ground clips unconnected, or preferably attach both ground clips to a good reference point such as a grounding point on the device under test.
I'm not an EE--what am I doing here?

Offline vk6zgo

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #3 on: September 04, 2012, 07:10:42 PM »
Do you have an operator's manual for the 'scope?
If not,have a look on this website:-
http://people.ece.cornell.edu/land/courses/ece5760/equipment/TEKtds1002.pdf

Pages 68 & 69 are the closest to what you want to do,but everything in the manual is a bit vague. ;D

Online rakesh

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #4 on: September 04, 2012, 07:17:41 PM »
thanks a lot for answering my question. do i have to change the coupling option to AC??

Offline vk6zgo

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #5 on: September 04, 2012, 07:57:28 PM »
Yes,I think it might be best,but as there shouldn't be any DC component,it may not matter.

Offline qno

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #6 on: September 04, 2012, 08:02:54 PM »
These type of controllers meant for battery motor control have a feature to detect ground faults.
The battery minus is not connected to the case.
If the resistance between the case and the battery minus is < 10k it is supposed to switch off.
Why spend money I don't have on things I don't need to impress people I don't like?

Online rakesh

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #7 on: September 04, 2012, 09:24:26 PM »
I connected across the two terminals of the vfd drive but am getting two waveforms which are switched pwm sine wave. My doubt is do we actually get two waveforms in the screen instead of one. And also do we have to set one of the channels as the reference to the other channel if so how do i do it in this oscilloscope.

when i measured the frequency at both the channels am getting a same frequency of about 16.6khz which is correct as mentioned in the VFD drive controller.

Offline Mechatrommer

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #8 on: September 04, 2012, 10:36:30 PM »
buy a differential probe... or build your own...
High Voltage 10-100MHz Differential Probe Investigated
« Last Edit: September 04, 2012, 10:42:33 PM by Mechatrommer »

Offline w2aew

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #9 on: September 04, 2012, 11:41:51 PM »
Short of buying or building your own differential probe, you can use the MATH feature of that scope to create a CH1-CH2 trace.  In this case, you would use two probes, one on each of the two nodes that you want to make a differential measurement of (with both probes' ground connections going to ground/earth of your circuit).  Then, use the MATH to create a math trace that is CH1-CH2.

The old analog scopes used to be able to this in hardware by selecting "ADD" as the vertical mode, and selecting "INVERT" on CH2...
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Offline grenert

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #10 on: September 05, 2012, 01:23:35 AM »
Tektronix has a nice document explaining differential measurement:
http://e-sites2.tek.com/cmswpt/tidetails.lotr?ct=TI&cs=pri&ci=13220&lc=EN

I can't recall if they discuss the two-channel method described above.  If not, I know that Tek has another document on floating measurements that discusses it (search for Tektronix and floating measurement).

Offline Mechatrommer

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #11 on: September 05, 2012, 02:24:09 AM »
i read a joke on another forum to tie you "floating scope" on fishing rod high up the roof and make the differential measurement, ie scope's gnd (floating) probe to the 2nd signal. or buy another handheld battery powered oscilloscope.

Online David_AVD

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Re: how to perform differential voltage measurement with an oscilloscope?
« Reply #12 on: September 05, 2012, 06:54:05 AM »
My doubt is do we actually get two waveforms in the screen instead of one. And also do we have to set one of the channels as the reference to the other channel if so how do i do it in this oscilloscope.

On a typical 2 channel analog oscilloscope, you'd put it into ADD mode (to get single trace) and INVERT one of the channels so you really get a subtract function.


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