Measuring Stray Voltage. Steady state

Transcription

1 Measuring Stray Voltage What to measure: >Steady state >Motor starting transients >Impulses September Steady state Where to measure: >All known cow contact points >Stanchions >Parlors >Ramps >Outdoor locations September

2 Impulses What to measure: >Cow trainer influence >Electric fencer influence >Crowd gate influence >Load switching influence September How to make stray voltage measurements My examples use the following instruments: Tektronix THS720P Fluke 123 Scopemeter Dranetz 658 Simple average reading DVM There are several other equally or better qualified instruments to use. These instruments were on hand at the time this presentation was prepared. September

3 Measuring steady state Items to consider: >Average versus true RMS readings >AC only versus AC+DC readings >Motor starting transients September Measuring impulses Items to consider: >Peak or Peak-Peak values >Measure the duration September

4 Comments on Duration Duration may have two (2) meanings: In stray voltage research work it is considered the time between zero crossings of an impulse. In industry it may be considered the time difference between the point an impulse reaches 90% of the peak value and when it decays to 50% of the peak value. September Before you measure make sure your equipment is functioning properly! The following examples are intended to show the limitations of equipment and methods to avoid measurement errors. September

5 Basic Equipment Testing During these tests we will investigate: >60 Hz sine, triangular and square waveforms >Frequency response >Crest factor >Accuracy >Sampling rate September Hertz sine waveforms with and without DC voltages The purpose of this test is to determine if your instrument can block DC voltages. Normally as a power supplier we are investigating the influence of 60 Hertz power line voltages. Meters that do not block DC voltages can result in erroneous readings. Be sure to do an initial check at the site for DC voltages and record the background level. September 2000 cforster@forstereng.com 10 cforster@forstereng.com 5

6 This is what an AC voltage with DC present will look like on an oscilloscope September Experiment with blocking DC September

7 60 Hertz Sine, Triangular and Square waveforms These tests will show the reason why a true RMS meter should be used. The reading errors will vary between instruments. I am using the sine, triangular and square waveform since these are the simplest to obtain with a function generator and they give a good indication of the meter variations that can occur. September 2000 cforster@forstereng.com Hertz Sine waveforms In this test our true RMS meter read 1.03 volts AC and my average reading meter agreed, indicating 1.05 volts AC. An average reading meter assumes all waveforms are perfect sine waves. September 2000 cforster@forstereng.com 14 cforster@forstereng.com 7

8 60 Hertz Triangular waveforms In this test our true RMS meter read volts AC and my average reading meter indicated 1.01 volts September Hertz Square waveforms In this test our true RMS meter read 1.54 volts AC and my average reading meter indicated 1.78 volts September

9 Pulse type waveforms and the CREST FACTOR When the ratio of the peak voltage to the RMS voltage exceeds about 3 times, most true RMS meters fail to measure correctly September 2000 cforster@forstereng.com 17 Experiment with Sine waveforms Triangular waveforms Square waveforms Crest factor September 2000 cforster@forstereng.com 18 cforster@forstereng.com 9

10 Instrument Bandwidth BW Every instrument should be able to measure 60 Hertz frequencies correctly. As we increase the frequency of the waveform the accuracy of the instrument may vary. In most cases the instrument measures a value lower than the correct value. September 2000 cforster@forstereng.com 19 Instrument Bandwidth BW. For the next test we will apply a sine wave of varying frequency from 100 Hertz to 2,000,000 Hertz. September 2000 cforster@forstereng.com 20 cforster@forstereng.com 10

11 Instrument Bandwidth BW. Here are the bench test results with approximately 0.5 volts rms input: Frequency Tektronix Fluke Dranetz 100 Hz ,000 Hz khz khz ,000 khz September Instrument Bandwidth BW. If we increase the frequency the errors become greater. For these bench tests the input was approximately 0.5 volts RMS input: Frequency Tektronix Fluke 10 MHz MHz MHz MHz September

12 Instrument Bandwidth BW. As we increase the frequency there are other factors that can cause erroneous readings. We will observe these phenomena during the lab tests. September Experiment with increasing frequencies September

13 Sampling Rates A digital instrument measures by sampling the varying input voltage or current and then calculating the result. If the rate of sampling is too low, the unit may miss a short impulse. September 2000 cforster@forstereng.com 25 Sampling Rates... Another problem with low sampling rates is aliasing September 2000 cforster@forstereng.com 26 cforster@forstereng.com 13

14 Measuring Impulses If the impulse you are trying to record is repetitive such as a fencer, cow trainer or crowd gate, the triggering of the instrument is simpler. Normally the impulse will be recorded each time it occurs or at least periodically. September 2000 cforster@forstereng.com 27 Measuring Impulses... If the impulse you are trying to record is a random occurrence or maybe just a single event, triggering can be a problem. I am assuming your instrument is fast enough to record the impulse. September 2000 cforster@forstereng.com 28 cforster@forstereng.com 14

15 Setting up the scope to trigger There are several decisions to make when setting trigger levels: Channel A, B or external Rising or falling slope Exact trigger level Exact trigger time September 2000 cforster@forstereng.com 29 Setting the Tektronix 720 Set vertical menu: Coupling-AC Invert-n/a Bandwidth-Full Position-n/a Probe-Important! September 2000 cforster@forstereng.com 30 cforster@forstereng.com 15

16 Setting the Tektronix 720 Set horiz. menu: Time base-main Trig. Pos.-~10% Display T -n/a September 2000 cforster@forstereng.com 31 Setting the Tektronix 720 Set trigger menu: Trig. type-pulse Source-Ch1 Polarity-Pos September 2000 cforster@forstereng.com 32 cforster@forstereng.com 16

17 Setting the Tektronix 720 Set acquire menu: Mode-Sample Stop-Single September Setting the Tektronix 720 Set Measure menu: Select for Ch1- Peak-Peak September

18 Setting the Tektronix 720 To record an impulse: Press-HOLD and the HOLD indication will switch to Trig? September Let s look at the Fluke 123 Scopemeter September 2000 cforster@forstereng.com 36 cforster@forstereng.com 18

19 Setting the Fluke 123 Set Ch A menu: Measure-Peak September 2000 cforster@forstereng.com 37 Setting the Fluke 123 Set meter options: Press V-Hz-A Press F1 September 2000 cforster@forstereng.com 38 cforster@forstereng.com 19

20 Setting the Fluke 123 Set Scope Menu: Press Scope Menu September Setting the Fluke 123 Set Scope Menu: Press Scope Menu Press F1 September

21 Setting the Fluke 123 Set Scope Menu: Press Scope Menu Press F2 September Setting the Fluke 123 Set Scope Menu: Press Scope Menu Press F2 Press Enter September

22 Setting the Fluke 123 Set Scope Menu: Press Scope Menu Press F3 September Setting the Fluke 123 To record an impulse: Press- HOLD/RUN and the Hold indication will switch to Wait September

23 In summary. To record single events you generally have to: Choose the Channel to trigger on. Set the magnitude level at which the unit will trigger Set estimated vertical and horizontal sensitivity Set type of input and scale multipliers Set bandwidth if adjustable Set pre-trigger position Choose type of waveform if selectable Choose pos. or neg. trigger mode Tell instrument to stop after one sweep September Let s measure an impulse... On the Tek 720P September 2000 cforster@forstereng.com 46 cforster@forstereng.com 23

24 Let s measure an impulse... On the Fluke 123 September 2000 cforster@forstereng.com 47 What happens as the lead lengths increase? The top waveform is the input voltage to the coaxial cable. The lower waveform is the voltage at the end of the cable with a 50 ohm terminating resistor. Note the phase delay Waveform shape is maintained September 2000 cforster@forstereng.com 48 cforster@forstereng.com 24

25 What happens as the lead lengths increase? Input impedance due to SWR lowers output The top waveform is the input voltage to the coaxial cable. The lower waveform is the voltage at the end of the cable with NO terminating resistor. Note the phase delay Waveform shape is changed September What happens as the lead lengths increase? The top waveform is the input voltage to the coaxial cable. The lower waveform is the voltage at the end of the cable with 500 ohm terminating resistor. A 500 ohm resistor does not solve the problem September 2000 cforster@forstereng.com 50 cforster@forstereng.com 25

26 The top waveform is the input impulse to the coaxial cable. The lower waveform is the impulse at the end of the cable with 500 ohm terminating resistor. What happens as the lead lengths increase? A 500 ohm resistor does not solve the problem September 2000 cforster@forstereng.com 51 Typ. Fencer How does an IMPULSE affect a cow? ,000 10,000 Phase Duration (microseconds) = time between zero crossings September 2000 cforster@forstereng.com 52 Reprinted with permission: C. Forster 7/5/00 cforster@forstereng.com 26

27 Is this Duration? Here is an impulse burst similar to that caused by secondary load switching Duration = 50 ns Magnitude = 2.5 v peak September 2000 cforster@forstereng.com 53 Or is this Duration? Here is the same impulse burst expanded 10 times Duration = 13 ns Magnitude = 1.0 v peak September 2000 cforster@forstereng.com 54 cforster@forstereng.com 27

28 Some conversion factors: 1 second = 1,000 milliseconds = 1,000,000 microseconds = 1,000,000,000 nanoseconds = 1,000,000,000,000 picoseconds September 2000 cforster@forstereng.com 55 Some conversion factors: second = 1 milliseconds = 1,000 microseconds = 1,000,000 nanoseconds = 1,000,000,000 picoseconds September 2000 cforster@forstereng.com 56 cforster@forstereng.com 28

29 Some conversion factors: second = milliseconds = 1 microseconds = 1,000 nanoseconds = 1,000,000 picoseconds September 2000 cforster@forstereng.com 57 Periods for sine waves: 1 Hertz = 1,000 milliseconds 60 Hertz = 16.7 milliseconds 1 khz = 1 millisecond 1,000 khz = 1,000 microseconds 1 MHz = 1 microsecond September 2000 cforster@forstereng.com 58 cforster@forstereng.com 29

30 Periods for sine waves: 1 MHz = 1 microsecond 10 MHz = 0.1 microsecond 10 MHz = 100 nanoseconds 100 MHz = 10 nanoseconds 200 MHz = 5 nanoseconds September 2000 cforster@forstereng.com MHz sine waves: September 2000 cforster@forstereng.com 60 cforster@forstereng.com 30

31 Now that you know how to interpret impulse duration, would either of the previous impulses affect a cow? September 2000 cforster@forstereng.com 61 Thanks! If you have any questions contact me: Chuck Forster cforster@mailbag.com September 2000 cforster@forstereng.com 62 cforster@forstereng.com 31