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Forum Index : Electronics : question for Gill
| Author | Message | ||||
Pt w/field Matt Senior Member  Joined: 24/02/2006 Location: AustraliaPosts: 105 |
hi Gill ive got a protec 506 meter like yours but when i do low revs on my 5hp petrol test bench it will go down to 0.220 khz and when i drop the revs lower it measure o.400 plus khz,you can tell by the amps and motor vibatoins that the meter is wrong whats going on? matt down south |
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Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
Hi Matt, The way I understand it is that these SmartDrives produce harmonics. That is to say in addition to the Fundamntal Frequency (in your case 220Hz)the sine wave is not pure and multiples of that Fundamental Frequency appear. The 1st harmonic being 440Hz, the 2nd harmonic being 660hz, the 3rd being 880hz and so on. When taking a frequency count at one time I had the meter jumping from one harmonic to another up to the 4th harmonic. The first time I saw a greater reading than expected, I thought it may have been caused by a high resistance (loose connection) in the dalta connections inducing/drawing a balancing current from another phase (I do have a slight diference of voltage on each phase[not good])but I did not expect to see more than the 2nd harmonic frequency. Well I've seen readings more than that as I've said. An oscilloscope would provide a picture to better diagnose the cause. Looking at the trace Dr. Chalko page I do not see a very good wave form at all. There seems to be breaks in the wave form and this may trigger another count. In short, I don't know what causes these higher (harmonic) frequency counts, but what I have done to aviod them is to attenuate the signal. That is to reduce the voltage. My thinking is that if the meter triggers a count on say a 2miliVolt rise, then any ripple in the AC waveform that exceeds that will give an extra count. By attenuating the 12V (whatever) down to a much lower voltage, the ripple will also be proportionately reduced and no longer trigger the extra counts. I use 2 x 100K 1W resistors. One on each test lead. This works for the Protek506 and the one I use most, the DSE_Q1476 Clamp Meter. The 3rd meter I use a DSE_Q1450 does not seem to need the resistors. My suggestion is to check all connections,check for voltage variation between each phase and add the resistors. Well I hope that's all helped. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Hmmm, I thought I saw a very good waveform when I put mine on a scope and if I recall correctly those discontinuous wave forms on Doc Chalcos page were related to diode switching? |
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| Gill Senior Member Joined: 11/11/2006 Location: AustraliaPosts: 669 |
Hello KiwiJohn, I think you are lucky indeed to have a good waveform. As I said I have no idea what causes the harmonics. It may indeed be diode switching related. Are you by chance using fast recovery Shotkey Diodes and we are not?? Here are some exerts from another forum (Forum Thread) on this subject that may shed more light on a more effective remedy than mine: Mick S "My biggest hastle at the moment is that I do not have any accurate method of measuring RPM's up on the tower, so I am guesing when I make any mofification that there is any improvement of if I have made things worse. I have a digital frequency meter which works well on electronic circuits but gives very confusing results whatever way I try to use it on my gennys. If anyone has ideas on suitable test probes, connection methods I would be very grateful." Flux "If you have a true digital frequency meter then you are unlikely to make any sense of the speed of an operating wind generator. Most of the multimeters with frequency range will do the job, but I suspect they are just f/v converters not digital time interval meters. In almost every case you will need some sort of filter to have any chance of measuring frequency of an alternator connected to a rectifier. You may measure the thing on open circuit without a filter. A filter consisting of 47k in series with .22uf across the meter terminals is usually enough to clean up the waveform so that you measure the fundamental, not the harmonics." Nando "seriously, Flux remark with the low pass filter ( 10 milliseconds) may remove the charging spikes from the line, sometimes you may increase the time constant to have better results. Another way is to use a current transformer, a home made is OK, like take a 20 or so watts transformer and place ONE turn in the core and place it in series with one of the 3 phase lines and load it ( using the 110 volts winding = 10 ohms resistor, wattage around 5 watts), the wave form may be distorted but you may be capable of reading the frequency and the current at the same time." I have just tried Flux' filter and it did not work for me. Matt, perhaps you could give it a go too? I'll have to stick to the resistors for now. was working fine... til the smoke got out. Cheers Gill _Cairns, FNQ |
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Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119 |
Come on Milliwatt Men! What about a low pass filter in conjunction with the voltage reduction suggested. Or probably better still, a simple digital rpm counter made up using black/white sectors on the shaft/rotor and a led/photodiode combination to count the reflected pulses, measured on a frequency/pulse counter meter? Latter alternative admittedly does require another three wires down the tower, but they're little ones. Megawatt Man |
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