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Thread: Reading Hz

  1. #1
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    Reading Hz

    Hello,

    I bought a UEI DL 389 multimeter several years ago as part of a tool kit through a community college. Liked it so much that I bought another, but that's not why you called.

    For the first time this week I tested out the duty cycle function on a SCR supplying radiant heat panels installed at my facility. Based on signal output I was reading +/- 3%.

    Decided to test the Hertz function on a VFD. In the past I've checked on standard voltage supplied to equipment (60hz +/-.5).
    Reading was all over the place and it seemed to affect the meter in a strange way. Which leads to my questions.

    Could the poor reading have to do with the meter? Am I misapplying the Hz function of the meter? If not, does the difficulty reading hz have to do with the drive artificially recreating the sine waves? Should the reading be taken phase to ground or phase to phase?

    And just for kicks, how to the drives read the hz output?

    Thanks in advance.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  2. #2
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    Scr is a solid state switch and need two of them to control ac voltage. Since they are pulsed in alot of cases like to control electric heater. Checking hz on the load side with your meter isn't fast enough to respond. If you look at a ceiling fan with a light kit and a remote dimmer it doesn't vary the voltage its turning the power on and off to the bulb ( remotes that use incandescent bulbs)

  3. #3
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    Thread Starter
    To be clear I tested the meter on two different machines.

    First checked the duty cycle on a 460 3ph 150A SCR supplying 3 banks of 277 1ph radiant heaters. As mentioned my meter tracked within 3% of the 5-10vdc signal. I did check Hz on the SCR outlet and read about 70hz. I figure that this had something to do with the meter picking up the switching in addition to the sine wave. On the SCR I'm pretty clear.

    Second check was on a VFD operating a 460 3ph 100HP Return Air fan. Meter gave no clear readings and began to beep and the display faded and came back on. Also picked up a small amp draw through the leads between phases and to ground when on the hz scale and the voltage scale. This is where my questions come in.

    I assume that the VFD pulsing the DC into a recreated sine wave prevented my meter from reading a clear frequency, but do not really know.

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  4. #4
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    A vfd slices the humps of the 60 hz sine wave. Where they connect to each other. Need a oscilloscope to see it.

  5. #5
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    Thread Starter
    Quote Originally Posted by servicefitter View Post
    A vfd slices the humps of the 60 hz sine wave. Where they connect to each other. Need a oscilloscope to see it.
    So most practically and analog oscilloscope?

    Sent from my SAMSUNG-SM-G891A using Tapatalk

  6. #6
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    Just to revisit:

    Seems that the duty cycle on my meter may not have been reading the SCR very well but just happened to read right around where the signal was at the time.

    Maybe this is a question for an electrical forum. Is there an instrument used to bench test SCRs to verify the actual duty cycle output?

    My boss speculated that they might just test based on the min and max signal and assume the relationship is linear in between. He bought a fairly basic multimeter that claimed it could read duty cycle but seems it cant on our SCRs.

    Also, why on my meter would duty cycle read 50% at 60Hz on the incoming line? Would it only read the positive side of the sine wave?

    Sent from my SAMSUNG-SM-G891A using Tapatalk

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