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  1. #1
    Join Date
    Feb 2007
    Location
    Central NJ
    Posts
    510

    Testing Hz with test leads or clamp?

    for a while now i've been looking to buy a new stand alone amp meter to replace my malfuctioning(i think?) fieldpiece clamp for the hs36 true rms. found a nice little one that will check both ac and dc amps. then i noticed that you can check Hz with it as well. due to my own nietivity i was unaware you could test Hz with a clamp(i was working in the industial field up until a month and a half ago, doing mostly hydronic work. now i'm going back to residential/light commercial, so i didn't get to see alot of variable speed fans in the field).

    my question is- which is the best way to check Hz on a variable speed fans, with a clamp or test leads from a true-rms dmm? what are your methods for checking the drives (from what i remember GE makes all the motors for a majority of the major manufacturers on the market, so i assume they all have the same plug/pin set ups) for speed issues or malfunctions? the outfit i'll be working for is a Bryant distributor.

    thanks alot, always get good info from everyone on this site, very grateful for all the knowledge!

    and GO DEVILS!
    You have to pay your due's before you pay the rent!

  2. #2
    Join Date
    Apr 2009
    Location
    Southeast Texas
    Posts
    2
    Quote Originally Posted by n-e-w Jerz! View Post
    found a nice little one that will check both ac and dc amps. then i noticed that you can check Hz with it as well. due to my own nietivity i was unaware you could test Hz with a clamp
    Yeah, pretty nifty! It's counting the number of times per second that the voltage reverses polarity (for voltage-sensing instruments) or that the current flow reverses direction (for current-sensing instruments). Take that number, divide it by 2, and display it on the meter's readout as "Hertz". So any meter that senses voltage or current could actually have the Hertz readout feature if the manufacturer wanted to put it in the meter's budget.

    Quote Originally Posted by n-e-w Jerz! View Post
    my question is- which is the best way to check Hz on a variable speed fans, with a clamp or test leads from a true-rms dmm? what are your methods for checking the drives
    Either a clamp-on or a traditional test lead meter should give accurate results measuring Hertz in power circuits. In many situations, the clamp-on probably has an advantage in speed-of-setup and safety, since you're not making a direct electrical connection with the power circuits as you would with traditional test lead meters. This is only my opinion, but I use test leads on power circuits only when circumstances won't let me use a clamp-on, and it doesn't happen often.

    Variable speed drives have a couple of issues that you may want to look at when comparing troubleshooting a single speed motor controlled by a full-voltage starter:

    1. Hertz measurements will be accurate since they basically consist of counting zero-crossings, however, current and voltage measurements are suspect until you know the details of the drive: there are a lot of drives that do not have a sinusoidal output, and so an RMS-reading meter will not be very accurate on current and voltage measurements.

    2. All early variable speed drives and also the inexpensive ones available today were "Volts per Hertz" drives. The voltage output of the drive and the frequency (Hertz) both changed proportionally to the setpoint: In the U.S., with the ordinary situation of 100% speed at 60 Hertz, and using a 480 volt motor as an example, if the drive's setpoint was 40% you would see an output from the drive of (0.40 * 480) = 192 Volts, at a frequency of (0.40 * 60) = 24 Hz. More or less: there are usually drive parameters that can be changed to alter the output a bit, but the point is that it's pretty much proportional. Now, here's the snag: modern variable speed drives use a number of different schemes to increase performance. An example of this is the "Vector Drive"; almost all drive manufacturers are producing examples of these drives. So you really need to know the particular kind of drive that you're working on before you can make more than the basic assumptions about troubleshooting.

    The good news is the basic troubleshooting stuff still works!

    1. If you forget about absolute accuracy and just compare the 3 phases going to the motor, all 3 should look pretty much the same.
    2. The motor itself can be eliminated as the source of the problem with 99% accuracy by doing two tests with the drive locked out: A) Does the motor spin freely by hand? and B) does the motor pass a complete megger test, with the megger attached to the drive-end of the cables?

    Hope this helps.
    Dave

  3. #3
    Join Date
    Nov 2008
    Posts
    1,673
    My meter's Hz function has a 50 kHz bandwidth so it responds to noise signals. If you have the same problem they might make a low pass filter as an accessory to fix this.

  4. #4
    Join Date
    Feb 2007
    Location
    Central NJ
    Posts
    510
    thanks guys, dave, great info i truly appreciate the information. welcome to the site.

    i haven't ordered the clamp yet but i did order GE's techmate pro with the adapter to check the newer 3.0 model motors and can't wait to try it. should speed up the diagnosis time for servicing faulty motors , at the very least.....
    You have to pay your due's before you pay the rent!

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