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Thread: Measuring AC current
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12th July 2018, 05:27 PM #1SENIOR MEMBER
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Measuring AC current
So i was tidying up the shed a bit and came across some current sensors. When i was still in South Africa i made and sold some products. One popular one was a submersible pump controller, i had a single phase unit that actually automatically switched the capacitors and "sensed" when the pump was running dry. While sorting the measurement of ac current i found that some single phase motors "distorted" the current waveform....this caused some accuracy issues in measurement. The method i used to measure was good enough to protect the motor...however i struggled to get it spot on with some motors. Now with a bit more experience i am going back to the design to hopefully make improvements. Reason for doing this is a true RMS current meter in some applications would be good. Also i have found many single phase ac motors fail because they depend mostly on the supply breaker for protection, the overcurrent device on some motors are not very good at protecting the motor. That was the reason i initially designed and made the pump controllers. Also it worked very well when i switched the start capacitors electronically. Added advantage is using the micro i can add some intelligence....
Possibly this or a version of this may be handy for a machine
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12th July 2018, 06:34 PM #2.
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I agree unless an often rather expensive current regulated contactor is used that SP motors are generally not well protected. My thickneser has one and my Bandsaw and DC came with such protection before I switched these to a 3P motor and VFD. My TS only has a thermal cut out switch on it which would not be fast enough as are the line breakers in some circumstances. All my other machines are 3P.
Yes, using a true RMS meter is important when measuring AC current and V. This becomes even more necessary when using 3P motors and VFDs since the VFD/motor combo really can mess up the wave form and the wide range of frequencies just ad to the problems. This had me stumped for quite a while. I even posted something on this back in 2014.
VFD current waveforms and values
A few years back I bought a Fluke current clamp and that has been invaluable and sorted out most problems
The problems of motor protection and use of (increasingly failing) motor caps is just another reason I switched to using 3P motors with VFDs. Provided they are correctly programmed VFDs are brilliant at protecting motors and themselves. VFDs do make extensive use of Caps so they do need to be able to protect themselves and you have to be a real klutz to mess it up - I have done it once on a used VFD although it might have already been on its way out.
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