3 phase VSD metering down to 30 Hz - Page 1
Forum section: electronics / projects

author: IoTguy
author_rank: Contributor
timestamp: April 04, 2017, 11:24:17 pm
content: Hi,A customer has asked me to quote to develop an instrument for measuring various AC parameters (3 phase) on a VSD source driving an induction motor as per the brief specification below. Initially the customer tried to use off the shelf panel meters but they did not work. I believe they didn’t work for 2 reasons:The lower end of the AC frequency for standard panel meters is 45 or 40 Hz at a pushThe output from the VSD has a high THD (chopped to form a sine wave)I don’t believe there is an off the shelf solution to their problem unfortunately.Basic requirements are:Will need to handle high voltage spikes and still give a true RMS reading with a THD of 30%.Nominal phase-phase voltage: 480 Volts +/-10% - 30%; 15-75Hz; THD and voltage transients of 1.4kV.Frequency: 30-60Hz. Needs to have a flat response to 30 Hz but be undamaged by operating down to 15 Hz.Current: 75 -250Amp. 5A secondary. Existing CTs rating 15-60HzPrecision: 1% All values.Auxiliary supply 230V AC 50Hz nominal or 24V DC.Interface: Modbus (RS485 or Ethernet).Transmitted values: P-P Volts; power; frequency.Update time: 1 second.Mounting: DIN rail or panel; IP30Ambient -10 to +40.The required number of units will be small so instrument cost is not a factor. I was hoping I could use a metering chip like the ADE9000 or other chip from the Analog Devices ADE range for the front end (open to suggestions). The datasheets for these metering chips do not seem to mention a lower frequency limit as such but only some of the performance graphs include 30 Hz where the accuracy is degrading, most of the performance graphs are for 40 or 45 Hz minimum. I presume these chips can still privide true RMS even with a THD of 30% since the can measure THD.I would probably isolate the micro and comms with an opto isolator between the metering chip and the micro. The micro side to talk to the metering chip (SPI ot I2C) and present the data over MODBUS is the easy bit for me. The unusual 3 phase power monitoring bit is slightly outside my comfort zone though.Can I use a metering chip at 30 Hz? I can add multi-point calibration tables in firmware to help compensate for the accuracy loss at lower frequencies. But maybe it will not be enough.Is there a better approach?

author: JamesAus
author_rank: Contributor
timestamp: April 05, 2017, 02:03:58 pm
content: Is there any particular reason you can't simply read the data straight out of the drive? I've yet to work with a VSD where this isn't possible.

author: IoTguy
author_rank: Contributor
timestamp: April 05, 2017, 03:00:30 pm
content: Hi James,Thanks for your interest.The customer will be using this instrumentation to test/monitor all parts of the system, including the VFD/VSD themselves.Also the motor here is actually driving a generator which is connected to a controllable load bank (restive load). They want to use this instrument to also measure stuff on the generator output side as well as the feed to the motor. I designed the voltage regulator on the generator for this (regulates by controlling the field coil current), this is when I noticed their panel meters were not working correctly. So they want to measure stuff at various points in the system. As the load is purely restive (and because of the inherent filtering of the M/G set) the signal on the load bank side is much cleaner i.e. pretty much a pure sine wave with unity power factor.The overall aim of the system is to test various drives with representative load conditions.

author: rstofer
author_rank: Super Contributor
timestamp: April 05, 2017, 04:47:20 pm
content: The GE Fanuc PowerTrac block does all of this stuff and a whole lot more but it doesn't run on Modbus or RS485 without something in the middle:http://www.cimtecautomation.com/documents/techsupport/Genius/Manuals/gfk-0450dGeniusPowerTRACBlockUser'sManual.pdfI have used a lot of these blocks on the Genius Bus where I had a network adapter board in a PC and the bus strung all over 26 unit substations spread over 50 acres.I'm sure there are other similar products that probably use Modbus or RS485 but I haven't used them.

author: IoTguy
author_rank: Contributor
timestamp: April 05, 2017, 05:15:35 pm
content: Hi rstofer,This device states:"0 to 120 VAC RMS at 47 to 63 Hz"I need 30 - 75 Hz . I also need 480 Volts +/-30%. There are also many other items that do not meet the required specification.I have looked at a great many power monitoring devices and none seem to support sub 45 Hz operation. I would be delighted to find one.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 05, 2017, 05:21:46 pm
content: Well then,here you go.QuoteThis device actually measures 512 samplesper cycleand is not limited to a fixed sample frequency, as most other meter are.Which is why other meters fail at out-of-spec frequency, since they can't reliably compute RMS anymore.Although I'm not sure if this meets your specs, I've only used the older version. Which is purely a logger/event recorder.You'll be looking at ~6 months to roll you own. They are not easy to develop unless you've done it before.*At work we design voltage regulators for generators for over 80 years now, you did not have to do that yourself*I also can tell you won't find many meters <40 hz. I've been looking for a 10 to 500 Hz panel frequency meter, but have not found one.So I made one myself. Only frequency is not that complicated.

author: IoTguy
author_rank: Contributor
timestamp: April 05, 2017, 06:40:46 pm
content: Hi Jeroen3,The Pcube3 does look to meet all requirements on first glance. Looks a very impressive and polished product. I've requested pricing. You might have caused me to miss out on 6+ months of work here ;-).Many thanks!