How can one measure the speed of a 3-phase generator electrically? - Page 1
Forum section: electronics / general

author: shamooooot
author_rank: Regular Contributor
timestamp: April 18, 2023, 09:25:49 am
content: Hello guysI am looking for some help to measure the RPM of an (AC permanent magnet synchronous generator) electrically through one of the output phases.. I was suggested this would be a better method than going through the hassle of mounting a sensor or encoder..Thank you

author: gf
author_rank: Super Contributor
timestamp: April 18, 2023, 09:34:51 am
content: Just measure the frequency of the generated AC voltage, it is proportional to RPM.

author: shamooooot
author_rank: Regular Contributor
timestamp: April 18, 2023, 09:45:15 am
content: Quote from: gf on April 18, 2023, 09:34:51 amDo I probe between two of the phases? can you please share with me the equation for this?

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 18, 2023, 12:21:56 pm
content: If you sense zero crossings on all 3 phases, you'll get faster response to changes since it would triple the frequency you're measuring.

author: jonpaul
author_rank: Super Contributor
timestamp: April 18, 2023, 01:40:51 pm
content: Bonjour à tousSimplest solution make an H field probe, ( magnetic field) zero cost:Any coax, with a BNC con, coax other end to 2..9 cm dia. wire loop, 10 turns any insulated wire.BNC to a scope or frequency counterapproach the motor or gen, position loop so one phase dominantes, thus a single phase sine wave, to get the frequencyRPM = 60X frequencyJon

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 18, 2023, 01:56:32 pm
content: A small correction to the prior answer. Frequency output (can be measured between any phase pair) is proportional to rpm. The proportionality constant will be an integer that depends on the pole configuration of the generator. 60 is most common but 120, 180 and other values are occasionally encountered. You can perhaps find a document describing yours, but the easiest method is to make a one time physical measurement of the rpm.

author: shamooooot
author_rank: Regular Contributor
timestamp: April 18, 2023, 03:08:46 pm
content: Thank you all for the valuable help..As I need to measure the RPM permanently and continuously, and log it through a DAQ (NI USB-6009).I think the best way is to probe the frequency between two phases through a frequency meter that can also output (0-10v/4-20mA) signal.. and find the proportionality constant to get the final RPM.Something like thishttps://www.amazon.com/Taidacent-Digital-Display-Frequency-Current/dp/B098S17Y9R?th=1

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 18, 2023, 04:34:52 pm
content: The Amazon meter you linked to is an analog meter that does not measure frequency, it accepts an analog signal and displays it as a frequency. It’s not measuring frequency, just voltage or current.Frequency only needs one phase, measuring two typically is used to provide the direction of rotation.You measure frequency by measuring time. You use a zero crossing detector on one of the generator phases and measure the time between the zero crossing events. The reciprocal of that time is the frequency. Your DAQ should measure the time between zero crossing pulses or gate (count) a clock signal that provides the necessary resolution.You can calculate the output frequency in a synchronous three phase system from:Rpm * Np / 120 = Output FrequencyWhereRpm = input rpm to generatorNp = number of poles in generator (usually 2 or 4)Typical 60 Hz values are 1800 rpm input to a four pole generator, 3600 rpm into a two pole machine.

author: mendip_discovery
author_rank: Super Contributor
timestamp: April 18, 2023, 04:46:38 pm
content: You can get some DIN rail stuff that could measure the voltage, current and the frequency. This would be worth logging as you are at it.For cheaper prices you could just use a clamp meter or a clamp probe and measure it that way.

author: shamooooot
author_rank: Regular Contributor
timestamp: April 19, 2023, 07:22:50 am
content: Can you please share with me a good option for this, I still need to use my NI data acquisition module. Would be nice if I find a way to get these values and transfer it to 0-10V signal for the DAQ.

author: shamooooot
author_rank: Regular Contributor
timestamp: September 25, 2023, 08:02:58 am
content: Unfortunately it didn't work..I tried with couple of frequency counters and they didn't pick the phase to phase frequency, which I think is due to the low AC voltage (0-12V)..I stepped up the voltage 30 times using a transformer but they again struggled to detect the frequency, they get stuck at a value and never recover until I turn them off and on again..I can't use any kind of sensors or modifications due to some limitations I have..The generator is Three-phase AC permanent magnet synchronous generator.Any ideas for a reliable circuit that I can use?I have a USB-6009 device.

author: soldar
author_rank: Super Contributor
timestamp: September 25, 2023, 09:09:29 am
content: Vaguely defined questions get generic answers which may help someone who is knowledgeable enough.If you want more detailed help I would suggest you spend some effort in specifying the question in much greater detail.This is an extremely simple problem for someone with basic knowledge. Is this work-related? Because you seem to be out of your depth.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 25, 2023, 09:55:09 am
content: For a <1 kHz generator, 48 kS/s is more then enough to extract the zero crossings from an analog input by software using interpolation.The algorithm is very simple. Find the two pairs of samples where the sign changes. Use the buffers sample numbers and store them.Interpolate the exact position of the crossing as percentage of the time between samples, store it.Convert the stored numbers to a sample count difference, and then to time using the samplerate and the interpolated percentage.Use the time between two interpolated crossings to determine the frequency. You may need to average the output for additional resolution.Accuracy depends on the samplerate and it's jitter.I have no clue how you do this with a NI DAQ, but if it can output a stable stream of samples this should be trivial in any programming language.

author: EPAIII
author_rank: Super Contributor
timestamp: September 25, 2023, 10:20:07 am
content: Some DMMs (Digital Multi Meters) have both a frequency function and the ability to send an output to a computer, usually via a USB port. The advantage of using such a meter would be that it is already set up to accept different input Voltage levels.https://www.google.com/search?q=DMM+with+frequency+scale+and+USB+output&client=firefox-b-1-d&sca_esv=568147601&sxsrf=AM9HkKlUxkl8yKY52V00sJnvWyihrw4j5A%3A1695637124472&ei=hF4RZbq5HOGrqtsPqN2Z0Ao&ved=0ahUKEwi6i-PDxMWBAxXhlWoFHahuBqoQ4dUDCA8&oq=DMM+with+frequency+scale+and+USB+output&gs_lp=Egxnd3Mtd2l6LXNlcnAiJ0RNTSB3aXRoIGZyZXF1ZW5jeSBzY2FsZSBhbmQgVVNCIG91dHB1dDIFECEYoAEyBRAhGKABSJo8UP8MWNEccAF4AJABAJgBjAqgAfYkqgEHNS0yLjAuM7gBDMgBAPgBAcICCxAAGIoFGIYDGLADwgIFECEYqwLiAwQYASBBiAYBkAYD&sclient=gws-wiz-serp