AC "load" simulation device with variable power factor - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: March 16, 2016, 08:31:36 pm
content: In order to demonstrate the various principles behind Power Factor i'd like to build a simulated AC "Load", which can have it's power factor adjusted. However, to keep it small and simple, i want the load to be essentially fake, and the voltage signal and the current signal to be separated. That way by adjusting the phase delay for the current signal i can vary the apparent power factor from 1(resistive) to 0 (capacitive).First thoughts are to use a simple transformer to step up 240Vrms 1ph 50 hz mains to say a max of 1000vdc, and then use an attentuated version of that signal to drive a current source, say 10A, but introduce a controllable delay into the current signal.This demo device would then be connected to one of my Power Analysers, and adjusting the phase delay would change the calculated power factor.By keeping the current source separate to the voltage source the actual power consumed can be very low, as the Voltage source will be into a high impedance load (>5MOhm) and the current source into a low impedance shunt (<1Ohm)Anyone got any nice ways to do this or suggestions as to a suitable circuit architecture?

author: mjkuwp
author_rank: Unknown
timestamp: March 17, 2016, 11:05:19 am
content: how about a function generator with two outputs?

author: max_torque
author_rank: Super Contributor
timestamp: March 17, 2016, 12:31:38 pm
content: Problem is i need to use the actual sensing devices the Power Analyser uses so i can calibrate the phase delay between those two devices. That means a LEM current transducer and a HV differential voltage probe. I can fool the LEM into reading a higher rating by winding multiple turns of the current loop around the transducer, but i need at least 500V for the voltage probe.I was wondering about using a transformer to take mains 50Hz 240Vrms, step it up say 2x, use that for the HV prove feed, then rerad an attenuated version of that voltage into a micro, that drives a DAC, with a controllable delay time, that controls the separate current loop. That would also enable me to simulate different load impedance as i could adjust the DAC output for any current vs voltage profile etc?

author: focovario
author_rank: Contributor
timestamp: March 17, 2016, 04:53:58 pm
content: max_torque,You could implement mjkuwp idea adding a pair of audio amplifiers, one amplifying the "volt" signal and the other the "current" signal. Naturally you will need one resistive load for each amp, but the power meters will not have any way to know that(Careful with the grounds!). A clamp on current probe might be the ticket here,You can do all kinds of crazy stuff by playing with the amplitude and phase between the signals, independent control of waveshapes, like sine volt signal and square current signal, etc.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 17, 2016, 06:19:11 pm
content: Simulating a capacitive of or inductive load is difficult, as there is momentarily power flowing out of the load. So you might need an active supply (e.g. 4 quadrant amplifier). It might be easier to have some kind of power capacitor decade, thus switching real capacitors instead of a simulated part. Just for testing the real load part might be a more or less normal electronic load behind a rectifier - more like a DAC giving a load pattern with a µC locked to line frequency. So not exactly an easy project.

author: woodchips
author_rank: Frequent Contributor
timestamp: March 18, 2016, 12:05:14 pm
content: Why not just use a transformer with the secondary having resistive, capacitive or inductive loads? A transformer will reflect the secondary load phase angle to the primary.

author: max_torque
author_rank: Super Contributor
timestamp: March 18, 2016, 08:47:45 pm
content: I want to simulate +-1000V and +- 500A! That's a big transformer. By separating the voltage and current signals i can drive the high voltage into a high impedance, and the high current into a low impedance, hence total power used will be tiny in comparison!If i drove a transformer with a typical hi-fi amp, that probably has say 50 to 100vdc rails, how would i go about working out the transformer spec to turn that into say +-1000v into say 100kohm?

author: focovario
author_rank: Contributor
timestamp: March 19, 2016, 01:01:12 am
content: Here is a proof of principle for the voltage part. I used 50 Hz sine from an AF generator, but it can be any frequency within the transformer/ amp capabilities. I used an old output transformer from a tube amplifier with an approx 30:1 turns ratio. I did not want to go above 600 V for safety reasons, but I could have gone higher. Amplifier is a custom built chip amp with BNC and banana connectors.Vin=19.60 V, Vout = 599.6 V.

author: albert22
author_rank: Regular Contributor
timestamp: March 19, 2016, 05:10:31 pm
content: What is the current that you need ? 10A or 500A ? 10A is rather easy you need just a low voltage transformer capable of circulating the required amps into the shunt. At 1ohm that is only 100W.For the voltage signal a high voltage low current transformer.The simplest way to implement the simulator that you want is to connect one of the transformers directly to mains (the biggest) and the other one to the same mains but through a simple analog variable phase delay network.The only problem with this is that the phase delay network needs to operate at mains voltage But that can be avoided by utilizing two transformers back to back.Voltage and current signals can be varied using variacs at the primary of the transformers.

author: Marco
author_rank: Super Contributor
timestamp: March 19, 2016, 05:29:22 pm
content: Quote from: max_torque on March 18, 2016, 08:47:45 pm100 kW of (apparent) power? (500A peak at UK mains voltage.) I think you need a paid consultant or photonicinduction and a lot of space for explosions.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 19, 2016, 06:27:00 pm
content: Maybe two microwave oven transformers driven off a large audio amp? Leave one unmodified for the voltage and replace the secondary on the other one with a single turn of copper tubing or bar that extends out so you can put the current sensor on it. Something smaller like a bug zapper transformer can also work for the voltage channel.

author: max_torque
author_rank: Super Contributor
timestamp: March 19, 2016, 06:49:14 pm
content: Quote from: Marco on March 19, 2016, 05:29:22 pmThe key word being simulate! I have to provide a +-1000V voltage waveform, but i can cheat on the current sensor, by winding multiple turns around the CT, so with 10 turns, just 10A becomes "100A"! And by using a very low current loop impedance i can do 10A at very low power

author: Marco
author_rank: Super Contributor
timestamp: March 19, 2016, 07:37:47 pm
content: Ah okay, well since you can simply put a shunt to ground below the coils carrying the current, the current source is trivial. Just need a high current closed loop amplifier, the shunt signal going to the inverting input (likethis). Use two all pass phase shifters with ganged potentiometers for the phase shift?

author: max_torque
author_rank: Super Contributor
timestamp: March 20, 2016, 02:23:16 pm
content: If i could wind a pair of custom transformers i could use a cheap s/h hi-fi amp to drive both voltage and current inputs!One transformer would be say a 1:10 voltage stepup, to turn the amps +-100V rails into +-1000V. The other transformer say a 10:1 step down tp turn say 10A at 100V into 100A at 10V??If i wanted them to work between say 50 and 500Hz, how would i go about designing / sourcing those transformers??

author: SeanB
author_rank: Super Contributor
timestamp: March 20, 2016, 02:33:26 pm
content: In the scrapyard last year there was exactly what you wanted, but I was not going to pay for the 30kg tabletop system, even though it had a few variacs and some large capacitors in it to provide the phase shifting and lower voltage using the mains, and a small induction motor as a demo load.

author: Marco
author_rank: Super Contributor
timestamp: March 20, 2016, 05:53:20 pm
content: The MOT works for the high voltage, bit overpowered though. You could put the 220V windings of some low power hipot mains transformers in series and the low voltage windings in parallelWhy stop at 10 windings to simulate the high current for your current clamp? (I assume you are testing devices with current clamps.) Try looping it through a 100 times and see what error it causes.