AC Load testing

[drkntz]

What type of setup do you use to test out systems providing a few A of AC power? For example, I'm working on a device that operates at 120VAC & up to 2A, and I'm testing it out with a few incandescent light bulbs drawing (total) 4+ amps RMS. For larger currents I use a household heater.

Is this really how it should be done? Feels hacky...

[NiHaoMike]

If you regularly need it to run for a long time, you can use a pair of induction motors and two slightly different size pulleys or gears. The motor that gets driven faster becomes a generator which feeds power back into the grid.

There are ready made electronic loads which basically do the same but they pretty much have to be used for hours daily to have the slightest chance of paying for themselves. (Or more likely, the value is in the capability to script automated testing with them.)

[Jeroen3]

What do you want to test?

[Hawaka]

AC load test can have a lot of form and features.  IMO the two most important points is what is the expected load of your product and your budget.

The load type is very important. Testing an inverter with a resistive load is not very interesting as the stability of the regulator will not be under stress. A better test would be with a phase cut dimmer or something that doesn't have a fix impedance during the period of your AC signal.

Your budget will tell you if your load will be a simple bulb or a regenerative programmable power supply/load.

[drkntz]

The test I was using this on was a shunt current monitor. The shunt's RMS rating is 4.5A and it's fused at 2, but the customer had some burn out... Turns out the issue was a transient problem (short circuit current) and not a progressive overload, like I was testing. Unfortunately, I'm not sure we'd have a very large budget for test rigs like this. I'm thinking about making a somewhat nicer version of the light bulb test, with a bank of light bulb sockets and a set of switches to add/remove bulbs. Not great for testing motor drives though.

Mostly, I was thinking my test setup was hokey and not very "clean": a bunch of lightbulbs screwed into sockets plugged into power strips which were wired to the board.

[NiHaoMike]

You can use a toroid transformer with a few turns of wire added to it as a current multiplier, then drive it with an audio amplifier. Now you'll be able to use Audacity or similar to generate custom inrush waveforms.

[mahyarJbr]

It's important to ensure safe and accurate testing when working with systems that provide AC power. While using incandescent light bulbs and household heaters can work as makeshift load testers, there are more controlled and professional ways to test devices that operate at higher currents. Here's a safer and more accurate setup you could consider:

1. Electronic Load:
Consider using an electronic load specifically designed for testing electronic devices. Electronic loads allow you to simulate various load conditions and precisely control current and voltage. They come in various power ratings and are suitable for testing a wide range of devices.

2. Variable AC Power Supply:
Invest in a variable AC power supply that can provide the required voltage and current. These power supplies allow you to control the output voltage and frequency, which can be particularly useful for testing devices designed for different regions or frequencies.

3. AC Power Analyzer:
Use an AC power analyzer to measure voltage, current, power factor, and other electrical parameters accurately. This ensures that you have a clear understanding of how your device is performing under different load conditions.

4. Safety Precautions:
Always prioritize safety. Make sure you are working within a proper electrical enclosure or workspace, and use appropriate personal protective equipment (PPE) when dealing with high voltages and currents. Use proper fusing and circuit protection to prevent overloads and short circuits.

5. Calibration:
Regularly calibrate your testing equipment to ensure accurate measurements. Inaccurate measurements can lead to incorrect conclusions about the performance of your device.

6. Documentation:
Keep detailed records of your testing procedures, including the settings of your testing equipment and the results obtained. This documentation can be helpful for troubleshooting and future reference.

7. Seek Professional Help:
If you're dealing with high currents and are unsure about the setup or safety precautions, it's always a good idea to consult with or hire a professional electrician or engineer who specializes in testing and measurement.

[pdenisowski]

I'm thinking about making a somewhat nicer version of the light bulb test, with a bank of light bulb sockets and a set of switches to add/remove bulbs. Not great for testing motor drives though.

Light bulbs are easy and cheap AC loads but be aware that incandescent and LED bulbs draw current in very different ways, with substantially different power factors and current harmonics.  Attached is a slide from a video that'll be posted to the Rohde & Schwarz YouTube channel in the next week or two.

I also made a "fun" video showing how a Keurig coffee maker operates as both a non-linear and linear load during normal operation

https://www.linkedin.com/posts/rohde-%26-schwarz_keurig-meets-the-rs-hmc8015-activity-7095809042442280960-xavF

[drkntz]

I'm thinking about making a somewhat nicer version of the light bulb test, with a bank of light bulb sockets and a set of switches to add/remove bulbs. Not great for testing motor drives though.

Light bulbs are easy and cheap AC loads but be aware that incandescent and LED bulbs draw current in very different ways, with substantially different power factors and current harmonics.  Attached is a slide from a video that'll be posted to the Rohde & Schwarz YouTube channel in the next week or two.

I also made a "fun" video showing how a Keurig coffee maker operates as both a non-linear and linear load during normal operation

https://www.linkedin.com/posts/rohde-%26-schwarz_keurig-meets-the-rs-hmc8015-activity-7095809042442280960-xavF

Hey Paul, actually just saw your video a few days ago, shared to me from Kory Stone. Anyhow, I am aware of the distorted current waveform of an LED (and CFL for that matter) bulbs. Since you're in the T&M world, I wonder what kind of approach you guys would use. I've done a couple PCBs involving a micro driving a triac and some sort of current monitoring, and usually end up testing with a couple light bulbs or an AC motor to get a fuzzy idea of "works" or "not works."

[pdenisowski]

Since you're in the T&M world, I wonder what kind of approach you guys would use. I've done a couple PCBs involving a micro driving a triac and some sort of current monitoring, and usually end up testing with a couple light bulbs or an AC motor to get a fuzzy idea of "works" or "not works."

To paraphrase Tolstoy*:  all resistive loads are alike, each reactive load is reactive in its own way.  

It might be possible to use a programmable electronic load to draw current (in CC mode) as per a user-define profile, e.g. draw 1 amp for 20ms, then .5 amp for 80 ms, etc. but I haven't personally tested that (yet). 

I think the potential issues here would be switching speed (how fast can the electronic load change the amount of current it's consuming) and the "transient response" (when it switches to a different current level, does the current change as a step-like function or is it smoother, is their "ringing", etc. etc.).

It's a really good question, but one I probably need to spend a little more time thinking about.


* "All happy families are alike; each unhappy family is unhappy in its own way." -- the opening line in Anna Karenina