Corona, me and the test bench

I told you already - or at least hinted - that I started a new “career” in light machine building. A little heavier than making clocks but nothing above what our 400 V three-phase 16 A fuses can handle.

I always wanted a motor test bench. There are few around. From schools, mostly and with mechanical scales, like the ones you found at the butcher’s shop. And, of course, with DC motors and in some cases with an electric brake, even.

Big, heavy and no way you can have a look at unbalanced (negative sequence) grid, starting transients, EDM and many other things where myths more than facts seem to prevail. The efficieny of the European IE3 and IE4 motors are also interesting to have a deeper look into. Not to mention EDM (bearing electric erosion) oils film break-through at different speeds and temperatures. And probably a lot of other things that I haven’t tought much about and will pop up once I get started.

Table top.
Low cost - use scrap.

Machining limits are:
Band saw up to around 4 or 5 inches (100 - 125 mm) steel and sigma c:a 70 um. Nice little Chinese guy.
Lathe with milling attachment and possible to run pieces up to around 100 mm diameter and 250 mm long. Works best on bronze and alu but can handle steel if you are careful and have hours to spend.
A little DC welder that I am a bit afraid of. Done some “Scrap Art” but nothing serious yet.

Project Status:
Just started. Finished a sturdy frame (buiilt with Al extrusions from a baldachin(?) that isn¨t needed any more, Some 20x20 mm square tube and some “self-made” nuts, screws and bolts.
Speed measurement. No Probs - done that most of my life - anything will work. From DC tachs via encoders and reciprocal measurements with simple pic-up.
Torque measurement. Pressductor/Torductor (ABB) but on a smaller scale, Strain Gauge perhaps but with a twist. One of the Amazon pressure transducers may also work - but not sure if I can get one working well in this application. This part is the real challenge. So far…
Time-wise, I am not in a hurry. The only guy getting impatiant is me, myself and Skogsgurra.
Unbalanced grid Will use either a Variac or a Squariac for one phase.
Harmonics and interharmonics A phase-locked power amplifier with suitable software and a 24 V secondary winding for up to around 10% THD. Need three amplifiers for that - could be costly - will scavenge the nearby scrap yard for som boomers (no, not that)
Power measurement Standard procedure - will use what I have available - and that is a lot.
Other ‘lectric’ measurements Also standard procedure.

Comments and suggestions are welcome (as Bob Pease used to say).

Stay safe!

1 Like

The motor test bench sounds interesting, Gunnar. As you progress, pictures please!

Good for you, Gunnar. It’s amazing how handy a small lathe and milling machine can be for an engineer. FWIW, I have my own youtube channel, and have posted some videos of small model engines running that I built at home.

DO NOT attempt to mount strain gages yourself. There are too many ways to screw them up or make them lie, or more commonly, drift in ways that are difficult to detect. Better to buy commercial load cells, or measure forces in more primitive ways, e.g. with die springs and mechanical indicators.

Thanks to all!

Smoked - yes doesn’t it look weird? Definitely so. But it needs very little place and it is STURDY. It didn’t cost me much (very little, actually). Looks aren’t everything. But I think that I will start with a steel brush and some paint. Next year, perhaps.

Ben - a link could be useful. I could probably find some way, but a link is easier.

Mike - very true. But I have some experience with HBM, even ran a full day instruction on their load cell elements, the preparation, glueing, connection and calibration. Done some installations and also some trouble-shooting on other’s installations.
The mechanical alternative is not of much use when you want to record on a scope what happens in sub-ms time scale.
Also worked with Orvar Dale’s torductor and it can be made faster if you raise the carrier frequency. But the ferrous material starts to behave bad already at around 1000 Hz, so I am not so sure if I am going that road.
As I mentioned, I will probably use a load cell “with a twist”. Like the original Drahtdehnungsmessgeber (the precursor of the glued tape sensor). That will by-pass all the problems, be cheap (piano wire on the shelf) and stable. Carrier frequency around 10 kHz and an AC differential amplifier will be my best option.
May also make myself an LVDT amplifier and measure with a contraption looking like a spring scale but with the necessary output signal for the scope/recorder. The “spring scale” will be stiff and can use 100 um full scale. Or less, but I see no need for a stiffer spring.

I am just beginning this and I am not in a hurry. So, we will see how it develops. I used to work with companies where we developed special measurement equipment and I did the electronics then. More fun to also test my budding machining skills.

Except for the frame, I started on couplings. TaperLock is too much and I have done some tests with Delrin, it MAY work, but will probably turn to bronze. The tapered threads to lock the couplings to the shafts is beyond my capabilities and looking for something simpler that doesn’t need a press fit. Try to avoid locking screws and such.

Pictures? I have some, but they really look like “Hey! Come and help me!” I may post them - but only if you promise to be kind to the budding machinist-wannabe.

Gunnar, see:

Hunh, somehow I’ve gotten 10 subscribers…gee, I guess I’m going viral? Ha!

1 Like