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Thread: VSD power tests
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25th March 2014, 11:07 PM #61
Good question, I think you should load it until it stalls, at each voltage. Joe has some shoes you can borrow...
What will happen is that as the load increases the slip increases and hence the current will increase. So the torque will stay pretty much the same, but the horsepower will decrease. ( because the rpm decreases with frequency)
When you get the half frequency the voltage will be around half and I think the horsepower will be about half, but the torque will be the same.. ( that's the prediction anyway..)
Ray
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26th March 2014, 01:07 AM #62
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lol
So you want power v rpm at each Hz setting? I guess thats unlikely to hurt anything on the 240V VSD.
Will have to see how well the "dyno" behaves, getting accurate numbers below maximum might be a little tricky.
Then again there might be so little power, who knows.
I might try test at spec rpm on the 415V VSD just to get a baseline.
Stuart
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26th March 2014, 09:39 AM #63
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26th March 2014, 11:27 PM #64
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Wow,,,,,,,,havent been out into my shed for a few weeks...... dam its a mess!!!!!!
Only got time for two roughish numbers today before my dyno melted. Who would have thought you cant use Telsta rope as a brake
Its not looking good 700g at 240V, 3.5kg at 415V. Thats 20%.
Stuart
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27th March 2014, 02:49 PM #65
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Dam!
I'm sure I read somewhere that telstra rope is good for everything!
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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27th March 2014, 03:51 PM #66
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Well I saw a car on a tandem with one piece of telstra rope around the back bupper I figured it would be right
Really I knew it wouldn't last on 1hp, but the leather I was using was working to well. I changed to the rope as a test to increase the "weight needed". The leather also "self energized" which didnt do much for the ease of measurement.
Stuart
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27th March 2014, 04:14 PM #67
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The problem with leather or nylon is that they are good insulators so the heat won't move away from the friction zone all that well and local temps will build up rapidly.
Maybe a spring steel strap and an ally disc would be be better?
Plus a fan to cool the disc?
I'm getting more and more tempted to make something as well.
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27th March 2014, 07:43 PM #68
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Sounds like a small prony brake is the go.
Phil
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27th March 2014, 08:20 PM #69
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Phil, you know the rules...
Michael
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27th March 2014, 08:54 PM #70
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Hi Bob,
The leather has done ok just is works to well... if I cut it to half the width it might work ok but then get to hot. I have some steel strip but that might need to much weight ........ and I I doubt the noise will be all that pretty. I'll have a look around the shed and see what I can come up with. Maybe I should have built a real brake.
Hi Phil
That's pretty much what it is.
Stuart
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27th March 2014, 10:57 PM #71
Go to the wreckers and find an old rear drum brake. I know at least the holdens used the main shoes for the hand brake. Then you have a cable activated brake that will never fail...
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27th March 2014, 11:31 PM #72
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yes. That may have to be the next model......thought how well a car brake will work at these loads I dont know. I guess I could just keep cutting away pieces of shoe until it was working badly enough to be useful. Remember we are only replacing a shoe
It also makes it a bigger build. but. if it has to be it has to be.
I could likely get my hands on brake shoe lining but havent bothered as yet for the same reasons....I only need something that can live with 750W for a few minutes at a time.
Stuart
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28th March 2014, 06:29 AM #73
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oops,
Hi Michael,
I thought I took a heap of pics just recently but can't find them.
I do have some though.
The last one is a scan of the exact same engine with the prony set up.
Phil
Davey Paxman (26).jpg Davey Paxman (25).jpgScan 3.jpg
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28th March 2014, 06:41 AM #74
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Thanks Phil. Always interesting to see how someone else does things. The water bath is a nice touch - I wouldn't have thought of including one but to get steady state conditions it's probably necessary.
Stuart, can you manage a waterbath or even just a pond pump running water over the brake wheel?
Michael
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28th March 2014, 09:47 AM #75
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Stu, some ideas for a diy dynamometer. How about you buy a DC motor next time one comes up cheaply on ebay. Then make a variable load for it. You measure current and voltage and know exactly how much energy you are dissipating. A variable can be made with a large power transistor and a heatsink. Another idea would be to use a car or truck alternator, feed its output into a fixed load like a bank of 12V lamps, and use an adjustable power supply to feed the rotor excitation in order to adjust the power taken up. Again by measuring voltage and current going into the load you know the power taken up.
Another idea to make a cheap high power variable load for a dyno generator was used long time ago to regulate speed on early Italian electric locomotives: a tank filled with saltwater with a plunger electrode, by adjusting how deep the electrode goes into the bath you can adjust the load. That is probably the simplest and cheapest high power electric load that can be built.
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