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General Category => General Discussions => Topic started by: Just on September 17, 2012, 06:51:33 PM

Title: [DIY] How to simulate a motor behavior?
Post by: Just on September 17, 2012, 06:51:33 PM

Hello All,

How to simulate a motor behavior from point of view of current consumption? Is there a way to build a power rheostat, which would dissipate 500W-1000W (the same amount of power as a motor consumes)?

Thank you!

Title: Re: [DIY] How to simulate a motor behavior?
Post by: Bikemad on September 18, 2012, 02:55:37 AM

Dmitryl, do you mean a load connected directly to the battery, or a load being supplied by the controller?

It should be possible to use a 3kW electric fire to load the battery directly, the current will vary according to the ratio between the fire's voltage and the battery's voltage.

3kW/240V = 12.5 Amps

240V/12.5A = 19.2 Ohms

48V x 19.2 Ohms = 2.5 Amps

48V x 2.5A = 120 Watts

So, if my theory is correct, a 240V 3kW resistive load should produce a 120W load on your 48V battery pack.

All you now need is eight electric fires (connected together in parallel) to produce a 960W load.  ???

(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)(http://www.electricshop.com/content/ebiz/electricshop/invt/842/842_m.jpg)


Alan
 

Title: Re: [DIY] How to simulate a motor behavior?
Post by: Lollandster on September 18, 2012, 08:06:21 AM

Just get yourself 10 feet of 16 AWG nichrome wire (it's about 0.26ohm per feet). That will give you P=(u*u)/R=(48v*48v)/(10ft*0.26ohm/ft)=886w. The wire will be red hot and about 700'C so try not to touch it. For less watts use longer wire and vice versa. Don't go shorter than 5 feet or you'll risk burning up the wire. If you want it cooler you'll need to use a longer wire of a lower guage (bigger cross section).
   At about $10 on ebay this would be much cheaper than buying 8 heaters. Still only resistive load though, no counter emf or any of that stuff.

edit/addon: to get desired length use the formula L=U^2/(P*R). To make it variable between 500 and 1000w you vary the length between 17.7ft and 8.9ft. Proof: L=U^2/(P*R)=48v^2/(500w*0.26ohm/ft)=17.7ft and L=U^2/(P*R)=48v^2/(1000w*0.26ohm/ft)=8.9ft. You can coil the wire to make it shorter (or buy one that is already coiled), but the heat calculation will be different.

also: do check with a multimeter that the resistance is the correct one before using it in case you got a short in you coil or the above data is incorrect. A pure resistance like this does in no way simulate a motors behavior, so unless you specify what you need the current consumption for I can't say if this will work for you. For testing a bms or a wattmeter it might work, but not for testing a motor controller.

Title: Re: [DIY] How to simulate a motor behavior?
Post by: Just on September 18, 2012, 05:10:12 PM

Thank you for the formula and explanation - very useful! Now I'm also a  mathematician :)