Re: From The Ground Up (Advice Thread)
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I have four 18v drills which need to be powered by 18v batteries, but I can't find any at the moment nimh wise.
http://cgi.ebay.co.uk/9-6V-3700mAh-NiMH ... 33572d80ba
Two packs of these connected in series will give you 19.2V and a decent running time.
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I need an led + resister, power switch and wiring.
Technobots for the LED. They do 12V and 24V ones that contain internal resistors, so you can just connect it directly to your battery supply. If using the packs above at 19.2V, get a 24V LED. They also do a selection of sizes of silicone wire that you pay for by the metre. Relatively cheap too.
For a power switch, just use a pair of Deans connectors and a small loop of wire. GiantCod has 10 pairs for £4, which is good value:
http://www.giantcod.co.uk/deans-style-u ... 03931.html
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But I don't understand speed controllers, I mean I know how they work, I just don't understand what the max ampage etc. is talking about. Is that the maximum drain from the motor on the battery? Or is that the maximum ampage it'll let the motor drain?
Without delving too much into physics, the equation for Power is Voltage x Current (P=IV). When you apply a voltage to a motor, the motor draws current (amps) from the source (aka a battery). This combination of volts and amps gives you power, which is visible to the eye in the form of the motor spinning. If you know the power rating of the motor, you can work out how much current it will draw when you apply a specific voltage to it. Rearranging P=IV to calculate current gives I=P/V.
Taking a Bosch 35 motor as an example; the motor's maximum power output is 35W and you want to run it at 12V. The current (I) is power divided by voltage;
So 35/12 = 2.91
This means your motor will be drawing 2.91A when running at 12V.
However, this is only when the motor is at its most efficient, i.e. it is spinning at a constant speed without any load being exerted on it. If you attach a wheel to the spinning output shaft and then create resistance by, say, pressing the wheel against a surface, you are applying a greater load to the motor, and it has to draw more current in order to maintain the power output. So when your motors are in your robot, they will draw more current when you try to push another robot than they will if you are driving along unimpeded in a straight line. Your motors will also draw more current when accelerating from a standstill and when changing direction rapidly.
Often during these instances, the current draw can spike up quite high for short periods of time. A figure of 4x the normal current draw is a good estimate of what the motors can be drawing. So if your drill motor draws 10A under normal conditions, then it could be seeing as much as 40A when rapidly changing the motor from forwards to reverse.
So when it comes to choosing a speed controller, you need to pick one that will be able to handle the amount of current being drawn from the battery by your motors. I like to use 10A as a standard current draw for drill motors (although this varies between each motor and could either be more or less), so you want a speed controller than can take at least 10A continuous, with the ability to handle higher currents for short (1-2secs) periods of time.
It's best to over-spec the speed controller slightly so that it's not performing right on its limits all the time. With the 10A example, I would use a controller with a 15A continuous current handling capability. Most speed controllers can cope with the larger bursts of current for a limited time but if you pull the high (stall) currents through your controller for long periods, that tends to be when the magic smoke comes out.
Re: From The Ground Up (Advice Thread)
http://architeuthis-dux.org/torquecalc.asp
One of the Banebots or Haborfreight setups should match your drills ...The amps pull may surprise you.
Re: From The Ground Up (Advice Thread)
Quote:
Taking a Bosch 35 motor as an example; the motor's maximum power output is 35W and you want to run it at 12V. The current (I) is power divided by voltage;
So 35/12 = 2.91
This means your motor will be drawing 2.91A when running at 12V.
Ummm ...NO
Nominal voltage 12 V
Nominal power 35 W
Nominal current 9 A
Maximum current 24 A
The 35 watts is nominal ( the power it can produce all day long without over heating ) and the efficiency at 35 watts OUTPUT is probably going to be around 30% ..meaning that your 2.91 amps is 30% of the actual amps consumed to give 35 watts OUTPUT ..you actually INPUT is 9 amps....with 6 amps being wasted as heat.
PEAK Watts is around 45 @ 12v.
http://www.boschmotorsandcontrols.co.uk ... /index.htm
http://www.boschmotorsandcontrols.co.uk ... nlinie.htm
Re: From The Ground Up (Advice Thread)
Oh well, explains why I had so many resits at uni :lol:
Re: From The Ground Up (Advice Thread)
Haha, thanks guys, I'll have a look on the weekend when I have time and hopefully buy a couple.
Re: From The Ground Up (Advice Thread)
Wait, I can't actually work out the amps from my 18v motors because I don't have the watts? And with one speed controller controlling two motors on each side I'm looking at 300 quids worth of speed controller if I'm using the bosch sample, that can't be right? Or am I being thick and two motors doesn't effect current draw? Urgh, I hate not being able to learn this stuff at school.
Re: From The Ground Up (Advice Thread)
Unfortunately doing the watts/volt = amps calculation doesn't give you a clear figure, beacuse no motor is perfect. Then there are different levels of draw for stall and nominal etc... And then over volting will increase the power of your motor and then your head explodes from it all!
I'm only just get my head round this now..
In short, if you can find a spec sheet for your motors (Google the product code) then that would give you the amperage's expected from your motor at stall, idle etc.....
Then put a fuse before your ESC rated at the maximum continuous current it can handle (or just below) so you don't destroy anything expensive.
And yes running 2 motors off your ESC connected in parallel will double the amp draw.
Re: From The Ground Up (Advice Thread)
Re: From The Ground Up (Advice Thread)
Just ordered a Sabertooth 25a dual, 6*8.4v batteries and a decent digital wall charger, should be decent for a 4*18v drill set up if my calculations were right
Re: From The Ground Up (Advice Thread)
Does anyone have any 4*4 wiring diagrams? It would be extremely useful as I'm not sure which parts work best in series or parallel. Thanks!
