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Author Topic: which amps?  (Read 4026 times)

bikerdude666

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which amps?
« on: February 07, 2012, 10:50:28 pm »
I'm trying to choose a battery pack and a correctly sized ESC to go with a motor in 1 of my models, this is the motor I've got... http://www.cornwallmodelboats.co.uk/acatalog/graupner-7306.html

I don't know which of the many numbers stated there it is that I've got to consider when choosing the correct ESC, I've got a couple of 10amps, a couple of 25amps and a 20 amp ESC, but don't know which will be best.

Also how do you know how much power the motor will use? As I want a decent run time, but don't want unnecessary weight from overly large battery packs....

Thanks for any help regarding what numbers it is I need to look at to find the answers I want.
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Shipmate60

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Re: which amps?
« Reply #1 on: February 07, 2012, 11:14:33 pm »
The stall current is quoted at 85 Amps.
The running current will depend on any binding of the shaft, inaccuracies in the mounting and the prop size and pitch.
If you load it more it will consume more current.
What type of craft and prop is it?
You can use any of the ESC's quoted as long as you fuse it at least 5 amps less than its rated current.
If it is not bib enough it will blow the fuse and you will be left with a dead boat!!

Bob
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Re: which amps?
« Reply #2 on: February 08, 2012, 08:27:05 am »
That's quite a pokey 500 size motor you have there, so I'm guessing you're going for a bit of speed. I would use at least a 20A ESC.

I use microgyros esc's for brushed motors, excellent controllers, very small and extremely well made. Will take 25A continuous.

http://www.microgyros.com
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bikerdude666

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Re: which amps?
« Reply #3 on: February 08, 2012, 11:34:38 am »
OK thanks, I'll try it with a 20 amp. Thanks for the microgyros link, but I've got several Mtroniks vipers, also got a 40amp viper somewhere as I ordered a 20amp and they sent a 40!

next silly question, where does the fuse go, between battery and esc, or esc and motor? And how do you decide which ESC you should use? As I don't want to have to pester you all everytime I do a new model....

Also is there a way I can get a rough estimate of how long a 1500mah 7.2V battery pack would last? So I can try and get an appropriately sized pack.



Ps. shipmate60, its for my duck, so 'hull' is flat bottomed and quite flat fronted, but will sit in the water stern down a little, hoping to get the front to raise a bit so it's not like trying to push a brick through the water...
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john s 2

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Re: which amps?
« Reply #4 on: February 08, 2012, 01:34:18 pm »
A 1500ma battery should give 1.5 amps for one hour, 3 amps for half hour, 6 amps for 15 min, so about 15  to 20 min run time may be possible. The word should is used because this assumes a perfect battery. no allowance for aging etc. By using a known load on a fully charged battery it is possible to check the capacity. If in doubt take a spare or two. John.
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bikerdude666

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Re: which amps?
« Reply #5 on: February 08, 2012, 01:37:10 pm »
right, well I don't have batteries for it, thats the last thing I need to buy, so it looks like I'll be going with twin 1500mah split packs. Thanks.
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john s 2

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Re: which amps?
« Reply #6 on: February 08, 2012, 02:23:10 pm »
As youre buying new batteries. May i suggest the Component shop? Depending on cell size, you may be able to buy a bigger capacity. John.
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bikerdude666

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Re: which amps?
« Reply #7 on: February 08, 2012, 03:42:06 pm »
Indeed, thats where I was going, used them previously, fast service, good prices and good batteries. They've got 7.2V 1500mah 'split packs' which will sit nicely either side of the prop shaft....

hopefully have it done by tomorrow, though will be waiting a while for batteries, but will do a bath test with another battery pack and upload a video...
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bikerdude666

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Re: which amps?
« Reply #8 on: February 08, 2012, 09:12:39 pm »
Quick update, got a slight leak from around the prop shaft, so had to try and seal that again (4th time lucky). However I wired everything up, it works fine using an Mtroniks viper 20, however (and I have no idea why) everytime I turn off my GiantCod radio set, and then turn it back on, the motor goes into full ahead, and you have to move the stick fully forwards, then fully back and then centre and its fine, like it needs setting up everytime! Anyway I had it in the sink and turned it on, motor went crazy and I managed to empty about 2 litres of water out of the sink and onto the surrounding worktops, an then floor! So it certainly is pretty powerful!
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john s 2

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Re: which amps?
« Reply #9 on: February 08, 2012, 09:30:49 pm »
I assume that you did set up your mtronic to start with? With trans on and throttle neutral. Switch on rec then within 2 sec press button to set neutral. Then full throttle press set button again , then pull back to full reverse. Sorry if you already know this . If still giving trouble a quick call to mtronics should resolve. John.
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RaaArtyGunner

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Re: which amps?
« Reply #10 on: February 08, 2012, 09:36:40 pm »
Quote from: John s on February 08, 2012, 01:34:18 pm
A 1500ma battery should give 1.5 amps for one hour, 3 amps for half hour, 6 amps for 15 min, so about 15  to 20 min run time may be possible. The word should is used because this assumes a perfect battery. no allowance for aging etc. By using a known load on a fully charged battery it is possible to check the capacity. If in doubt take a spare or two. John.

John,

Am trying to understand why with bigger capacity, ie doubling the battery size, from 1.5 to 3 to 6 amps, the run times are halved.
 Or is the example about drawing double the load, 3 & 6 amps from the same sized battery, namely 1500ma, then the run times will halve.

Thank you.
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Shipmate60

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Re: which amps?
« Reply #11 on: February 08, 2012, 09:54:00 pm »
That is right for the GiantCod set.
It is originally set up for an aircraft throttle so needs a little "wiggle" to programme neutral.
I usually do this before switching on the receiver and all is fine.

Bob
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john s 2

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Re: which amps?
« Reply #12 on: February 08, 2012, 11:07:30 pm »
Im sorry that i did not make myself clear. What i was talking about was taking increased amps from the same capacity battery. Its the same as having a pint, the quicker you drink  the quicker it goes (sadly). The same with a battery , the more amps you draw the quicker it is discharged. John.
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RaaArtyGunner

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Re: which amps?
« Reply #13 on: February 08, 2012, 11:30:11 pm »
Quote from: John s on February 08, 2012, 11:07:30 pm
Im sorry that i did not make myself clear. What i was talking about was taking increased amps from the same capacity battery. Its the same as having a pint, the quicker you drink  the quicker it goes (sadly). The same with a battery , the more amps you draw the quicker it is discharged. John.

Thanks  :-)) :-)) :-))
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bikerdude666

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Re: which amps?
« Reply #14 on: February 09, 2012, 06:22:27 pm »
Quote from: shipmate60 on February 08, 2012, 09:54:00 pm
That is right for the GiantCod set.
It is originally set up for an aircraft throttle so needs a little "wiggle" to programme neutral.
I usually do this before switching on the receiver and all is fine.

Bob

Ah thanks, so if in future I turn the TX on and do the full forward, full reverse, back to the middle, then turn the RX on, the motor won't go crazy? That's good to know.
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Dreadstar

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Re: which amps?
« Reply #15 on: February 10, 2012, 12:00:02 am »
Bikerdude,you will need to turn on the RX as well before you 'wiggle' the sticks.
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Netleyned

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Re: which amps?
« Reply #16 on: February 10, 2012, 06:06:14 am »
If it is a Giant Cod T4U wiggle the stick before turning Receiver on.

Ned
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bikerdude666

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Re: which amps?
« Reply #17 on: February 10, 2012, 08:30:08 am »
Quote from: netleyned on February 10, 2012, 06:06:14 am
If it is a Giant Cod T4U wiggle the stick before turning Receiver on.

Ned

Not absolutely sure, but I think it says something like that on the front. It was the cheapest 4 channel set that giant cod sell. Will have to remember that so I don't have propellors spinning very fast uncontrollably out of the water.
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Netleyned

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Re: which amps?
« Reply #18 on: February 10, 2012, 08:49:07 am »
It will be T4U or T4EU but they are the same animal.

Ned
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