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Power supply

math

Jan 2, 2017
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Hi guys,

First, sorry my english is not very good so if you don't understand, just ask I'll trh to explain differently!

Okay so I'm building a small CNC machine! I'm doing it the cheap way so I don't want to spend 400$ for the spindle! I've read some forum that you can use RC brushless motor with low kv. So I choose this one :
https://hobbyking.com/en_us/turnigy-aerodrive-sk3-6364-245kv-brushless-outrunner-motor.html

It's a 70A and 37V max motor! At the local electronic shop, they sell 12V 50A power supply and 24V 25A power supply... it's the same power at the end but I want to know if one is going to be better then the other! When I will take a pass with the mill it's gonna take a lots of amp! but I need a good rpm too!

And just to be sure, these are similar to 3phases AC but they can run on AC? need to buy a DC power supply and a ESC?

Thanks guys, hope.you can help me :)
 

Harald Kapp

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Nov 17, 2011
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At the local electronic shop, they sell 12V 50A power supply and 24V 25A power supply... it's the same power at the end but I want to know if one is going to be better then the other!
From the specs (70 A, 37 V) the resistance of the motor is ~0.53Ω.
At 24 V it will draw ~12 V / 0.53 Ω = 23 A.
At 24 V it will draw ~ 24 V / 0.53 Ω = 46 A.
The power drawn does not depend on the power supply but on the motor and the load from the CNC action. The power supply must be able to supply that power. If not, the power supply may shut down, limit the current or whatever, depending on the design of the power supply. Therefore the 24 V / 25 A power supply is not powerful enough for this motor at full load. The 12 V / 50 A on the other hand is a waste of ressources and will deliver only 1/4 of the power a 24 V / 50 A power supply could deliver.

need to buy a DC power supply and a ESC?
Yes, you'll need a controller explicitly suited for BDC motors. This controller will generate the necessary voltages for the motor from a DC source. The controller needs to be rated for the voltage and current you're going to use.
A typical ESC for model applications wil be controlled from the RC receiver. For your CNC application you will need a controller with manual speed control.

Note that the actual current drawn depends on the load. When not engaged the CNC machine will draw much less current than when the tool is engaged into working on the material.

I don't want to spend 400$ for the spindle!
Won't you need a spindle anyway, regardless of the type of motor?

For this application you're probably better of buying a mains operated motor. The guy on this woodworking website is using scavenged motors from all sorts of appliances for building woodworking machines. That same technology may work for you, too.
 

math

Jan 2, 2017
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Therefore the 24 V / 25 A power supply is not powerful enough for this motor at full load. The 12 V / 50 A on the other hand is a waste of ressources and will deliver only 1/4 of the power a 24 V / 50 A power supply could deliver.
I'm not sure if I understand. The 24V/25A isn't powerful enough but the 12V/50A is? sorry for not understanding english is my secondary language.
Thanks a lot! :)
 

Harald Kapp

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The 24V/25A isn't powerful enough but the 12V/50A is?
Mathematically 12V*50A=24V*25A. Your misconception is that the power delivered to the motor is defined by the voltage source. It isn't (as long as the voltage source is within its normal operating range, which I will assume for the moment).
Using a fixed resistance as a model for the motor (which in fact would have to be modeled in a more compelx way, but let us ignore this for the moment), th current through the motor is given by I=V/R. The voltage is the voltage from your source, so it would be either 12 V or 24 V.
The resistance I calculated from the motor limit parameters as R=37 V / 70 A 0 0.53 Ω.
The power at the motor is given by P=I*V and with I=V/R this is P=V²/R.
For V=24 V this results in P=(24 V)²/0.53 Ω = 1078 W
For V=12 V this results in P=(12 V)²/0.53 Ω = 272 W which is only 1/4 of the power available from 24 V.
Even if the 12 V source is able to deliver 40 A, the motor will not draw these 50 A, it will draw only 23 A, see above.

Compare this to mains power from a wall socket:
Assume the socket is fused with e.g. a 16 A fuse, so it is capable of delivering up to 16 A. When you plug a charger for a smartphone into this socket, the charger will not draw 16A! It will draw only as much current (e.g. ~50 mA for a 5 V / 1 A charger on 110 V mains voltage) as is required to charge your phone.
 

math

Jan 2, 2017
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Okay!! I'll have to find for a 24V power supply. If I look at your first reply, I'll need 46A at full load so I'll need a 24V/50A power supply. Is there any more options? no AC in these? even if they look like mini 3 phases.

and ken, I've look on Ebay I don't have a link but if you type "cnc 8mm collet" you'll get the collet that can be put on the shaft. not sure if I like the idea of the pressure screw with a 10k rpm device... the gravity point isn't gonna be in the middle of the shaft that will cause vabrition and use the bearings! I may tap the collet and use a die to cut a external thread on the shaft to make sure they're gonna balance! I prefer doing this because the device in your link is 300W and I'm not sure if 300W is gonna be enough to cut in aluminuium and I'm sure it will not cut steel. the motor I put in the post will take at full load 1000W and that may cut steel. Maybe not too ahaha but its 3x more powerful! maybe it have less torque... I will have to look ;) but thanks a lot for your help it's realy appreciate!
 

KMoffett

Jan 21, 2009
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I think you may be dreaming. Sticking that extension on looks like asking for problems. Even a small difference in shaft and bore diameters is going to create a lot of run out. At high speeds and small diameter cutters, I think you would regret that decision. Machining steel...you are talking a mill not a router.

Ken
 

math

Jan 2, 2017
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I'm studing in industrial maintenance, I have machining classes. The fitting issue isn't a big deal, I know it's chinesium stuff but as said above, I've gonna machine a peice a steel that will make them fit if they don't! Of course it's gonna take me a lot of time but I have a lot of time but not a lot of money! anyway these motor are so nice to have! If we take 80% eff, the mecanical output is like 1.2hp at 24V/45A! 1.2hp in the palm of your hand! ahahah I'll keep you inform of my final decision! I have to find a 24V/50A power supply!
 

Harald Kapp

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I'll need 46A at full load so I'll need a 24V/50A power supply.
I'm still not sure this is a good option. At 50 A you'lll need very thick wires to keep losses in chedck. Using this calculator I'd recommend 16 mm² or 6 AWG for the minimum wire diameter.

I don't think the motor you want to use will stand up to 1 kW permanent load. Datasheet specs are max. You'll have at least to add a generous amount of cooling, otherwise the motor is sure to die from overheating very quickly. There's a reason 1 hp motors are typically much larger: dissipation of excess heat.

Have you ever looked at used motors on ebay? You can get them for less money and without the need for a BLDC controller and DC power supply.
 

math

Jan 2, 2017
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I don't know if they can handle it too! But these doesn t have a lot of torque! I've talk with a CNC building compagny near my house and he told me that these just doesn't have enough torque for cutting hard stuff! they are very good for wood but not aluminuium and steel! He told me to go with an induction motor with a panetary gear to have more speed at the end since these AC usualy runs at only1750rpm! these are very torquy!! even at low speed! He also told me that electric saw motor would be good! I don't think I'll go DC, power supply aren't cheap and I don't want chine stuff! but still need to make a king of bearing holder because I dont want to use the motor's bearing! Thanks for your advice! of course 60A is a lot harder to handle than the 15A that came out from my wall :p
 
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