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  #1  
Old 11-17-2012, 08:13 PM
OSUSeniordesign OSUSeniordesign is offline
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Current Limit for Motors

I know that electric motors have a current limit on them in reality. Is there a way to set a current limit on electric motors in RealFlight? This is for a design class in which we are limited to 20 Amps.
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Old 11-17-2012, 08:19 PM
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For the electric motor you can adjust:
KV
IO (AMP)
Motor resistance (Ohm)
For the battery you can adjust:
Internal Resistance (Ohm)
Capacity (mAh)
And you can set a discharge curve.
For the ESC you can adjust:
Current limit (Amp)
Resistance (Ohm)
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Old 11-18-2012, 02:33 AM
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Actually in reality motors DO NOT have a "current limit" which is why if you prop it wrong you'll blow the motor. There is no limit to how much current the motor will attempt to draw.

Real motors can be said to have a "heat" or "thermal ceiling" instead.

If you let the motor draw too much current, you'll burn out the coils, destroy the glue holding the magnets or demagnetize them.

Realflight doesn't model this.
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Last edited by opjose; 11-18-2012 at 11:40 PM.
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Old 11-18-2012, 09:53 AM
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DO not forget motor weight can also be adjusted. This is absolutely essential when setting up a custom motor. Otherwise your start adding lead that does not belong. I've drawn 950watts on a eflite power15 motor. Rated for about 1/3 that much. It flew a plane for about 2 minutes before the ESC started shutting down due to overheating. Seriously hot stuff.
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Old 11-18-2012, 11:41 PM
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Good magnets and magnet bonding agents used on that motor.

I've had magnets delaminate from the bells when the motors were run at 20% over rated limits for 30 seconds.
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Old 12-04-2012, 02:01 AM
OSUSeniordesign OSUSeniordesign is offline
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Thanks for all the responses. Haven't checked in a while because of finals but I have a few more questions.

For real RC motors that come with a specific power rating, is that the max it can draw or is that what it will provide under its operating current?

If we are trying to minimize the current can we step up our voltage to drop the current it's running at for a given power or am I completely delusional?
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Old 12-04-2012, 08:23 AM
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No, you are on the right track. Current is a better indication of a better-quality motor. I always stay within my current limitations rather than voltage. Example: My Eflite motor (power 15) is advertised at 2-4S. Continuous current at 34 amps. max amps is 42 for 15 seconds. prop range advertised is 10x6 - 13x6.5. I'm currently running a 14x4 prop on 3S and barely drawing 30A full-throttle. Its all about sizing your battery and propping accordingly to stay within the power limitations of the motor.
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Old 12-04-2012, 10:53 AM
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Quote:
Originally Posted by OSUSeniordesign View Post
If we are trying to minimize the current can we step up our voltage to drop the current it's running at for a given power or am I completely delusional?
On this part you are however delusional. If you up the voltage to the motor, it will up the current, not drop it. That in turn will up the overall power.
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Old 12-04-2012, 11:59 AM
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Quote:
Originally Posted by Madratter View Post
On this part you are however delusional. If you up the voltage to the motor, it will up the current, not drop it. That in turn will up the overall power.
If you raise the voltage, your Amp's will decrease for the same amount of watt's which is your power output. Simple formula is Volts x Amps= Watt's.

22v x50A = 1,100w output
36v x 30.5A= 1,100w output

Still getting the same amount of work (watts) done but less amps by increasing the voltage.
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Old 12-04-2012, 12:10 PM
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Quote:
Originally Posted by csgill75 View Post
If you raise the voltage, your Amp's will decrease for the same amount of watt's which is your power output. Simple formula is Volts x Amps= Watt's.

22v x50A = 1,100w output
36v x 30.5A= 1,100w output

Still getting the same amount of work (watts) done but less amps by increasing the voltage.
That is true. That is why I said the watts would go up.

The relevant fact here is that the motor has an internal resistance and Ohm's law applies.

I * R = V or in the form we care about here I = V / R

So as you increase V you increase I (current) and you likewise Increase power. This is why your 7s version of my F2b has more oomph (power).

What all this means in practice is that if you wish to increase Voltage and decrease amperage to get the same power, you will need a motor with more internal resistance.
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Old 12-04-2012, 02:28 PM
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That is why there are two methods to extend flight time.
1: Bigger battery (mAh) but gives more weight
2: lower Kv rating on the motor and prop up but its a real balancing act on which prop to use.
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Old 12-04-2012, 02:39 PM
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Quote:
Originally Posted by OSUSeniordesign View Post
Thanks for all the responses. Haven't checked in a while because of finals but I have a few more questions.

For real RC motors that come with a specific power rating, is that the max it can draw or is that what it will provide under its operating current?
Neither.

The specific power rating of a motor is a RECOMMENDATION by the manufacturer.

The motor is said to be able to tolerate the power level the manufacturer is stating.

If you change the prop, you can cause the motor to exceed the ratings given.

In turn how the motor reacts to this, is the result of a combination of things.

e.g. if Cooling is VERY good, you may be able to get away with higher draws for longer periods of time...

However in most cases if you exceed the recommended power draw levels, you'll blow the motor due to heat.

Conversely there is usually NO problem if you prop the motor to draw LESS than the recommended wattage/amperage.

Again, remember it is the PROP that determines how much power the motor draws.

Quote:
Originally Posted by OSUSeniordesign View Post
If we are trying to minimize the current can we step up our voltage to drop the current it's running at for a given power or am I completely delusional?
The latter...

As already mentioned because

watts = amps x voltage

The motor will spin faster because of the increased voltage causing the PROP to demand more power from the motor.

---

You can however decrease the prop size after increasing the voltage, to try to keep the power draw within the manufacturer's recommended range.

That is why a watt meter is essential.

However there is no way of making the motor handle more power, short of pulling the windings and putting on larger wire with fewer turns... but even that produces limited improvements.

Effectively the max amperage at the stated voltage ( aka Wattage ) is the most you can get.


That said I've purchased Receiver Ready planes that say they are equipped with a 700watt motor, but then I check the specs on the motor from the manufacturer only to find the included motor is rated for 900 watts. Needless to say I was quite happy about the vendor "underrating" the included electronics.... I wish more vendors would do the same.
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Last edited by opjose; 12-05-2012 at 01:29 PM.
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