First, I want to state that I am new to the hobby. Quick question regarding the motor and ESC:

Why is the ESC supplied different than the recommended ESC when buying a replacement motor???

motors and esc company are different and there for sell them seperate from say buying a different motor but make sure you get a large enough esc tho that will handle the motor. what are you looking at for motor or esc

I am replacing the stock motor on my Apprentice 15e with a Turnigy G15.

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The current (amps) stated for the motor is the most that it can handle. But what it draws will depend on how many volts (ie. how many cells in your battery) and what size propellor.

If the replacement motor has the same 'kV' rating as the stock motor, and you are using the stock prop, then you will draw roughly the same amps, and therefore the stock ESC will be fine.

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Originally Posted by Harrow

The current (amps) stated for the motor is the most that it can handle. But what it draws will depend on how many volts (ie. how many cells in your battery) and what size propellor.

If the replacement motor has the same 'kV' rating as the stock motor, and you are using the stock prop, then you will draw roughly the same amps, and therefore the stock ESC will be fine.

KV rating is pertaining to rpm, it is not a big factor in amp draw of the motor. You can have a 1000 kv motor that draws 60 amps and a different size 1000 kv one that draws 15 amps. KV is how many rpm's the motor will turn per volt of output from your battery.Two otherwise identical motors with different kv ratings will draw slightly different number of amps but not usually significantly different.

Originally Posted by bobly

KV rating is pertaining to rpm, it is not a big factor in amp draw of the motor. You can have a 1000 kv motor that draws 60 amps and a different size 1000 kv one that draws 15 amps. KV is how many rpm's the motor will turn per volt of output from your battery.Two otherwise identical motors with different kv ratings will draw slightly different number of amps but not usually significantly different.

Just to nitpick a little!

KV=rpm per volt with no load

Originally Posted by bobly

KV rating is pertaining to rpm, it is not a big factor in amp draw of the motor. You can have a 1000 kv motor that draws 60 amps and a different size 1000 kv one that draws 15 amps. KV is how many rpm's the motor will turn per volt of output from your battery.Two otherwise identical motors with different kv ratings will draw slightly different number of amps but not usually significantly different.

Sorry, I have to respectfully disagree. With all other things being equal (battery and prop) the 'KV' rating of a motor is the single most important factor in determing how many amps it will draw. If you are using the same battery and propellor, but one motor has 30% higher 'kV' rating than another, then provided the motor is powerful enough, that motor will spin the propellor 30% faster and draw roughly 30% more current.

I'm still trying to figure it all out. I know the prop you hang on the motor is a huge factor, that's why I pointed out the 'no load' thing. Harrow, what you say about the relation of kv and power draw makes sense.

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Originally Posted by xmech2k

I'm still trying to figure it all out. I know the prop you hang on the motor is a huge factor, that's why I pointed out the 'no load' thing. Harrow, what you say about the relation of kv and power draw makes sense.

It's all pretty straight forward. If you only change one thing at a time, then you can use the following rules of thumb. (All of which are intuitve, but I have also verified by measurement with a high quality ammeter in many situations.)

Amps will vary with....

1. Changing number of cells. Go from 3 cell battery to 4 cell, amps go up by 33%.

2. Change the KV of the motor. Go from a 500 to a 600kV motor, amps go up 20%.

3. Change the pitch of the prop. Go from a 6x4 to a 6x5, amps go up by 25%.

4. Change the diameter of the prop. Go from 6x4 to a 7x4, amps go up by the CHANGE IN SURFACE AREA. Therefore the SQUARE of the change diameter, which in this case is (7^2)/(6^2) = 49/36 = 33% increase in amps.

So, if you change props from a 6x4 to a 7x5, the amps will increase by 25% for the pitch increase, and a further 33% for the diameter increase, which is a total 66% increase. (1.25 x 1.33 = 1.66) Go to 4 cells instead of 3, and can multiply that again x 1.33 = 121% increase. (1.66 x 1.25 = 2.21)

Now I won't argue with Retiredtech about using an ammeter; I always do! But when you are doing a scratchbuild and need to work out what components to buy when putting together your power system, (or just replacing a motor or upgrading up an existing model), you can get a very useful estimate using the guidelines above.

All this does depends on the motor and battery being able to deliver the power. Overloading the motor will result in the linear relationship of the calculations breaking down (as well as the motor )

Originally Posted by Retiredtech

Evidently ‘Watts’ is the most important factor in determining AMP draw:
1. Current draw is directly related to Watts when voltage is constant (Amps=Watts/Volts)
2. Power is rated in watts, not Kv (1 HP=745.7 watts)
3. When determining motor size, E-flite uses watts; nowhere is Kv mentioned.
4. Castle Creations says to use ‘watts required’ when ordering their motors.

But all of this is mute if one does what one should do … USE AN AMMETER and measure current at full-throttle!

RT

If you are choosing a motor for a new plane, then yes, you will determine the watts required based on the size of the plane and flying style (3d or scale), you then choose battery voltage and propellor size to suit. However, the watt rating of the motor will still not determine the amps that it draws in a given installation any more than the amp rating of the motor will determine how many amps it draws. Put a very small propellor on a 500W motor and it will only draw a couple of amps. It is the kV of the motor, the battery voltage and the propellor size combined that determine how much power (or how many amps) will be drawn by the motor.

In this context, where we are discussing a suitable replacement motor, the battery voltage and propellor size remain constant, and the kV rating of the motor is the variable that determines the amps drawn.

That's great info Harrow! I'm copying that and saving it for future reference. Now if only someone could explain EDF's that simply!

+1 on the wattmeter! If you're gonna be the least bit serious about this hobby, you need one.

All these posts are the reason I just 'buy & fly'... simply Easier and less taxing on this old brain.