What Determines a Motor Power

[Nigel Heather]

Say I have two motors, one a 3648-900 and the other a 4050-900.  
 

The 4050 is quoted as being more powerful, more watts but if I attach the same prop and connect the same battery, the prop will rotate at the same speed because they motors have the same kV.

 

So what cause the 4050 to be more powerful than the 3648.  Is it because it can swing a bigger, pitchier prop.  But if you swing the same prop is it a waste, are they same power motor in that case?

[Shaun Walsh]

I'm no expert but I'll give it a go:

The 4050 motor has approx 60% larger volume than the 3648, more room for more/thicker windings.

The greater length and diameter allow larger magnets to be fitted producing greater torque.

Greater torque will allow you to turn a bigger prop but if you put a small prop on, it will not exceed its kV x V max rpm, it will just draw fewer amps and develop less thrust.

Edited October 4, 2023 by Shaun Walsh

[Nigel Heather]

Yep what I was thinking but my question is if they are both capable of swinging a 12x7 within spec will they create the the same thrust (flight power), I assume they will and will they draw the same current (electrical power).

 

So in other words will they both be the same - you are just taking full advantage of the 4050.

 

Cheers,

 

Nigel

[Shaun Walsh]

9 minutes ago, Nigel Heather said:

Yep what I was thinking but my question is if they are both capable of swinging a 12x7 within spec will they create the the same thrust (flight power), I assume they will and will they draw the same current (electrical power).

 

So in other words will they both be the same - you are just taking full advantage of the 4050.

 

Cheers,

 

Nigel

If you post details of the motors and how many cells in the battery it can be worked out.

[Graham Bowers]

15 minutes ago, Nigel Heather said:

Yep what I was thinking but my question is if they are both capable of swinging a 12x7 within spec will they create the the same thrust (flight power), I assume they will and will they draw the same current (electrical power).

 

So in other words will they both be the same - you are just taking full advantage of the 4050.

 

Cheers,

 

Nigel

My understanding is "almost".

 

The Kv value represents what the motor could spin at, per volt, in a no load condition. It's synchronous speed.

 

When a propeller load is applied, the prop slows the motor down from it's synchronous speed. The difference between the synchronous speed and actual speed being known as "slip".

 

My expectation is that the bigger motor, having greater torque capability, will experience less slip.

 

Therefore I'd expect the 4050 to spin the notional 12x7 prop a little faster than the 3648. That'd take a bit more current.

 

I imagine we're talking 5% ish difference, though.

Edited October 4, 2023 by Graham Bowers

[Cuban8]

39 minutes ago, Graham Bowers said:

My understanding is "almost".

 

The Kv value represents what the motor could spin at, per volt, in a no load condition. It's synchronous speed.

 

When a propeller load is applied, the prop slows the motor down from it's synchronous speed. The difference between the synchronous speed and actual speed being known as "slip".

 

My expectation is that the bigger motor, having greater torque capability, will experience less slip.

 

Therefore I'd expect the 4050 to spin the notional 12x7 prop a little faster than the 3648. That'd take a bit more current.

 

I imagine we're talking 5% ish difference, though.

Not an easy thing for many folks to get their head around. When I specify my motors from data sheets I allow a slip figure of around 25-30% in rpm from no load to desired prop working speed and hence motor/prop power.  Although seemingly somewhat arbitrary % from an engineering standpoint, I find that the final motor/prop/battery/ predicted static thrust/current figures to always be pretty close to my original estimates.

Certainly within 10%, which I'm happy with considering the many variables that are involved.The only exception has been one motor from a well known supplier that turned out to be way off its advertised KV figure and gave a bit of head scratching for a while. Probably not labelled correctly at the factory so a simple error - used it for another project.

 

 

Edited October 4, 2023 by Cuban8

[Nigel Heather]

I notice that some motors quote a minimum prop - what does that mean - what would happen if you put a lesser prop on the motor?

 

Cheers,

 

Nigel

[Nigel Heather]

2 hours ago, Shaun Walsh said:

If you post details of the motors and how many cells in the battery it can be worked out.

Ahh! I see, the small one goes up to 4S but the bigger one goes to 7S.

 

So say I wanted to run on 4S, could I get the bigger motor, it would then run the same as the smaller motor but under-utilised but at a later date if I wanted to increase the power I could use 5S on the bigger motor.

 

So getting the bigger motor would be more flexible for the future. 

[Outrunner]

To put it simply without going into the numbers the bigger the magnets and the bigger the windings the bigger the power. The numbers allow you determine the actual power output and what speed it will run at. Try putting a 15" prop on a 10 gram motor it ain't going to turn. Put a 4" prop on a 60mm motor it won't go far.

[Mike Blandford]

The larger motor has larger magnets so a stronger magnetic field. To achieve the same kv value, it will therefore require fewer turns of the wire, so a shorter length of wire. Since there is also more space available for the wire turns, the wire is thicker. Shorter and thicker wire means less resistance so less loss.

Using the same battery and propeller, the larger motor will spin a bit faster as more of the power from the battery will reach the propeller. Since both motors have the same kv, the same current will produce the same torque. To spin the propeller faster will require more torque, so more current.

I would estimate (using Motocalc) a 5% increase in RPM and a 10% increase in current.

 

Mike

[PatMc]

The motor is really only a power converter. The power it extracts from the battery is basically governed by the size of the magnets & the weight of copper in the windings. In practice you can estimate a motor's power output from any given battery by it's total weight.

To a lesser extent there are some design features that effect the mechanical & electrical efficiency but these generally result in diminishing returns relative to cost.      

Edited October 4, 2023 by PatMc

[Frank Skilbeck]

15 hours ago, Nigel Heather said:

Ahh! I see, the small one goes up to 4S but the bigger one goes to 7S.

 

So say I wanted to run on 4S, could I get the bigger motor, it would then run the same as the smaller motor but under-utilised but at a later date if I wanted to increase the power I could use 5S on the bigger motor.

 

So getting the bigger motor would be more flexible for the future. 

Basically yes, the penalty now is more weight and cost. I've done this in reverse, used an existing motor that was larger than required, but I had it so why not. 

[Geoff S]

48 minutes ago, Frank Skilbeck said:

Basically yes, the penalty now is more weight and cost. I've done this in reverse, used an existing motor that was larger than required, but I had it so why not. 

 

If you're converting a glow design to electric, the extra weight of a bigger motor is often an advantage in my experience.

[MattyB]

Lots of good posts above explaining some of the technical detail, but for those who prefer simple rules of thumb...

• The motor Kv, prop and input voltage (which is directly influenced by the internal resistance of the pack) are the primary factors in determining how much current is pulled and power is generated at full throttle. However...
• The max power a given combination of these factors will generate is NOT the same as the maximum power the motor can sustain!
• To determine that for the a typical low end brushless motor in a standard tractor sport model, take the mass of the motor in grams, multiply x3* - the figure you have is a conservative estimate of the maximum power in Watts the motor can safely sustain.
• Carrying a bigger motor and running it at less it's maximum peak sustained power has no real negatives in a practical sense other than the additional noseweight (whcih in many cases will be useful). Doing the opposite (i.e. running a motor that is too small beyond it's max sustained peak power capability) definitely does!

* - For higher end motor and in other types of models (e.g. EDFs) where cooling is very good, this multiplier can be increased safely, but stick with 3 until you have sufficient expereince to know when you can push the envelope safely.

 

Edited October 5, 2023 by MattyB

[PatMc]

Sorry Matty, that's not correct. There is no absolute max power for a motor, however there is a max current it can take. Since power is the product of voltage & current the max power can only be stated with reference to the voltage that is being supplied.

The max current that a motor can take is subject the length of time it's applied, how well it's cooled etc. 

The voltage will be limited to the practicality of the prop size at the motor's kv. 

[MattyB]

32 minutes ago, PatMc said:

There is no absolute max power for a motor, however there is a max current it can take. Since power is the product of voltage & current the max power can only be stated with reference to the voltage that is being supplied.

 

Fair point, I should have stated "The max power a given combination of these factors will generate is NOT the same as the maximum current the motor can sustain!"

[Frank Skilbeck]

All depends on how.good the motor wiring is at stopping magic smoke leaks 😀

[Simon Chaddock]

To answer Nigel's direct question.

With the same battery voltage, the same prop and both motors having the same kV then they will each run at the same rpm so generating the same thrust.

There will be a small difference due to different mechanical and electrical losses providing the smaller motor is not over loaded by the prop. The only real diufference will the motor's weight.

 

To cover the point about a 'minimum' prop. This will depend on the motor's structural "burst" rpm. Most brushless motors can be safely run without a prop. They are in principle constant speed motors. The difference between no load and most effcient 'power' rpm is only about 15%. Note this does not apply to series wound brushless motors which can easily exceed their burst speed with no load. If a brushless motor is being used within its voltage specification its burst rpm will not be attainable however run a 4s specified motor on 12s and you likely wil!

[PatMc]

5 hours ago, Simon Chaddock said:

To answer Nigel's direct question.

With the same battery voltage, the same prop and both motors having the same kV then they will each run at the same rpm so generating the same thrust.

There will be a small difference due to different mechanical and electrical losses providing the smaller motor is not over loaded by the prop. The only real diufference will the motor's weight.

No they won't, the larger motor will run faster & take more current. See Mike Blandford's reply above.

 

5 hours ago, Simon Chaddock said:

 

To cover the point about a 'minimum' prop. This will depend on the motor's structural "burst" rpm. Most brushless motors can be safely run without a prop. They are in principle constant speed motors. The difference between no load and most effcient 'power' rpm is only about 15%. 

 

Note this does not apply to series wound brushless motors which can easily exceed their burst speed with no load. If a brushless motor is being used within its voltage specification its burst rpm will not be attainable however run a 4s specified motor on 12s and you likely wil!

What point about minimum prop was made ? Where has a "motor's structural "burst" rpm" ever been specified ? 

As far as the motor's concerned there is no minimum size prop but you won't get far powering, for example, a Wot 4 with a 4" prop.

All of our brushless motors can be safely run without a prop.

 

We don't use series wound motors. Most of our brushless motors are "Delta" wound a few are "Star" (AKA "Y") wound.

From the electrical POV there really isn't such a thing as specific voltage limitation but it wouldn't be practical to use a voltage that can only drive an impractical size prop within the current limit of the motor.

  

 

Edited October 7, 2023 by PatMc

[Simon Chaddock]

.Nigel wrote "I notice that some motors quote a minimum prop". That is the where the regerence came from and he firther asked what would happen if a smaller one was used.

No reference was made to practicality.

 

[PatMc]

OK, I missed the minimum prop size question but answered it anyway and addressed the follow on question by using an extreme example to point out the obvious impracticlity that could result

[Geoff S]

5 hours ago, PatMc said:

We don't use series wound motors. Most of our brushless motors are "Delta" wound a few are "Star" (AKA "Y") wound.

From the electrical POV there really isn't such a thing as specific voltage limitation but it wouldn't be practical to use a voltage that can only drive an impractical size prop within the current limit of the motor.

  

 

 

Not sure what a 'series wound' brushless motor might be. Series/shunt or compound wind refers to brushed motors with wound poles (ie not permanent magnets).  IIRC series wound have their greatest torque at zero rpm and were typically used on the trolley buses that replaced the trams on the route that passed our home, though I guess there was also some element of shunt winding, too.

 

As Pat writes, the voltages we typically use are fairly low and unlikely to break down the insulation in any of our motors. The main limiting factor is the current and the resistance of the wires which contributes to the heat generated and hence the inefficiencies that may lead to failure.

[Dickw]

6 hours ago, PatMc said:

................... Where has a "motor's structural "burst" rpm" ever been specified ? ............................... 

 

It is not common I agree, but several brands do specify a maximum rpm - mainly on geared in-runner motors. For example Jeti and Leomotion specify 70,000 rpm for the sizes I use, and Neu claim similar values depending on the motor.

It might sound silly rpm, but I have briefly seen over 80,000 rpm (on board data logger) when a prop came off and I know of in-runner rotor failures with the magnets breaking free under similar circumstances.

 

Not likely to be an issue for most users though, and hence the lack of "max rpm" specified for most motors - although I guess that is what max voltage is probably aimed at rather than any insulation issues.

 

Dick

[PatMc]

The speed of series wound motors is roughly inversely proportional to the current, if they are run with little or no load they can reach dangerously high speeds. Typical damage would be the commutator disintegrating then the debris flying from it. These motors were (are ?) normally solidly connected to their load, never belt driven.

 

Ref my 60 year old copy of Electrical Technology by Edward Hughes & my memory. If only I could as easily recall where I last left my specs. 🙄  

[Martin Harris - Moderator]

Bit confused…

 

Can you have a series wound brushless motor?  With a commutator?