Nov 13, 2013, 03:17 AM Registered User Joined Sep 2013 6 Posts What decides the Maximum Current Draw? Hi, If a motor is connected to an ESC that is rated for more than the motor requirement then what decides the maximum current that will be drawn by the combination? Let me clear it up with an example: Suppose I have an ESC rated for 50Amps and a compatible motor rated for 20 Amps Max now as we know ESCs control max current based on the width of pwm pulse they receive on their signal input or in other words they allow current as commanded by throttle channel. so now if I move the throttle to max would the ESC dump full 50 Amps to the motor (Obviously burning it ) or would it limit it to 20Amp (As defined by winding resistance of the motor)? So far I have a feeling the no matter how much you increase the throttle the max current draw depends on the maximum current draw by motor and not by max rating of the ESC. But I don't want to burn up my motor verifying it! does anyone know ?
 Nov 13, 2013, 03:35 AM Wake up, feel pulse, be happy! United States, AK, Fairbanks Joined Aug 2009 11,814 Posts Neither. The current drawn by the motor depends on the shaft load (prop) and input voltage. Current is never "pushed" through a system. The ratings you see (e.g. 50A ESC, 20A motor) are just ratings. They tell you how much current you can safely pass through the device without burning it out. They do NOT tell you anything about how much current will actually flow. You might have an ESC rated for 50A and a motor rated for 20A, but with no load on the shaft you might see a peak current of only 0.75A. Conversely, if you hold the motor's shaft still, the current might be hundreds of amps, immediately burning out the motor, ESC, or both. The actual winding resistance of a motor is extremely low; the motor's induced back voltage is what results in the net drop (and hence current draw) being low under a no-load condition and very high when the load is large. A greater load forces the motor to spin more slowly, reducing the back voltage relative to the input and increasing the current draw.
Nov 13, 2013, 06:13 AM
Registered User
Joined Sep 2013
6 Posts
Quote:
 Originally Posted by C₄H₁₀ Neither. The current drawn by the motor depends on the shaft load (prop) and input voltage. Current is never "pushed" through a system. The ratings you see (e.g. 50A ESC, 20A motor) are just ratings. They tell you how much current you can safely pass through the device without burning it out. They do NOT tell you anything about how much current will actually flow. You might have an ESC rated for 50A and a motor rated for 20A, but with no load on the shaft you might see a peak current of only 0.75A. Conversely, if you hold the motor's shaft still, the current might be hundreds of amps, immediately burning out the motor, ESC, or both. The actual winding resistance of a motor is extremely low; the motor's induced back voltage is what results in the net drop (and hence current draw) being low under a no-load condition and very high when the load is large. A greater load forces the motor to spin more slowly, reducing the back voltage relative to the input and increasing the current draw.

So how much throttle should be safe to apply?
100% 80% or 60%?
 Nov 13, 2013, 07:43 AM WAA-08 THANK FRANK! Las Cruces, New Mexico, United States Joined Jun 2002 6,721 Posts -54%. No more, no less.
Nov 13, 2013, 09:27 AM
Jack
USA, ME, Ellsworth
Joined May 2008
15,601 Posts
Quote:
 Originally Posted by allahjane So how much throttle should be safe to apply? 100% 80% or 60%?
The key signal if you have no other instrumentation is the temperature of the windings. The rate at which the temperature rises, and the peak temperature, will vary with the load (i.e., the size of the prop).

So if you put a prop on and go to full throttle for 10 seconds, then check for heat with a finger, you can get a feel for how much of a load you are putting on the motor. The best place to check is the back plate of the motor as the heat from the windings will go into the bearing tube and then the back plate.

Keep doing progressively longer tests (10s, 20s, 30s, etc.) and when you get to the point where you cannot keep your finger in contact with the base plate because it is too hot (that will be about 130F/54C) that is about as hot as you want the motor to get. And it will not have been damaged yet.

To test for continuous full throttle operation with the best power you would have to find the load that will let the motor get hot but have a stabilized (i.e., not still rising) temperature. When I do that, I want to see the temperature leveled off at around 120-125F or so and not rising at all after 2 to 3 minutes of full throttle operation.

In flight the propeller will generally (but not always) unload a little bit, the current will drop a little, and the motor will probably have better cooling air flow. So anything that is OK in a static test should normally be OK in flight unless the cooling air flow is more constrained.

Jack
 Nov 15, 2013, 03:57 PM Dean Pappas central NJ Joined Feb 2008 6 Posts Hello, The real answer is buried in the beginning of Jack's response; get some instrumentation! in this case, you want a current meter, or Wattmeter so that you can see how many amps you are pulling when trying out different propellors. Blindly trying heavy prop loads can destroy good equipment, senselessly. Regards, Dean Pappas
 Nov 15, 2013, 04:42 PM Registered User Mexico, BC, Mexicali Joined Aug 2004 4,814 Posts Greatly influences the motor's ability to dissipate heat, Here we include the installation and type of model that is installed. A standard motor in a 3d environment, can dissipate up to 1 watt per gram of weight, but in an atmosphere of EDF that can dissipate up to 3 watts per gram of weight. Manuel V.
Nov 16, 2013, 01:36 PM
Registered User
Chattanooga, Tennessee, United States
Joined May 2003
26,695 Posts
Just to add to the comments on a motor's inability to dissipate more than about 1W/g, here is a temperature plot for a 30 second static run of a Littlescreamer Park Jet motor running at well below the maker's "optimistic" max. amp recommendation....... even while running the motor temperature (red line) keeps increasing ("thermal runaway").. once the run was stopped motor temperature kept on increasing, reaching maximum a minute after the run.

And a second graph for a Turnigy 3530 showing the same (dangerous) pattern.

Cheers, Phil

Images

Last edited by Dr Kiwi; Nov 16, 2013 at 01:41 PM.
 Nov 16, 2013, 06:16 PM Wake up, feel pulse, be happy! United States, AK, Fairbanks Joined Aug 2009 11,814 Posts Phil, were those numbers recorded in flight or on the bench?
 Nov 17, 2013, 09:59 AM Registered User Chattanooga, Tennessee, United States Joined May 2003 26,695 Posts Just static on the bench.. obviously worst-case scenario as far as cooling goes. I assume that things would usually get better in-flight.... but if you have your motor inside a cowling, then this might be what you get.
 Nov 17, 2013, 02:51 PM 2012 NZ Speedcup - 231 MPH New Zealand, Tasman, Richmond Joined Mar 2006 1,722 Posts We're running outrunners at our speedcup meetings cooled by being mounted on the front of the plane at 10+Watts/g, so there can be a vast difference between what is possible on the bench and in flight. Having said that, we're getting 300+ km/h airflow through the motor so Dr Kiwi's rule of thumb is the best place to start from.
 Nov 17, 2013, 06:17 PM Jack USA, ME, Ellsworth Joined May 2008 15,601 Posts Are those flown in laps? And at pretty much continuous full throttle? What is the duration at full throttle? Jack Latest blog entry: Motor Rewinding & Modification...
 Nov 17, 2013, 06:26 PM 2012 NZ Speedcup - 231 MPH New Zealand, Tasman, Richmond Joined Mar 2006 1,722 Posts No, passes through speed traps, so usually up to say 6 seconds WOT then throttle off as you do a reversal and repeat.
 Nov 18, 2013, 09:24 AM Jack USA, ME, Ellsworth Joined May 2008 15,601 Posts OK, that explains why they can survive at 10W per gram. It is sort of like those gliders that have the throttle on an on/off toggle switch. They are OK for 20 seconds and at 25s the motor is burned up. Jack Latest blog entry: Motor Rewinding & Modification...
Nov 18, 2013, 07:23 PM
Registered User
Australia, VIC, Melbourne
Joined Feb 2010
1,355 Posts
Quote:
 Originally Posted by Dr Kiwi once the run was stopped motor temperature kept on increasing, reaching maximum a minute after the run.
This has to be a result of where you are measuring temp (I assume the probe is not inside the stator?).

But yes, the only reason most of these motors survive at anything like the power they do is because of the short duty cycles at full power. Consider a 1 HP industrial motor for a size comparison (a good one, not an inefficient AC induction version).